***PET RFQ Phase I***

***Project Report***

***Volume I: RFQ Accelerator***

*Edited by Ralph J. Pasquinelli*

*4/9/98*

**Contents**

**Chapter** **Page**

Preface\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....
3

Executive
Summary\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
4

Introduction
\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
5

Chapter
Summaries\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....
17

1\. Pre 1995 Project
History\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....
20

2\. Ion Optical Design of the RFQ PET
Accelerator*\...\...\...\...\....*\...\.... 26

3\. RFQ Tuning Systems
\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\... 54

4\. Low Level RF
(LLRF)\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
63

5\. High Level RF
(HLRF)\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
68

6\. Charge
Doubler\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....
73

7\. Machine Operatons and
Performance\...\...\...\...\...\...\...\...\...\...\...\... 82

8\. Beam Instrumentation and Diagnostic
Systems\...\...\...\...\...\...\...\... 92

9\. Equipment
Interlocks\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
101

10\. Radiation Safety &
Shielding\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
107

11\. Magnet Power Supply Systems
\...\...\...\...\...\...\...\...\...\...\...\...\..... 130

12\. Vacuum & Mechanical
Systems\...\...\...\...\...\...\...\...\...\...\...\...\..... 135

13\. Water Cooling
Systems\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....
155

14\. Control System
Hardware\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....
161

15\. Control
Software\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...
163

Phase I Acclerator Performance
Parameters\...\...\...\...\...\...\...\...\..... 173

**Preface**

The PET RFQ project is nearing the end of the first phase of what was
envisioned to be a three phase project. This document is a full report
on the accomplishments and difficulties encountered during this phase.
As such it is important in the assessment of whether or not to proceed
to the succeeding phases and, if so, is how to proceed.

This report contains documentation from the efforts of building the RFQ
accelerator for production of PET isotopes. It is meant as a summary of
design, construction, performance and improvements. Each chapter
contains references to more detailed resources when appropriate. The
contributors listed below are part of a staff approaching one hundred
individuals that have worked on this project for over two years. Their
efforts and dedication have created a functional ^3^He^++^ accelerator.

*Contributors to this report include:*

Ralph Pasquinelli, Roy Rubinstein, Chuck Schmidt, Ding Sun, Kris
Anderson, Bob Webber, Dave Peterson, Brian Chase, Joe Dey, Frank
Bieniosek, Del Larson, Nancy Grossman, Dan Holslin, Howie Pfeffer, Duane
Voy, Elliott McCrory, Bruce Hoffman, Mike Shea, Richard DeHaas, Phil
Young.

**Executive Summary**

The goal of this project was to accelerate ^3^He^++^ for PET isotope
production. Utilization of ^3^He does not require enriched target
material and was believed to reduce neutron radiation flux. The starting
point was an existing Strategic Defense Initiative Organization (SDIO)
funded 8 MeV RFQ accelerator that had reliability and performance
problems. This report describes the redesign/rebuild of almost all
elements of that accelerator, including raising the energy to 10.5 MeV.
The project design goals are:

-   Peak Beam Current=12 milliamperes (mA electrical , 6 mA particle
    current)

-   Average Beam Current = 300 microamperes (µA) (150 µA minimum)

-   Duty Cycle= 2.5%

-   Beam Uniformity on Target= +/- 25%

-   Beam Reliability/Availability= 50% of scheduled time

-   Minimal Radiation Shielding Outside the Target Vault, i.e. not a
    radiation area

-   Extensive Radiochemistry Targetry Studies of ^11^C,^13^N,^15^O,^18^F

-   Technology Transfer for Potential Clinical Use

To date the accelerator has achieved 10.5 MeV ^3^He^++^ beams with the
following typical characteristics:

-   Peak Beam Current= 5 mA (electrical, 2.5 mA particle current)

-   Average Beam Current = 100 µA

-   Duty Cycle= 2.0%

-   Beam Uniformity on Target= +/- 50%

-   Beam Reliability/Availability= 25-30%

-   Entire Accelerator Foot Print is a Radiation Area

The collaboration believes that the reliability of the accelerator will
improve, but that the minimum design average beam current of 150 µA will
not be demonstrated before the beginning of radiochemistry experiments.
The achieved performance is adequate to begin the radiochemistry program
in January 1998. The cost to date is \$8 million out of the original
\$10 million budgeted. It is unlikely that the technology transfer goals
will be pursued in view of the remaining funds.

**Introduction**

This introduction will try to serve as a narrative describing how the
project was conceived, the initial goals, what actual performance was
achieved, some of the short comings and the lessons learned. More
specific technical details are described in subsequent chapters of this
report.

**Project Description**

The PET RFQ project (or "PET project") is a collaboration between
Fermilab (FNAL, Batavia, Illinois), Science Applications International
Corporation (SAIC, San Deigo, California), Biomedical Research
Foundation (BRF, Shreveport, Louisiana), and the University of
Washington (UW, Seattle, Washington) to create a linear accelerator for
the production of Positron Emission Tomography (PET) isotopes utilizing
Radio Frequency Quadrupoles (RFQs) . ^3^He is the particle to be
accelerated to 10.5 MeV for the production of the four main radio
isotopes used in PET, ^18^F,^11^C,^13^N,^15^O. Most of the accelerator
development has taken place at Lab G on the Fermilab campus. Originally,
the project had three phases. The first phase was to design, construct
and commission the ^3^He linear RFQ accelerator. The second phase was to
carry out radiochemistry experiments. The final third phase was to
investigate the possibility of technology transfer to commercialize the
application of RFQ accelerators for PET isotope production. A number of
difficulties and setbacks were uncovered during the design and
commissioning of the accelerator complex that have made the possibility
of phase three improbable with the current funding profile.

This report will concentrate on phase one of the project. Subsequent
chapters of this report will detail the major technical problems
encountered and their solutions. A second volume under separate cover
will concentrate on the radiochemistry aspects of the project to date.

**History**

In 1988, the Strategic Defense Initiative Organization (SDIO) of the US
Department of Defense requested proposals for new accelerator
technology, under development for military purposes, to be applied to
medical research.^1^ Science Applications International Corporation
(SAIC) was awarded funding in 1989 to develop an 8 MeV ^3^He^++^ RFQ
accelerator for the production of PET isotopes. There were several
advantages noted for this technique. The proposed RFQs were small and
lightweight, and required little neutron shielding (compared to the more
commonly used proton/deuteron cyclotrons). In addition, the targets for
PET isotope production using a ^3^He^++^ beam do not require enriched
isotopes.

Dabiri et al^2^ described in 1990 the proposed SAIC ^3^He^++^
accelerator. It included a ^3^He^++^ ion source (then currently under
development at SAIC) and two 425 MHz RFQs for accelerating the particles
to 8 MeV. The projected peak electrical current was to be 15 mA, with
166 microsecond pulses at 120 Hz for a 2% duty factor. The RFQ
transmission efficiency was predicted to be 97%. In October 1990, SAIC
announced^3^ the achievement of a 0.5 MeV proton beam in their RFQ
prototype.

At the Targetry 91 workshop, Hagan et al^4^ updated the design
parameters. The ion source was now to produce singly charged ^3^He^+^,
followed by a 212 MHz RFQ accelerating to 1 MeV and then a charge
stripper; this was followed by a two tank 425 MHz RFQ accelerating to 8
MeV. The output was to be 360 Hz, 55 microsecond pulses, giving 2% duty
factor; pulse current was to be 15 mA (electrical), which corresponds to
an average electrical current of 300 µAmps . The original ^3^He^++^ ion
source was felt to be too developmental, and there were also significant
changes to the earlier RF system design. At the end of October 1991, a 9
mA peak pulse current had been achieved^4^ of 1 MeV ^3^He^+^.

In July 1992^5^, tests were carried out on the 8 MeV 3He^++^
performance. A beam of 2.5 mA peak, 120 Hz, 33 microsecond pulses was
achieved, giving an average electrical current of \~10 µAmps.

A letter in March 1993^6^ described the then state of the accelerator,
following termination earlier that year of the PET RFQ program at SAIC.
It noted that there were still significant technological challenges
remaining in the RF system, and that several other accelerator systems
would have to be improved in order to achieve the design current. A very
rough estimate of the costs to bring the machine to design
specifications was of order \$1M.

A 1993 report^7^ summarized the results achieved before project
termination. A current of 4 mA electrical of 8 MeV ^3^He^++^ was
obtained, at a 1.6% duty factor (these numbers correspond to about 65
µAmps electrical average current). Some reasons for the low current were
given, including a charge doubling efficiency of only 20%, but the
system was very unreliable.

In late 1995, the continuation of the project was funded by the DOE with
\$10 million. Fermilab was chosen as the managing collaboration
institution. Earlier work (under the SDIO funded project) had shown a
need for an increase in beam energy over the 8 MeV initial design value.
The increase in beam energy would allow for a thicker target window and
improved isotope yields. Since the funding of the project was limited
and did not allow for major system redesign, it was decided to increase
beam energy by adding a fourth RFQ to the accelerator chain. Mechanical
production capabilities limited the fourth RFQ energy gain to 2.5 MeV,
hence the new beam energy was chosen as 10.5 MeV.

The project has been technically reviewed three times over the last two
years, twice by DOE and once by an independent review team appointed by
the Fermilab Director. Each review committee consisted of experts on
RFQs, accelerator technology, and radiochemistry. Suggestions from these
committees were carefully scrutinized and, when possible, implemented in
the redesign of the accelerator. Very little of the original SDIO funded
accelerator remains with the current design. These details will be
addressed in the following report.

The original intention was to complete machine design and commissioning
at Fermilab followed by dismantling the accelerator complex and shipping
to BRF in Shreveport, Louisiana. A facility in the physical plant at BRF
has been completed should this option still be part of the
collaboration's plans. The decision whether or not to ship the
accelerator will be made during Spring 1998. The radiochemistry program
could also be carried out at Fermilab should the collaboration decide to
keep the accelerator at its current location in Lab G. If the Fermilab
option is chosen, some infrastructure modifications to lab G will be
necessary to allow more efficient operations for the radiochemists.

Current status of the accelerator commissioning has the machine
operating at Lab G under the stewardship of SAIC personnel. Chapter 16
will contain the details of operations at Lab G during a trial period
with minimal FNAL intervention. The intention of this period was to
simulate day to day operations of the accelerator in a remote location,
an important factor to consider should shipping the accelerator to BRF
be chosen.

**Accelerator Layout**

![](media/image1.wmf){width="6.1097222222222225in" height="2.71875in"}

*Schematic representation of the accelerator systems . Each of the major
components will be discussed in more detail in subsequent chapters of
this report.*

**Resuming the Project**

There was an extensive period of litigation between government agencies
as to when transfer of the existing accelerator hardware could be made
between DOD and DOE. During this period, the accelerator had been moved
to BRF from San Deigo for the dedication of the new Biomedical Research
Institute (BRI). The project was officially resumed in September 1995,
with a \$10 million funding profile. At this time, there was no detailed
design report available. The original machine concept described in
Reference 2 was not the machine that was designed and built under the
SDIO contract. What was built under the SDIO contract was a three RFQ
system where the first RFQ operated at 212.5 MHz and was an integral
part of what became a 1 MeV doubly charged ^3^He source.

It was early on at Fermilab that the collaboration decided to increase
the energy of the beam to 10.5 MeV. This was mostly based on the need
for more robust target windows. The original thin (less than 0.6 mil)
target windows could not be guaranteed to be pin hole free by the
manufacturer. Nearly 1 MeV of energy was deposited in these thin
windows, leaving barely 7 MeV beam for radiochemistry experiments. A
thicker window was desirable, hence more energy would be necessary. The
additional beam energy would allow for more specific activity of the
target. (Specific activity is the ratio of radioactivity from a compound
of interest (usually one radionuclide is involved) to the mass of that
same compound in the sample of interest. It is usually expressed as
Curies per millimole or Megabequerels per millimole.) The value of 10.5
MeV final energy was derived from the longest RFQ that could be machined
with conventional machine shop tools. (SAIC had experienced some past
difficulties in building their patented RFQ design if lengths much
beyond a meter were considered.) It was also believed that with the
limited funding of the project, that costs would be prohibitive to use
any different accelerating structure (i.e. drift tube Linac). In
retrospect this may not have been the case.

The first course of action after unpacking the boxes shipped from
Shreveport, was to evaluate the hardware. Several tests were the
preliminary goal, 1. To understand the performance of the RFQs, 2. to
assemble the singly charged ^3^He ion source, and 3. to experiment with
the SAIC gas jet charge doubler. Four to six months was necessary to
assemble the test stand to carry out these investigations. By spring of
1996, work on all three objectives was well underway. Several very
important discoveries were made during this period.

The x-ray emissions from the 212 RFQ greatly exceeded the minimal level
required for continuous occupancy. Numerous lead blankets were wrapped
around the RFQ to reduce the radiation level to allow continuous
occupancy. The RF system used to drive the RFQ was one of the surplus
FNAL Linac stations that had recently been decommissioned. Even though
the RF station was adequate for the test, it clearly lacked controls and
interfaces necessary for integration into a reliable accelerator (all
the old Linac interfaces, interlocks and controls were not part the
surplus hardware). Because these tests were only to 1 MeV, there were
none of the problems of phase lock and stability that would be
discovered as subsequent RFQs were put on line. The testing continued
very smoothly through the summer of 1996.

An additional test was carried out on one of the 425 MHz RFQs once
surplus hardware was procured from the canceled GTA project at Los
Alamos. It was fortunate for the PET project that the GTA utilized the
same 4616 tetrode in one of their RF drivers. The PET project purchased
three 4616 amplifier cavities and four solid state amplifier drivers.
These early tests of the 425 MHz RFQ showed some sparking of the RFQ
vanes when driven with 73 kWatts, the estimated correct value for
achieving the gradient to accelerate ^3^He^++^ from 1 to 5 MeV. There
was no real spark protection in the test setup. Later inspection of the
RFQ showed discoloration and loss of copper plating on the vane tips in
the middle of the RFQ structure. (In a subsequent inspection of these
vanes by Gerry McMichael of Argonne National Lab, he disclosed that the
damage to the vanes was not detrimental to their future use.)

The tests on the ion source showed a number of design flaws that needed
attention. Initial testing indicated excessive usage of ^3^He gas. At
\$115 per gaseous liter, this would become a serious operational expense
if not remedied. The source was totally disassembled and revealed a
number of "holes" in the source chambers that passed the He gas through
to the vacuum pump unnecessarily. Once plugged, gas consumption was
immediately reduced. The arc modulator power supply was not robust or
reliable. A redesign of a pulse forming network power supply similar to
units used at Fermilab was pursued.

The gas stripper/charge doubler was initially based on a static gas
cell. Striping efficiency was low and could be improved by a gas jet, a
totally new design. The valve used for the SDIO project was piezo
electric and very expensive. It had a limited lifetime estimated at
1-100 million cycles. With the estimated accelerator design duty cycle
of 2.5% running at 360 Hz, this translates to 0.77 to 77 hours of
operation, clearly an unacceptable replacement interval. A new design
based on an automobile fuel injector was adopted. The gas load necessary
to strip ^3^He^+^ to ^3^He^++^ was such that a new vacuum system with
ample pumping was necessary. In addition, these first tests experimented
with a number of different gases to test stripping efficiency, including
helium, nitrogen, argon, carbon dioxide and xenon.

**First DOE Review**

The first nine months of effort were spent trying to understand exactly
what the characteristics of the inherited project were. It was at this
same time that the first formal DOE review was conducted on May 29-31,
1996. The project was closely scrutinized by the review team and the
following action items were disclosed: 1. Perform an ion source to
target particle tracing with space charge effects included, i.e. an end
to end simulation. 2. Set up a detailed project management scheme to aid
tracking of project progress. 3. Develop a specific set of performance
standards that would be agreed upon by all collaborating institutions.
Meet these minimum requirements before shipping the accelerator back to
BRF. 4. Hold a second review of the project in six months.

Action items 1 and 3 were absolutely critical to the technical success
of the project. It was indeed true that no full simulation had been
carried out by the SDIO project. By the same token, we were discovering
the performance of the existing hardware and it became evident that some
reverse engineering would be necessary. The end to end simulation was
part of this process. Upon rebirth of the project, it was believed that
the lack of a reliable RF system was a major cause for poor operations.
At this stage, this simple assessment was not true as many subsystems
required attention. In the period between DOE reviews, extensive work
was done on the simulations and redesign of hardware support systems
continued in parallel. The performance goals for the project were
optimistic as are most goals of accelerator design. A minimum acceptable
performance was drafted and agreed upon by all collaborators on August
30, 1996. This document has been included as an appendix.

One of the areas of greatest concern was the design of the Medium Energy
Beam Transport (MEBT) with beam at the 1 MeV energy. The original SDIO
design utilized an RF bunch rotating cavity after the stripper to match
the beam longitudinally between the 212.5 and 425 MHz RFQ string. The
bunch rotation proved ineffective due to the charge neutralized state of
the beam after stripping. As such, a new means of matching was
necessary. It was also at this time that the physical foot print of the
accelerator was a main concern. The accelerator was originally designed
to be very light weight, requiring little shielding. Such a design could
easily be located almost anywhere. The collaboration had the intention
of locating the accelerator on the second floor of the newly dedicated
BRI in Shreveport. (In fact the SDIO designed machine was set up on the
second floor for the BRI dedication.) The room on the second floor had
limited space that barely could house the original footprint. The
addition of the 4th RFQ would not easily allow the accelerator to be
installed on the second floor if the accelerator remained an in line
design.

A number of MEBT designs were considered. The one chosen was the idea of
Delbert Larson, an SAIC consultant. The new MEBT would consist of an
isochronous 540 degree magnetic lattice to preserve bunch length. An
added benefit of the design was that the accelerator now folded back on
itself essentially shortening the overall length and easing the siting
problem. (As it turned out, the project's future home was prudently
relocated to the BRI physical plant because the unknown nature of the
shielding requirements could have made location on the second floor
impossible.) There were serious concerns over the space charge effects
of the beam in the MEBT. Extensive modeling was conducted, but
understanding any nonlinear space charge effects would have to wait
until actual beam tests.

Once adopted, the design for the MEBT was pursued vigorously. The
lattice required the design and construction of two large 270 degree
bending dipoles and seven quadrupoles of various strengths. All magnets
were designed and fabricated at the Fermilab Technical Support Division.
This effort continued through November 1996. There were a few technical
problems that required some rework, but for the most part the magnets
were produced on schedule and have performed remarkably well.

**Second DOE Review November 6-8, 1996**

A great deal of activity took place between the DOE reviews of the
project. The following is a paraphrasing of findings of the second
review committee and subsequent actions by the PET design team.

The original ion source operated well in preliminary testing. The optics
for the ion source were redesigned and required a new vacuum vessel.
From this point on in the project, the reliability of the ion source
filament became a source of concern. The components for the ion source
contain some organic plastic insulators and the entire source was not
stored in an inert environment. It is believed that some form of
contamination is the culprit for filament failures. The filament
continues to be problematic, but new and improved designs and procedures
are being followed to improve performance and reliability. Emittance
measurements were performed with equipment borrowed from the FNAL Linac
department. Two recommendations were presented. Relocating the steering
magnet and use of consistent units of emittance. Both recommendations
were or are being acted upon.

The transmission efficiency of the 212 MHz RFQ was typically 65% in
actual testing at lab G. Further simulations with the RFQ code PARMTEQ
supported this transmission efficiency. In the period between DOE
reviews, the 212 MHz RFQ sprang a leak between the vacuum and water
cooling passages of the vanes. The leak was caused by the corrosive
action of water on the aluminum alloy (7075-T6) from which the vanes are
fabricated. This problem was temporarily repaired with an impregnation
technique to stop the leak (more details in chapter 12) and allowed
continued testing. Three recommendations were presented. 1. Additional
probes to measure fields in the RFQs should be added. A total of 12 such
probes were added to all of the RFQs. 2. Continued modeling of the RFQ
should utilize higher order term analysis software. This should be
completed before making new replacement vanes. It was decided to take
this opportunity to not only improve modeling, but also redesign the RFQ
in an attempt to improve transmission efficiency. 3. X-ray production
endpoints should be conducted to truly understand the intervane voltage.
These tests were conducted on the 212 MHz RFQ with conclusions that show
agreement between the theoretical fields and the extinction level of the
x-rays produced.

The charge stripper testing with the fuel injector was successful in
prototype testing to 60 Hz repetition rates. No measurable emittance
dilution was observed at stripping efficiencies approaching 83%. The
final stripper design was able to utilize a single fuel injector
operating at the full duty cycle of 2.5% at 360 Hz. Additional pumping
was added to the final stripper, a total of 4000 liters per second.
Final stripping efficiencies at full duty cycle are approximately 70%.
Argon gas showed the highest stripping efficiency, but its high
molecular weight and low thermal conductivity caused excessive heating
of the turbo molecular pumps at full duty cycle. Nitrogen gas is now
utilized daily with only a few percent degradation in stripping
efficiency, but vastly improved vacuum performance. Operating at the
higher duty cycle reduced stripping efficiency because the pumps are not
able to fully recover vacuum between 360 Hz cycles.

The MEBT received the highest level of scrutiny. The review committee
was convinced that the MEBT performance as designed was questionable at
best. The recommendations were to look into developing contingency plans
in case the MEBT was a failure. Other suggestions were: addition of an
isolation vacuum valve to the MEBT diagnostic box, use TRACE3-D to
evaluate magnet settings, add as many diagnostics as possible, try to do
additional modeling utilizing the code PARMILLA and WARP3D. At this
stage in the project, the PET team was committed to the MEBT design. All
the magnets in the lattice were close to completion, those that were
complete met the specifications. More modeling did take place as
suggested. The diagnostic box was jammed full of diagnostics including
segmented Faraday apertures, a current toroid, and beam profile monitors
for use at low duty cycles during commissioning. As it turned out, it
was mechanically very difficult to include a vacuum isolation valve in
the MEBT, hence that recommendation was not followed. Some back up plans
were discussed, but if the MEBT did not work, there would be a big delay
in the project. It was a bit of a gamble, but a victory for the project.
The MEBT has worked very close to design performance and has been very
reliable.

The HEBT comments included lack of instrumentation in the beam line,
unclear how 10.5 MeV is verified, use of a dog leg to reduce neutron
flux in the reverse direction. The committee was impressed with progress
on the target window development. Recommendations included
characterizing the HEBT magnets and look into an insertable profile
monitor. The magnets were indeed sent to the magnet test facility for
characterization. They were also carefully studied for cooling
efficiency since the HEBT magnets were designed to focus 8 MeV beam not
10.5 MeV. As a result, they would need to be run at higher currents. The
magnets passed the thermal testing at design current for 10.5 MeV.
Because of the high beam energy in the HEBT, a profile monitor of the
wire type would not survive for very long. Instead a set of movable
collimator jaws was designed and built that could handle the full beam
power if necessary. A software application was written to do collimator
scans to test for beam uniformity on the target window. This program has
worked well and has been verified against carbon foil irradiations. (A
carbon foil is irradiated, cut into strips, then very carefully counting
for residual activity to measure beam uniformity.)

Recommendations for the RF systems was to improve the arc protection
protocol and add RF field probes to all RFQs. The PET team has indeed
incorporated a systematic means of controlling RF fields in the RFQs
after arcing occurs. Because each of the RFQs needed to be rebuilt due
to water corrosion of the vanes, the probes were easily added to all
RFQs.

Action items from the second review: 1. Continue modeling of RFQs. 2.
Appoint single lead subsystem mangers. 3. The Fermilab Director should
appoint an independent advisory committee to review work on Accelerator
construction and plans for future radiochemistry experiments. 4. Conduct
another DOE review if the accelerator has not met project milestones by
spring of 1997. Modeling continued and was completed. Single subsystem
managers were appointed, an advisory committee was chartered, and there
was no need for the next review as 10.5 MeV beam was accelerated in
April of 97.

**Director's Advisory Committee 5/22-23/97**

By the time the advisory committee convened, the accelerator complex was
very close to completion. Beam had already been accelerated to 10.5 MeV.
A number of useful comments on operations were presented and
appreciated. A large part of the review concentrated on the targetry
program and safety issues related to relocation of the accelerator to
Shreveport. As the targetry and Louisiana safety regulations are not
part of this report, nothing further will be mentioned here.

**Onward Toward 10.5 MeV Beam**

All of 1997 was devoted to final construction, assembly and
commissioning of the accelerator complex at Lab G. Even though a number
of serious problems had been addressed and corrective action was taken,
there were still more unforeseen problems ahead.

One of the more serious delays was caused by the problems encountered
with machining and plating of the RFQs. The water corrosion problem
meant that all four RFQs would have to be built by the project. (Only
the 425 MHz RFQ C was in the original plan.) There were several
iterations with machine shops to find a company that could precisely
machine the vane contours. The major headache was finding a plating
house that could successfully apply the copper plate to the aluminum
alloy. Each of the RFQ vanes and the RFQ C housing experienced plating
problems and were returned to the vendor. A new plating vendor was
finally found that could complete the job successfully. By April of
1997, all major components were in place and 10.5 MeV beam was first
accelerated.

As commissioning ensued, one problem after another surfaced. The
filaments in the ion source were failing after one to two weeks of
operation. Previous work with the ion source showed filament lifetimes
exceeding two months. This performance was not repeatable. The RFQs were
slow to condition, the RF power systems had several infant mortality
problems. As duty cycle was increased to the full 2.5 %, the RFQ
coupling loops began failing due to overheating. The design of the
coupling loop was such that the vacuum sealing o-ring was located in the
region of highest electric field. As a result, they overheated and
caused vacuum failure. The loops were redesigned to relocate the o-ring
to the interior of the center conductor. This fix was effective as no
new loop failures occurred. The water systems for the project were
specified internally, but procured on the outside. They experienced
cooling compressor failure, valve control failure, flow transducer
failure, and improper control loop parameters. All have been fixed and
the system has been performing reliably since. As is often the case with
Murphy phenomena, all of the failures occurred separately, few in
parallel, with the end result of extended down time necessary to fix the
problems. These types of difficulty plagued the project for the last
half of 1997 and made commissioning and operation of the accelerator a
harrowing endeavor.

Radiation safety issues were constantly being addressed. The original
design of the accelerator indicated very low radiation losses due to the
linear directed nature of the beam and low beam energy (8 MeV). The
accelerator was not expected to require any special shielding except for
the target vault. With the increase in energy from 8 to 10.5 MeV,
accelerator shielding issues became less clear (see Section 10). Once
the RFQ x-ray problem was mitigated, accelerator shielding was studied.
Careful modeling of the RFQ transmission characteristics indicated beam
losses approaching 20% in the one centimeter drift space between RFQs in
the 425 MHz string. These losses did indeed materialize and are the
source of high radiation production outside the target vault. Initial
measurements were made at low duty cycle and scaled appropriately. It
was clear that the losses would create a radiation environment and
interlocking the accelerator enclosure would be required. The major
concern at this stage was whether or not the physical plant at BRI could
be shielded sufficiently to allow open occupancy on the mezzanine above
the accelerator and outside the building. Several possible solutions
were discussed and tabled until further measurements were made. With the
uncertainty of shipping the accelerator to Shreveport, little further
attention has been paid to the shielding problem at BRI. Sufficient
shielding was already in place at lab G to not be a concern on the
Fermilab campus. As duty cycle of the accelerator increased,
measurements showed that the beam losses at the RFQ B&C interface would
require additional shielding at lab G to keep the control room area
classified for continual occupancy. An 8 inch polyethylene wall was
constructed on a movable cart and placed near the RFQ junction, hence
mitigating the problem.

A number of attempts to start preliminary radiochemistry experiments
began in late May 1997. The first few attempts were focused on
operational procedures and shakedown of the hardware. By the end of the
year, most of the low beam current tests had been performed and the need
for sustained beam currents approaching 100 uAmps was desirable. By
December 1997 most subsystems were operating reliably. There was one
major component that was still outstanding, the fast valve trigger. A
fast valve (one able of closing in 7 msec) is located in the HEBT. Its
primary purpose is to close rapidly in the event a target window
shatters. Earlier experiments showed that the fast valve trigger sensor
supplied with the valve was so sensitive as to trip from outgassing of
the target window with moderate beam current levels. A new sensor was on
back order for many months. A backup sensor was employed, but not as
fast as one would like. The bottom line was that a window broke in
December, the fast valve stopped most of the debris, but operation of
the accelerator is not like horse shoes, close does not count. The
material from the window contaminated the RFQ C and returning to full
gradient became impossible. The RFQ was disassembled and clearly had
signs of serious arcing damage. The debris deposited in the RFQ was also
now radioactive, making for a contaminate waste dilemma. The RFQ was
cleaned, reassembled, and put back under vacuum before the Christmas
holiday. In early January, the RFQ was reconditioned and beam
commissioning resumed. By this time, the new fast valve sensor was
delivered, inspected, installed and tested before the next chemistry
experimentation period.

With most of the problems of reliability solved, one main headache
remained, keeping the RFQs, all four of them, tuned on resonance and at
full gradient. For some as yet unexplained reason, the conditioning of
the RFQs has remained a mystery. Several weeks are required to
recondition and recover from maintenance periods that bring the vanes up
to atmosphere. Even when conditioning has been reached, the acceleration
of beam exacerbates the spark rate in the 425 MHz RFQ chain. The beam
losses between RFQs approach 20% and seem to increase the spark rate.
The detuning of the RFQs approaches several kilohertz per second after a
spark. The thermal tuning loops are unable to respond quickly and the
RFQ frequencies start to wander. When there are four nomadic RFQs, it
can take 20-30 minutes to settle down. This proves to be a very
frustrating environment to perform chemistry experiments. A solution to
this problem is now in progress. The original SDIO accelerator utilized
mechanical tuning paddles for resonance control. By themselves, they
were not capable of maintaining resonance control and were abandoned
early in the project. A thermal control loop was adopted, but lacked the
response time needed to recover quickly from sparks. The hybrid system
now being implemented uses both thermal and mechanical tuning. It is
expected that resonance control recovery will now be seconds not
minutes.

**Success at Last**

In early February 1998, the first serious radiochemistry experiments
were successfully carried out. With numerous failures and fixes in the
past, the accelerator was able to deliver sustained beam currents of
50-75 uAmps to the target. The radiochemists were able to obtain twice
as much useful data as in all previous attempts put together. While the
resonance tuning problems continued, they were not so severe as to stop
the experiments. A March 1998 run was very successful, with the new
tuning paddles providing prompt recovery from arcs. Plans are to install
tuning paddles for the remaining 212 and 425 C RFQs.

**Lessons Learned**

As is evident from this introduction, the project was not without
difficulties. There were a number of designs that needed re-engineering.
The project clearly exceeded the initial time estimate for completion of
the accelerator and the cost estimate. As progress on the accelerator
continued, many times it felt like opening Pandora's Box. Although this
project utilized some of the hardware from the SDIO program and surplus
RF stations from the FNAL Linac, the design of this accelerator has
provided a number of novel hardware contributions and important changes
to software programs utilized in designing accelerators. The MEBT design
may be a useful mechanism for matching RFQs to subsequent accelerating
stages. High quality 270 degree bending magnets were designed and
implemented. The simple but effective use of a fuel injector for a gas
stripper provides a robust and reliable gas jet target. The extensive
modeling of the accelerator revealed a number of fundamental errors in
codes that have been used for years in the field of accelerator design.
All of these contributions and more will be described in subsequent
chapters.

It was initially believed (SDIO) that the beam loss between RFQs could
be tolerated because of optimistic goals for accelerator components. In
reality, no part of the accelerator achieved the original design
parameters. The operating parameters that have been achieved are listed
in the table below. If the use of a ^3^He beam for producing PET
isotopes proves to be superior to proton cyclotrons, a better means of
accelerating the beam would be to use the same 1 MeV source and
stripper, but to follow the MEBT with a short drift tube Linac. The
drift tube section would have a larger aperture, lower losses, and
higher reliability over the RFQ structures used in this project.

*Table of Specifications and Performance of the ^3^He^++^ PET RFQ
Accelerator*

**[Item Design Value Achieved Value]{.underline}**

Peak Beam Current 12 mA (electrical ++) 5 mA (electrical ++)

Average Beam Current 300 uA (electrical ++) 100 uA (electrical++)

Ion source Current 25 mA peak 21 mA peak

Ion source emittance 0.76 pi mm.mr. (90%) 1 pi mm.mr. (90%)

Duty Cycle 2.5% 2.0%

Stripping efficiency 70% 68-70%

MEBT transmission efficiency 100% 70%

MEBT input emittance 0.8 pi mm.mr. (90%) 0.9 pi mm.mr. (90%)

MEBT output emittance ^a^ 0.9 pi mm.mr (90%) 0.8 pi mm.mr. (90%)

RFQ 212 transmission ^b^ 80% 80%

RFQ 425 A-B-C transmission 43% 34%

HEBT beam uniformity +/- 25 % +/-50 %

Personnel occupancy Not a radiation area Accelerator footprint is

radiation area

\(a\) Scraping is observed to lower emittance in the MEBT

\(b\) - Accurate measurements of emittance and Twiss parameters exist
only for

the old 212 RFQ. The new 212 produces more beam (18 mA=75% ?) but much
of

the additional current is lost in the MEBT/stripper. Losses may be
caused

by scraping due to a different exit angle (Twiss parameters), or
increased

output emittance. Unfortunately we did not have the opportunity to
remeasure

output emittance and Twiss parameters for the new RFQ.

\(c\) Initial \'design value\' numbers were very aggressive and perhaps
unrealistic.

**References**

1\. See the introduction in Reference 7.

2\. A. E. Dabiri et al., Nuclear Technology, Vol. 92, p. 127, October
1992.

3\. Announcement of Proton Beam in SAIC Radio Frequency Quadrupole
Accelerator.

4\. W. Hagan et al., Proceedings of the IVth International Workshop on
Targetry and Target Chemistry. PSI Villigen, Switzerland; September
9--12, 1991; edited by R. Weinreich, p. 19.

5\. Memo K. Krohn and J. Link to P. Young and W. Hagan, 15 July 1992,
subject: Trip report for July 6--9, 1992.

6\. P. Young to K. Krohn, March 29, 1993.

7\. K. A. Krohn et al., Proceedings of the Fifth International Workshop
on Targetry and Target Chemistry. Brookhaven National Laboratory,
September 19 to 23, 1993; edited by R. Dahl, R. Ferrieri, R. Finn, D.
Schlyer, p. 20.

**Chapter Summaries**

*Chapter 1. Pre 1995 Project History.* A short description of the
project before it was restarted with the current collaboration.

*Chapter 2. Ion Optical Design of the RFQ PET Accelerator.* The design
of a doubly charged ion source was beyond the scope and budget for this
project. A singly charged ion source original to the SDIO project has
been modified and improved. Filament problems have plagued this source
since the early stages of the project. When it appeared that the
problems with filament burnout were overcome, they resurfaced in the
last 3-6 months. A serious effort is now underway to understand the lack
of reliability. Currently produced filaments typically last only 2-3
weeks. Filaments have been produced that lasted 2-3 months, an
acceptable lifetime. This level of reliability must be re-established.
The ion source does deliver the design beam current when the filament is
operational.

The RFQs for the project are basically the same design as the original
RFQs under the SDIO funded program. All RFQs needed to be rebuilt
because of water corrosion problems. It was decided to take this
opportunity to redesign the first RFQ in the chain to improve its
capture efficiency. In addition a fourth RFQ was added to increase the
energy of the beam to 10.5 MeV. All four RFQs were designed, built,
tuned and tested within the current collaboration.

The original accelerator design consisted of two RFQs that accelerated
3He++ to 8 MeV. While still under SDIO management, the accelerator was
reconfigured to utilize three RFQs, the first being a 212.5 MHz capture
RFQ to accelerate 3He+ to 1 MeV (the energy at which the second electron
is stripped, essentially creating a 1 MeV doubly charged ion source),
followed by two tandem 425 MHz RFQs. The need to strip to double
positive charge and momentum match into subsequent 425 MHz RFQs for
acceleration was not realizable with an RF bunch rotation cavity due to
charge neutralization of the beam caused by gas leaking from the charge
doubler. A novel 540-degree-bend isochronous and achromatic matching
section, the Medium Energy Beam Transport (MEBT), was designed (in the
current project) to maintain the bunched characteristics of the beam
while separating out the singly charged and electron remnants. This was
the most controversial aspect of the project during early DOE reviews as
there were fears of debilitating space charge effects. In fact the MEBT
did not exhibit the predicted space charge or focusing problems and
works reliably. The matching technique demonstrated in the MEBT may also
be an attractive means of matching low energy beams in other
applications. The Low Energy Beam Transport (LEBT) and High Energy Beam
Transport (HEBT) sections are similar to those in the original project
and were also carefully modeled and studied. A complete end to end beam
transport simulation was performed.

*Chapter 3. RFQ Tuning Systems.* The resonance tuning system employed in
the SDIO program exhibited a number of reliability problems associated
with mechanical paddle tuners. The complete system was redesigned
utilizing thermal tuning via water cooling/heating.

*Chapter 4. Low Level RF.* The LLRF system is a completely new design
based on the latest electronic techniques. Extensive digital signal
processing is employed to give the system a high degree of flexibility.

*Chapter 5. High Level RF.* The High Level RF of the original SDIO
program was a serious reliability problem. The use of planar array power
amplifiers was a constant source of operational down time. At the outset
of the current project, it was believed that the surplus FNAL Linac RF
stations could be modified to provide the RF power. These surplus
stations had a 25 year high reliability operational history. The
stations were indeed modified and are now operating with minimal
maintenance. Additional 425 MHz surplus hardware was purchased from Los
Alamos National Lab.

*Chapter 6. Charge Doubler.* The 425 MHz RFQs are designed to accelerate
doubly charged ^3^He^++^. Calculations and analysis for removing the
second electron showed the need for a gas jet charge stripper versus a
stripping foil. The technique from the original SDIO project of
utilizing a piezo valve for the gas jet proved unacceptable due to short
lifetime and poor performance of the piezo valve. A new gas jet stripper
utilizing an automobile fuel injector was designed, built, and tested.
This device now routinely provides 70-80% charge stripping efficiency.

*Chapter 7. Machine Operations and Performance.* The chronology of
events leading up to present day operations and current performance is
presented.

*Chapter 8. Diagnostics.* A complete suite of diagnostic instrumentation
was designed for the accelerator. Some of the devices include charge
integrators, wire profile monitors, Faraday apertures and cups, and a
beam intercepting longitudinal monitor. An emittance scanner was
borrowed from the ion source group at FNAL to do emittance scans at 1
and 5 MeV. The equipment has worked extremely well, allowing for ease in
commissioning all sections of the accelerator.

*Chapter 9. Equipment Interlocks.* Virtually all of the hardware
associated with the accelerator is new to the current project. An
extensive hardware protection interlock system was designed and built
based on commercially available Programmable Logic Controllers (PLCs).
This system has already proven valuable in protection of RF, vacuum, and
water subsystems.

*Chapter 10. Radiation Safety and Shielding.* Radiation safety for this
machine demanded a careful in-depth effort. A number of serious problems
were encountered, analyzed and mitigated. Although one of the selling
points of a linear particle beam was low radiation levels, the opposite
was found to be true. The initial SDIO effort concentrated more on light
weight design than on low radiation levels. As such the RFQ vanes,
housings and vacuum vessels were designed from aluminum alloys. The
x-ray radiation levels of the RFQs in the aluminum vacuum vessel
exceeded 10 Rads/hr on contact. New Stainless steel chambers 1-5/16 inch
thick were designed, installed and eliminated the x-ray hazards. The
neutron production from 8 MeV ^3^He^++^ on aluminum was difficult to
estimate. Radiation levels near the RFQ junctions were found to exceed
100 mrem/hr due to particle beam losses associated with the gaps between
adjacent RFQs. This required a radiation interlock system to be
installed. The entire accelerator enclosure is now an interlocked
radiation area. Additional shielding will be necessary for relocation of
the machine to Shreveport. (This problem is currently being addressed.)
The radiation levels in the target vault exceed the shielding
capabilities of the current vault design. While this does not present a
problem in the lab G location at Fermilab, a new or modified vault
design will be necessary for BRF in Shreveport.

*Chapter 11. Magnet Power Supply Systems.* With the addition of the MEBT
to the project came the need for nine additional powered magnets.
Together with existing magnets for LEBT and HEBT, the system now has in
excess of twenty power supplies. The MEBT has the additional requirement
for stable current regulation that exceeds standard catalog power supply
specifications. A complete remotely controlled and monitored power
supply system has been designed, built, installed, and runs reliably.

*Chapter 12. Vacuum and Mechanical Systems.* The complete mechanical
systems for the accelerator were redesigned from the ground up,
literally. New stands for supporting the accelerator structure were
designed to allow easy assembly and maintain ten mil alignment
tolerances. A new vacuum system was carefully specified, analyzed,
designed and installed. All mechanical designs have taken "mobility"
into consideration so that the accelerator could be easily relocated to
Shreveport. A great deal of mechanical design was required to create the
functional Charge Doubler described above. Water corrosion problems in
the RFQs required a temporary impregnation fix as well as coating new
RFQs with Teflon for long term protection.

*Chapter 13. Water Cooling Systems.* The design of the accelerator calls
for self contained cooling systems. The water systems for the project
are based on commercially manufactured components. The systems include
three types of cooling loops, Low Conductivity Water (LCW), four
independent RFQ cooling loops, and a central heat exchanger loop. After
some initial infant mortality of components, the water systems are
functioning well. Some water temperature control problems are being
addressed as the factory settings on the heat exchanger control loop
were not optimized for our system.

*Chapter 14. Control System Hardware.* The control system hardware is
the spinal chord of the accelerator. Clearly the design of this
accelerator held many challenges. One system where challenges were not
welcome was the control system. It was decided to adopt the field proven
design of the FNAL Linac control system. The hardware did not require
extensive redesign, hence reducing cost to the project. The control
system has indeed provided reliable operational experience.

*Chapter 15. Control Software.* Although the control system hardware
existed, a great deal of application code needed to be developed to
support accelerator commissioning and operations. In addition, it was
decided to create a user friendly graphic control environment to
facilitate daily operations and diagnostics. Software was written
utilizing commercial and in house applications.

**1. Pre 1995 Project History**

The project to develop a He-3 accelerator for use in the production of
short lived isotopes (C, F, O, N) was first proposed in 1988 in response
to a Broad Agency Announcement (BAA) from the Strategic Defense
Initiative Office (SDIO). The SDIO was at the time interested in helping
to develop spin off products from technologies that had been
significantly invested in by the core SDIO program. Accelerator
technology and in particular Radio Frequency Quadrupoles (RFQ)
accelerators was one of these technologies. At the same time there was
significant interest by the Positron Emission Tomography (PET) community
in finding smaller, cheaper, lighter sources for the short lived
isotopes that are of interest to this community. As a result, one of the
items in the BAA was a solicitation for new and innovative ideas for
production of PET isotopes.

In response, SAIC teamed with the University of Washington to propose
using 3He to produce the PET isotopes. This was a very interesting idea
for several reasons. In addition to using a technology that SDIO had
made significant investment in, this approach was believed to hold other
advantages such as:

-   Small size low weight, Linacs lighter than cyclotrons

-   Single particle, cyclotrons for PET are often dual particle

-   No need for enriched targets, enriched targets are expensive can be
    difficult to obtain

-   Cyclotrons use protons or deuterons as the projectile and require
    enriched target material

-   Low neutron production, deuteron cyclotrons require large shielded
    enclosures and at 8 MeV. 3He is below the coulomb barrier of high Z
    materials so few neutrons are produced in the accelerator and only
    the target needs to be shielded.

Following the review of all the proposals, the SDIO funded two programs
one being the SAIC He-3 approach. The characteristics of that first
proposal are summarized in the following table:

  --------------------------------- -------------------------------------
  Output Energy                     8 MeV, He-3++

  Beam Current Electrical           15 mA (electrical)

  Ion Source                        He-3 ++

  Duty Factor                       2%

  RF Power                          400 kW
  --------------------------------- -------------------------------------

As originally envisioned the accelerator would be composed of a doubly
charged He-3 ion source followed by a matching section, two RFQs, and
the high energy beam transport.

In October of 1989 work began on the program. The first part of the
project was to investigate the critical technologies and move the design
from concept to preliminary design. As part of the critical technology
investigation, a study was initiated to look into the difficulty of a
doubly charge ^3^He^++^ ion source. Ion source expertise was obtained
from Los Alamos and Lawrence Berkeley Laboratory in order to research
this problem. This roughly yearlong study resulted in a doubly charged
source that produced approximately 1.5 mA of doubly charged beam, far
below that which was required. This outcome resulted in a rethinking of
the layout of the entire machine. An ad hoc design group looked into the
other possibilities for producing the desired 8 MeV beam. The resulting
report recommended the machine be changed so that a singly charged ion
source could be used followed by acceleration to an energy where the
stripping cross sections were favorable for creating a doubly charged
beam of sufficient current. This is the approach that is currently being
used.

This new design was then presented to the projects sponsor, SDIO, and
rejected due to the increase in weight over the simpler doubly charged
source approach. In retrospect the decision by the SDIO not to approve
the changes was somewhat misguided by their belief that the increase in
weight would make the technology unattractive to the medical community.
Later discussions with General Electric Medical Systems revealed that
this small increase in weight was not critical to the acceptance of the
RFQ technology by the medical users. Nonetheless, the first stop work
was issued on the project. A series of meetings was held to restart the
program and in February of 1991, a preliminary design review was held
where the new design was presented. The following table (taken from PET
note 3, viewgraph 02-05-91 and PET note 2, Preliminary Design Report)
shows this design. (PET notes are a series of reports generated from the
original project and available from current project management.)

  ------------ --------------- ------------------------------------------
  Item         Technology      Characteristics

  Ion Source   Duoplasmatron   3He+ , 20 keV, \>20 mA, 200 usec, 120 Hz

  LEBT         Electrostatic   Emittance \< 105 pmm x mrad

  212 MHz RFQ  Modified 4 vane 1 MeV, \> 15 mA (Tr 75%), Emittance 20.6
                               pmm x mrad

  Charge       Gas jet         3He+ to 3He++ , 80% strip efficiency, 24
  Doubler                      mA out

  MEBT                         23 mA electrical (Tr 95%)

  425 MHz RFQ  \(2\) 4 vane    8 MeV, 3He++ , 285 kW, 15 mA (Tr 65% total
               RFQs            through chain)

  HEBT/ Target                 Water or Carbon
  ------------ --------------- ------------------------------------------

This design was still relatively compact. The total length was about 5
meters. In order to keep the weight as low as possible (still a
sensitive issue with the SDIO) an all aluminum RFQ structure was used, a
light weight einzel lens was used for focusing in the LEBT, and the
vacuum tanks were fabricated from light weight aluminum.

The program then went into final design and fabrication (a series of
approximately 80 reports were written which document the design of the
SDIO system, PET 1 -- 83, i.e. PET notes). A number of issues became
clear as each stage was constructed. First, the einzel lens that had
been built for the LEBT focusing into the first RFQ proved to be
incapable of focusing the high current and partially neutralized beam
from the ion source beam. The decision was made to use a solenoid lens
for focusing, going back to the lens that had been used on the ion
source development effort and abandoned due to weight. Second, it became
clear that a fast way for controlling the resonant frequency of the
RFQ's was needed. This led to the implementation of paddle tuners (PET 8
May 12, 1992 SDIO progress report). Third, the design of a close coupled
RF system which had been originally proposed to keep the entire system
compact was abandoned as too developmental and a more conventional
planar triode array was used(PET 2, 2/6/91). There were also a number of
other less major changes to the design as the system was developed.

The end of 1991 and the first half of 1992 were focused on the
construction as the following table shows.

October 1991 1 MeV beam out of the 212 MHz RFQ

February 1992 5 MeV beam out of the first 425 MHz RFQ

March 1992 8 MeV beam out of the final RFQ

July 1992 First attempt at radiochemistry

During this same period of time and in accordance with the original
goals of the program, SAIC sought out commercial interest in this
technology. The major commercial entities involved in the development of
the PET market were General Electric Medical Systems (GEMS) and Siemens
Medical. GEMS had shown a keen interest in this technology due to its
potential to "leap frog" the market. GEMS had already purchased the
controlling interest in the PET cyclotron business of Scandatronix, and
after searching all other technologies, believed that the He-3 approach
was the future for isotope generation for PET. In 1992 SAIC and GEMS
negotiated an agreement whereby SAIC licensed its patent to GEMS and
both companies participated jointly in the development. Throughout the
last half of 1992, SAIC and GEMS worked together to develop a program
whereby this technology could be developed and commercialized.

The original funding for the project, 3.6M from the SDIO, was fully
expended in mid 1992. Support for the continuation of the program came
from SAIC and GEMS. The total funding through the end of 1992 was around
4.9M dollars.

In November of 1992, GEMS and SAIC held a concept review whereby the
progress of the program was reviewed and the plans where discussed for
the development of the first prototype unit (the existing SDIO funded
system was the proof of concept unit). The following table summarizes
the performance to date as of this review (CASPR Concept Review).

  -------------------------- --------------- -----------------------------
  Location                   Transmission    Achieved Current (11/12/92)

  Ion Source (3He+)                          22 mA peak

  LEBT                       80              17 mA peak

  212 MHz RFQ                68              12 mA peak

  Charge Doubler and MEBT                    13 mA peak

  425 MHz A and B RFQ        23              3 mA peak

  HEBT to Target             95              50 microamps Average
  -------------------------- --------------- -----------------------------

A few notes are important with regards to the operation of the systems
at that point in time. The peak average current that had been achieved
was 50 microamps. Unfortunately this current could not be maintained on
a regular basis due to the unreliability of the RF power system. Also,
even though stripping efficiencies better than 80% had been seen, the
valve that was used to pulse the gas was unreliable and did not have
good long-term stability. An automotive fuel injector valve was proposed
to replace the existing piezoelectric valve, but never implemented.

The MEBT , a linear combination of permanent magnetic quadrupoles (PMQ)
and an RF buncher, was difficult to analyze and tune, since one could
not change the value of the PMQ's and the combination of singly and
doubly charge beam confused the measurement. The PMQ's were necessary
due to the very limited space that could be allowed for transverse
focusing unless a second longitudinal RF buncher was used. The operation
of the RF buncher in the presence of the high gas loads from the charge
doubler was emerging as one the areas of difficulty in this MEBT
approach. Since making a longitudinal emittance measurement at these low
energies is very difficult, the operation of the buncher could not be
analyzed without the 425 MHz RFQ in place. This difficulty combined with
the slow turn around on the varying of the PMQ fields (3 weeks), made
the MEBT/charge doubler the focus of most of the development efforts
during this period.

At the close of 1992, the PET market was going through a significant
slowing, as was health care in general, due to the uncertainty over the
direction that the President's health care reform initiative would take.
This uncertainty resulted in GEMS canceling all of their internal
research programs with paybacks that were in excess of 2 years. The 3He
RFQ project fell into this category and in February of 1993 the program
was stopped again. For about six months after that the project continued
at a low level on internal SAIC funding until July when the system was
shut down. Most of the effort during this last period was focused on the
buncher in the MEBT.

In late 1993 and early 1994 the program was again started by the
Department of Energy which resulted in the present collaboration between
Fermilab, SAIC, Biomedical Research Foundation of Northwest Louisiana,
and the University of Washington. As the program was reviewed and
proposed for DOE funding, it became clear that the PET market had
significantly changed since the original SDIO funding. The role of 8 MeV
3He was considered to be too marginal in terms of its predicted
production of the PET isotopes. The final energy of 8 MeV had originally
been selected as a trade-off between accelerator length (compactness,
weight, etc.) and isotope production. The radiochemistry part of the
collaboration, the University of Washington, believed that the machine
needed to achieve at least 10 MeV to be able to make a significant
contribution to the body of knowledge on PET targetry. This then changed
the machine into its present status of 10.5 MeV. The table below, which
is part of the Interface Control Document written for this program,
gives the energy and current requirements for which the existing machine
was designed.

The program as it currently stands is significantly different from that
system proposed in 1988. As its configuration has changed so has its
role. Today we have a very powerful source of 3He, probably the most
intense one in the world. That is not the only achievements of this
machine however as the following list attests:

-   Worlds most intense source of accelerated 3He

-   First time three RFQs have been operated in series, or for that
    matter four

-   First successful isochronous MEBT to match two RFQs, no RF bunching

-   Resonant control of four RFQs operating in series has never been
    attempted before

-   Demonstration of magnetic spreading of beam to form a uniform beam
    on target.

-   Demonstration of the concept of using automotive fuel injectors for
    stripping cells. First proposed at the SSC but never tested.

-   Demonstration of Segmented Vane RFQ structure (212 MHz RFQ), small
    cross section could be an enabling technology for low frequency RFQs

-   Development of thin, high energy density target windows

-   Development of 3He targetry

-   Shielding studies for a 10.5 MeV beam of 3He had not been performed
    before and may have relevance to other low energy accelerator
    systems.

LONGITUDINAL TWISS AND SYSTEM PARAMETERS

+----------------+--------+---+----+-----+------+-----+------------+
|                | Ebeam  | T | I  | al  | bl   | el  | Comments   |
|                |        | r | el |     |      |     |            |
|                |        |   | ec |     | deg. | d   |            |
|                |        | \ | t. |     | /keV | eg- |            |
|                |        | % |    |     |      | keV |            |
|                |        |   | mA |     |      |     |            |
|                |        |   |    |     |      | 6\* |            |
|                |        |   |    |     |      | rms |            |
+----------------+--------+---+----+-----+------+-----+------------+
| Ion Source Out | 20kV   |   | 30 | \-  | \-   | \-  |            |
+----------------+--------+---+----+-----+------+-----+------------+
| Prestripper    | 20kV   | 8 | 25 | \-  | \-   | \-  | Nov. 92    |
| RFQ In         |        | 3 |    |     |      |     | PREPET.PAR |
|                |        |   |    |     |      |     | Input      |
+----------------+--------+---+----+-----+------+-----+------------+
| Prestripper    | 1.     | 8 | 20 | .   | .828 | 880 | WDC        |
| RFQ Out        | 003MeV | 0 |    | 283 |      |     | archives   |
+----------------+--------+---+----+-----+------+-----+------------+
| \*             | 1.     | 7 | 28 | -.0 | 1.54 | 1   | Optimised  |
| Poststripper   | 003MeV | 0 | \* | 420 |      | 800 | input DSun |
| RFQ A In       |        |   | \* |     |      |     | 10/95      |
|                |        |   | \* |     |      |     |            |
+----------------+--------+---+----+-----+------+-----+------------+
| Poststripper   | 5.     | 8 | 23 | .0  | .381 | 2   | 28-JAN-93  |
| RFQ A Out      | 047MeV | 2 |    | 194 |      | 500 | PARMTEQ    |
+----------------+--------+---+----+-----+------+-----+------------+
| Poststripper   | 5.     | 9 | 22 | .   | .385 | 2   | 28-JAN-93  |
| RFQ B In       | 047MeV | 5 |    | 116 |      | 500 | PARMTEQ    |
+----------------+--------+---+----+-----+------+-----+------------+
| Poststripper   | 8.     | 8 | 19 | .   | .229 | 3   | 28-JAN-93  |
| RFQ B Out      | 025MeV | 5 |    | 417 |      | 200 | PARMTEQ    |
+----------------+--------+---+----+-----+------+-----+------------+
| Poststripper   | 8.025  | 9 | 18 |     |      |     |            |
| RFQ C In       | MeV    | 5 |    |     |      |     |            |
+----------------+--------+---+----+-----+------+-----+------------+
| Poststripper   | 10.539 | 8 | 15 | .1  | .198 | 4   | 10-JAN-96  |
| RFQ C Out      | MeV    | 5 |    | 404 |      | 478 | PARMTEQ    |
|                |        |   |    |     |      |     | D.Sun      |
+----------------+--------+---+----+-----+------+-----+------------+

\* NOTE: FROM "Prestripper OUT" to "Poststripper RFQ A In" the frequency
of the bunch doubles so that one must double the emittance value and the
beta function to convert from 212.5 MHz units to 425 MHz units.

\*\* NOTE: In the BRI PET project the x and y coordinate axis have been
moved 45 degrees from the SDIO PET project so that now the x axis is in
the vertical plane and the y axis is in the horizontal plane

\*\*\* NOTE: Electrical currents doubles in the MEBT due to the
stripping and charge doubling

TRANSVERSE TWISS PARAMETERS\*\*

<table>
<colgroup>
<col style="width: 13%" />
<col style="width: 12%" />
<col style="width: 8%" />
<col style="width: 8%" />
<col style="width: 7%" />
<col style="width: 9%" />
<col style="width: 9%" />
<col style="width: 11%" />
<col style="width: 0%" />
<col style="width: 20%" />
<col style="width: 0%" />
</colgroup>
<tbody>
<tr class="odd">
<td></td>
<td>Ebeam</td>
<td>av</td>
<td><p>bv</p>
<p>mm/mrad</p></td>
<td><p>ev</p>
<p>pmm-mrad</p>
<p>un-norm</p>
<p>6*rms</p></td>
<td>ah</td>
<td><p>bh</p>
<p>mm/mrad</p></td>
<td colspan="2"><p>eh</p>
<p>pmm-mrad</p>
<p>un-norm</p>
<p>6*rms</p></td>
<td>Comments</td>
<td></td>
</tr>
<tr class="even">
<td>Ion Source Out, Demo Electrodes, 15mA/90%</td>
<td>20 kV</td>
<td></td>
<td></td>
<td>180 ±15</td>
<td></td>
<td></td>
<td>180 ±15</td>
<td colspan="3"></td>
</tr>
<tr class="odd">
<td>Ion Source Out, PET Electrodes,</td>
<td>20 kV</td>
<td>-2.60</td>
<td>.450</td>
<td>100-150</td>
<td>-2.60</td>
<td>.450</td>
<td>100-150</td>
<td colspan="3"><p>TRACE2D 31/jan/91</p>
<p>At exit of extraction electrodes</p></td>
</tr>
<tr class="even">
<td>Prestripper RFQ In</td>
<td>20kV</td>
<td>1.62</td>
<td>.0284</td>
<td>192</td>
<td>1.62</td>
<td>.0284</td>
<td>192</td>
<td colspan="3">Measured FNAL 6/96</td>
</tr>
<tr class="odd">
<td>Prestripper RFQ Out</td>
<td>1.003MeV</td>
<td>-1.17</td>
<td>.130</td>
<td>30</td>
<td>-1.08</td>
<td>.112</td>
<td>30</td>
<td colspan="3">Measure FNAL 6/96</td>
</tr>
<tr class="even">
<td>Poststripper RFQ A In</td>
<td>1.003MeV</td>
<td>2.00</td>
<td>.0460</td>
<td>33</td>
<td>-1.42</td>
<td>.0371</td>
<td>33</td>
<td colspan="3">Calculated D.Larson MEBT 96</td>
</tr>
<tr class="odd">
<td>Poststripper RFQ A Out</td>
<td>5.047MeV</td>
<td>1.51</td>
<td>.0638</td>
<td>8.3</td>
<td>-1.24</td>
<td>.0507</td>
<td>8.3</td>
<td colspan="3">28-JAN-93 PARMTEQ</td>
</tr>
<tr class="even">
<td>Poststripper RFQ B In</td>
<td>5.047MeV</td>
<td>.370</td>
<td>.0221</td>
<td>8.5</td>
<td>-2.46</td>
<td>.134</td>
<td>8.5</td>
<td colspan="3">28-JAN-93 PARMTEQ</td>
</tr>
<tr class="odd">
<td>Poststripper RFQ B Out</td>
<td>8.025MeV</td>
<td>-1.07</td>
<td>.0359</td>
<td>5.5</td>
<td>2.96</td>
<td>.159</td>
<td>6.0</td>
<td colspan="3">28-JAN-93 PARMTEQ</td>
</tr>
<tr class="even">
<td>Poststripper RFQ C In</td>
<td>8.025 MeV</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td colspan="3"></td>
</tr>
<tr class="odd">
<td>Poststripper RFQ C Out</td>
<td>10.539 MeV</td>
<td>-.3883</td>
<td>.0398</td>
<td>6</td>
<td>1.8295</td>
<td>.1489</td>
<td>6</td>
<td colspan="3">10-JAN-96 PARMTEQ D.Sun</td>
</tr>
</tbody>
</table>

1.  \*\* NOTE: In the BRI PET project the x and y coordinate axis have
    been moved 45 degrees from the SDIO PET project so that now the x
    axis is in the vertical plane and the y axis is in the horizontal
    plane

**2. Ion Optical Design of the RFQ PET Accelerator.**

**Comparison Between End to End Design and Performance.**

**Introduction**

The BRF-FNAL-SAIC-UW PET Accelerator is now operational, and much has
been learned about its operation.

The initial expectations for device performance were:

-   150 µA (acceptance criterion) to 300 µA (design) of average 3He++
    beam current at an energy of 10.5 MeV

-   Beam uniformity at the target considerably better than what would be
    expected from a Gaussian beam

The actual performance to date (March 1998) is:

-   Up to 120 µA of average 3He++ beam current at an energy of 10.5 MeV

-   Beam uniformity at the target considerably better than what would be
    expected from a Gaussian beam

This report section will present a comparison of the design and
performance for the device, treating each section individually. As will
be discussed further below, the low and high energy matching lines have
worked very well, the medium energy transfer line and stripper is not at
its design goal, and the RF and/or RFQs have not functioned up to their
initial design expectations.

**Review of Accelerator Design Parameters.**

The PET Accelerator consists of a ^3^He ion source, a low energy beam
transport (LEBT) region, a 212.5 MHz RFQ, a medium energy beam transport
(MEBT) region, a 425 MHz RFQ string, and a high energy beam transport
(HEBT) region. To achieve the most efficient acceleration of the ^3^He
ions to the energy needed for isotope production, it is desirable to
doubly ionize the ^3^He beam. This is accomplished by initially
accelerating the 21.5 keV ^3^He^+^ source beam to 1 MeV through the
212.5 MHz RFQ. At this energy the ionization cross sections are such
that a gas jet stripper can be used to remove the second electron from
the ^3^He^+^. The resulting ^3^He^++^ ions are then transported and the
transverse and longitudinal beam parameters manipulated for matching
into the subsequent RFQ. The ^3^He^++^ beam can now be efficiently
accelerated to approximately 10.5 MeV through the 425 MHz RFQ string.
The MEBT, which has a principal function of matching the beam into the
425 MHz RFQ string, was anticipated to be the most challenging section
to design and implement. Due to space and vacuum constraints, a design
based on a near-symmetric, 540 degree isochronous triple focus bend was
adopted.

The final summary design report for the PET accelerator is entitled "End
to End PET Design" and is available on the PET WWW page.^1^ Table 2.1
presents an overview of the end to end design, specifying peak current,
normalized emittance, and the analysis codes used for each section of
the device.

The end to end design uses typical peak current values throughout. To
convert to average current values one must multiply by the design duty
factor of 2.5%. Using this conversion factor, the design electrical
average current output at the end of the accelerator is 300 microamps.
Quoted emittances are normalized, 90% (5 RMS).

The study of the PET accelerator has been divided into various sections.
For this project the source is defined as the portion of the device
between the plasma and the ground electrode; the LEBT is defined as the
portion of the device between the source and the 212.5 MHz RFQ; the MEBT
is defined as the portion of the device between the 212.5 MHz RFQ and
the first 425 MHz RFQ; and the HEBT is defined as the portion of the
device between the last 425 MHz RFQ and the target. A diagram of the
accelerator appears in Figure 2.1.

+--------------------+--------------------+--------------+--------------+
| **Table 2.1.       |                    |              |              |
| Parameters of the  |                    |              |              |
| End to End Ion     |                    |              |              |
| Optics Design.**   |                    |              |              |
+--------------------+--------------------+--------------+--------------+
| **Beam Section**   | **Analysis Codes   | **Peak       | **Output**   |
|                    | Used**             | Output**     |              |
|                    |                    |              | **Transverse |
|                    |                    | **Electrical | Normalized   |
|                    |                    | Current**    | Emittance**  |
+--------------------+--------------------+--------------+--------------+
| Source             | EGUN               | 24 mA        | 0.76 p mm-mr |
+--------------------+--------------------+--------------+--------------+
| LEBT               | TRACE 2-D          | 24 mA        | 0.76 p mm-mr |
+--------------------+--------------------+--------------+--------------+
| 212.5 MHz RFQ      | PARMTEQ            | 20 mA        | 0.8 p mm-mr  |
+--------------------+--------------------+--------------+--------------+
| MEBT               | TRACE 3-D, SCAT,   | 28 mAa       | 0.9 p mm-mr  |
|                    |                    |              |              |
|                    | TRANSPORT          |              |              |
+--------------------+--------------------+--------------+--------------+
| 425 MHz RFQ        | PARMTEQ            | 12 mA        | 0.49 p       |
|                    |                    |              | mm-mrb       |
+--------------------+--------------------+--------------+--------------+
| HEBT               | TURTLE, TRACE 2-D  | 12 mA        | N/A          |
+--------------------+--------------------+--------------+--------------+

a - Stripping occurs which doubles the input electrical current within
the MEBT.

b - Intentional scraping of the beam occurs reducing normalized
emittance.

![](media/image2.wmf){width="5.985416666666667in"
height="2.6798611111111112in"}

*Figure 2.1. Schematic Diagram of the PET accelerator.*

**Section by Section Comparison of Design vs. Performance as of October
1, 1997.**

Improvements to the PET accelerator are ongoing, and here we will use
data logged up to October 1, 1997. The operation up to that point had
been worked on diligently, with each device section tuned to obtain
maximum accelerator achievement.

*Achieved vs. Design Beam Current***.**

Table 2.2 compares the design values of beam current with those achieved
to date for the various sections of the PET accelerator. Table 2.3
converts the beam current design values into percentage transmission
coefficients for each accelerator section. Table 2.4 compares the design
and performance duty cycle and the design and performance average
current for the total project.

  ----------------------- ----------------------- -----------------------
  **Table 2.2.                                    
  Performance vs. Design                          
  for Each Section of the                         
  PET Accelerator.**                              

  **Beam Section**        **Design Output Peak    **Achieved Output Peak
                          Electrical Current**    Electrical Current**

  Source                  24 mA                   24 mA

  LEBT                    24 mA                   24 mA

  212.5 MHz RFQ           20 mA                   15 mAb, 14 mAc

  MEBT/stripper           28 mAa                  21 mAa,b, 14 mAa,c

  425 MHz RFQ             12 mA                   7 mAb, 5.5 mAc

  HEBT                    12 mA                   7 mAb, 5.5 mAc
  ----------------------- ----------------------- -----------------------

a - Stripping occurs which doubles the input electrical current within
the MEBT.

b - Results recorded for low repetition rate (60 Hz) operation.

c - Results recorded for 360 Hz operation.

+-----------------------+-----------------------+-----------------------+
| **Table 2.3.          |                       |                       |
| Performance vs.       |                       |                       |
| Design Transmission   |                       |                       |
| Coefficients**        |                       |                       |
|                       |                       |                       |
| **for Each Section of |                       |                       |
| the PET Accelerator - |                       |                       |
| Peak Currents.**      |                       |                       |
+-----------------------+-----------------------+-----------------------+
| **Beam Section**      | **Design Transmission | **Achieved            |
|                       | Coefficient**         | Transmission          |
|                       |                       | Coefficient**         |
+-----------------------+-----------------------+-----------------------+
| Source                | 24 mA                 | 100%                  |
+-----------------------+-----------------------+-----------------------+
| LEBT                  | 100%                  | 100%                  |
+-----------------------+-----------------------+-----------------------+
| 212.5 MHz RFQ         | 83%                   | 63% b, 58% c          |
+-----------------------+-----------------------+-----------------------+
| MEBT                  | 70% a                 | 70% a,b, 50% a,c      |
+-----------------------+-----------------------+-----------------------+
| 425 MHz RFQ           | 43%                   | 39%                   |
+-----------------------+-----------------------+-----------------------+
| HEBT                  | 100%                  | 100%                  |
+-----------------------+-----------------------+-----------------------+

a - Stripping occurs which doubles the input electrical current within
the MEBT.

b - Results recorded for low repetition rate (60 Hz) operation.

c - Results recorded for 360 Hz operation.

  ----------------------- ----------------------- -----------------------
  **Table 2.4.                                    
  Performance vs. Design                          
  for Duty Cycle and                              
  Average Current**                               

  **Quantity**            **Design Value**        **Achieved Value**

  Duty Factor             2.5%                    2.0%

  Average Current         300 uA                  110 uA
  ----------------------- ----------------------- -----------------------

As can be seen from the above tables, the source, LEBT and HEBT have
reached their design goals. The MEBT/stripper has equaled its peak
design goal when run at low duty cycle (60 Hz), but achieves only 72% of
its design transmission goal when running at 360 Hz, as stripping
efficiency falls off at the higher repetition rate. The original 212.5
MHz RFQ is the furthest away from meeting its design goal, achieving 76%
of its peak design transmission at low duty cycle, 70% of its peak
design transmission at 360 Hz. The 425 MHz RFQ string meets 91% of its
peak design transmission at 360 Hz. All RFQs are operating at 80% of
their design duty factor, causing a further reduction in achieved
average current.

Much of the falloff in beam transmission as the repetition rate increase
from 60 Hz to 360 Hz occurs in the stripper/MEBT, and is due to several
factors. Because of the presence of neutralizing electrons, it is
difficult to precisely apportion losses between the stripper and the
MEBT. But our best understanding of the efficiencies is as follows.
Measurements made on the spectrometer test stand showed a maximum
stripping efficiency of 83% in argon. When operating in nitrogen under
typical operation, the typical stripping efficiency is expected to be in
the range of 75% at low repetition rate. Thus 15 mA beam current into
the stripper becomes 22.5 mA of He^++^ beam. The transmitted current
through the MEBT at 60 Hz is 21 mA when the MEBT is tuned for maximum
throughput. The difference (7%) is likely due to beam scraping in the
MEBT. When the MEBT is tuned for maximum 10.5 MeV beam, the transmitted
current falls to typically 18 mA. In this case, MEBT transmission is
about 80%, with the remainder of the beam presumably lost to scraping in
the MEBT. At 360 Hz, with MEBT tuned for maximum 10.5 MeV beam, the
transmission through the MEBT is 14 mA ( an additional drop of 22%). The
majority of the additional loss is probably due to the stripper, as the
gas throughput is limited to the maximum capacity of the vacuum pumps.
Other contributing factors include a small drop in RF power and RFQ
output as repetition rate increases to 360 Hz, and a possible small
charge-exchange loss in the MEBT as the background gas pressure rises.

The performance of the device at 360 Hz can be summarized as:

Source - 100% of peak design current obtained.

LEBT - 100% of peak design current obtained.

212.5 MHz RFQ - 70% of peak design current obtained.

MEBT/stripper - 72% of peak design transmission obtained.

425 MHz RFQ string - 91% of peak design transmission obtained.

HEBT - 100% of peak design transmission obtained.

Duty cycle - 80% of design rate achieved.

The overall efficiency, 1 x 1 x 0.7 x 0.72 x 0.91 x 1 x 0.8 = 0.367,
leads to 110 uA output current out of our 300 uA design.

*Achieved vs. Design Beam Uniformity.*

The HEBT section of the PET accelerator was designed to fold in the
tails of the vertical beam distribution in order to make a more uniform
beam at the target window. This was done in order to spread the heat
load of the beam more evenly on the window, in order to enhance its
survivability. Evidence is that we have achieved a beam uniformity of
plus and minus 50% about a mean value over the desired window aperture.
This achievement is in reasonable agreement with the TURTLE design
studies. The achievement of 50% about the mean is twice the value of 25%
that we had proposed as a shipping criterion for the accelerator.

The specifications for delivery of the PET accelerator were arrived at,
as they usually are, by first doing an extremely careful design analysis
of the entire accelerator and then by estimating how closely the actual
implementation would come in achieving that design. The careful design
study involved use of EGUN for source modeling, TRACE 2-D for LEBT
modeling, TRACE 3-D for MEBT modeling, PARMTEQ for RFQ modeling, and
TURTLE for HEBT modeling. While the design value for the average current
output from the device could be fairly accurately determined as a
function of beam emittance from the PARMTEQ work, the beam uniformity
specification could not easily be determined. This is because 1) the
TURTLE codes used in HEBT analysis were only capable of simulating
10,000 rays and 2) the beam distribution entering the HEBT was unknown.

The limitation in the number of rays simulated meant that the number of
rays entering any individual pixel was in the neighborhood of 5-15, and
statistical variations due to the small number of rays per pixel made an
accurate assessment of the beam uniformity difficult. Also, there is
debate generally, and for the PET project specifically, about what sort
of beam distribution one assumes for the input into the HEBT beamline.
While it is popular to assume a Gaussian particle beam distribution in
each of three uncoupled phase space planes, actual beam distributions
are often parabolic, uniform, or hollow and coupling between the phase
space planes is often relevant. Due to the uncertainties in beam
distribution entering the HEBT, the approach taken was to use the 10,000
ray TURTLE code to model the HEBT assuming a Gaussian, uncoupled, three
dimensional beam at the HEBT input.

The plan decided upon was to use quadrupole and multipole magnets
inherited from the previous SDIO project in the HEBT. The magnetic
fields of these magnets were measured and those measurements were used
in the TURTLE study. Drift lengths in the HEBT were varied until the
beam size and uniformity on target achieved the desired characteristics.
The desired uniformity characteristic was that the beam have a uniform
vertical profile and a Gaussian horizontal profile. (See the end to end
design report, available on the WWW.)

Modeling predicted, and experiment has verified, that the quadrupole
magnet can be adjusted to vary the width of the horizontal Gaussian
distribution. Thus, when the specifications for the machine were drawn
up, there was a tradeoff between the amount of current one could deliver
to the chosen target and the uniformity of the beam at that target. (One
could simply defocus the beam horizontally to achieve a wider, flatter
Gaussian, throwing away the beam in the tails. Indeed, this has been
verified experimentally, as we have been able to achieve 25% uniformity
of the beam, but at the expense of throwing away about half of that
beam.) To date we have yet to achieve 150 µA of average current even
when the beam is focused tightly at the target. For that reason we do
not have the luxury of discarding beam at the target location, and we
have a less uniform beam than specified originally.

*Beam Studies.*

It was strongly suggested by DOE and national lab reviewers that
detailed beam studies be done at every point along the machine. The
source and LEBT beam emittance was studied in great detail, as was the
beam emittance with and without a stripper at 1 MeV. The 1 MeV beam
throughout the MEBT was carefully studied using wire profile monitors, a
spectrometer magnet, an emittance scanner, and a spectrum analyzer. 5
MeV beam studies were also done. At each step the measurements are in
quite good agreement with the final beam optics design.

*Pointer to More Detailed Documentation.*

This report is meant only to convey the nominal design of the PET
accelerator, and is not meant as an all encompassing detailed study. For
more detailed information on the various beam sections the reader is
referred to the documents entitled: "LEBT 96"; "MEBT 95"; "MEBT 96";
"Report on End to End Simulation of RFQ A, B and C"; and "HEBT 96". All
reports are on the PET Project\'s World Wide Web Site or can be obtained
from the office of the PET project manager.

**Operations with the New 212.5 MHz RFQ.**

The original 212.5 MHz RFQ was replaced in October, 1997. The primary
reason for the replacement was corrosion in the vane water passages.
There were grave doubts that the original 212.5 MHz RFQ would survive
through 1998. Since the 212.5 MHz RFQ could only produce 70% of its
expected peak output current, a redesign of the vane structure was also
done. The new vane structure was predicted to lead to greater beam
output from the 212.5 MHz RFQ, but it was also predicted to lead to more
scraping of beam in the MEBT, since the MEBT was designed assuming
output conditions from the original RFQ.

Accurate measurements of emittance and Twiss parameters exist only for
the old 212.5 RFQ. Measurements to date indicate that he new 212.5 MHz
RFQ produces significantly more beam (18 mA peak) than the old RFQ but
much of the additional current is lost in the MEBT/stripper.
Unfortunately we did not have the opportunity to re-measure output
emittance and Twiss parameters for the new RFQ, due to budget and
schedule constraints. Without such measurements it is difficult to
ascertain specific performance characteristics. It has been noted that
beam transmission through the MEBT peaks at significantly different
magnet settings than the total 10.5 MeV beam transmission peaks. This
may be indicative of beam scraping, which could be caused by either a
larger output emittance in the new 212.5 MHz RFQ, or a mismatch in Twiss
parameters. Some additional scraping, due to a mismatch in Twiss
parameters, was expected.

**Where and Why the PET Accelerator Fails to Meet Expectations -
Summary.**

*Original 212.5 MHz RFQ Operation - Identifying Where Expectations Fell
Short.*

With the original 212.5 MHz RFQ the PET accelerator had clearly measured
problems in the 212.5 MHz RFQ transmission, MEBT/stripper performance,
and 425 MHz RFQ string performance. In each case, failure to achieve
design goals can be attributed to overly aggressive initial design
parameters. At each step of the design process numbers were chosen that
represented the upper limits of possible theoretical designs. The
project was originally sold through the proposal process as 300 mA
electrical ^3^He accelerator, and the design had to be very aggressive
in order to achieve this.

As shown in detail above, those sections which do not meet the design
are:

Duty cycle - 80% of design rate achieved.

212.5 MHz RFQ - 70% of peak design current obtained.

MEBT/stripper - 72% of peak design transmission obtained.

425 MHz RFQ string - 91% of peak design transmission obtained.

It was expected that the duty cycle of the RF power units would be in
the range of 2 - 2.5%. The higher (more aggressive) value was used in
our design study; but the lower limit was what we have been able to
achieve in practice.

The pre-stripper RFQ design originated with the earlier SAIC/UW project.
The earlier project design specified a 20 mA output from the
pre-stripper RFQ. Operations during that project were reported to be
about 18-20 mA, although the documented beam currents from that project
were less than this number. Based on the oral reports of 1992
performance and modeling, 20 mA was chosen for our design.

The MEBT was predicted to transport between 55% and 100% of the beam,
depending on space charge considerations. The stripper was measured to
convert 80-85% of beam into doubly charged ^3^He when operating at 60 Hz
or less, and 70% when operating at 360 Hz. Again, the most aggressive
parameters were chosen for design values.

The 425 MHz RFQ string was predicted to have between 16.8% and 60%
transmission, strongly depending on input emittance. Measurements
excluded the 60% value from possibility, so the most optimistic value of
43% was chosen for our design.

The transmission designs led to a prediction that the PET accelerator
could achieve up to 300 uA of average current. Realizing the
aggressiveness of the design, a value of 150 uA average current was
chosen for the shipping criterion of the accelerator.

*Underlying Reasons for Lower than Desired Transmissions.*

In summary, we have identified three major causes for the lower than
desired transmission.

1\. [Overly-aggressive goals]{.underline}. It is clear that the present
project made aggressive goals for beam emittance and transmission in
order to achieve the 300 uA of average current stated in the original
proposal. The value for transmission of the old 212.5-MHz RFQ of 83%
quoted in the end-to-end simulation was based on the original 1990
design. It was not confirmed in experiment or simulations performed in
the present project. The RFQ design used a value for the assumed beam
emittance of 0.5 p mm-mr, 90% normalized. Under such an assumption, beam
transmission through all of the RFQs becomes much greater (see below),
and 300 uA of average current is readily obtained. Unfortunately,
measured emittances from the LEBT through the MEBT are significantly
higher.

2\. [Falloff in performance from 120 Hz to 360 Hz]{.underline}. We have
experienced a larger drop off of MEBT/stripper transmission than
expected as we go from 120 Hz to 360 Hz. For example we have seen 47 uA
of steady average current at 120 Hz, but only 110 uA of steady average
current at 360 Hz. As described above, the majority of the shortfall is
due to increased gas loads in the stripper. Despite the pulsed nature of
the stripper, at 360 Hz the gas load on the vacuum pumps reaches their
pumping capacity. This fact requires us to lower gas flow into the
stripper region, below that which would be experimentally optimal as
scaled from 120 Hz operations.

3\. [Duty factor]{.underline}. Finally, duty factor (beam pulse length)
has been limited by the RF equipment to 2% instead of the originally
proposed 2.5%, a significant limitation.

**3He+ Duoplasmatron Ion Source**

**Introduction**

The ion source for the PET Project is a standard duoplasmatron type ion
source operated at high peak power (7.5 kWatt) and modest duty factor
(\~3%, i.e. 360 Hz, 80 microsecond pulse) with helium. Twenty-five
milliamps of 3He beam at 21.5 kV is extracted from the source for
injection into the first RFQ. 4He is used for startup and maintenance of
the source to save on 3He cost.

![](media/image3.wmf){width="3.111111111111111in"
height="2.3055555555555554in"}

*Fig. 2.2 Pet Duoplasmatron Ion Source*

**General operation**

The source uses a heated oxide coated (barium and strontium oxide)
filament as an emitter of electrons. A pulsed \"arc\" voltage between
the filament (-) and source anode (+) extracts and accelerates the
electrons into the He gas, ionizing the gas within the source and
producing a dense plasma while the arc pulse is on. The produced plasma
diffuses throughout the source. A strong magnetic field between the iron
poles of the intermediate electrode and the anode converges on the anode
aperture. This field pinches the plasma through the small aperture and
into the expansion cup from which ions are extracted. A small aperture
(0.5-mm dia. x 0.25-mm long) is important for minimizing the gas flow
from the source to conserve the usage of 3He, maintain a good vacuum in
the extraction region, and minimize vacuum pumping. Typical pressure in
the plasma generating region is 1-1.5 Torr and several times 10-6 Torr
in the extraction region. With the source at +21.5 kV and the extraction
electrode at ground potential a 21.5-keV 3He+ beam is extracted. Just
before the extraction electrode is an electron trap electrode (-1 to -4
kV) which traps electrons within the beam to assist neutralization and
prevents electrons from being accelerated back onto the source. This may
also improve the intensity slightly and give some focusing to the beam.

**Performance**

For the needs of this project the source is expected to have:

• 25-mA 3He+ beam (peak),

• 80-µsec pulse width,

• 360-Hz repetition rate,

• low emittance,

• low gas consumption,

• long lifetime (3 months), and

• reliable operation.

The source has shown that it can meet these objectives, with the
exception of long lifetime in recent history.

The source typically produces 23-25 mA of 3He beam at the extraction
voltage of 21.5 kV which is the injection energy for the following RFQ
accelerator. A beam of \~26 mA is the theoretical (Child-Langmuir) limit
for this extraction gap (10.6 mm), aperture (6.6 mm) and voltage (21.5
kV). The estimated beam emittance for the initial 21.5-keV 3He beam,
extrapolated from the emittance measured at the end of the 21.5 keV line
(LEBT) is approximately 0.8-1.0 p mm mr for the normalized (90%)
emittance.

For accelerated beam in the RFQs and isotope studies the source operates
on 3He gas. To conserve 3He and operational cost, the source is started
and operated on 4He whenever beam is not needed for extended periods of
time. Switching from 4He to 3He or back requires 3-4 minutes and is done
by opening and closing two valves through the control system.

The source operates at the full 360 Hz, with an \~80 microsecond pulse
width (\~2.8% duty-factor). It has operated for at least two long
periods of 3-3.5 months without failure. During these periods the source
was on at least 90% of the time and only turned off when necessary for
accelerator work.

Unfortunately the source hasn\'t always been this good in terms of
lifetime. There have been periods when the source didn\'t operate or
failed within a few days due to filament failure. Recently it has
operated for only 1-3 weeks before the filament loses emission and fails
due to poisoning from a contaminant. However, during these times the
other objectives of the source were essentially met so long as the
source was given sufficient attention to maintain operation. It is
expected that 3-4 months of reliable and easy operation can be achieved
with further investigations and improvements.

**Modifications**

The original source was a commercial duoplasmatron for producing
hydrogen ions at a low duty-factor (\<1%). For the PET Project several
modifications were necessary. It could not operate at 360 Hz with a
pulse length of 80 µsec without over heating the filament. A large gas
consumption due to a larger aperture and several misplaced vented screws
were also found. A change in some of the electronics was also desirable.

*Filament*

To achieve a useful lifetime (a few months), it was necessary to make
significant changes to the filament. The original filament, made of
nickel wire mesh, would not withstand the heating by the plasma at full
duty factor (The accelerator is designed for 2.5% duty cycle and the
source provides a wider beam pulse for a duty cycle of 2.8%). Heating
and helium bombardment would make the nickel brittle so that it cracked,
developed voids, and broke within a few days. Nickel, being an active
metal, absorbs helium and becomes brittle. Changing to a molybdenum wire
mesh for the filament overcame this problem. With molybdenum there is
significantly less damage from helium bombardment so the mesh remains
flexible throughout its lifetime. The higher melting point of molybdenum
gives more latitude for conditioning and heating the filament and allows
operation at the full duty-factor.

![](media/image4.wmf){width="3.3048611111111112in"
height="1.3743055555555554in"}

*Fig. 2.3 Source filament.*

To use molybdenum, new assembly techniques needed to be developed. Spot
welding molybdenum is rather difficult due to the high melting
temperature. However, spot welding molybdenum to nickel works well, so
the molybdenum mesh is spot welded to nickel rods utilized for current
leads and support. Since the rods are relatively large they do not get
hot enough to melt. In places where the molybdenum mesh is folded for
additional strength, a thin strip of nickel is placed between the fold
to strengthen the weld. This has made working with molybdenum filaments
rather simple and easy.

In the duoplasmatron source it is customary to use an oxide coated
filament to produce the necessary electron emission, at a reasonable
temperature, for high plasma production, hence it is important to
understand the oxide coating on the filament. The initial coating
material is a powdered mixture of BaCO3 and SrCO3 - purchased
commercially and called a \"radio mixture\" in the electrochemical
industry. It is mixed with a solvent to form a slurry with which the
final shaped mesh can be coated. After the filament has fully dried it
is heated in air to \~300oC to burn away the solvent. The final
conditioning, to convert the carbonates to oxides, is done in a good
vacuum. The carbonates decompose starting at \~950oC converting to an
oxide which is complete at \~1100oC. Thus it is important to reach a
sufficiently high temperature to fully produce the oxide coating. While
outgassing the solvent, H2O and CO2 develop. The heating should be
controlled so the pressure does not exceed 0.2 Torr. Once the outgassing
ends and the vacuum is 10-5 Torr or better, activation is done by
placing several hundred volts between the filament (-) and a close metal
surface (+). When fully activated the emission should be several hundred
milliamperes at 100 V or less. Following conditioning, storage should be
in an inert atmosphere. The filament should be in air for a minimum time
to reduce contamination and conversion of the oxide to a hydroxide
(Ba(OH)2). Once placed in the source the filament will need to be
outgassed and conditioned again, but for a much shorter time.

*Gas consumption*

To minimize the usage of 3He gas, it was necessary to close all
unnecessary openings across the anode surface. This required removing a
few openings that vented the gas to the high vacuum region. In addition
it was possible to reduce the anode aperture from 1 mm to 0.5 mm. The
present helium gas flow for operation is 2-3 SCCM (standard cubic
centimeters per minute). This represents a 3He consumption of \~1
liter(stp) per an 8-hour period (1 liter(stp) of 3He gas costs \~\$100).

*Electronics*

Using a combination of ideas from Fermilab and SAIC, an arc modulator
supply was developed that was simple, efficient and has been very
reliable. The arc modulator was reduced to essentially three parts. A
PFN (pulse forming network) to store and provide a minimum of energy
during the pulse, a charging power supply to recharge the PFN between
pulses, and an IGBT (insulated gate bipolar transistor) solid state
device to switch the pulse on and off. A modulator of this type has been
operating on the source for over a year switching 50-100 A, 100-300 V at
360 Hz without failure. The complete unit is in a 9-inch-high standard
chassis and produces little heat. The triggering unit requires a 5-V
gate to turn the IGBT on and off.

![](media/image5.wmf){width="3.1659722222222224in"
height="1.9583333333333333in"}

*Fig. 2.4 Basic arc modulator circuit*

**Improvements**

*Source*

A significant difficulty with long term operation (\~3 months or more)
is the filament lifetime due to poisoning from contamination. As the
oxide coated filament becomes poisoned it must run hotter to compensate
for what would be a loss in emission. As the temperature increases the
filament eventually destroys itself. Contamination has been found to
come from several areas.

Water vapor was found coming from the plastic support insulators holding
the source and extraction electrodes after they were in air for a few
months during a summer shutdown. When vacuum was reestablished the LEBT
chamber supporting the source would only pump down in the 10-6 Torr
range and the filament failed to achieve good emission when installed in
the source. Very large amounts of water vapor and some hydrocarbons were
observed in the gas spectra. After the chamber insulators and parts were
cleaned and vacuum baked, the vacuum improved to the low 10-7 Torr and
the source operated well. The chamber support insulators and epoxy parts
can be used but they must be carefully maintained and outgassed for good
source operation. Ideally it would be desirable to change to ceramic
insulators and remove all internal gassy parts from the vacuum system.

Recently the gas handling system was relocated. This was useful for
removing the valves and gas controls from the high voltage enclosure
which reduces the electronic components at high voltage and their
potential for failure. It also moved the He bottles from the ion source
electronics rack and placed the gas assembly in a convenient space at
ground potential and closer to the source. Connecting to the source
required approximately a 15-foot length of tubing to prevent discharges
through the gas line. The end result decreased the time to change from
4He to 3He (or back) from \~15 minutes to \~3 minutes. Unfortunately an
SO2 contamination was recently observed which appears to be coming from
the long Teflon-type gas feed line. Sulfur, among many other things, is
a known contaminant for oxide coated filaments. It slowly poisons the
filament, requiring it to run hotter and causing it to fail after one to
three weeks from over heating. Different tubing and possibly glass is
being considered for this line.

If the source were to be used for long term or in a clinical
environment, significant redesign should be considered. The filament is
poorly supported since the feedthroughs are weak and ceramic beads are
used to keep the filament shield from sagging and touching the
intermediate electrode. Unfortunately the filament parts connect to the
intermediate electrode base plate. This plate is small and held by
screws to the intermediate electrode making it difficult to modify. A
better arrangement is to mount the filament from an insulated base plate
on strong feed through insulators. A threaded collar rather than screws
would facilitate filament changes.

Several other areas of the source could be improved. The intermediate
electrode has shown some erosion and should be designed so its front can
be replaced. A smaller anode aperture made of tungsten should be placed
over the source anode. Since this source is a commercially produced item
there are no complete drawings available. Before the source could be
used by untrained personnel considerable rework and possibly a new
design with drawings, would be needed.

*Electronics*

The arc modulator is a Fermilab-SAIC designed unit that operates with
excellent reliability. The magnet supply is a commercial power supply.
The source pressure is measured by a barotron gauge. These units are all
at high voltage and are monitored and controlled through the control
system, which is also at the extraction voltage (21.5 kV).

The current to heat the filament comes from an ac power transformer
which is manually controlled with a Variac. Since the filament current
is seldom adjusted, once the source is operating, it has been adequate
but there is no remote control or monitoring. Remote control and
monitoring of this function would be desirable.

The filament temperature should be remotely monitored. Since the
filament is at the arc potential (\~150 V, 50 A), the thermocouple
signal, a few tens of millivolts, has considerable noise. A useful
signal of the temperature has not been implemented in the control
system. Instead a panel meter shows some measure of the filament
temperature.

Improvement of the filament supply and filament temperature monitoring
are desirable but do not prevent present operation. Complete spare
supplies are planned in the future.

**Summary**

The duoplasmatron source is capable of achieving the objectives of the
present PET Project. It has on several occasions fully met the
objectives. On several other occasions, however, it failed to have a
desirable or even useful lifetime primarily due to contamination
problems. In one case this was found and corrected by cleaning and
baking insulators. The present case is under investigation but a
contamination from the gas feed tube appears likely. Contamination is a
serious problem for ion sources. Following good vacuum practices and
using good vacuum material are mandatory. Contamination has been the
greatest difficulty with using the PET source but it is an obstacle that
can be overcome.

Several things which could be done to further improve the source have
been mentioned, such as: ceramic support and electrode insulators;
removing or isolating parts in the LEBT vacuum chamber; modifying the
source assembly for better filament support and easier changing; and
improving the intermediate electrode and anode. These are relatively
expensive changes and are not necessary for current operations with
experienced personnel. For the present project these would probably be
more disruptive than beneficial. However, if this project were to go
into a further accelerator development phase, these modifications would
be beneficial.

The ion source design is discussed in more detail in the document
entitled "LEBT 96" (available on the WWW). The beam optical performance
of the ion source is in excellent agreement with its design.

**Low Energy Beam Transport (LEBT) Section.**

The LEBT design is discussed in the document entitled "LEBT 96"
(available on the WWW). The beam optical performance of the LEBT is in
excellent agreement with its design. Essentially 100% beam transmission
is obtained, and the emittance has been measured to be 0.8-1.0 p mm-mr
at the end of the LEBT.

As expected, some minor difficulties occurred during LEBT commissioning,
and these have been solved over time. The magnetic center of the LEBT
solenoid was measured to not coincide with the physical center. This
required the solenoid to be moved somewhat away from center. In its
original incarnation the beam pipe that fit inside the solenoid was too
large to allow for sufficient solenoid movement. The original beam pipe
was replaced by a smaller one, and that allowed the LEBT to function as
designed. A second issue is space charge neutralization within the LEBT.
As discussed more fully in "LEBT 96", the ion optics of the solenoid
appear to be best modeled under an assumption of full space charge
neutralization. This neutralization of course depends on gas pressure in
the LEBT. We have experimented with a technique that bleeds small
amounts of argon gas (1-2x10-5 Torr) into the LEBT region to assist
space-charge neutralization. Throughput of the 212.5 MHz RFQ and
subsequent components increases approximately 10% by this technique. As
the reliability of the remainder of the accelerator improves, this
technique may be revisited as a potential performance enhancer.
Transported beam currents quoted above are in the absence of LEBT gas
injection.

**Pre-stripper and Post stripper RFQ**

**Introduction**

The PET RFQ accelerator consists of four RFQs (pre-stripper RFQ,
post-stripper RFQ A, B and C). In this chapter, general RF structure and
tuning, design, basic operation results, existing problems and possible
future improvement of these RFQs will be discussed. Since the project
started at Fermi lab, all three original RFQs (pre-stripper RFQ and
post-stripper RFQ A and B) have been re-built and a fourth RFQ
(post-stripper RFQ C) has been added. The re-built post-stripper RFQs A
and B are the same as the previous ones. The contour of the vane tips of
the re-built pre-stripper RFQ was slightly modified, but the basic RF
structure was kept the same. The basic RF structure of the new RFQ C was
also kept the same as the RFQs A and B. The decision not to change the
RF structure such as the aperture is due to several reasons: the
available RF power, the time schedule and budget limit of this project
which has been reported in the general introduction section of this
report.

**RF Structure and Tuning**

The pre-stripper RFQ has a segmented 4-vane structure: "windows" were
cut through the vanes1 to reduce the radial dimension of the RF cavity.
There are three "windows" in each quadrant: one is at the middle and two
are at the ends of the vanes (Figure 2.5). One extra benefit of this
segmented 4-vane structure is that it moves the frequency of the
TE210-like mode (the operating mode) below that of the TE110-like mode
(the dipole mode). However these windows disturb the longitudinal
uniformity of the field which has been confirmed by both field
measurement and calculation (using computer code MAFIA).

The post-stripper RFQs are typical 4-vane type RFQs: there are no
windows. All three post-stripper RFQs are connected together. (There is
an end wall between each other.) These RFQs are individually powered by
separate RF amplifiers.

The vanes and cavities are made of copper plated aluminum. RF joints
between the vanes and the cavity are made by pure pressured contact.
Each vane is pulled against the cavity housing by 39-42 screws to
maintain the integrity of the contact. The vanes are adjustable: they
can be pulled down, pushed up or tilted by adjusting these screws
(Figure 2.6).

All RFQs were tuned by mechanically adjusting the vanes while the RF
field was measured by a field perturbation method. The final goal for
tuning is that the dipole component of the field must be less than 2% of
the quadrupole mode. Actually this ratio reached 1-1.5% after tuning.
The perturbation method used for tuning the post-stripper RFQs was to
insert a metal cylindrical bar through the holes on the cavity housing
into the outer region of the cavity. A better perturbation method was
developed for tuning the pre-stripper RFQ: a small bead sitting inside a
hollow plastic straw was inserted into the cavity until the straw
touched the vane. Two types of beads were used, plastic and metal. The
advantage of this method is that it can obtain exclusive field
information near the critical region - the vane tip. A beadpull method
was also implemented in this region, however the thread used in the
beadpull introduced a large perturbation which imposed an impractical
restriction on the accuracy of thread positioning. It also was proven by
the measured field data that the straw-bead method is better than the
beadpull method.

RF power is coupled into the cavities through a 1-5/8 inch coax line and
a magnetic coupling loop (drive loop). Each RFQ has only one drive loop.
The vacuum window for the drive loop is made of Rexalite with o-ring
seals.

In each quadrant, there are 23-25 holes (diameter of 0.5 inch) on the
wall of a cavity housing. These holes are used for maintaining the
vacuum inside the cavity. Three of these 23-25 holes are used to insert
a small pickup loop. They are located at the middle and near the ends of
the cavity. All pickup loops in a single RFQ are adjusted to have the
same coupling value to within 0.25 dB (nominal values range between -47
\~ -50 dB). These pickup loops provide the operator with a tool to
monitor the RF field at 12 positions (3 pickup loops in each quadrant)
during operation. Currently only two loops are used in each RFQ cavity
for feedback.

![](media/image6.wmf){width="3.1243055555555554in"
height="1.1520833333333333in"}

*Figure 2.5. Schematic Drawing of Pre-stripper RFQ (segmented vanes, RF
cavity and vacuum tank)*

![](media/image7.wmf){width="3.7354166666666666in"
height="4.027777777777778in"}

*Figure 2.6. Adjustable Vanes and RF Cavity*

**Pre-stripper RFQ**

*Design goal*

The original pre-stripper RFQ was designed and built at SAIC. The one
used now is a slightly modified one. This RFQ is designed to accelerate
3He+ ions from 21.5 keV to 1 MeV with a transmission efficiency of 80%
(or better) for an injected current of 25 mA.

*Design parameters*

The computer program PARMTEQ (SAIC version) was used to do beam dynamics
design and simulation and the computer program RFQVG (SAIC version) was
used to generate the data file of the contour of the vane tips. The
pre-stripper RFQ was designed with constant aperture (r0) and constant
transverse radius. (Here the aperture (r0 ) is a commonly used term in
RFQ design back to the 1970s. It is not the minimum radius (a) of the
vane tips. Rather it may be regarded as a characteristic average radius
of the RFQ vane tips. The designed r0 is 0.25 cm while the minimum
radius of the vane tips is 0.174 cm.)

The detailed design parameters are listed in Table 2.5. The data of the
contour of the vane tips is in document drawing #905A30141-905A30142.

*Table 2.5. Design Parameters of the new Pre-stripper RFQ*

+-------------------------------------------------+--------------------+
| Particle                                        | 3He+               |
+-------------------------------------------------+--------------------+
| Frequency (MHz)                                 | 212.5              |
+-------------------------------------------------+--------------------+
| Aperture ro (cm)                                | 0.25               |
+-------------------------------------------------+--------------------+
| Modulation (at end of buncher)                  | 1.78               |
+-------------------------------------------------+--------------------+
| Input energy (MeV)                              | 0.0215             |
+-------------------------------------------------+--------------------+
| Output Energy (MeV)                             | 1.0                |
+-------------------------------------------------+--------------------+
| Inter-vane Voltage (kV)                         | 57.0               |
+-------------------------------------------------+--------------------+
| Maximum E Field (MV/m)                          | 32.2               |
+-------------------------------------------------+--------------------+
| Maximum E Field (MV/m) (at middle)              | 41.2               |
+-------------------------------------------------+--------------------+
| Q0 (MAFIA)/measured                             | 7600/6100          |
+-------------------------------------------------+--------------------+
| Power (kW) (MAFIA, without beam)                | 65.3               |
+-------------------------------------------------+--------------------+
| Power(kW)(needed, without beam)                 | 81.4               |
|                                                 |                    |
| (power x Q0 )(MAFIA)/(measured Q0)              |                    |
+-------------------------------------------------+--------------------+
| Pulse Width (ms)                                | 70                 |
+-------------------------------------------------+--------------------+
| Repetition Rate (Hz)                            | 360                |
+-------------------------------------------------+--------------------+
| Inject Current (mA)                             | 25                 |
+-------------------------------------------------+--------------------+
| Transmission                                    | 80%                |
+-------------------------------------------------+--------------------+
| Number of cells                                 | 151                |
+-------------------------------------------------+--------------------+
| Length (cm)                                     | 103.965            |
+-------------------------------------------------+--------------------+

*Design note*

Both the experimental and simulation results showed that the
transmission of the original pre-stripper RFQ was about 65%. Therefore a
modification on the contour of vane tips was made to improve the
transmission when the pre-stripper RFQ was redesigned. However there was
a restriction on the modification: the total length of the vanes must be
kept the same as before in order to fit in the vacuum vessel. The
buncher section was made longer and the inter-vane operating voltage was
increased from 54 kV to 57 kV. This operating voltage corresponds to a
32.2 MV/m maximum surface field (2.1 times Kilpatrick limit) which
should not cause a sparking problem. (The formula used to calculate the
Kilpatrick limit is the one used at Los Alamos Lab: f = 1.643E2 e-8.5/E
, where f is frequency in MHz and E is breakdown field in MV/m. This
formula was transformed from the original Kilpatrick formula at Los
Alamos lab. The Kilpatrick limit is only an empirical criterion
predicting the possibility for electrical breakdown to occur at a given
frequency and field. Tests at various accelerator labs have shown that
electric fields as high as several time the Kilpatrick limit can be used
without causing breakdown.)

The RF power needed to get the designed inter-vane voltage is \~81.4 kW
(without beam). (This is calculated by comparing the power and Q
calculated by the 3D RF design program MAFIA with the measured Q: power
(needed) = power(MAFIA)\* Q(MAFIA)/Q(measured).) The total RF power
needed for accelerating 25 mA beam to 1 MeV is 106.4 kW.

The RF field in the pre-stripper RFQ is not longitudinally uniform (it
is 28% higher at the middle of the RFQ) due to the introduction of
windows in the vanes. This has been proven by measurement and agrees
with simulation. Therefore the surface field at the middle of the RFQ is
about 41.2 MV/m (2.7 times the Kilpatrick limit) at the designed power
level (81.4 kW without beam). The corresponding "inter-vane voltage" at
the middle of the RFQ is 73 kV.

One should bear in mind that the field and power quoted here are based
on the modeling results using the MAFIA code. Due to the complicated
geometry of the RFQ, it is not unusual that such modeling results in 10%
or higher inaccuracy.

In the design, the last four cells of the vanes are modified (the
aperture was gradually increased while modulation was gradually reduced
to 1) to make the transverse phase spaces of the output beam close to
each other (diverging) in both the x-x' and y-y' planes. This is to
reduce the large divergence originally present in one plane 3.

Due to machining difficulty caused by the position of the water cooling
channels, the contour of the vane tips at the exit end of all vanes were
slightly modified from the data file generated by RFQVG to raise the
vane tip by \~0.0-0.025 inch. The data in final drawing files 905A30141
and 905A30142 reflect this change.

*Simulation results*

Simulation results show that the transmission is 80% for the input
parameters at the designed inter-vane voltage (57 kV) with no
significant emittance growth. If the operating voltage is increased to
20% above nominal, the transmission can be 87.6%. Shown in Fig 2.7 is
the transmission versus the inter-vane voltage. Detailed simulation
results have been reported in other documents4,5.

Since the RF field is not longitudinally uniform, a modified version of
PARMTEQ was developed at SAIC which can incorporate such an effect into
the simulation. The results from this version of simulation show that
there is no significant increase of beam loss.

![](media/image8.pct){width="407.0in" height="263.0in"}

*Figure 2.7. Transmission Efficiency of the Pre-stripper RFQ (simulation
results) for 0.5 π mm mrad.*

*Operation results*

Operation results with the new RFQ show that an output current of 18 mA
has been obtained out of 21 mA injected current at 120 Hz of repetition
rate. This 85% transmission efficiency is better than the designed
transmission efficiency (80%). The RF power required for this
performance is about 94-100 kW which is also in agreement with the
designed value (99.4 kW for 18 mA). The injected emittance, however, is
60-100% larger than the value chosen for the design. This means this RFQ
actually exceeds the design goal. The output emittance, of course, is
also larger than the designed value for next stage of RFQs. Note that
the currents into and out of the RFQ are measured on current toroids
only, and that we have not done measurements with an emittance scanner
or a suppressed Faraday cup. Therefore these measurements are not as
accurate as one would like.

A frequently asked question is how the operating voltage was determined.
The operating voltage (inter-vane) was determined by power scanning:
gradually adjusting RF power and looking at the transmission efficiency.
From the scanning, the power needed to start accelerating beam at high
transmission efficiency (beginning of a plateau) can be determined. This
power is compared with the power needed (calculated from MAFIA results
and measured Q) to determine the operating voltage. For example, recent
data show that the operating power is 94 kW to get 18 mA output current.
This means that the power for the RF field is \~76-77 kW . Compared with
the 81.4 kW power needed from the MAFIA simulation (adjusted by the
measured Q), the 76 kW power gives an operating voltage 0.97 times the
designed operating voltage. Taking into account the possible deviation
from the MAFIA simulation to the real field, we believe the 76 kW is
very close to the designed inter-vane voltage (57 kV +/- 5%).

The X-ray spectrum was also used to calibrate the operating inter-vane
voltage. X-ray measurements at an RF power level \~120 kW show that the
majority of the photon energy is around 67 kV (plus and minus 20%) while
the high end of the photon spectrum is at 90 keV. Using 120 kW and 90
keV high end energy, a simple calculation shows that the highest
"inter-vane voltage" at 76 kW is 71.6 kV (at middle of the RFQ) which
corresponds to a minimum 56 kV inter-vane voltage in the rest of the
RFQ.

This RFQ has been run at designed or higher power level for a long time
without a sparking problem. This fact proves that the operating voltage
is not very close to the sparking limit.

**Post-stripper RFQs**

*Design goal*

The original design of the post-stripper RFQs had two RFQs to accelerate
3He++ ions from 1 MeV to 8 MeV. Later on, a decision was made to add a
third RFQ to increase the final energy to 10.5 MeV. The total
transmission efficiency (from entrance of RFQ A to exit of RFQ C) is 60%
with aforementioned emittance.

*Design parameters*

All three post-stripper RFQs have similar RF structure. They have only
acceleration sections with constant aperture (r0) and constant
transverse radius. (The r0 is 0.18 cm and the minimum radius of the vane
tips is 0.1097 cm for all three post-stripper RFQs. The definition of r0
has been explained in a previous section.)

The basic design parameters are listed in Table 2.6. The data of contour
of vane tips are in document drawings #905A35101-905A35104 and
905A35163-905A35164.

*Design note*

The length of vanes for all RFQs is limited by the fabrication
capability (\~ 148 cm) of the available computer controlled milling
machine. The corner at each end of the vane tip was chamfered (1/4
circle with radius of 0.5 cm) to facilitate a smooth end. The gap
between the end of a vane to the end wall of the cavity is 0.5 cm. The
thickness of the end wall is 0.16 cm. Adding all these lengths together,
there is a 2.16 cm drift space between each post-stripper RFQ (Figure
2.8). Strictly speaking, this drift space is only an approximation since
it is not a pure (field free) drift space. However the fact that the
transverse RF field in this region is much smaller than in the
acceleration region warrants the approximation. It should be pointed out
that the thickness of the end wall is only 0.16 cm and the rest of drift
space is for RF structure. Therefore the total 2.16 cm space is almost
the minimum space needed for connecting two separate RFQs.

The designed inter-vane voltage is 58.4 kV (RFQ A and B) and 59 kV (RFQ
C) corresponding to a \~39.9-40.3 MV/m maximum surface field
(\~2.0-2.025 times Kilpatrick limit) which should not cause a sparking
problem. The RF power needed to get the designed inter-vane voltage is
71-77 kW (without beam). The method to estimate this power has been
described in pre-stripper RFQ section.

*Table 2.6 Design Parameters of the Post-stripper RFQ*

+-------------------------------+------------+-----------+------------+
| Particle                      | 3He++      | 3He++     | 3He++      |
+-------------------------------+------------+-----------+------------+
| Frequency (MHz)               | 425        | 425       | 425        |
+-------------------------------+------------+-----------+------------+
| Aperture r0 (cm)              | 0.18       | 0.18      | 0.18       |
+-------------------------------+------------+-----------+------------+
| Modulation                    | 2.1        | 2.1       | 2.1        |
+-------------------------------+------------+-----------+------------+
| Input energy (MeV)            | 1.0        | 5.05      | 8.025      |
+-------------------------------+------------+-----------+------------+
| Output Energy (MeV)           | 5.05       | 8.025     | 10.54      |
+-------------------------------+------------+-----------+------------+
| Inter-vane Voltage (kV)       | 58.4       | 58.4      | 59.0       |
+-------------------------------+------------+-----------+------------+
| Maximum E Field (MV/m)        | 39.9       | 39.9      | 40.3       |
+-------------------------------+------------+-----------+------------+
| Q0 (MAFIA)/Q0(measured)       | 9500/7800  | 9500/8900 | 9500/8900  |
+-------------------------------+------------+-----------+------------+
| Power (kW) (MAFIA)            | 63.0       | 67.0      | 68.3       |
+-------------------------------+------------+-----------+------------+
| Power(kW)(needed, without     | 76.7       | 71.5      | 72.9       |
| beam)                         |            |           |            |
|                               |            |           |            |
| (power\*                      |            |           |            |
| Q0)(MAFIA)/Q0(measured)       |            |           |            |
+-------------------------------+------------+-----------+------------+
| Pulse Width (ms)              | 70         | 70        | 70         |
+-------------------------------+------------+-----------+------------+
| Repetition Rate (Hz)          | 360        | 360       | 360        |
+-------------------------------+------------+-----------+------------+
| Inject Current (mA)           | 28         |           |            |
+-------------------------------+------------+-----------+------------+
| Transmission                  | Table 2.7  | Table 2.7 | Table 2.7  |
+-------------------------------+------------+-----------+------------+
| Number of cells               | 86         | 61        | 51         |
+-------------------------------+------------+-----------+------------+
| Length (cm)                   | 137.1      | 146.04    | 145.58     |
+-------------------------------+------------+-----------+------------+

![](media/image9.wmf){width="5.69375in" height="3.207638888888889in"}

*Figure 2.8. Transition Region between Post-stripper RFQs, arrows
direction of Beam.*

*RFQ vane tips are depicted.*

*Simulation results*

Due to the special configuration of the post-stripper RFQs, beam loss
can not be neglected. Therefore extensive simulations were carried out
to study the details of beam loss in the post-stripper RFQs. In addition
to the SAIC version of PARMTEQ, another version of PARMTEQ from Argonne
National Lab was used to check the results. This version is
substantially different from the SAIC version in that it uses a finite
element method to establish RF fields and includes higher order terms of
expansion coefficient of the RF field. The detailed results of these
simulations have been reported before4,5.

> Shown in Table 2.7 is the total transmission efficiency at the end of
> RFQ A, B and C. The table shows that total transmission from entrance
> of RFQ A to the exit of RFQ C is 60% (when inter-vane voltage is
> increased to 1.1 times designed value). Shown in Figure 2.9 is the
> simulation results of total transmission efficiency versus the
> operating inter-vane voltage.
>
> A simulation using Argonne Lab's PARMTEQ yields higher transmission
> through the post-stripper RFQ A, but 10-20% lower or higher total
> transmission depending on the input parameters.

The beam loss in post-stripper RFQ A is a little higher than that in the
acceleration section of a normal RFQ (see Table 2.7). Simulation results
show that this is mainly due to longitudinally mismatch and the larger
transverse emittance. In a normal RFQ, when beam arrives at the
acceleration section, it has been already bunched: the half length of
the bunch is about 30 degrees for good transmission. However the input
beam to RFQ A has a half bunch length of 65 degrees. If the bunch length
can be shortened, the transmission can be increased to above 90%. Of
course, if smaller transverse emittance or current is used, the
transmission can be above 90% .

Simulation results show that the drift space between post-stripper RFQs
is the major source for the beam loss in RFQ B and C since beam is
transversely mismatched after drifting through this space. A possible
solution to avoid this drift space problem is to join all three RFQs
together. However this would require more study on possible problems
such as mode coupling and power distribution which was beyond the scope
of this project.

*Table 2.7. Simulation Results using the SAIC PARMTEQ code of Total
Transmission of RFQ A, B and C with various transverse emittance*.

(input current 28 mA, 65∞ half bunch length.)

  ----------------------- ---------------- -------------- ----------------
  Emittance               Transmission at  Transmission   Transmission at
                          End of A         at End of B    End of C

  0.5 π mm mrad           0.865            0.655          0.600

  0.8 π mm mrad           0.675            0.490          0.377

  1.05 π mm mrad          0.520            0.290          0.168
  ----------------------- ---------------- -------------- ----------------

*Operation results*

Post-stripper RFQ A was commissioned first and the output current was
measured. Then the whole string of RFQ A, B and C were commissioned. The
beam current out of RFQ C was measured while the beam current out of B
has never been measured due to the tight commissioning schedule.

![](media/image10.wmf){width="5.99375in" height="3.948611111111111in"}

*Figure 2.9. Transmission efficiency of Post Stripper RFQ (Simulation
Results) for 0.5 π mm mrad.*

During the commissioning of RFQ A, a peak output current of 12 mA was
obtained while the injected current was 20 mA (60% transmission
efficiency.) The injected emittance, however, was 60-100% (0.8-1.07 p
mm-mrad) larger than the value chosen for the design. The repetition
rate of the RF pulses was 120 Hz. With these transverse emittances, the
simulation results showed that the transmission efficiency would range
from 68% (0.8 p mm-mrad) to 52% (1.07 p mm-mrad). Therefore this RFQ did
perform as the simulation predicted. Note: the simulation does not
depend upon the RF duty cycle.

An output current of 7 mA has been obtained at the end of RFQ C while
the injected current into A was 18 mA (39% transmission efficiency). The
injected emittance, again was 60-100% (0.8-1.07p mm-mrad, normalized,
90%) larger than the value chosen for the design and the repetition rate
of the RF pulses was 120 Hz. With these transverse emittances and
injected current, the simulation results showed that transmission
efficiency would be \~34% (0.8 p mm-mrad) and \~32% (1.07 p mm-mrad).
Therefore in terms of transmission efficiency, the whole string of post
stripper RFQs did perform as the simulation predicted or even better.
The operation at 360 Hz repetition rate is not stable at the time of
this writing due to a variety of problems (see the operation section of
this report). However, the data at the 120 Hz repetition rate is valid
for the comparison between the design and the achieved value since the
RF field and the beam parameters should be the same at both the 120 Hz
and 360 Hz repetition rates.

RF power (with beam) used for the aforementioned current and
transmission are: \~107 kW for RFQ A, \~90 kW for RFQ B, and \~85 kW for
RFQ C with beam. These numbers correspond to the power levels (without
beam): \~80 kW for RFQ A, \~75 kW for RFQ B and \~78 kW for RFQ C. This
means that the RF fields in the post-stripper RFQs were \~1.02-1.03
times higher than the nominal values which is also in agreement with the
design. There was no x-ray spectrum measurement performed on these RFQs
to determine the operating voltage, since we were convinced by the x-ray
measurements on the pre-stripper RFQ that the operating power
measurement could give relatively accurate information.

**Summary of Performance, Problems and Future Improvements**

As with all RFQs in operation, a natural (and frequently asked) question
is: does this machine achieve designed values? To answer this question,
first it has to be pointed out that currently the real beam emittances
are 1.6-2 times the emittances chosen by the original design. Then the
answer is: even with this emittance the new pre-stripper RFQ does
provide transmission efficiency of as much as 85% which is higher than
the designed goal, although measurement problems make this number
suspect. The total transmission efficiency of the post-stripper RFQs is
39% which is comparable to the value predicted by the simulation design
study reported before 2,3. To facilitate understanding of the
performance of these RFQs, the comparison of performance and
design/simulation are listed in tables 2.8 and 2.9 (at 120 Hz of
repetition rate).

*Table 2.8. Designed and utilized inter-vane voltage*

+-------------------+----------+------------+-----------+------------+
|                   | Pre-     | Pos        | Post      | Pos        |
|                   | stripper | t-stripper | -stripper | t-stripper |
|                   |          |            |           |            |
|                   |          | A          | B         | C          |
+-------------------+----------+------------+-----------+------------+
| Inter-vane        | 57 kV    | 58.4       | 58.4      | 59.0       |
|                   |          |            |           |            |
| voltage (design)  |          |            |           |            |
+-------------------+----------+------------+-----------+------------+
| Inter-vane        | 57-60 kV | 58.4-61.3  | 58.4-61.3 | 59.0-62.0  |
| voltage           |          | kV         | kV        | kV         |
|                   |          |            |           |            |
| (utilized)        |          |            |           |            |
+-------------------+----------+------------+-----------+------------+

*Table 2.9. Achieved Transmission Efficiency and Simulation Results*
*real emittance, 0.8-1.07* p *mm-mrad, normalized transverse emittance.*

+-----------------------+-----------------------+-----------------------+
|                       | New Pre-stripper      | Post-stripper A-C     |
|                       |                       |                       |
|                       | (21 mA inject         | (15.5 mA inject       |
|                       | current)              | current)              |
+-----------------------+-----------------------+-----------------------+
| Transmission          | 80 %                  | 32-34%                |
| (simulation)          |                       |                       |
+-----------------------+-----------------------+-----------------------+
| Transmission          | 85 %                  | 39%                   |
| (achieved)            |                       |                       |
+-----------------------+-----------------------+-----------------------+

What are the existing/remaining problems and can a better machine be
designed and built? They can be summarized as follows.

1\. The transverse beam emittance of 0.5 p mm-mrad chosen by the
original design was too optimistic. Accordingly, the RFQs were designed
with little room left for a beam with larger emittance. Larger emittance
will impose a more severe challenge for an RFQ at this energy and
frequency.

2\. The change in frequency from 212.5 to 425 MHz between pre and post
stripper RFQs causes longitudinal beam dynamics problems (bunch length
is effectively doubled) that are not totally solved by the MEBT. The
buncher in the original design did not achieve the designed bunching
effect due to beam charge neutralization while the alpha magnet MEBT is
a good bunch length keeper instead of a real buncher. Therefore the
bunch length is still too long. In a conventional RFQ, the half bunch
length of a well bunched beam in the acceleration section is about 30
degrees. The half bunch length of the beam injected into RFQ A is 65
degrees.

3\. Since it was not practical for this project to build a single long
(\~ 4.3 meters) post-stripper RFQ, three independent RFQs are used and
this creates two drift spaces. Both drift spaces cause transverse
mismatch and beam loss as has been verified with radiation surveys.

Based on these factors, a future design may wish to study the following
improvements:

1\. Reduce the emittance of the beam injected into the pre-stripper RFQ.

2\. An RF structure for bunching the beam injected into post-stripper
RFQ A should be considered. This buncher should compress the half bunch
length from 60-70 degrees to at least \~40 degrees. An innovative option
may be to design an extra entrance section for post-stripper RFQ A to
serve as a buncher. Alternatively, one might attempt to bunch the beam
more tightly at the exit of the pre-stripper RFQ, since the MEBT serves
as a bunch length conservator, and the tighter bunching can be made
prior to MEBT entry. Lastly, one could consider an RF buncher near the
end of the MEBT, where vacuum does not have to compete with gas
stripping, and may therefore be adequate to allow for RF bunching of the
beam.

3\. All post-stripper RFQs could be joined into one single RFQ. Drift
space between RFQs is undesirable. Multi-meter long RFQs are being
developed at Los Alamos National Lab and Lawrence Berkeley National Lab.

4\. The average aperture of the RFQs may be increased to accommodate
larger emittances. However this approach is facing a limit. The radial
focusing force of an RFQ is approximately proportional to l2v/ r02 where
l is the wave length, v is inter-vane voltage and r0 is the average
aperture. From this expression, one can see that when the aperture
increases, the focusing strength decreases. In order to keep the
focusing strength, the inter-vane voltage has to be increased much more
than the aperture. The consequence is that the maximum surface field
(which is proportional to v/r0 ) now is increased and will soon reach
the sparking limit.

5\. Based on the aforementioned ideas, a preliminary design exists which
shows that if average aperture is increased to 0.2 cm, inter-vane
voltage is increased to 77 kV and modulation is decreased to 1.7, such
an RFQ may have transmission efficiency of 95% for 28 mA 3He++ with
transverse emittance of 0.8 p mm-mrad and half bunch length of 50
degrees. Even with the half bunch length of 65 degrees ( which means the
buncher is still not in use) the transmission may still be 91%. The
transverse emittance of 0.8 p mm-mrad is close to what is being injected
into the post-stripper RFQs. For this design, the maximum field is 53
MV/m which is 2.7 times the Kilpatrick limit. This level of RF field is
high and challenging, but it may not be impossible based on the results
of RF breakdown tests performed in other labs. Therefore at the very end
of this project, a careful test of the RF field limit could be conducted
using our RFQs. Also the shape of the transverse section of the vane
tips needs to be studied. The power needed for such an RFQ would be
about 500-600 kW (without beam).

**Medium Energy Beam Transport (MEBT) and Stripper Section.**

The MEBT section of the PET accelerator was the most challenging to
design and build, and was, by far, the most scrutinized portion of the
device during reviews and scientific meetings. Due to its difficult
nature, the MEBT was studied in great detail, as discussed in the
documents entitled "MEBT 95" and "MEBT 96", both available on the WWW.

The problem posed by the MEBT region was to match the rapidly diverging,
rapidly debunching beam emanating from the pre-stripper RFQ into a
second RFQ where the beam must again be bunched and be very small. In
addition to the matching constraint, the 3He+ beam must be stripped to
form 3He++. Since the beam has an energy of only 1 MeV, foils would not
survive and a gas stripper had to be used. The 1992 project learned that
gas from the stripper would short out an electric RF buncher, and an
alternate bunching strategy therefore had to be found for the present
project. In addition to matching and stripping the beam, it was desired
that the MEBT region leave the PET accelerator as short as possible. A
schematic diagram of the chosen alternative appears above as Figure 2.1.

The MEBT design is based on the fact that faster particles take outward
trajectories in bends to arrange for a system that passes all particles
in the same time. (The additional time required to traverse the extra
distance in the bend is exactly offset by the time saved due to the fact
that the particles are moving faster.) Introduction of bending magnets
into the beamline implied that the MEBT would have a dispersion
function. (The dispersion function h(s) is used to determine the
horizontal offset, xh, of a particle from the ideal orbit because of its
off-momentum, xh = h(s)(dp/p), where dp is the difference between the
momentum of an arbitrary particle and the ideal beam momentum p.) In
order not to introduce undue emittance growth, which would lead to beam
loss in the downstream RFQ string, it is required that both the
dispersion, and the dispersion gradient, be reduced to very low levels
by the time the beam enters the downstream RFQ string. This led to the
consideration of a MEBT design with a three dimensional waist at the
center of the drift between the two bending magnets. By using such a
symmetry, the dispersion function, and the gradient of the dispersion
function, would both be near zero at the exit of the second bending
magnet.

A first concern regarding the MEBT design was space charge.
Implementation of the MEBT design required a beamline of considerable
length (about 6 meters). At this length, and with the low velocity (1/40
c) of the 3He particles, and with the large (28 mA) peak design current,
space charge was a major concern. An intense effort was made to model
the MEBT section. The original design was done using SCAT, a 2D code
that includes some 3D effects (SCAT calculates dispersion as well as the
energy at which a beam line is isochronous). Next, the 3D nature of the
MEBT was studied using TRANSPORT, a well known, widely used code. Since
TRANSPORT does not include space charge in the analysis, the MEBT design
was also studied using TRACE 3-D. Comparison between TRACE 3-D and
TRANSPORT showed a discrepancy. The root of the problem was traced to a
disagreement in the R56 matrix element used in the codes. Analysis from
first principles showed that TRACE 3-D was the code with the bug; the
author was notified and the code was fixed. Once fixed, TRACE 3-D agreed
with TRANSPORT on the zero space charge limit optics. TRACE 3-D was then
used to model both the linear and non-linear space charge effects on the
beam optics. The results of the study indicated that full space charge
in a Gaussian beam would lead to beam loss of about 40-50%. (An
efficiency of 50%-60%.) The linear component of space charge would move
the final focal point (which should coincide with the entrance to the
second RFQ) as a function of beam current. Pulse to pulse and intra
pulse beam current stability of 5% was deemed necessary to control
fluctuations of the position of the focal point to an acceptable level
under full space charge situations. It was realized that space charge
neutralization could offset the problems of space charge, and that a
parabolic or uniform beam distribution would also be beneficial. For
these reasons, an efficiency of transfer in the MEBT was estimated at
between 80% and 85%. For full details, please consult the documents
"MEBT 95" and "MEBT 96". The efficiency of the stripper was also
estimated at between 80% and 85%, for a combined MEBT/stripper
efficiency of 70%. Since the particles come in singly charged, and leave
doubly charged, the electrical current doubles per particle leaving the
combined MEBT/stripper design efficiency as 140%.

A second concern regarding the MEBT design was the quality of the
magnetic fields. In the MEBT beam sizes range from under one mm to 43
mm. Such a large ratio of beam size puts a premium on magnetic field
quality where the beam is large. Tracking studies (with a code
specifically written for this project) were done that indicated a
required magnetic tolerance of plus or minus 1.6 G out of the 4.1 kG
fields in the dipole bends. This in turn led to a machining tolerance of
0.5 mil for the pole pieces of the dipoles, which also used a
multi-conic section design to achieve the desired field uniformity.

A third concern regarding the MEBT design was tunability. The uncertain
amount of space charge neutralization was one source of uncertainty in
the design. Modeling was done with both full and zero space charge, and
a range of magnetic strengths were determined that would allow a match
over the entire range. In the MEBT, the quadrupoles used to confine the
beam were very short, with magnetic cores about two inches long in the
beam direction. The pole piece separation is about equal to the length
of the magnet, making 3-D effects a dominant contributor to the beam
optics. The uncertainty in accurately modeling for 3-D effects, and the
uncertainty in the level of space charge neutralization, led to a desire
to have as many tunable magnets present in the system as there are
variables one wishes to tune for.

A fourth concern regarding MEBT design was compactness. A guiding goal
of the overall PET accelerator was to keep the device small and
lightweight. Since the MEBT folds the beam over, it contributes
significantly to compactness. But in order to keep things light, the
dipole magnets were designed to have a bending radius as small as
possible, which in turn limited the amount of straight section space
available for the MEBT diagnostics, pumping and vacuum hardware. This
presented a challenging engineering problem which was solved during the
construction of the MEBT.

Space charge had a significant effect on the design of the MEBT magnets,
and the magnets were designed so that the MEBT could be tuned to meet
its matching criteria in the presence of either full or zero space
charge forces. Optimizing the design required a different pole face
rotation angle for the dipoles depending on the space charge assumption.
We chose the rotation angle appropriate for full space charge. Operation
has shown that zero space charge is more appropriate. Thus the dipoles
could probably be improved by shimming the pole pieces. This improvement
might bring about a 10% increase in MEBT/stripper transmission.

The 0.5 mil machining tolerance of the dipoles proved to be difficult
for the vendor. When the half cores came in, three of the four were
measured to meet the specification. The fourth was out of tolerance and
sent back to the vendor. It was worked on again, sent back again, and
still out of tolerance. Since they had made no progress, the decision
was made to put the dipoles together and measure the fields. The out of
tolerance half core led to magnetic measurements that were just barely
acceptable.

When the MEBT beam pipe was received it was found that the pipes (304
stainless steel) had become magnetic at the welds, despite our efforts
inquiring about non-magnetic welding techniques. POISSON studies
indicated that magnetized welds would cause unacceptable field errors.
It was determined that annealing might restore the stainless steel to a
non-magnetic state; a vendor was found to do this heat treatment, and
the beam pipes were then acceptable (more details in chapter 12).

In the first phase of MEBT commissioning it was found that optimum
operation was obtained with the last quadrupole, Q5, excited to the
voltage limit of its power supply, with an associated current of 285 A
(300 A is the current limit of the supply). At this power level the
magnet also became hot, as the upper limit of the expected power had
been reached. It had been realized that such an excitation might be
required, but due to the uncertainties in space charge and fringe
fields, the design went forward with a less robust quadrupole than the
worst case would require. Once experiment showed that more strength was
indeed required, the decision was made to add 10 additional partial
laminations to the quadrupole pole tips. (The original design uses 35
full laminations. An alternative solution would have been to acquire a
different power supply, but that alternative would have led to even more
required power, further heating of the quadrupole, and would have been
very difficult as the quadrupole iron was beginning to magnetically
saturate.) This was a straight forward fix which resulted in successful
MEBT operation. The present optimum quadrupole excitation is 202 A with
the operation temperature considerably below the klixon protection value
of 70 degrees Centigrade.

Operation of the MEBT has been excellent, with beam transmission meeting
the design specifications at low duty cycle. (The fall-off of the
MEBT/stripper transmission at 360 Hz is due to the stripping, and
possibly recombination losses, not the beam optics or magnets.) By
appropriately adjusting each of the seven quadrupoles (five main and two
trim quads) in the MEBT, the emittance was measured to be slightly less
when it leaves the MEBT than its entry value. (Presumably this is
because of scraping of a portion of the beam in the MEBT region. This
scraping could be reduced by shimming the dipole entrance/exit edge
angles as discussed above.) Operation at current levels up to 21 mA
shows that space charge is not the problem it was calculated to be; this
may indicate the presence of beneficial neutralization caused by
electrons trapped within the beam. No emittance growth also indicates
that the stripper caused no determinable emittance growth. This latter
effect was unanticipated, since a naïve calculation based on widely used
formulas indicated that an observable emittance growth was expected.

While the design of the MEBT was quite challenging, and its design
scrutinized in utmost detail with grave concerns expressed, in practice
it has operated with excellent reliability, and was commissioned on
time. The magnets were measured to meet their tight specifications in
all cases, and were delivered on schedule.

The development of the MEBT is an important success of this project, and
it led to the following achievements:

-   Identification and correction of a bug in TRACE 3-D, thereby
    enhancing the accelerator physics tools of the accelerator
    community.

-   Identification of a weakness in the prevailing interpretation of a
    widely used formula for multiple scattering caused emittance growth.
    (Jackson's next edition of his classic text will more fully express
    caveats about the formula in question.)

-   Observation of minimal space charge problems in a 1 MeV, 20 mA 3He++
    beam. Suggestive of neutralized transport due to electrons trapped
    in the beam.

-   Development of a new means to match bunched beam accelerators, which
    might be used in the future to match RFQs to linacs.

**High Energy Beam Transport (HEBT) Section.**

The HEBT design is discussed in the document entitled "HEBT 96" and is
available on the WWW. The beam optical performance of the HEBT is in
reasonable agreement with its design.

As expected, some minor difficulties occurred during HEBT commissioning.
The quadrupole and multipole magnets have to be physically moved in
order to center the beam depending upon their excitations. (They
presently steer.)

Early on in the project there were some concerns about the HEBT magnets
overheating. Increasing the beam energy to 10.5 MeV meant that the HEBT
magnets had to be energized more for the present project than for the
1992 efforts, and the HEBT magnets were cooled by a cooling plate, not
water cooled magnet wire. Magnet measurements were made as a function of
excitation current and it was determined that the HEBT magnets could be
used.

The beam uniformity produced by the HEBT is a reasonable match to the
TURTLE design studies. Beam uniformity of plus and minus 50% about the
mean has been obtained experimentally for all pixels within the desired
radiochemistry target window area. While this exceeds the specification
of plus and minus 25%, it is adequate to begin targetry studies. Further
improvements are expected with experience.

**References**

1\. http://www-linac.fnal.gov/pet

2\. C. W. Schmidt and M. Popovic, \"Duoplasmatron Source Modifications
for 3He+ Operation\". 1997 International Conference on Ion Sources,
Taormina, Italy. To be published in Rev. Sci. Instrum. (1998).

3\. W.D. Cornelius and P.E.Young, Design of a 1 MeV 3He+ RFQ for the
SAIC PET accelerator facility, Nuclear Instrument and Methods in Physics
Research B79(1993) p.933

4\. D. Sun, Report on End to End Simulation of RFQ A, B and C, Internal
Report, (08/1996)

5\. D. Sun, Report on End to End Simulation of PET RFQs Using PARMTEQ
with "8-term Potentials", Internal Report, (12/1996)

1.  

## 3. RFQ Tuning and Resonance Control System

**Introduction**

1.  This chapter describes the history, recent design, operational
    performance, and present state of the PET Radio Frequency Quadrupole
    cavity resonance control system. Prior to the beginning of the PET
    project at Fermilab in 1995, three of the PET RFQs had been operated
    in tandem to accelerate ^3^He^++^ to 8 MeV by the SAIC/UW
    collaboration. RFQ cavity resonance control had been accomplished by
    a combination of mechanical tuning paddles and water flow control.
    During that time, paddle shaft failures due to RF heating were
    experienced. That problem was addressed and thought to be solved by
    the time the RFQs arrived at Fermilab, although operating time at
    high RF power and high pulse repetition rate had been quite limited.
    An SAIC report^1^ documents early measurements (1992) of the
    sensitivity of the 212 MHz RFQ resonant frequency to RF power and to
    water flow.

### System Design at Fermilab 

### *System Requirements*

1.  Requirements of the resonance control system for the four PET
    accelerator RFQs appeared straightforward:

-   The RFQs must be able to be powered up individually and
    independently from a standby state to nominal RF power levels at
    360 Hz without need for manual intervention.

-   The resonant frequency of each RFQ must be able to be independently
    controlled and maintained at its nominal frequency of 212.50 or
    425.00 MHz.

-   The RFQs must reach nominal frequency within a small number of
    minutes after startup or major perturbation.

-   The RFQs must recover gracefully from perturbations, e.g. cavity
    sparks, without manual intervention.

### *Initial Operation*

1.  Operation of the 212 MHz RFQ began at Fermilab in late 1995. Initial
    operation was not to accelerate beam, but rather to test the
    Fermilab RF power amplifiers which were adapted to replace the
    failure prone amplifiers used previously at SAIC and characterize
    RFQ performance and x-ray production. The RFQ water flow valves and
    controller, paddle motor controller, and interface electronics
    previously used were neither readily available nor immediately
    compatible with the Fermilab machine control system hardware. RFQ
    operation at Fermilab began without the presence of anyone who had
    the experience of the previous operation.

    For initial operation, the RFQ tuning paddles and water flow and
    temperature were fixed and a manually controlled RF frequency input
    to the amplifier was used to drive the RFQ on resonance; the drive
    frequency, not the cavity resonant frequency, was controlled. RFQ
    cavity cooling was provided by a NESLAB recirculating chiller
    unit^2^ which supplied constant temperature water to the RFQ. This
    mode of operation proved only marginally satisfactory even for low
    RF power, low pulse rate operation. At higher average RF power
    levels it was apparent that manual frequency control was inadequate
    to cope with the sensitivity and dynamics of the RFQ detuning due to
    RF heating.

    A control system for the PET machine had already been chosen. It was
    based on the system used in the Fermilab Linac and selected because
    of its ready availability and proven performance. The control system
    had a history of supporting software feedback loops to control
    various parameters in the Linac amplifier and cavity systems. This
    existing feature provided the most direct and expedient solution to
    the initial resonance tracking problem.

    Phase comparison of a RFQ field probe signal and an amplifier output
    forward power directional coupler signal provided a measure of the
    difference between the RF drive frequency and the RFQ resonant
    frequency. The phase detector output, applied to a standard control
    system sample/hold and digitizer channel, served as the input to a
    software feedback algorithm. Running at a 10 Hz rate, this algorithm
    (called the VCO, voltage controlled oscillator) controlled a DAC
    (digital to analog converter) to adjust the RF drive frequency to
    quickly and accurately track the RFQ resonance variations. This
    immediately resulted in a stable operating mode suitable for
    amplifier testing. It provided the additional benefit that the
    standard control system now contained the information necessary for
    systematic measurements of the resonance control problem.

### *Measurements*

1.  Operating in this mode, the dynamics of the 212 MHz RFQ detuning as
    a function of RF power level and pulse repetition rate were
    measured. At very low average RF power levels, where RF heating was
    insignificant, this method also provided a means to study the
    sensitivity of resonant frequency to other variables such as RFQ
    cooling water temperature.

Results similar to those reported by SAIC were found. When RF power is
applied to the RFQ, the initial detuning rate is minus 10 KHz per second
per kilowatt. At the nominal operating power of about two kilowatts,
this detuning can exceed the cavity bandwidth in less than three seconds
if not controlled. When RF power is applied, a double time constant
characteristic is observed with the resonant frequency initially moving
lower with a time constant of about 20 seconds, then returning partially
back with a time constant of nearly four minutes. These characteristics
are shown in Figure 3.1. The sensitivity to uniform temperature change
of the whole RFQ structure was measured to be 6 KHz per degree
Centigrade.

The large and rapid detuning of the RFQ upon application of RF power was
believed to be due to differential heating between the vanes and the
housing of the structure. This theory was supported by the double time
constant behavior; the vanes and housing had different heat dissipation,
different cooling, and different thermal masses. The smaller heat
capacity of the vanes indicated that the vane temperature was the fast
component.

Separate cooling water paths for the vanes and the housing provided a
means to control and quantize this effect. A commercial cartridge
electric heating element was introduced into the vane water inlet path
and a 208 volt AC controller was fabricated to power the heater up to
4500 watts. The controller^3^ consists of a solid state relay operated
by a pulse width modulator running with a 0.5 second period (short
compared to RFQ thermal time constants) and provides for linear power
and therefore linear water temperature rise control. Thermocouples were
used to monitor the vane inlet, housing inlet, and combined return water
temperatures. The NESLAB delivered a constant temperature water flow of
3 to 4 gallons per minute which was typically split two parts to one
between the vanes and housing respectively.

![](media/image11.pct){width="384.0in" height="314.0in"}

*Figure 3.1 212 MHz RFQ Resonant Frequency Response to Change in RF
Power*

Measurements of the 212 MHz RFQ resonant frequency as a function of the
differential vane to housing temperature using the heater showed a
sensitivity of 70 KHz/C˚ with a time constant of about 40 seconds. The
measurements demonstrated that the differential vane to housing
temperature is indeed the important variable. They also showed that the
vane inlet water temperature provides an effective handle to control the
resonant frequency. Utilization of the heater to control the resonant
frequency appeared to be an attractive alternative to the active
mechanical tuning previously used. Elimination of moving parts would
presumably result in a more reliable system and questions as to whether
the tuning paddle problems had been adequately solved would be mute.

At this time a second software feedback algorithm, the resonance control
loop, was installed to control resonant frequency by adjusting the water
heater power. While the VCO loop forced the amplifier drive frequency to
track the varying RFQ resonance, the resonance loop on a slower time
scale compared that drive frequency to the desired frequency and drove
the difference to zero. Tests showed that this combination of loops
permitted automatic turn-on at full RF power and attainment of the
desired operating frequency within a small number of minutes of turn-on.
Once up to power and "on frequency", RFQ operation was very stable and
the resonance control system needed to make only small slow corrections.

This test system formed the basis on which the resonance control system
for the four RFQs in the PET accelerator would be designed and
implemented. When the 425 MHz amplifiers and RFQs became available for
measurement they were found to exhibit detuning sensitivities several
times smaller than the 212 MHz structure; presumably because the 425 MHz
vanes lacked the large cut-out of the 212 MHz vanes and they had
different cooling water channel placement. As a result of these
measurements and the operational successes, the mechanical tuning
paddles would be eliminated from all the structures in favor of the
water temperature tuning mechanism.

The benign view of steady state operation experienced at this point of
the project would turn out to be optimistic once beam acceleration with
multiple RFQs was attempted, RFQ voltages were pushed to the limit, and
beam induced sparking became common. Had these operational problems been
foreseen at the inception, some form of mechanical tuning to improve
response time may have been warranted.

### Simulations

The initial operations and measurements described were performed running
only one RFQ at a time. In this mode, the precise RFQ resonant frequency
was not an issue so long as the drive frequency tracked. Beam
acceleration through multiple RFQs however requires phase coherence of
the accelerating fields from one RFQ to the next. This is achieved by
driving all the RFQs from a single fixed frequency reference source; 425
MHz is derived from 212.5 by a frequency doubler circuit. A simulation
of the complete RFQ cavity, amplifier, heater, and water system was
undertaken to understand the non-trivial issues of switching under power
from a variable frequency RF source (required for startup) to a fixed
frequency RF source (required for beam acceleration) and maintaining
resonance control in the process. The simulations were done using the
EXTEND^4^ software package.

The measured sensitivities and time constants formed the basis for the
simulation. Significant effects like water flow time delays, RFQ cavity
bandwidth, and the fact that error signals can vanish if the RFQ is
driven too far off resonance were included. The VCO and resonance
control software feedback loops of the operating system were included
and complemented in the simulation by additional loops. The added loops
serve to establish a stable "stand-by" mode for RF turn-on and to
provide resonance control in the fixed frequency drive mode. A logic
block orchestrated switching the various loops on and off between
stand-by operation, variable frequency (VCO) operation, and fixed
frequency (MASTER) operation.

###  The simulation results closely matched performance which had been measured to date. This gave confidence that the simulation could be relied upon for the overall system design. The simulation indicated that a satisfactory and stable system could be designed using the vane water heater to control the RFQ resonant frequency. The simulation proved to be a useful, powerful and effective tool. It led the way to the final system design which ultimately resembled the simulation very closely in both implementation and performance.

### Design Decisions

1.  

Following the initial operation, measurements, and simulations, the
overall multi-RFQ resonance control system design proceeded based on the
following decisions.

-   Each RFQ would have its own closed loop cavity cooling water system
    with a NESLAB chiller providing temperature control and circulating
    water. Each NESLAB would be operated and utilized as a source of
    constant temperature water.

-   Tuning control for each RFQ would be achieved by heating the NESLAB
    water at the vane inlets. The housing would receive the NESLAB water
    directly.

-   The mechanical tuning paddles were considered unnecessary and would
    be eliminated.

-   Software feedback loops would provide the required active control
    functions.

-   The RFQs would be operated with variable frequency drive at turn-on
    and during major perturbations in order to track fast and large
    resonance variations.

-   Once up and in a stable operating state near the nominal resonant
    frequency, the RF drive frequency would be switched to the fixed
    reference at exactly 212.50 or 425.00 MHz frequency.

## System Description

### *Tuning and Resonance Control System Hardware*

1.  The hardware of the resonance control system is quite simple. Each
    RFQ cavity is on its own separate closed loop water system with
    NESLAB chiller, heater and heater power supply, and three
    thermocouples as described in the *Measurements* section above. The
    NESLAB provides constant temperature water. The heater modulates the
    temperature of the water seen by the RFQ vanes. The thermocouple
    measuring the combined vane and housing return water temperature is
    used for active feedback in the system standby mode; the other two
    are used for passive monitoring only. An over temperature switch on
    the heater housing and a water flow interlock protect the heater
    from burnout due to insufficient water flow. The same two mechanisms
    protect the RFQ from damage due to overheating caused by lack of
    water flow or failure of the NESLAB cooling system.

Four major software components are important to the tuning and resonance
control system. Multiple instances of a code module which performs
generic proportional, integral, and differential feedback operations
(PID loop) are at the heart of the real-time system. Five PID loops are
used for each RFQ: the Return Water Temperature Control PID, the RF
Frequency Control PID, the VCO Resonance Control PID, the Master
Resonance Control PID, and the Average Power Feed Forward PID. A
real-time logic module (Switch Logic) controls the state of the PID
loops for each RFQ system. LabView applications provide graphical
interfaces for monitoring and manual control of the various parts of the
tuning systems. LabView code also controls sequenced RF turn-on and
turn-off procedures. The standard Linac control system parameter page
provides access to all tuning system parameters and controls.

The PID processes run in control system Internet Rack Monitor nodes. At
a 10 Hz rate this code compares the value of an input variable to its
desired value and generates proportional, time integral, and time
differential error signals. These three components with individually
programmable gains are summed with each other and with an optional
direct input to produce the module's output. This output is then
available for application to a control point to form a closed loop
feedback circuit. The PID includes logic inputs to disable operation
which permits "wire or-ing" multiple PID instances to the same control
point.

The Switch Logic module is application specific code which orchestrates
the operation of five PIDs for the resonance control task. There are
three possible states of operation of the tuning and resonance control
system. The states are named Standby, VCO, and MASTER. The Switch Logic
module controls transitions among the states and its function is best
understood in the context of the system operation described below.

Figure 3.2 shows the LabView interface to the tuning and resonance
control system for one RFQ. It depicts the system hardware on the right
side and the software processes on the left. Several switches and
setting controls are available in this interface. A different LabView
interface provides sequenced RF on and RF off switches. These sequences
initialize the startup RF drive frequency and send the proper signals to
the low level RF system and the Switch Logic module for startup from
standby.

1.  ![](media/image12.wmf){width="6.757638888888889in"
    height="4.563888888888889in"}

### *Figure 3.2 LabView RFQ Tuning and Resonance Control Interface for 212 MHz RFQ*

### *Standby State*

1.  In the standby state, direct water temperature regulation maintains
    the RFQ resonant frequency at a known stable value. The Return Water
    Temperature PID adjusts the water heater to control the temperature
    of the combined vane and housing return water to a programmable
    value. This temperature alone does not uniquely determine the RFQ
    resonant frequency; but given a constant supply water temperature
    from the NESLAB, known flow rates to the vane and housing, and the
    absence of RF heating, the return water temperature does translate
    to a predictable frequency. The VCO and Master Resonance Control
    PIDs are disabled in this state.

### *Variable Drive Frequency VCO State*

1.  The variable RF drive frequency or VCO state is the active state
    during RF startup and during recovery from tuning perturbations
    outside the range of the MASTER state. In this state the RF
    Frequency Control PID adjusts the low level RF system drive
    frequency (transparently through the Switch Logic module) to track
    the RFQ resonance in response to the cavity tuning error signal.
    This error signal is provided by the phase detector comparing
    amplifier forward power to RFQ cavity RF. Simultaneously, the
    Frequency Control PID output serves as the input to the VCO
    Resonance Control PID. This loop controls the water heater to force
    the RFQ resonant frequency and therefore the drive frequency to the
    nominal value. Loop gains are adjusted so that the VCO Resonance
    loop slowly nudges the resonant frequency to where it belongs while
    the Frequency Control loop deals with fast tuning changes. The
    Return Water Temperature and Master Resonance Control PIDs are
    disabled in the VCO state.

    The Average Power Feed Forward PID was included to provide a fast
    input to the tuning system in response to possible sudden changes in
    RF power. In fact, this feature proved to be of little value due to
    the fact that a fixed gradient is required to accelerate beam in the
    RFQs, hence this PID is generally not used and left switched off
    and/or set to zero gain.

### *Fixed Frequency Drive MASTER State*

1.  In the MASTER state, the Switch Logic maintains the low level RF
    system frequency at a constant and precise 212.50 or 425.00 MHz.
    This results in RF drive to each of the RFQs at a fixed phase
    relative to the common reference frequency source. The Master
    Resonance PID controls the water heater in response to the cavity
    tuning error signal from the phase detector. This feedback corrects
    any resonant frequency variations within the bandwidth of the
    heater/RFQ system to maintain RFQ resonance at the nominal 212.50 or
    425.00 MHz. All other PIDs are disabled or ignored.

### *State Control*

The Switch Logic code for each RFQ supplies the RF drive frequency
setting to the low level RF system and handles PID state control based
on a combination of manual switches and active run-time inputs.

The STANDBY state is entered either by actions of the LabView RF
shutdown sequence or by manual selection of the Temperature position of
the Resonance/Temperature switch shown on Figure 3.2. The Switch Logic
enables the Return Water Temperature PID and disables all other PIDs
upon entrance to this state.

The LabView RF startup sequence selects the Resonance position of the
Resonance/Temperature switch and the Switch Logic defaults the system
into the VCO state where it can track large resonance variations
resulting from RF turn-on. The Auto/VCO switch determines whether the
system is forced to remain in the VCO state or is permitted to
automatically transition to the MASTER state. The Auto/VCO switch must
be manually selected by the operator.

In the Auto mode, transitions between VCO and MASTER states are
controlled by the Switch Logic based on the cavity tuning error signal
and the RF Frequency Control PID output. A switch from VCO to MASTER
will occur if both the tuning error signal and the frequency error are
smaller than preset tolerances. This indicates that the RFQ is being
driven on resonance and that the resonant frequency is close enough to
the nominal 212.5 or 425 MHz to jump the drive frequency to exactly that
value. The system then remains in the MASTER state so long as the cavity
tuning error signal remains in tolerance. The tuning error going out of
tolerance indicates a perturbation beyond the capacity of the MASTER
state to handle and causes a transition back into the VCO state. This is
necessary since the cavity tuning error signal is based on a measurement
using a RFQ voltage signal which can vanish if the RFQ is driven at a
frequency too far from its current resonant frequency. In the VCO state,
the system can track any large resonant frequency variations and settle
back into a stable condition until ready again for MASTER operation. The
speed of resonant frequency tracking in the VCO state is limited by the
10 Hz update rate of the RF drive frequency setting, whereas the speed
of resonant frequency control in the MASTER state is determined by the
40 second time constant of the RFQ response to vane water temperature
changes.

1.  Since each RFQ tuning system operates independently of the other
    three, it is only via the common reference frequency oscillator that
    the phase coherence required among the four RFQs to accelerate beam
    is achieved. This common reference condition exists only when all
    systems are operating in the MASTER mode.

## Operational Performance and Present Status

Operationally the RFQ tuning and resonance control systems perform well
within design specifications. Using the LabView controlled turn-on
sequence any one or all four RFQs can be powered up from STANDBY with
the click of a single switch. In less that five minutes from turn-on at
full power, all systems achieve stable operation in the MASTER state.
Rarely do the automatic RFQ tuning systems "get lost" and require manual
intervention, even under extreme operating conditions such as RF pulse
repetition rate switches and periods of RFQ sparking.

Imperfections in the low level RF system which resulted in a coupling of
RF amplitude to RF drive frequency caused some complications for the
tuning systems in the early stages. Switching from the VCO to MASTER
state would produce a step change in RF power delivered to the RFQ which
would in turn cause a RFQ detuning step. This step could be so large as
to be out of the range of the MASTER resonance control loop, so the
system would quickly revert to the VCO. The instability was overcome by
careful setup of the low level RF hardware to minimize the coupling,
however it did point out the most serious shortcoming of the resonance
control system design - limited response bandwidth in the MASTER state.

Though the tuning and resonance control systems perform as designed, the
design has been less than optimum for the operational mode of the PET
machine to date. Cavity sparking has been a persistent problem in
typical daily operation. The sparking has two primary causes: 1) the RFQ
operating voltage is normally pushed as high as possible to maximize
beam transmission, and 2) the beam itself induces fits of sparking. The
sparking and response of the RFQ spark protection system cause
variations in the average RF power applied to and absorbed by the
affected RFQ. These variations produce sufficiently large resonant
frequency changes in a short enough period of time that the heater is
not able to overcome them and the affected system must revert to the VCO
state. Depending on the cessation of sparking, recovery to the MASTER
state may take seconds to minutes. Since any one RFQ not in the MASTER
state prevents stable beam acceleration and all four RFQs are operated
with a high likelihood of sparking, significant beam downtime is
suffered.

1.  The tuning and resonance control system of course is not able to
    affect the frequency of occurrence of cavity sparks, but lost beam
    time might be reduced if the tuning system included a mechanism to
    facilitate faster response to the effects of that sparking. This is
    where the mechanical tuning paddles may have proved beneficial had
    they been maintained in the system design. Their response time could
    have been seconds, compared to the tens of seconds achieved by water
    temperature modulation.

## PostScript on Tuning Paddles

In March 1998, after the body of this section was written, interruptions
of stable beam due to RFQ sparking and resultant cavity detuning
continued to be unacceptable. A decision was made to install one tuning
paddle (two tuning paddles were used in the original SAIC SDIO design)
in each of the 425 MHz RFQ's A and B. The single paddle provided a ±25
KHz prompt tuning adjustment range.

Automatic control of the paddle was integrated into the existing system
of software feedback loops. In the VCO state, the paddle rests at the
middle of its control range and operation is exactly as previously
described. In the MASTER state, a Paddle Control loop adjusts the paddle
position in response to the cavity tuning error signal from the phase
detector. Within its range, this control maintains the resonant
frequency at the desired nominal with a speed of response at least an
order of magnitude faster than the water heater system alone. The Master
Resonance PID has been reconfigured to monitor the paddle position and
control the water heater to maintain the average paddle near the center
of its range. This combination of paddle plus water heater results in a
system with both large dynamic range and fast response.

The positive effects of the new arrangement were both immediate and
dramatic. The ability to respond quickly to changes in cavity tuning,
whether due to beam loading, cavity sparking, or RF power variations,
greatly reduced the likelihood that an RFQ system would switch off the
MASTER state and significantly shortened the recovery time when it did.
This in turn broke the vicious cycle of coupled instabilities in which
beam transients caused by sparking or tuning errors in one RFQ system
induced similar problems in another RFQ. Subsequently a tuning paddle
was re-installed in the 212 MHz RFQ. This further stabilized the whole
machine making possible steady beam currents above 100 microamps for
long periods. Plans are presently underway to design a tuning paddle for
the 425 MHz RFQ C.

## References

1.  1\. Wayne Cornelius and Steve Ringler, [Pre-Stripper Water Flow
    Experiments]{.underline}, SAIC PET Note #62, 1992.

    2\. Model HX-150, NESLAB Instruments, Newington, New Hampshire.

    3\. Fermilab drawing #9700-EB-348007.

4\. EXTEND modeling code by Imagine That, Inc., San Jose, CA.

1.  

**4. Low Level RF (LLRF)**

**Introduction**

The PET Low Level RF (LLRF) System regulates the accelerating voltage in
the four RFQ cavities by controlling the frequency, phase and amplitude
of the RF signal driving the High Level RF system. The central LLRF
hardware for all four cavities resides in one VXI crate. This crate
supports a CPU card, a clock decoder card and two Baseband Processor
modules. Each Baseband Processor (BBP) module supports two RFQ RF
systems. Each BBP contains one embedded SHARC DSP^1^ (Digital Signal
Processor) that controls all board functions. All system and subsystem
functionality may be broken down into the following groups; control
system interface, timing interface, RF waveform generation, frequency
control, feedback regulation, and cavity spark protection. These
functions and the system specifications are derived from the following
system requirements.

-   One 212.5 MHz and three 425 MHz LLRF. All systems have a common
    frequency reference source based on a 212.5 MHz crystal oscillator.

-   RF repetition rate of 360 Hz and control system rate of 10 Hz.

-   Separate amplitude and phase control for each station with
    regulation of transient beam loading.

-   Full control of RF waveform for each station.

-   Detection of cavity sparks with fast RF kill, slow cavity protection
    and recovery.

-   Cavity resonance detection and frequency modulation of RF pulse.

-   Full access to all important system waveforms through the control
    system at a 10 Hz rate

**Control System Interface**

The PET control system is based around the Internet Rack Monitor (IRM).
For the LLRF system, an IRM's CPU card and timing card are installed
directly into the LLRF VXI crate. Standard IRM processes run in this CPU
as well as several LLRF specific applications. These include downloading
DSP code to the BBP modules, parameter settings and readings, waveform
acquisitions from the DSP, cavity spark recovery process, and error
handling. This interface is described in detail in chapters 14 and 15.
The CPU provides data to console programs and data loggers, as well as
other IRMs requiring data for local process control. This interface has
adapted well to changes in the DSP code for the BBP modules.

**Timing**

The timing card provides triggers to the CPU and BBP modules based upon
an event plus delay. The LLRF system responds to the 10 Hz control
system trigger and the RF start trigger. The 10 Hz trigger synchronizes
the CPU and the BBPs with the rest of the control system. The start LLRF
waveform trigger may be aligned at the optimal relationship with the
high level RF. This trigger starts the Direct Access Memory (DMA) and
spark detection routines running.

Baseband Processor

The Baseband Processor (BBP) module is the central component of the LLRF
system. This module can be thought of as a two channel vector signal
modulator and a two channel vector signal analyzer with the processing
done by a SHARC DSP. Each BBP regulates two cavity RF systems. A block
diagram of the module is shown in figure 4.1. Four AD9026 12 bit 26 MHz
ADCs and four AD9713B 12 bit high speed DACs are connected directly to
the 48 bit data bus of the SHARC.

![](media/image13.wmf){width="5.999305555555556in"
height="2.915277777777778in"}

*Figure 4.1 Baseband Processor*

The accelerator operates with a nominal repetition rate of 360 Hz and a
maximum RF pulse width of 80 micro-seconds. When the SHARC receives a
start trigger from the timing card it starts a DMA process that will
play out and acquire RF waveforms. Two of the 10 DMA channels in the
SHARC IO processor have hardware DMA request and grant lines. By
configuring the DMA controller, the IOP (Input Output Processor) will
play and acquire waveforms without external hardware or memory. The IOP
is configured to play and acquire at one half the DSP clock rate. The
high speed, high spectral purity digitizers are combined with signal
analysis software in the DSP to provide the building blocks for a vector
signal analyzer. The combination of the DACs and DSP software is the
basis for a vector signal modulator.

The analog IO from the ADCs and DACs is connected to two general purpose
daughter cards. PET daughter cards perform RF IQ modulation and
demodulation. Two models have been built to support 425 MHz and 212.5
MHz RF These cards frequency translate baseband vector signals up to and
down from the local oscillator frequency. With IQ modulation, the DSP
has complete control and feedback regulation of RF amplitude, frequency,
and phase.

**RF Waveform Generation**

At the 360 Hz rate, two 1024 word waveforms are played to the RF
modulators. At the 10 Hz rate of the control system, a trigger from the
timing card starts the waveform processing. This processing is done by
the SHARC DSP by routines written in "C". The program waveform array is
built from the parameters of pulse length, rise time, amplitude, RF
frequency and phase. With feedback off, the program waveform array is
written to memory to be played out by the DMA controller. With feedback
on, the complex arrays are processed as shown in the feedback diagram,
figure 4.2. The processing includes vector array multiplication,
addition, filtering, and limiting. Group delays in the RF system are
taken out by adjusting array indexes The complex time domain signal is
transformed by FFT to measure the resonant frequency for input to the
cavity tuning control loop. Ten 256 sample waveforms per system are
returned by the control system for 10 Hz display update. This processing
is done for each LLRF subsystem and completes in about 50 ms.

![](media/image14.pct){width="427.0in" height="274.0in"}

*Figure 4.2 PET Feedback*

Early tests of the feedback system went well. Before a RFQ was available
for testing, the LLRF was run closed loop with the HLRF (High Level RF)
driving a dummy load. Closed loop performance was as predicted by the
simulations done with "Extend", a commercial time domain software
simulation program. Next the HLRF was connected to the cavity. At this
point closing the feedback loops was hampered by several nonlinearities.
The worst being a spurious oscillation in the PA (Power Amplifier). This
oscillation would grow during the 80 micro-second pulse to about 20% of
the nominal drive level. The root cause of the oscillation is traced to
the interaction of the RFQ and the output cavity filters in the PA.
Outside its 30 KHz bandwidth, the RFQ reflects all energy back to the
PA. With this reactive load the PA output cavity provides little
filtering for the tube. The network based on the tube, output tank,
drive line length, and the RFQ is marginally stable at many frequencies.
One way to deal with this would be to add circulators in the drive line.
Circulators for a 212 MHz system are too large and too costly for this
project. Instead the system was made stable by careful mistuning of the
output stage of the PA. While this made operation possible, it limits
the peak output power and dynamic range of the HLRF. The 212 MHz HLRF
also was found to be very nonlinear . No output was measured until the
low level drive was about 40% of nominal. All stages of the station
operate in class C mode so they all are inherently nonlinear. The
largest nonlinear stage turned out to be the solid state amplifier. By
increasing the drive level and padding the output of the amp,
nonlinearity was minimized. Cavity gradient regulation is shown in
figure 4.3 and fast IQ loop response is shown in figure 4.4.

![](media/image15.wmf){width="4.846527777777778in" height="3.8625in"}

*Figure 4.3 Cavity Gradient Regulation for 100 seconds. Plotted relative
volts vs. time.*

![](media/image16.wmf){width="4.835416666666666in"
height="3.7090277777777776in"}

*Figure 4.4 Cavity Amplitude, Phase, I and Q Error Signals. Plotted
volts vs. sequence time during pulse.*

**Spark detection**

The original spark detection circuit works by monitoring the reflected
power signal from the cavity drive. The output of a diode detector is
compared to a reference voltage. The output of the comparitor is "or-ed"
with the comparitor of the other RF system in the module. This generated
an interrupt to the DSP that killed the RF to both systems. This
coupling between pairs of RFQs caused stability problems at high
repetition rates combined with high spark rates. This problem is fixed
by a redesign of the RF daughter card that adds an RF switch to the
output of each IQ modulator. The logic for controlling the switch is in
hardware while spark counting and inhibit for high spark counts is still
done in software. One other problem found with the spark detection is
false detection of sparks with high reflected power from an off resonant
cavity. A false spark causes the RF to be shut off for a short time
which allows the cavity to go further off resonance, cascading the
problem. The solution is the placement of a high pass network on the
output of the diode detector. This allows high sensitivity of the fast
rise time spark waveforms while providing attenuation of the slow
waveform from the off resonance condition. The DSP will shut off the RF
and start a spark recovery process when the number of sparks occurring
in a 100 ms period exceeds a set value.

**Other Changes**

The PA has a high phase dependence on many factors such as screen
voltage, average power, and power line voltage. Large changes in loop
phase are difficult for IQ regulation to handle. A Slow phase loop was
added that regulates a phase offset term. This regulation insures that
modulation and demodulation vectors are always in proper alignment.
During conditioning of the cavities the spark rate is so high that the
feedback learning may become unstable. A slow feedback loop was added to
regulate the RF amplitude set point based on the cavity gradient signal.
This loop has good regulation of the average cavity pulse and will work
with the fast feedback loop turned off.

**Future Options**

At this point in the project, the biggest area that the LLRF might
improve the accelerator's over all performance is spark recovery. One
improvement would be real time loops that would better handle gain
changes in the HLRF during heavy sparking. This could be done with a
redesign of the RF daughter cards. The other possibility is to improve
the regulation of cavity resonance by providing fast control of the RF
power dissipation in the RFQs. This power would be controlled by pulse
width modulating the RF waveform. This may reduce the time required to
return the RFQ to its nominal frequency after heavy sparking.

**Reference**

1\. ADSP-2106x SHARC User's Manual, First Edition, © 1995 Analog Devices
Inc.

**5. High Level RF (HLRF)**

**Introduction**

The requirements for High Level RF (HLRF) are shown in Figure 5.1. The
Full Duty Factor of 2.5% for the project is found from a 70 µs "flat
top" time with a repetition rate of 360 Hz. The right-hand column of
Figure 5.1 gives actual measured data for each of the stations into a 50
Ω Dummy Load. The other goal of the HLRF was to establish reliability
for each of the Power Amplifier Stations. This reliability issue would
not only be required in each of the different Power Stages within each
station but also in the internal system components of each station.
Finally, recommended future upgrades will be listed.

  --------------- --------------- --------------------- ----------------------
  **Power         **Frequency**   **Total Power         **Power Observed into
  Amplifier**                     Required**            Dummy Load**

  **Station**                     **at Full Duty        **at Full Duty
                                  Factor**              Factor**

  **PA 212**      **212.5 MHz**   **120 - 143 kW**      **170 kW**

  **PA 425 A**    **425 MHz**     **136 - 156 kW**      **200 kW**

  **PA 425 B**    **425 MHz**     **120 - 140 kW**      **200 kW**

  **PA 425 C**    **425 MHz**     **105 - 125 kW**      **200 kW**
  --------------- --------------- --------------------- ----------------------

*Figure 5.1: HLRF Design Parameters and Actual Measured
Performance***.**

**General High Level Performance Issues**

In order to achieve the power goals each station would require a
solid-state stage, a Burle 7651 Power Tube stage, and a final Burle 4616
Power Tube stage. A block diagram of these stages is shown in Fermilab
Drawing \# 9700-ED-348023. The stations were created from four Fermi
National Accelerator Laboratory 200 MHz Power Amplifier that were
surplussed after a 1993 upgrade to the LINAC accelerator. All of the
stations required an upgrade of internal system components for
reliability and remote control.

![](media/image17.wmf){width="3.5694444444444446in" height="2.875in"}

*Figure 5.2: 425 MHz Solid-State Amplifier.*

The input from the Low Level RF is a nominal 10 dBm (decibel referenced
to one milliWatt, 0 dBm = 1 mW) for each of the stations. The
Solid-State stage has a capability of up to 400 W. In PA212 this is done
with two amplifiers, a Mini-Circuits ZHL-5W-1 and a Microwave Power
Devices PCA 201/1973A. The Mini-Circuits amplifier was required because
of the gain capabilities of the Microwave Power Devices amplifier. Each
of the 425 MHz stations use a single Microwave Power Devices PCA
425-82/10075A Solid-State amplifier as shown in Figure 5.2. Four
Solid-State 425 MHz amplifiers were purchased from the Los Alamos SDI
surplus equipment.

*Solid-State Problems Encountered and Reliability*

The reliability of the Solid-State Drivers has been very good. There has
been only one instance of failure since receiving the Microwave Power
Devices PCA 425-82/10075A from Los Alamos. This occurred on July 30,
1996 and was repaired. The repair entailed the replacement of a LS logic
chip on the pulse input of the amplifier.

![](media/image18.wmf){width="3.5555555555555554in"
height="2.9027777777777777in"}

*Figure 5.3: 425 MHz Burle 7651 Power Tube Amplifier.*

The second stage is a Power Amplifier using a Burle 7651 Power Tube.
This stage is capable of producing up to 4 kW of power. The change from
200 MHz to 212.5 MHz in this stage of PA212 required nothing more than
adjusting the finger stock short on the output section of the 7651
stage. To modify the 200 MHz Power Amplifier to 425 MHz for the other
three stations required some hardware modifications. The paper
"Modification of the LINAC 7651 driver to operate at 425 MHz" contains
details.^1^

*7651 Stage Problems Encountered and Reliability*

One Burle 7651 tube has been replaced (It failed due to extensive
operational hours in the LINAC). A problem with the -14 VDC Kepco supply
to the 7651 Bias Modulator caused sagging during its 70 µs "flat top"
time at 360 Hz. This was discovered in late May of 1997. The solution
was to improve power supply regulation by the addition of a 200 Ω, 14 W
resistor across the -14 VDC Kepco supply so that it would draw and
regulate current in its proper quadrant.

A second problem encountered in changing from the LINAC's Duty Factor of
0.68 % to the PET Duty Factor was found in the middle of August 1997. A
100 Ω, 2 W resistor in the 7651 Bias Modulator was being overheated over
an extended period of time by the longer duty factor for PET. After the
circuit was analyzed, the proper higher wattage resistor was installed.

![](media/image19.wmf){width="3.5555555555555554in"
height="2.861111111111111in"}

*Figure 5.4: 212.5 MHz Burle 4616 Power Tube Amplifier***.**

The third stage of the power amplifier uses a Burle 4616 Power Tube.
This stage is capable of producing at full duty cycle up to 170 kW @ 212
MHz and 200 kW @ 425 MHz. For PA 212 (Figure 5.4) the only modifications
from the LINAC station to PET station were to move the finger stock
shorts on the input and output section and adjust the input and output
coupling to 50 ohms. For the 425 MHz stations three Burle 4616 power
amplifiers cavities were purchased from Los Alamos SDI Surplus
Equipment. The three LINAC surplus stations were then modified to accept
these new amplifiers.

*4616 Stage Problems Encountered and Reliability*

On May 24, 1996 it was discovered when commissioning the second 425 MHz
4616 Power Amplifier that the RF output would dip during the 70 µs
pulse. Upon further investigation it was discovered that the finger
stock short on the output section of the amplifier was too small in
diameter and did not deflect the finger stock correctly. Thorough
measurements showed a need for a new output short. This power amplifier
now resides in PA425A and has worked flawlessly. This problem was not
seen on any of the other 425 MHz Power Amplifiers.

In the spring of 1997 it was discovered that the line voltage in Lab G
would sag when the Tevatron in Fixed Target mode would ramp. This would
cause the DC anode voltage of all the 4616 amplifiers to dip from 18 kV
by as much as 10 %. This problem did go away when the Tevatron was
turned off for the Main Injector upgrade in September 1997, and should
not present a future problem for the project.

In late March of 1997 a 25 µF, 20 kV 4616 anode capacitor in PA425A
failed. It was replaced with a spare from the LINAC and the voltage
across the capacitor was dropped from 20 kV to 18 KV by moving the tap
points on the Anode Supply transformer. The changing of tap points to 18
KV was also done on the other three Anode Supplies. This problem has not
been seen again.

In the middle of June 1997 we lost a 4616 Power FET Screen Supply in
both PA425B and PA425C. The cause of failure was from exceeding the
average power rating of resistor R7A and R7B in Fermilab Drawing \#
0230.00-EC-60663. Upon further investigation it was found that a SCR was
missing on the 4616 Screen Modulator Regulator card in both stations.
This would allow the screen supply to stay on for longer than 70 µs and
change the average power across the resistors. This SCR was checked in
the remaining two stations. All four 4616 Power FET Screen Supplies also
received a Dale 25 Ω, 25 W wire wound resistor to replace R7A and R7B
and the supplies controller card had a potentiometer turned to reduce
the maximum "ON" time to 110 µs.

In the middle of July 1997 two Burle 4616 Power Tubes failed. One in
PA425C with a grid short and the other in PA425A with a screen short.
These tubes were sent back to Burle for failure examination and repair.
The report from Burle is that failure was caused by the use of Hasteloy
B2 material in manufacturing the filament-cathode assembly. This failure
mechanism has been noticed in tubes returned by other facilities after a
nominal 3000 hours of operation. New 4616 tubes are now manufactured
with an improved version of the Hasteloy material and occurrences of
grid shorts have not been reported.

In late August 1997, the bias supply for the screen voltage for the 4616
Power Tube was increased to improve screen voltage regulation in all
four stations. A noticeable improvement in RF power regulation resulted
from this modification.

**Station Improvements**

The original 200 MHz LINAC stations needed to have their control systems
updated to allow for more monitoring capabilities and the ability to
send status bits to the Internet Rack Monitor (IRM) installed in each of
the stations. Some of the new capabilities are to monitor RFQ water
flow, RFQ vacuum, and cooling water conductivity. In using this new
control system an operator is able to quickly interpret the current
status of the station either from the front control panel or remotely by
computer.

Another function of the IRM is to read back via analog channels the
power levels of each of the different stages in a station. The last
important function of the IRM is to provide programmable timing signals
to a station.

The cooling water system for the 4616 Power Tube required a new
manifold. This upgrade allowed for the installation of three water flow
meters and the ability to easily remove the 4616 Stage from the station
for shipment to Shreveport.

The final improvement was the addition of remote control of the 4616
screen voltage. Remote adjustment is useful when trying to turn on a
station that has repeated anode current overloads or when a new 4616
Power Tube is sparking internally during conditioning.

**Recommended Upgrades**

In the course of running the accelerator it was discovered that
increasing the pulse period to 80 µs of "flat top" time, beam current
could be increased. This change in duty factor caused the 4616 DC Anode
Supplies to drop from 18 KV to between 17 and 16 KV depending on the
station. Thus, the maximum power out of each of the stations was
decreased. It was then proposed to raise the anode voltage to 19.5 KV
using a variable transformer (Variac) inserted into the 480V - 3 phase
primary on one of the Anode Supplies. The test showed an increase of
13.5% in output power with the anode voltage at 19.5 KV. A recommended
Anode Supply upgrade is to replace the existing 20 KV 25 uF cap with a
25 KV unit (\$2000 per station), to replace the transformer and install
a new one with higher ratings (\$3000 per station), and to install a new
SCR primary controller (\$1500 per station). The reason for the SCR
controller is to aid in system turn-on without Anode Supply current
overload.

![](media/image20.wmf){width="3.5416666666666665in"
height="2.888888888888889in"}

*Figure 5.5: 4616 18 KV DC Anode Supplies.*

**References**

1\. PET Web page, http://www-linac.fnal.gov/pet

**6.** **Charge Doubler**

**Introduction**

The charge doubler converts 3He^+^ to 3He^++^ by means of a pulsed
gas-jet stripper. Stripping fractions are typically in the range of
70-80%. Fractions up to 83% have been observed under certain conditions.
Since the beam-particle charge is doubled, the electrical output current
is larger than the input current.

A summary description of the charge stripper was presented at the 1997
Particle Accelerator Conference^1^, which is also posted on the PET web
page. This document provides a more thorough description of the device,
and contains additional information concerning design approach, problems
encountered, and operating modes.

A stripper design based on a pulsed gas jet appears to be preferable to
alternative approaches, all of which suffer from serious difficulties.
(a) Development of a plasma-based stripper (He++ ion source) was deemed
too technically challenging given the scope of this project; (b) a solid
foil is not expected to survive under the beam intensities in use; (c) a
static-fill cell suffers from spill over of gas to adjacent regions of
the accelerator and beam heating of the gas; and (d) by utilizing a
pulsed gas jet synchronized with the beam it is possible to
significantly reduce the gas load on the vacuum pumps, and the pressure
rise in adjacent regions of the accelerator.

Because of charge-exchange processes , a non-plasma-based charge
stripper requires operation at an energy in the range of 1-MeV or higher
for good stripping efficiency.^2^ The charge stripping process is a
competition between the desirable ionization process He+ -\> He++ and
the undesirable charge-exchange processes He++ -\> He+ and He+ -\> He0.
For efficient stripping the He+ beam ions must attain sufficient energy
to significantly exceed the electron velocity corresponding to the
binding energy of the He+ ion, which is relatively large, 54.4 eV ^3^.
At 1 MeV, the beam ion velocity is the same as the velocity of a 180-eV
electron. Although some cross-section information does exist for
stripping helium beams on carbon foils, ionization cross sections for
3He are in most cases not well known, but can generally be expected to
favor heavy molecules, such as N2, CO2, Ar, etc., over light molecules
such as He and H2. The required gas line density for stripping is
approximately 3-6 x 1016 molecules/cm2.

A differentially-pumped static gas cell was utilized in the original
SAIC project, after initial attempts to operate using a piezoelectric
pulsed gas valve. One problem with a static fill is that pressures in
adjacent regions of the accelerator are high enough, for example, to
short out the electric fields in a conventional buncher. This concern
lead to the development of the novel MEBT design utilized in this
project. Another problem is that the stripper gas is heated by the He+
ions passing through the gas.^4^ An early estimate of this effect is
posted on the PET web page ("Thermodynamics of gas stripper cell"). In
particular, the beam heating of the background gas can be expected to
cause a localized density reduction in the gas, which then leads
directly to a reduction in stripping efficiency. In fact, attempts to
operate the stripper cell as a static gas fill observed erratic
stripping rates, as well as difficulty in obtaining high stripping
efficiencies . Pressures in the RFQ and downstream of the stripper were
high. In a gas-jet target, the refresh rate of the gas is high enough
that the gas is relatively unaffected by the beam.

**Injector and nozzle**

Because of difficulties with a piezoelectric pulsed gas valve, we
investigated mechanical fuel injectors^5^. Automotive-type fuel
injectors are rugged and fast mechanical valves. We chose for operation
in the stripper the valve with the fastest response among several valves
tested, a Nissan fuel injector, Part No 16600-40F01. A MOSFET-based
pulser provides 75-V, 200-300 µs pulses to the injector. The electrical
response of the injector is predominantly inductive, with current rising
linearly during the electrical pulse until the injector opens at about 1
A. The pulser rapidly shuts off the current at the end of the pulse,
which closes the valve to minimize on-time of the injector and to
prepare for the next pulse. The voltage level of the pulse is sufficient
to allow rapid operation of the injector, while remaining well below the
breakdown strength of the insulation. Injectors have been shown to
operate reliably at 360 Hz for 109 pulses. The throughput of the pulser
is approximately 150 Torr-liters/sec of gas in a 300-µs pulse. Pumping
capacity of 3000 liter/sec is required to remove this amount of gas at a
pressure of 5 mTorr and repetition rate of 360 Hz.

The injector injects gas into a converging-diverging nozzle.^6^ The
nozzle was designed to match the pressure of the injected gas in the
converging section through a throat to a diverging section. The nozzle
converts the energy of thermal motion of the injected gas to directed
motion in a gas jet. If the nozzle is properly matched, the gas flow
undergoes a theoretical transition between subsonic and supersonic flow
at the throat. The gas cools as it expands, and may reach very high
directed velocities. It is possible for the beam to condense into
clusters containing large numbers of molecules. All of these properties
(e.g. super cold gas, cluster formation) tend to reduce the pressure of
the gas jet. We chose a .020" (0.51-mm) diameter throat, and a diverging
section which opens to the full diameter of the beam tube, 15-mm.
Although sophisticated diagnostics were not available to probe in detail
the properties of the gas jet, measurements of relative pressures in the
stripper cell and the adjacent regions (RFQ and MEBT) indicated that
under the appropriate conditions, a reasonably well-directed gas jet was
formed.

**1-MeV Test**

A 1-MeV beam test was made to investigate performance of the stripper
cell. A top view of the test chamber is shown in Figure 6.1. Total
pumping capacity in the test cell was 1000 liter/sec. A spectrometer
magnet bends the 1-MeV He+ and He++ beam ions. The magnet is a small
trim dipole magnet borrowed from the Antiproton Source. The
triangular-shaped drift chamber allows the 3 charge states He0, He+, and
He++ to separate. They are collected at 3 biased Faraday plate
collectors at bend angles of 0^o^,11.5˚ and 23.6˚ . In addition 2 wire
profile monitors were installed between the collectors to measure the
effect of the stripper gas on beam profiles.

The greatest stripping efficiency was obtained with argon gas,
surpassing 80% in some cases, followed closely by nitrogen. The
stripping efficiency in He gas was less than 40%. As noted elsewhere in
this report, due to heating and slowing down of the turbopump vanes when
using argon gas, we normally operate the stripper cell in nitrogen gas,
despite the slight loss in stripping efficiency. A plot of stripping
efficiency as a function of time delay of the beam pulse with respect to
the stripper pulse is shown in Figure 6.3. Roughly 400 µs from the time
of the beginning of the electrical pulse to the injector is required for
the gas to appear at the interaction region. The gas in injected for a
period of roughly 400 µs, then it is pumped out until the next beam
pulse. Stripping measurements were made in a variety of gases including
Ar, He, CO2, N2, and Kr. The results of some of the measurements of
stripping efficiency are summarized in Figure 6.2.

![](media/image21.wmf){width="6.388888888888889in" height="2.44375in"}

*Figure 6.1. 1-MeV beam test.*

![](media/image22.wmf){width="6.388888888888889in"
height="4.083333333333333in"}

*Figure 6.2. Performance of gas-jet stripper on He+ beam in 1-MeV test*.

![](media/image23.wmf){width="6.388888888888889in"
height="4.083333333333333in"}

*Figure 6.3. Yield of He+ (dashed) and He++ (solid) in the Faraday
collectors as a function of stripper delay time in the 1-MeV beam test
for argon gas at 60 Hz.*

Emittance measurements of the 1-MeV beam are described on the PET web
page. Emittance growth of the beam due to scattering on the gas in the
stripper cell was initially predicted to be a matter of concern. However
measurements showed no significant beam size increase due to gas in the
stripper cell. A calculation based on the work of Sigmund & Winterbon
(SW)^7^, agrees with the measured lack of emittance growth due to
scattering.

SW give for the scattering angle α

![](media/image24.wmf){width="1.5138888888888888in"
height="0.4305555555555556in"}

where Z1=2 (helium ion), Z2=7 (nitrogen atom), e is the electronic
charge, ε0 is the permitivity of free space, E=1 MeV is the beam energy,
a = 1.4•a0•Z2-1/3 is the atomic radius from the Fermi-Thomas screened
potential model, and a0 is the Bohr radius (0.53•10-10 m). The parameter
αs is a dimensionless parameter derived from Figure 6.3 of SW as a
function of τ = π•a2•nl, where nl= 6 x 1020 atoms/m2 is the atomic path
length of the stripper gas . The result of the calculation is that the
expected scattering angle is about 0.9 mrad. The RMS size of the curves
above is about 4 A (He+), which corresponds to about 30 mrad for this
geometry, assuming a point source. The increase in beam size is thus
predicted to be negligible, as observed in the experiment. It should be
noted that the beam profiles (Figure 6.4) are not purely Gaussian, but
have somewhat broader tails than a Gaussian distribution. The enhanced
tail on the He++ profile at large magnet currents is due to the residual
He+ beam ions remaining after stripping.

![](media/image25.wmf){width="6.388888888888889in"
height="4.083333333333333in"}

*Figure 6.4. Wire scans of beam profile for stripped and unstripped
beams. He+ is in the absence of gas in the stripper, He++ is in the
presence of gas in the stripper.*

**Operational performance**

Details of the stripper cell are described elsewhere in this report. The
injector and nozzle geometry remain as described above for the 1-MeV
test. Pumping capacity is 3000 l/s in the lower section (turbo) and 1000
l/s in the upper section (scavenge). Gas is directed down into a
stripper chamber for interaction with the beam, and into a chamber
evacuated by two large turbo pumps. Operational performance of the
stripper cell is summarized in figures 6.5 and 6.6. The pressure in the
turbo section is a measure of the gas injected by the jet toward the
turbo pumps. The pressure in the scavenge section is a measure of the
gas scattered back into the scavenge section. Note that the pressure in
the scavenge section is much lower than the turbo pump pressure,
indicating that well over 90% of the gas is injected into the turbo
section As a result the bulk of the gas flows are isolated from the
other regions of the accelerator. In particular the 212-MHz RFQ adjacent
to the stripper cell operates at a pressure of less than 1 x 10-6 Torr.

For a given back pressure on the injector, the amount of gas injected
increases rapidly up to a peak as measured by the pressures at the two
pressure gauges in the stripper cell. Above the peak, the amount of gas
injected drops. The position and amplitude of the peak is a function of
the back pressure on the injector. For injector pulse lengths below the
peak, gas throughput decreases as back pressure increases. Above the
peak the reverse relationship holds true. The stripping efficiency as
measured by the amount of current detected at the end of the MEBT also
peaks when the pressures peak. In order to prevent

\(a\)

![](media/image26.wmf){width="5.845833333333333in"
height="3.736111111111111in"}

\(b\)

![](media/image27.wmf){width="5.858333333333333in"
height="3.747916666666667in"}

*Figure 6.5. Performance of the stripper cell at 120 Hz as a function of
injector pulse length for (a) 20 psig and (b) 30 psig nitrogen gas back
pressure on the fuel injector and 13 mA He+ beam out of the 212-MHz
RFQ.*

overloading the pumps at the peaks of the curves, these plots were taken
at 120 Hz. Time was not available to do a more thorough investigation of
this performance, or to relate it to theoretical parameters of the
nozzle and injector.

The performance of the stripper as a function of time depends on the
operating point chosen. Figures 6.6a and 6.6b show stripper performance
as a function of time. Note that in contrast to the 60 Hz data shown
above, there is a large floor in the stripping fraction before the
injector is turned on. This is due to the presence of gas in and near
the stripper cell that has not been pumped out. Figure 6.6a shows beam
current out of the MEBT for a point below the peak. The beam current
slowly increases with time, indicating a slowly forming gas pulse,
reaching a peak at about 1200 µs. Figure 6.6b shows the same the beam
current again, this time taken above the peak of Figure 5. The stripping
efficiency, hence the gas injection rate, increases much more rapidly,
reaching a peak at about 900 µs. Since the time available to pump out
the stripper cell between beam pulses is very short (2.78 ms), the
above-peak operating point appears to be modestly more advantageous to
operation because an additional 300 µs is available to pump out the cell
after the injected gas pulse is over. At least as important is the fact
that the beam intensity is more stable from pulse to pulse. This is the
nominal operating point.

At 360 Hz there is a roughly 10% decrease in stripping efficiency from
the efficiency obtained at the lower repetition rates. This is at least
partly due to the fact that it is necessary to operate at less than
maximum throughput to prevent overloading the pumps. Although direct
measurements of the stripping efficiency are not possible, it is likely
that the stripping efficiency drops from about 80% at 120 Hz to about
70% at 360 Hz. The implied efficiency of transport of beam through the
MEBT is about 70%, based on the overall measured efficiency on the
stripper-MEBT combination of about 50%.

**Effects on MEBT performance**

Typical operating pressure in the MEBT is in the range of 10-5 Torr.
This is a sufficiently high pressure to generate charge-neutralizing
background electrons due to ionization of the background gas. On the
other hand, charge exchange on background gas, if allowed to take place,
could deplete the beam current by creating He+ beam ions, which would
then be lost in the MEBT. Expectations are that the gas density in the
MEBT remains below the density required for significant amounts of
charge exchange to take occur. But the cross sections and detailed
pressure distributions in the MEBT are not well known. Future
investigations are required to determine whether charge-exchange losses
in the MEBT, or some other mechanism, contribute to the observed
shortfall in stripping efficiency at 360 Hz.

These and other sources of electrons in the stripper ensure that there
is a copious supply of electrons for charge neutralization in the MEBT.
As described in Reference 1 and elsewhere in this report, space-charge
neutralization due to the presence of electrons is a problem for
reliable operation of conventional bunchers, but is probably
advantageous to operation of the MEBT.

\(a\)

![](media/image28.wmf){width="5.8902777777777775in"
height="3.7645833333333334in"}

\(b\)

![](media/image29.wmf){width="5.8902777777777775in"
height="3.7645833333333334in"}

*Figure 6.6. Beam current out of the MEBT as a function of stripper
pulse delay at 360 Hz in nitrogen gas. (a) 10 psig back pressure, 229 µs
pulse length (below peak of figure 6.5), (b) 20 psig back pressure, 292
µs pulse length (above peak).*

**References**

1\. F. M. Bieniosek, K. Anderson, R.J. Pasquinelli, M. Popovic, C.W.
Schmidt, R. Webber, D.J. Larson, P. Young, Charge stripper and MEBT for
the 3He RFQ accelerator, Proc. 1997 Particle Accelerator Conference.

2\. The 3He RFQ PET tracer production system, Preliminary Design Report,
Nov. 26, 1990, Internal Project Report PET 2.

3\. M. Gryzinski, Classical theory of atomic collisions, Phys. Rev. 138,
A336 (1964).

4\. J. Gorres, K.U. Kettner, H. Krawinkel, C. Rolfs, The influence of
intense ion beams on gas jet targets, Nucl. Instrum. Meth. 177, 295
(1980).

5\. JL Dobson and BRF Kendall, Controllable leaks using electrically
pulsed valves, J Vac Sci Technol. A 1990, p 2790

6\. B. Franzke, N. Angert, Ch. Schmelzer, Charge exchange of fast iodine
ions in a dense jet of carbon dioxide, IEEE Trans. Nucl. Sci. 19, 266
(1972).

7\. P. Sigmund, K.B. Winterbon, Small-angle multiple scattering of ions
in the screened coulomb region, Nucl. Instrum. Meth. 119, 541 (1974).

**7. Machine Operations and Performance**

**Introduction**

After two years of intense effort building the PET accelerator at
Fermilab, it\'s worth pausing briefly to review how we got where we are,
examine the accelerator\'s current performance, and assess our
operational status. The following topics will be covered: operations
history, accelerator performance, and targetry sessions. We cover
activities and performance results up through Mar. \'98.

**Operations History**

The time line presented in Figure 7.1 is a synopsis of major milestones
based on data retrieved from the project log books. The first phase was
the design of the new PET accelerator. This ran roughly from January
1995 to October 1996. Part of the design phase included completing
experiments to test several design concepts. Three major efforts were
pursued in parallel, 1. Getting the ion source functional, 2.
Characterizing the 212.5 MHz RFQ, 3. Testing the performance of a gas
jet stripper. During this first year, project staff designed and built
much of the final hardware. A detailed series of target window break
tests were performed to begin understanding the dynamics of the HEBT.
Toward the end of this phase we discovered vane damage in the 425 A RFQ
due to arcing during RF conditioning. Water corrosion within the RFQ
vanes was first observed at this time and resulted in leaks between the
vanes and the vacuum. The collaboration now faced not only building a
new fourth RFQ for acceleration to 10.5 MeV, but the need to replace the
vanes in the three existing RFQs.

First beam from the new ion source took place on October 14, 1996. One
MeV beam was accelerated two weeks later and tests of the new stripper
hardware began on December 12. MEBT installation commenced in
mid-January \'97 and was completed by the end of the month. First beam
through the MEBT occurred on January 27, 1997. After the delays and
setbacks experienced so far, the rapid progress and quick success of the
MEBT was a pleasant surprise. Given the complexity of the task and the
unproved beam physics, most members of the team predicted a longer and
more difficult process in getting the MEBT on line.

Installation of the 425 A RFQ was initiated with 5 MeV beam being
accelerated by March 11, 1997. Our rapid progress and relative success
during this phase led to the decision to install 425 B & C
simultaneously. Installation of B and C started on April 10. A vacuum
leak in RFQ 425 C caused a short setback. Once fixed, the first 10.5 MeV
beam was accelerated on April 17, 1997. Elapsed time was six months from
setting up the new ion source to accelerating 10.5 MeV beam. After the
initial setbacks with the RFQ vanes, progress had been fairly rapid. The
collaboration expected to conclude development quickly and move to
Shreveport by the summer 1997. This was not to be the case.

A number of activities took place in parallel during the 425 B &C
commissioning phase. The HEBT was installed, a number of radiation
surveys were conducted, and work continued daily on tuning and improving
accelerator performance. The HEBT was installed April 28 with subsequent
delivery of 10.5 MeV beam to the target vault by May 2. The
radiochemists performed their first target studies on May 21. The
constant efforts on improving accelerator performance were paying off.
Fifty µA of beam was delivered by May 23. Radiation surveys indicated
alarming high levels of radiation along the 425 RFQ chain. Reasons for
this were not completely understood at the time. On June 9 we had our
first RF coupler window failure. This turned out to be a warning signal
of more problems to come.

After the initial successes, the project entered probably the most
difficult phase in June. This is the portion labeled \"Beam and
Accelerator Improvement\". In the drive to get the beam up to 150 µA ,
the accelerator needed to be run at a higher duty cycle approaching the
2.5% design value. A number of frustrating breakdowns of RF coupling
loop windows occurred. RF losses in the window caused overheating of the
o-ring seal leading to vacuum failure. The coupling loop window and
o-ring seal were redesigned, fabricated and installed in all 425 RFQs
and no further failure occurred. The successive series of vacuum
failures had a significant impact on RFQ cavity conditioning. What
normally could have been done in two weeks, took closer to twelve.
During this time the project also experienced a series of failures of
various RF electronics modules. These infant mortality type failures
held up progress on increasing beam current until mid-August.

After the installation of the new RF coupler loop windows on July 24,
rapid progress in conditioning the RF cavities occurred. A beam current
of 100 µA was achieved by early September. This was followed by detailed
HEBT tuning to create a homogeneous distribution of beam on the target
window. The next targetry session began September 24. The higher power
of the beam caused overheating of one of the HEBT collimator jaws and a
water leak in one of the cooling channels forced the removal of the
target vacuum vessel that contained the collimators. A tungsten aperture
plate was installed at the B & C RFQ junction in an attempt to stop beam
from hitting the RFQ C vanes at the same time. It was hoped that this
would reduce the radiation emitted from this location. The plate did
intercept beam current, but did not reduce the radiation at that
junction by a commensurate amount. The test was unsuccessful as can be
seen in figure 7.7, and the plate was removed after the test.

The final phase of accelerator commissioning was installation and
commissioning of the new 212 RFQ. First 1 MeV beam through the new RFQ
occurred on October 16. Repairs to the HEBT collimator system were
completed and the target diagnostics system was reinstalled by October
31. HEBT scans resumed on November 9 and a targetry session began on
November 10. Results of this short session indicated we had made
progress on the accelerator. Current levels and stability were improved.

Targetry sessions began in earnest on 12/11. We experienced a window
break early in this session. Our fast valve, designed to prevent foil
from migrating into RFQ C, was not armed at the time. The fast valve
sensor was too sensitive for the type of target we were using. We were
getting premature triggering of the fast valve due to outgassing from
the targets and decided to run with it disarmed. Foil contamination in C
caused excessive sparking which limited the accelerator\'s performance
for the rest of this session. Plans were made after this session to
reconfigure the fast valve sensor system to make it less sensitive.

Our current and final phase of accelerator operations started in 1998.
This is the PET chemistry phase. The original planned configuration of
the accelerator is complete. Our primary focus now is to provide beam as
needed for the radiochemists. Modifications and adjustments to the
accelerator are done based on results of chemistry sessions. We\'ve had
three chemistry sessions in 1998\....all of them relatively successful.
Improvements to the accelerator this year to date include reconfiguring
the fast valve sensor system to make it less sensitive, overhauling the
water skid to fix a series of control problems, and installation of
mechanical tuning paddles in the 212 and A and B cavities. The paddles
have had a marked improvement on machine stability.

+----------+----------+------------------------------------------------+
| **1995** | 11/7     | **OLD PET CONFIG./STRIPPER STUDIES**           |
|          |          |                                                |
| **1996** | 6/26     | 1st accelerated beam with old pet config.      |
|          |          | 12/20                                          |
|          |          |                                                |
|          |          | window break tests                             |
|          |          |                                                |
|          |          | start stripper tests 3/15                      |
|          |          |                                                |
|          |          | discover burned vanes in 425A 6/18             |
|          |          |                                                |
|          |          | complete source and stripper tests 6/26        |
|          |          |                                                |
|          |          | discover corrosion leaks in 212 vanes 9/26     |
+----------+----------+------------------------------------------------+
|          | 10/4     | **START NEW CONFIG./1 MeV SYSTEM ON-LINE**     |
|          |          |                                                |
|          | 12/10    | new source on line 10/14                       |
|          |          |                                                |
|          |          | **1 MeV beam 12/5**                            |
|          |          |                                                |
|          |          | stripper tests started 12/10                   |
+----------+----------+------------------------------------------------+
| **1997** | 1/20     | **MEBT COMMISSIONING**                         |
|          |          |                                                |
|          | 2/9      | MEBT installation complete 1/27                |
|          |          |                                                |
|          | 3/1      | **MEBT beam 1/27**                             |
|          |          |                                                |
|          |          | MEBT studies                                   |
|          |          |                                                |
|          |          | Install 425A                                   |
+----------+----------+------------------------------------------------+
|          | 3/21     | **425A COMMISSIONING**                         |
|          |          |                                                |
|          | 4/10     | **5 MeV beam 3/11**                            |
|          |          |                                                |
|          |          | MEBT & 425A tuning studies                     |
|          |          |                                                |
|          |          | Install 425 B & C                              |
+----------+----------+------------------------------------------------+
|          | 4/30     | **425 B & C COMMISSIONING**                    |
|          |          |                                                |
|          | 5/20     | **10.5 MeV beam 4/17**                         |
|          |          |                                                |
|          |          | Accelerator tuning and beam studies            |
|          |          |                                                |
|          |          | HEBT installed 4/28                            |
|          |          |                                                |
|          |          | Beam to vault 5/2                              |
|          |          |                                                |
|          |          | **1st targetry session 5/21**                  |
|          |          |                                                |
|          |          | 50 µA beam 5/23                                |
|          |          |                                                |
|          |          | RF window failure on 425 A (1st of several)    |
+----------+----------+------------------------------------------------+
|          | 6/9      | **BEAM & ACCELERATOR IMPROVEMENT**             |
|          |          |                                                |
|          | 6/29     | RF window seal system redesign / silicone      |
|          |          | o-rings installed 6/19                         |
|          | 7/18     |                                                |
|          |          | **target diagnostics installed 7/3**           |
|          | 8/7      |                                                |
|          |          | series of failures in RF electronics modules / |
|          | 8/27     | redesign successful                            |
|          |          |                                                |
|          | 9/16     | **targetry session**                           |
|          |          |                                                |
|          | 10/6     | major source of radiation at 425 B-C interface |
|          |          | verified 7/16                                  |
|          |          |                                                |
|          |          | new RF window system installed 7/24            |
|          |          |                                                |
|          |          | cavities finally condition to 360 Hz           |
|          |          |                                                |
|          |          | HEBT beam scans restarted 7/25                 |
|          |          |                                                |
|          |          | rapid increase in current levels               |
|          |          |                                                |
|          |          | beam tuning studies                            |
|          |          |                                                |
|          |          | beam to 100 µA level                           |
|          |          |                                                |
|          |          | **chemistry session 9/24 - 9/27**              |
|          |          |                                                |
|          |          | collimator failure                             |
|          |          |                                                |
|          |          | tungsten aperture installed 9/29 removed 10/7  |
+----------+----------+------------------------------------------------+
|          | 10/26    | **NEW 212 COMMISSIONING**                      |
|          |          |                                                |
|          | 11/15    | **new 212 on line 10/13**                      |
|          |          |                                                |
|          | 12/5     | HEBT repairs completed                         |
|          |          |                                                |
|          |          | new HEBT scans started 11/9                    |
|          |          |                                                |
|          |          | **targetry session 11/10 - 11/13**             |
|          |          |                                                |
|          |          | **120 µA current with new 212**                |
|          |          |                                                |
|          |          | **targetry session 12/11 - 12/16**             |
+----------+----------+------------------------------------------------+
| **1998** | 1/13     | **PET CHEMISTRY EXPERIMENTS**                  |
|          |          |                                                |
|          | 1/28     | maintenance week on vacuum. systems            |
|          |          |                                                |
|          | 2/10     | RF window failure in C                         |
|          |          |                                                |
|          | 2/24     | new fast valve configuration in                |
|          |          |                                                |
|          | 2/27     | new 4616 tube installed in 425 A PA            |
|          |          |                                                |
|          | 3/16     | **successful chemistry run (2/10-2/15)**       |
|          |          |                                                |
|          | 3/23     | 60 µA on target/some stability problems        |
|          |          |                                                |
|          | 3/31     | mechanical tuning paddles installed in 425 A & |
|          |          | B                                              |
|          |          |                                                |
|          |          | series of tuning studies started\...18 mA out  |
|          |          | of 212                                         |
|          |          |                                                |
|          |          | Fermi operator support underway                |
|          |          |                                                |
|          |          | HEBT repairs completed                         |
|          |          |                                                |
|          |          | new HEBT scans started 11/9                    |
|          |          |                                                |
|          |          | **targetry session 3/16 - 3/20**               |
|          |          |                                                |
|          |          | accelerator performance much improved          |
|          |          |                                                |
|          |          | tuning paddles installed in 212                |
|          |          |                                                |
|          |          | **125 µA current, improved stability**         |
|          |          |                                                |
|          |          | **targetry session (3/31 - 4/4)**              |
+----------+----------+------------------------------------------------+

*Fig. 7.1 PET Accelerator Development History*

The accelerator\'s performance in March has been vastly improved.
Current levels, stability, and reliability have all gone up.
Installation of the mechanical paddle tuning system solved the resonance
control problems. Fermi\'s operations group assigned two operators to
help support PET in late Feb. The additional staff support has enabled
us to run longer hours and recover more quickly from occasional problems
that come up. We also now have a broader base of trained operators
available.

Figure 7.2 shows blocks of time allocated to the major phases of the
accelerator\'s development. Interspersed within these large blocks of
time are shorter tasks devoted to repairing and maintaining hardware
already installed. For example, an average of four days minimum per
month were spent working on the ion source. These overhead type tasks
are discussed below under reliability.

**Accelerator Performance**

*Beam Current*

The project goal was a minimum of 150 µA average beam current. Figure
7.3 shows progress toward that value. As noted in the figure, increasing
beam current at 10.5 MeV took place in three stages. After rapid initial
progress, a period of hardening systems and conditioning the RF cavities
occurred. Progress resumed in late August with 120 µA of beam achieved
readily. Commissioning the new 212 RFQ involved conditioning a new
cavity and, in essence, meant starting the commissioning process for the
accelerator all over. The new 1 MeV beam needed to be properly matched
into the MEBT. Progress again was fairly rapid with sustained average
current above 100 µA. We\'ve continued to make improvements to the
accelerator between beam chemistry sessions. Beam stability at higher
current levels is much improved. We\'re now able to run regularly at 120
µA at around a 70 usec. pulse. It appears that we should be able to
obtain in excess of 130 µA at full RF pulse width of 80 usec.

![](media/image30.wmf){width="5.9944444444444445in"
height="4.186805555555556in"}*Figure 7.2. Operations weeks expended on
major PET project tasks*.

![](media/image31.wmf){width="5.763888888888889in"
height="3.9722222222222223in"}

*Figure 7.3 Evolution of average beam current versus time*.

Operations at higher current levels revealed a constant problem with
resonance control. Sparking in one of the cavities often caused a loss
of resonance in that cavity. This in turn changed the beam loading in
cavities downstream resulting in loss of resonance in them as well.
Resonance control problems would migrate through the 425 chain,
interrupting beam. We decided to install mechanical tuning paddles
developed by SAIC during the San Diego phase of the project. These
paddles would act as a fast response system, adjusting cavity resonance
quickly in the event of sparks or major changes in beam loading. We
completed installation of paddles in 425 A and B by Feb. 24. The
accelerator recovered quickly, with marked improvement in performance.
Our chemistry session the week of 3/16 was the best ever, as a result of
these improvements. We installed a paddle in the 212 RFQ the week of
3/23 and plan to complete the sequence with a paddle in RFQ C in mid
April.

*Beam Quality*

The major function of the HEBT is to take the beam that emerges from the
425 C RFQ and shape it into a form useful for targetry. The nominal
target size is 2 x 10 cm with a goal of +/- 25% maximum beam current
density over that area. Beam hot spots shorten the life of the target
windows. The density distribution is controlled by two magnets in the
HEBT. A quadrupole controls the horizontal distribution and a multipole
controls the vertical. Fig. 7.4 presents the best current distribution
achieved to date, with +/- 50% uniformity. The actual target window
occupies the zone from 3.5 to 6.5 cm on the horizontal scale and 1 to 11
cm on the vertical scale. This distribution in the target zone runs
roughly from 20 to 100 in scaled units.

![](media/image32.wmf){width="3.5131944444444443in" height="4.31875in"}

*Figure 7.4 . Beam distribution on the 2 x 10 cm target window.*

*Accelerator Reliability*

Reliability is another critical measure of the accelerator\'s potential
utility. The reliability data presented here is based on daily
operations reports and log book entries starting June 2, 1997. By this
date, most of the critical systems had been commissioned. Fig. 7.5 shows
days per month of beam on vs. scheduled maintenance and unscheduled
maintenance. Unscheduled maintenance represents down time due to system
failures. Scheduled maintenance includes items such as planned upgrades
and installation of new equipment. Two days per month were allocated to
the ion source for planned maintenance. Figure 7.6 shows beam available
as a percent of operating time for the month. The dip in October being
due mostly to recommissioning involved with the new 212 RFQ. It was also
during this period that daily operations and trouble shooting was
delegated to the SAIC staff on site.

  ------------------------- -------------------- ----------------------------- ----------------------------- -------------------------
  **[month]{.underline}**   **[BEAM              **SCHEDULED\                  **UNSCHEDULED\                **[TOTAL]{.underline}**
                            ON]{.underline}**    [MAINTENANCE]{.underline}**   [MAINTENANCE]{.underline}**   

                                                                                                             

  June 97                   3.5                  5.5                           11                            20

  July 97                   8                    9                             7                             24

  August 97                 9.5                  2.5                           8                             20

  Sept. 97                  10.5                 2.5                           6                             19

  Oct. 97                   8                    9                             8                             25

  Nov. 97                   8.5                  1.5                           10                            20

  Dec. 97                   8                    2                             3                             13

  Jan. 98                   7                    6                             7                             20

  Feb. 98                   7.5                  5.5                           8                             21

  Mar. 98                   [13.5]{.underline}   [3]{.underline}               [5.5]{.underline}             [22]{.underline}

  **total**                 **84**               **46.5**                      **73.5**                      **204**
  ------------------------- -------------------- ----------------------------- ----------------------------- -------------------------

*Fig. 7.5 Beam available in days per month.*

![](media/image33.wmf){width="5.75in" height="3.625in"}

*Figure 7.6. Percentage of "beam on" time per month.. (Beam
on/(Total-Sched Maint))*

Improving accelerator reliability involves assessing where problems
exist. The unscheduled maintenance noted above breaks down as follows:

Ion Source 19.5

High Level RF 17

HEBT 6

Water cooling skids 8

RFQ hardware 6

Vacuum Systems 5

Neslab Chillers 4.5

Controls/Software 4

Low Level RF [\_3.5\_\_]{.underline}

73.5

This data is not meant as a criticism of the design or efforts of any
particular group. It is more a reflection of the complexity of the
various subsystems and the time and resources available for getting them
on line. Also, significant improvements have been made in all of these
areas over the past year.

Beam current is shown as a function of pulse width for three different
accelerator configurations in figure 7.7. The middle curve (labeled Old
212 RFQ) represents the original configuration in effect until September
29, 1997. The tungsten aperture data was described earlier. The curve
labeled New 212 RFQ represents current performance.

![](media/image34.wmf){width="5.736111111111111in"
height="3.361111111111111in"}

*Figure 7.7. Average beam current versus pulse width for three operating
conditions.*

Optimizing beam current and beam quality are a major part of operations.
Beam current improvements are made through tuning routines, i.e. magnet
settings, phases and cavity gradients are systematically adjusted while
watching overall beam current and stability. The process requires
successive iterations. Targeted beam quality improvements are
accomplished by doing HEBT scans, involving setting up the four
collimator jaws in the HEBT so a small (typically 7 mm x 4 mm) pixel is
formed. Beam transmitted through this open rectangle is collected in a
faraday cup and the integrated value is recorded in a database. Scan
data over a target sized zone provides a picture (such as Figure 7.4) of
the beam\'s current density across the window.

*Operations Radiation Safety Procedures*

Accelerator operations are conducted in full compliance with Fermilab's
safety procedures and are monitored by the project's Radiation Safety
Officer (RSO). Operations staff are radiation worker trained and have
had crew chief training. There is a designated crew chief whenever the
beam is up. Daily operations plans are listed on a white board in the
control room. Operations progress reports are e-mailed to the
collaboration daily. Lockout-tagout procedures as relevant are part of
everyday activities. Staff is trained and competent in handling
radiation monitoring equipment such as the Ludlams, wallflowers, and
friskers. Parts and equipment removed from the accelerator and taken out
of the enclosure are checked with friskers on site for contamination.

*Operations Staff*

The on site SAIC staff took over solo operations in mid-October 1997
after the new 212 MHz RFQ went on-line. SAIC\'s operations group
includes an electrical engineer (also site manager), a full time
operator, and a physicist here about 2/3 time. In addition a full time
mechanical engineer is available to support targetry work and mechanical
engineering related items, plus part time support from an
engineering-physicist who is experienced in RFQ technology. Members of
this team have an average of about seven years experience in accelerator
projects. Three members were principals in the highly successful
implementation of the Loma Linda project.

Fermi operations staff began supporting PET in late Feb. Currently two
trained Fermi operators are assigned to PET on a rotating basis. Shifts
include day (8 to 4) and evening (4 to midnight). Control room operators
on duty over the owl shift (midnight to 8 am) monitor RF and vacuum
system performance on monitors in the control room.

**Targetry Sessions**

There have been eight sessions of targetry experiments with the PET
accelerator to date. The first five sessions, taking place in 1997, were
limited in results by the accelerator\'s uncertain performance and
incomplete development. The first and second target sessions (May 21,
1997 & July 9, 1997) consisted of a shakedown of the control/hardware
systems and initial foil irradiations. The foil irradiations are a means
of cross checking the beam density distribution. Beam current levels
were 30 µA average.

The third session (September 23, 1997) was planned to be the first
chemistry session with the accelerator fully operational. Beam currents
approaching 100 µA were available, but it was also during this period
that the HEBT sprang a water leak due to overheating. The vacuum problem
caused by the leak was overcome in a couple of days and testing of Havar
target windows was initiated. It was apparent that more HEBT tuning was
necessary due to excessive hot spots on the target window.

The fourth targetry session began November 10, 1997. The new 212 RFQ was
installed at this time, but beam currents approaching 100 µA had not yet
been reestablished (60 µA was delivered). The radiochemists completed
three carbon foil counts during this period to characterize the HEBT
beam. After some adjustments in the HEBT, beam uniformity measured by
radiating foils was comparable with HEBT collimator scans. A new gridded
target window support structure was also tested successfully.

We started the fifth chemistry session on 12/11. We had 80 µA of beam
current available initially. A foil window break resulted in
contamination of RFQ C with some bits of conductive foil which led to
severe sparking in C. This limited the accelerator\'s performance for
the duration of this session. We were able to collect data for a gas
target and complete several tests of different types of foils.

The three sessions held so far (Jan. through Mar.) in 1998 have been
much more productive. We\'ve generally been able to supply beam in the
amounts needed, as needed, with much improved beam stability. The
chemists have successfully tested a variety of gas, solid, and water
targets and proceeded with successful isotope yield studies and
extraction.

**Summary**

The effort to date on building the accelerator has exceeded the original
estimate by six to eight months and about \$2 million. As has been
discussed in the report, numerous difficulties unforeseen at the outset
of the program have caused this delay and extra expense. The team\'s
constant efforts and improvements to the accelerator appear to have
produced good progress over the past few months. The accelerator\'s
performance and reliability have improved significantly. At this point
we fully expect to be able to successfully support the radiochemistry
program.

#  

# 8. Beam Instrumentation and Diagnostics Systems

## Introduction

The machine as delivered to Fermilab in 1995 included little beam
instrumentation equipment deemed suitable for this project. The only
beamline components recovered were three Pearson transformers for
measuring beam current. Of the limited signal processing electronics
provided, only the beam current integrating instrument for "beam on
target" was compatible with the machine requirements and suitable for
integration into the control system. The beam instrumentation systems
have been completely re-built as a part of this project.

## System Design at Fermilab

### *Considerations and Requirements*

Uncertainties in the expected performance of individual RFQs, novel
charge stripper and MEBT designs, limited space in the machine layout
for important diagnostics, and beam power capable of damaging certain
instrumentation devices necessitated a staged beam commissioning
process. This presented a wide variety of overall beam instrumentation
requirements.

Transverse emittance measurements were desired at 20 keV, 1 MeV, and
5 MeV. Energy and charge state characterization of the 1 MeV beams out
of the 212 MHz RFQ and the charge stripper were required. MEBT
commissioning required transverse beam profile and position measurements
at several locations and a longitudinal bunch length diagnostic at the
MEBT downstream end. HEBT and target commissioning called for detailed
transverse beam current distribution measurements of the full power 10.5
MeV beam. A calibrated Faraday Cup/beam stop is necessary for
operational tune-up and to protect the chemist's target during tune-up
and between runs. An accurate "dose on target" measurement capability is
required for chemistry experiments. Beam current measurement throughout
the accelerator was important at all stages.

### *Design Decisions*

Assessment of the perceived requirements in conjunction with budget and
scheduling considerations led to a plan for provision of beam
instrumentation and diagnostic systems.

-   Non-intercepting beam current transformers would be permanently
    installed along the machine where appropriate and space allowed.

-   A water cooled charge collector/beam stop suitable as a temporary
    fixed diagnostic for full current at any of the beam energies of
    concern would be built for the project.

-   Transverse emittance measurements would be accomplished utilizing a
    system borrowed from the Fermilab ion source development lab.

-   A simple 1 MeV spectrometer using a borrowed Fermilab magnet would
    be fabricated.

-   Multiwire or harp style monitors would be designed and fabricated
    for transverse beam profile measurements at 1 MeV in and following
    the MEBT. These would not be actively cooled and therefore would be
    useful only at low beam pulse repetition rates (\<= 10 Hz).

-   Faraday apertures, beamline apertures constructed of segmented
    charge collecting electrodes, would be used as non-destructive beam
    size and position indicators at numerous locations in the machine.

-   The MEBT would be constructed with three insertion locations
    designed to accept either a dual plane multiwire profile monitor
    assembly for commissioning or a Faraday aperture assembly for high
    duty cycle operations.

-   A beam intercepting wire would be used as the source for relative
    longitudinal bunching signals at the output of the MEBT.

-   No room exists between the three 425 MHz RFQs in the final layout
    for any type of beam instrumentation device.

-   A collimator with four water cooled, independently and remotely
    controlled, isolated charge collecting jaws would be designed and
    built to be installed at the end of the HEBT for 10.5 MeV beam
    collimation and diagnostics.

-   A properly designed Faraday Cup would be built to be located between
    the collimator and the chemistry target.

-   New signal processing electronics designs and fabrication would be
    kept to a minimum.

-   Beam instrumentation signals would interface directly with the
    existing Control System Internet Rack Monitor analog input hardware.

## System Descriptions and Operational Performance

### *Beam Current Measurement*

Five current transformers are installed - at the ion source output, at
the 212 MHz RFQ output, at the charge stripper output, at the 425 MHz
RFQA input, and in the HEBT. Pearson Model 2100 transformers, 0.5 mV/mA
into 50 ohms, are used throughout. Signals are processed by the toroid
and charge collector electronics described below.

A credible ion source beam current measurement is obtained when the
source magnet is at a nominal current and the LEBT solenoid and steering
magnets are de-energized. With the LEBT magnets energized and the source
magnet adjusted for maximum beam current at 1 MeV from the 212 MHz RFQ,
the ion source transformer signal becomes distorted and its calibration
ambiguous. This is believed to be due to stray magnetic fields and/or to
electron currents due to beam striking the transformer. Even under these
conditions the signal is a useful diagnostic always on display on an
oscilloscope in the control room.

The remaining transformers provide reliable signals accurate to within 1
milliamp over a wide range of normal operating conditions.

### *Emittance Measurement*

A slit and collector emittance measurement system, including actuator
and control and signal processing electronics, was borrowed from the
Fermilab ion source development lab. This was used to make measurements
on the PET machine beam at 20 keV, at 1 MeV (with and without the charge
stripper and before and after the MEBT), and at 5 MeV (with only 425 MHz
RFQA in place). Only minor mechanical modification to the slit and
collector head and to the vacuum interfaces was required. All emittance
measurements required dedicated mechanical and vacuum setups of the
machine. No emittance measurement capability remains in the operational
accelerator. Results of the emittance measurements are published
elsewhere^1^. The internal papers section of the PET Web page^2^
contains several other articles on the emittance measurements.

### *Spectrometer*

A specially designed fan shaped vacuum chamber coupled with a borrowed
Fermilab magnet served as a spectrometer to make measurements of the 1
MeV beam. The chamber was terminated with three isolated disks serving
as Faraday charge collectors and two isolated vertical wires between the
Faraday collectors. The disks, each sized to accept the full beam spot,
were placed at angles of 0, 11.5 and 23.6 degrees. Installed downstream
of the charge stripper, this spectrometer arrangement permitted
measurement of straight-ahead beam with the magnet de-energized and
simultaneous measurement of singly and doubly charged helium beams with
the charge stripper and analyzing magnet operating.

Signals from the Faraday collectors facilitated energy analysis and
charge stripper efficiency measurements. Signals from the vertical wires
provided horizontal beam profile measurements as the beams were scanned
across the wires with the analyzing magnet. This proved valuable to
watch for beam blowup during stripper development using different gases
and at different pressures.

A diagram of the spectrometer and additional information can be found
elsewhere in this report^3^.

### *Profile Measurement*

Multiwire (harp) beam profile monitors were specially built for use
during MEBT commissioning. Each monitor provides for both horizontal and
vertical measurements with nine 0.002" gold plated tungsten wires in
each plane. These monitors were located at the entrance to the first
MEBT alpha magnet, at symmetrical locations in the MEBT cross-over arm
between alpha magnets, and at the RFQA match point downstream of the
MEBT.

Space in the MEBT is extremely tight, limiting design options of the
profile monitors. Wire heating due to absorbed beam energy was also a
serious concern. A compact wire mounting scheme with built-in tensioning
lever arms was conceived and implemented to limit wire sag and
distortion at elevated temperatures. The design was tested by heating
wires to \>1000 K with electrical current. Beam operation was limited to
10 Hz pulse repetition rate while the profile monitors were installed.
In spite of these precautions, considerable discoloration of the wires
was observed when the monitors were removed and several wires were found
broken.

Signal wiring was arranged to utilize the same multi-channel electronics
as the emittance measurement system. Up to four planes of profile
monitoring could be processed simultaneously. The profile measurement
capability proved invaluable for commissioning and tune-up of the novel
MEBT beamline.

### *Faraday Apertures*

Each Faraday aperture consists of four isolated electrodes (top, bottom,
right, and left) which form an aperture limit in the local beamline.
Signals from the electrodes in conjunction with steering and focusing
scans of appropriate magnets provide some information on beam size and
position. These appeared attractive as small, simple, inexpensive
devices which would provide important diagnostic capability. Apertures
were designed for the input of the 212 MHz RFQ, the input of first MEBT
alpha magnet, two symmetrical locations in the MEBT cross-over arm
between alpha magnets, and the input of the 425 MHz RFQA.

Originally, it was thought that this type of device alone might be
sufficient for MEBT commissioning. Fortunately, further consideration
led to the decision to construct the MEBT with three insertion locations
designed to accept either a multiwire profile monitor assembly or a
Faraday aperture assembly. As it turned out, the Faraday apertures alone
would have been woefully inadequate to provide the diagnostics required
for MEBT commissioning. Even after commissioning, the Faraday aperture
upstream of the 212 MHz RFQ and the three in the MEBT have been of
limited value.

The Faraday aperture immediately upstream of the 425 MHz RFQA, however,
has proved operationally essential for a reason not imagined during the
machine design. During commissioning it became apparent that the beam
path length and therefore transit time through the MEBT is extremely
sensitive to MEBT tuning. Phasing between the 1 MeV beam bunches and the
425 MHz RFQs was tightly coupled to MEBT magnet adjustments. A 0.1%
change in alpha magnet current resulted in a 90 degree relative phase
shift between the beam and the 425 MHz RF. This coupling made tune-up
quite difficult.

The Faraday aperture provides a solution. The small fraction of the beam
impinging on the aperture yields a 212.5 MHz signal which represents the
beam phase entering 425 MHz RFQA. This must be held constant relative to
the 425 MHz RFQ cavity voltage. The 212.5 MHz signal is amplified,
frequency doubled and phase compared to a sample of the 425 MHz RF. The
result is used as feedback to adjust the 212 MHz RFQ phase and thus
stabilize the beam phase entering 425 MHz RFQA. A trombone delay line in
the beam RF signal path facilitates setting the desired operating point.

This aperture arguably has served another purpose. The electrodes were
originally made of copper. During the rebuild of MEBT Q5 magnet, the
electrodes were exposed for inspection. It was learned that the
electrode on the right side, as the beam sees it, had at some time been
heated to the point of puddling the copper and causing a short circuit
of that one electrode to ground. Investigation using information in the
datalogger showed a dramatic change in that electrode's signal during a
period in which beam had been run at 360 Hz with some MEBT quadrupole
magnets not energized. The aperture may have protected the RFQ from
damage. Subsequently the copper electrodes of this aperture were
replaced with ones made of tungsten.

In addition to the RF signal processing used for phase feedback control,
each Faraday aperture electrode at each location is served by a channel
of the toroid and charge collector electronics.

### *Longitudinal Measurement*

The MEBT is designed to reflect at its output the longitudinal phase
space parameters of the input beam. The degree to which this imaging is
achieved is sensitive to MEBT magnet strengths. Commissioning thus
required a longitudinal diagnostic to determine relative bunch length at
the 425 MHz RFQA match point to confirm design performance and to find
the correct operating point.

Field spreading due to the low velocity of the 1 MeV beam rules out the
use of non-intercepting monitors for the required high frequency
measurement. An intercepting wire which provides prompt signals from
some combination of secondary emission and charge absorption is
suitable. Flat response over a large bandwidth is not required for
relative bunch length measurements. The relative amplitude of the 212.5
MHz signal and one or more harmonics can be measured with a spectrum
analyzer under different MEBT tuning conditions and the relative bunch
lengths of each condition inferred.

A dedicated intercepting wire was originally planned for this purpose,
though it was found during commissioning that a suitable signal was
available from the multiwire profile monitor installed downstream of the
MEBT. It was also found that the wideband response of this device was
adequate to provide a time domain signal which yielded unambiguous bunch
length information when viewed on an oscilloscope. The network analyzer
and oscilloscope methods produced results in agreement with each other
and both were used to set up the MEBT.

### *Collimator and Faraday Cup*

The HEBT collimator and Faraday cup are located immediately upstream of
the target vacuum isolation valve at the end of the HEBT. The primary
function of the collimator is to limit the beam spot size on the
chemistry target. The Faraday cup serves as a full power beam absorber
and a calibrated beam current monitor. In combination, the collimator
and Faraday cup serve as a sort of pinhole camera diagnostic to measure
the beam current density distribution directed at the chemistry target.

The collimator consists of four flat rectangular jaws each fabricated as
a sandwich consisting of a thin tantalum face plate and thicker copper
back plate. To minimize neutron production, only the tantalum is exposed
to the beam. The back plate has machined channels for cooling water and
is soft soldered to the face plate.

Each jaw is independently and remotely controlled with a stepper motor
and is equipped with a position readout potentiometer. The right/left
jaws are situated slightly upstream of the top/bottom jaws to avoid
mechanical interference and permit either pair to be nearly fully
closed. Electrical limit switches prevent opposing jaws from colliding.
Each jaw is electrically isolated to allow electrical biasing and to
provide a beam charge collection signal. Thermocouples on each jaw
provide for temperature measurement.

The Faraday cup is a water cooled, concave rectangular box-shaped beam
stop. Like the collimator, it is fabricated of sandwiched tantalum and
copper though the bonding method is different^4^. The back plate
provides channels for water cooling. The cup is pneumatically actuated
for insertion into and withdrawal from the beamline. Redundant
microswitches provide position status to the control system and to the
radiation safety system. A thermocouple provides for temperature
monitoring. The cup is electrically isolated to allow electrical biasing
and provide the beam charge collection signal. A separate electron
suppresser diaphragm is installed immediately upstream of the Faraday
cup to improve the cup's charge collection efficiency and accuracy.

The collimator and Faraday cup are housed in a common vacuum chamber.
This assembly is a complicated mechanical design with moving parts,
cooling water hoses, switches, instrumentation, and electrically
isolated elements all in a compact volume which must operate under high
vacuum. It is the product of a considerable and careful mechanical
design effort.

For machine commissioning the Faraday cup served as the primary high
energy beam absorber and beam current monitor.

Beam current density distribution measurements, necessary to commission
the HEBT for targetry work, are made by stopping the collimator down to
an aperture small relative to the beam size and measuring the
transmitted beam current on the Faraday cup. As the aperture is moved
across the beam, the current distribution can be mapped out. A Labview
application running on the control system computer provided coordinated
aperture scanning control and Faraday cup signal acquisition. This fully
automated method provides for quick and clean first order measurements
of beam distribution as a function of HEBT magnet settings. A great deal
of time was saved and personnel radiation exposure was significantly
reduced over the alternative beam density determination method of
cutting and counting the activation of a carbon foil exposed to the
beam. Initial specifications for the collimator called only for variable
horizontal and vertical aperture with fixed center position. The
decision to adopt the more flexible design was well rewarded in the
reduced HEBT commissioning efforts.

For chemistry experiment operation, the Faraday cup is used as the beam
stop for tune-up and protects the chemist's target from exposure during
tune-up and between runs.

### *Chemistry Target* 

With the exception of electrical biasing and charge integration
electronics, target instrumentation is provided by the radiochemists.

A plastic interface ring, attached to the vacuum valve at the end of the
HEBT, is provided for target mounting. This ring also provides
electrical isolation of the target and contains an electron suppresser
electrode. Accurate collection of the beam charge striking the target is
important to the radiochemists for recording target dose. The suppresser
electrode serves to reflect electrons which may otherwise scatter off
the target and reduce the charge capture efficiency. The suppresser
connects to a channel of the toroid and charge collector electronics and
receives from that a negative 45 volt bias.

The electrically isolated target connects to a commercial current
integrator, the BIC^5^, which presents a low impedance input. A box
containing two 45 volt batteries connects in series with the BIC input
to provide a positive 90 volt bias on the target. Beam current
measurement agreement among the BIC, Faraday cup, and HEBT current
transformer is better than 5%.

### *Toroid and charge collector electronics*

A single electronics module design^6^ was conceived to handle signals
both from beam current transformers and from charge collecting
electrodes including Faraday apertures, collimator jaws, and the Faraday
Cup. Each NIM format module contains eight channels, four dedicated to
charge collecting electrode signals and four which may be configured at
time of board loading for either charge collector or transformer
signals.

A current transformer signal channel provides 46 dB gain with ≥ 100 KHz
bandwidth to amplify signals up to levels suitable for control system
IRM sample and holds and digitizers. Output is scaled to 100 mV/mA for
Pearson Model 2100 transformers.

A charge collecting electrode signal channel provides DC coupled charge
amplification. A current-limited, user-selectable polarity, electrode
bias voltage is provided by optional batteries housed within the module,
e.g. the 45 volt Eveready Type 415. Two signal outputs are generated - a
wideband (\> 1 MHz) output scaled at 250 mV/mA for monitoring signal
modulation during the beam pulse and a narrow band (0.5 Hz) output
scaled at 20 mV/µAmps for monitoring average electrode current. A
user-activated 10 µAmps current source is included for test/calibration
purposes.

## Present Status

Figures 8.1 and 8.2 show the location of beam instrumentation devices in
the low energy end of the machine and in the HEBT respectively. No beam
instrumentation exists between the upstream end of 425 MHz RFQA and the
downstream end of 425 MHz RFQC.

All beam current transformers and electronics are fully operational and
integrated into the control system.

All emittance measurements require dedicated mechanical and vacuum
setups of the machine. No emittance measurement capability remains in
the operational accelerator. The hardware and electronics has been
returned to the Fermilab ion source development lab. It remains
available if needed for any reason at a later date.

The spectrometer setup has been dismantled and the magnet returned. The
fan shaped vacuum chamber, Faraday collector disks, and associated
hardware remains with the PET project.

No profile measurement capability exists in the operating machine. The
multiwire monitors have had any broken wires repaired and, along with
the signal cables, remain with the PET project. The signal processing
electronics used was part of the emittance measurement system and has
been returned to the Fermilab ion source development lab. It remains
available if needed for any reason at a later date.

The Faraday apertures at the locations described are fully operational
and integrated into the control system. Experience has shown them to be
of little utility for their intended purpose. The Faraday aperture
immediately upstream of the 425 MHz RFQA remains crucial to stable
machine operation serving as a beam RF phase pickup. The phase signal
processing and phase detection circuitry in the present operating
machine remains a poorly organized and packaged assemblage of cables and
components. For now the system performs satisfactorily in spite of its
state, however it requires further attention before the machine should
be declared finished.

Aside from the Faraday aperture used as the phase pickup, the operating
machine has no dedicated longitudinal beam measurement systems.

The collimator and Faraday cup are currently being re-installed in the
machine following a failure of one of the collimator plates. Apparently
the plate was over-heated by the beam due to lack of cooling. The soft
solder joint between the tantalum and copper gave way creating a leak
from the water channel to the vacuum. This leak has been repaired.
Changes are being implemented to assure adequate cooling water flow, to
assure better temperature monitoring, and to provide suitable
interlocks. At the same time, several other mechanical changes including
new water fittings have been implemented to improve reliability. When
this assembly was removed for repair it was also observed that the
Faraday cup had experienced some extreme conditions. The tantalum
surface showed signs of superficial melting or sputtering. Circumstances
which may have caused this have not been determined; it is conceivable
that the beam may have been run with the cooling water to the cup valved
off. Flow interlocks are to be installed to prevent this possibility in
the future. However, since the circumstances under which the damage
occurred are not

![](media/image35.wmf){width="6.166666666666667in"
height="4.2756944444444445in"}

*Figure 8.1 Low Energy and MEBT Beam Instrumentation Layout*

*(MIM supports either Segmented Faraday Aperture or Multiwire Profile
Monitor)*

definitely known, the condition of the Faraday cup surface should
continue to be
monitored.![](media/image36.wmf){width="6.027083333333334in"
height="2.9993055555555554in"}

The BIC target current integrator, target electron suppresser electrode,
biasing and signal processing electronics are fully operational as
described and integrated into the control system as required.

The Toroid and Charge Collector electronics is performing as designed
and as required. Four modules serve the needs of the operational
machine. One spare module resides with the project.

All beam instrumentation required for normal operation of the machine is
presently operational and integrated into the control system. The
instrumentation is functioning satisfactorily and, with the exception of
the beam phase circuitry, is in a desirable final state. The special
commissioning instrumentation has been returned to its rightful owner or
set aside for future use in the PET project. If the machine is relocated
and must go through extensive re-commissioning this instrumentation
remains available.

## References

1.  1\. E. McCrory, M. Popovic, C. W. Schmidt and P. Young, [Emittance
    Measurement Techniques Used in the 1 MeV RFQ for the PET Isotope
    Linac at Fermilab]{.underline}, submitted to the 1996 International
    Linac Conference in Geneva, Switzerland, Aug. 26-30, 1996.

    2\. http://www-linac.fnal.gov/pet

    3\. See the "Charge Doubler" section in Chapter 6 of this report.

4\. See the "HEBT" section in Chapter 12 of this report.

1.  5\. Model 1000C Current Integrator, serial number 3845, Brookhaven
    Instruments Corporation, Austin, Texas.

    6\. Fermilab drawing #9700-EC-348015.

**9. Equipment Interlocks**

## Introduction

This report covers the equipment interlocks for vacuum systems, power
supplies and other related systems. Personnel safety issues such as
electrical protection and radiation interlocks are covered in other
reports.

### System Overview

The PET Accelerator contains a wide variety of equipment encompassing
many different requirements for control and status signals. Two main
areas requiring reliable intelligent control are the vacuum systems and
the magnet power supplies. Information on water conductivity and
temperature are needed by a variety of systems. Other signals just need
a simple interface to the control system. In order to provide maximum
flexibility and ease of installation and use, an industrial programmable
logic controller (PLC) was chosen for these tasks.

### Reasons for using a PLC

#### Commercial Availability

This project provided an opportunity to use many commercially available
products. The PET accelerator is small compared to other installations
at Fermilab so the trade-offs between purchased and custom built
solutions were easy to calculate. The possibility of transferring the
accelerator to another institution required that the installation and
maintenance utilize standardized, readily available parts and equipment.

#### Variety of Devices and Signals

Each section of the accelerator has a complete vacuum system consisting
of a roughing pump, thermocouple gauge, foreline valve, turbomolecular
pump and full range gauge (FRG). The gauges provide analog signals, the
valves are controlled by 24 VDC solenoids, the pumps require 24 VDC
signals or relay contacts for control and provide 24 VDC signals or
relays contact for status. The five different types of turbo pump
controllers require different combinations of 24 VDC signal and relay
contacts for control and status even though the units all perform the
same general function. The PLC allows each vacuum system to be presented
to the control system in a uniform fashion.

The power supplies require relay contacts or DC voltage to permit
operation. Each of the magnets is protected against over-temperature and
some are water cooled. One signal representing the water conductivity
affects many power supplies and RF systems.

Other signals which require conditioning or distribution by the PLC
include operator switches, water flow, water temperature and other
assorted thermocouple readings.

## PLC Systems Requirements

#### Analog Inputs

The vacuum controls require 40 analog input channels in addition to the
variety of 24 VDC and relay signals. The selection of a PLC for this
project was partially driven by availability of analog input modules.
The 405 series by PLC Direct^1^ has appropriate analog input modules at
reasonable cost.

#### Communications protocol

The PLCs must be remotely controllable and readable through the standard
parameter pages and graphics pages of the PET control system. This
dictated that at least one Fermilab Internet Rack Monitor computer (IRM)
be able to communicate with the PLCs. The non-proprietary DirectNET
protocol^2^ used by the PLC Direct products was well suited to be
incorporated as an IRM routine. Many other PLC manufacturers have
proprietary protocols which require special hardware or software for
external communications.

### Vacuum PLC

#### Electrical Distribution

The variety of sensors, relays and solenoids require distribution of 24
VDC power either directly to the sensor or to the PLC contacts to
activate the device. The typical vacuum valve actuator requires 150 mA
for activation. The vent valve solenoids require almost 1 ampere each.
Each vacuum PLC chassis has a bulk 10 ampere 24 VDC supply to provide
the necessary power. Since all of the wiring to field devices uses 24
gauge wire, the distribution of 24 volts to the actuators is protected
against overload by positive temperature coefficient (PTC) resettable
fuses. Each branch is limited to less than 2 amps. A separate 1.75 amp
24 VDC supply is used at each PLC to provide clean power for the full
range gauges.

#### Types of Equipment

The roughing pumps have no provision for remote control and no status.
Although solid state relays were originally installed for controlling
the pumps, it was found that only the target roughing pump needed to be
controlled. The thermocouple gauges on the roughing line provide
indirect status on the state of the roughing pump. The foreline valves
require 24 VDC control and have either one or two limit switches for
status. The turbo pumps have On/Off control and return status for On and
\>80% nominal speed. The turbo valves have 24 VDC control and two limit
switches for status. Full range gauges require 24 VDC power and provide
log scaled analog voltage reading of the pressure. The isolation valves
have 24 VDC control and two switch readback. Vent valves have only 24
VDC control. The fast valve requires 3 relays for control; Open, Close
and Arm, and provides status for Open/Closed, Local/Remote and
Tripped/Ready.

#### Modules used

Since the central accelerator racks were designed to be separated for
shipment, the vacuum PLC uses two chassis. The main chassis in rack
PETR07 contains the CPU, two 32 bit 24 VDC input modules, one 32 bit 24
VDC output module, two 8 channel relay modules, two 8 channel analog
input modules and a master module for remote chassis communications. The
slave chassis in PETR15 contains the slave remote I/O module, one 32 bit
24 VDC input module, one 32 bit 24 VDC output module, three 8 channel
relay modules, one 8 channel analog input module, one 8 channel type J
thermocouple input module and one 8 channel type K thermocouple input
module.

#### Communications

The vacuum PLC has a 19,200 baud serial port connection with IRM node
586. 40 words of analog status and 16 words of digital status are read
during each transfer.

Communications with the power supply PLC involve exchange of just a few
bits. Water conductivity, heartbeat bits and RF Permit use 24 VDC
signals on a multiconductor cable between the two PLCs.

#### PLC Logic

Traditional relay ladder logic and more advanced state machine functions
are used. The full logic diagram is available on the PET Web site ^3^.
In addition to the status of the individual devices in the vacuum
systems, the operator receives an indication of the present state of the
pumping sequence. The PLC states are; Off, Roughing Pump Check, Turbo
Accelerating, Waiting to Pump, Pumping Down, Vacuum OK, and Venting. The
sequence of operation of the vacuum pumps and valves depend upon the
state of the vacuum in the section of interest. Given the command to
pump down, the PLC must first determine the level of the vacuum in the
chamber. The full range gauge (FRG) provides a voltage output
proportional to the log of the pressure and is the primary means by
which the PLC measures the vacuum.

If the chamber pressure is above about 10^-3^ Torr, the system can be
rough pumped without concern about back streaming. First the roughing
pump is checked by looking at the thermocouple gauge voltage. When the
foreline vacuum is within the proper range the foreline valve is opened
and the turbo pump is started. The vacuum is considered normal when the
turbo is above 80% of nominal speed, the valve and roughing status are
correct and the chamber pressure is below a predetermined limit.

If the chamber pressure is below what the roughing pump can provide, the
turbo pump is started and the foreline valve is not opened until the
turbo is above 80% of nominal speed and the roughing pressure is
correct. This assures that there will be no back streaming of oil or
other contaminants from the roughing lines.

Many of the vacuum sections in the accelerator are not isolated from
each other and require coordination of the vacuum pumping sequence to
provide proper pumping and to prevent damage to the pumps or oil
contamination of the accelerator components. The three beam line valves
not only isolate the vacuum in the accelerator but can also intercept
the beam. Proper sequencing of the valves offers equipment protection
and reduction of prompt radiation during a vacuum fault.

A mild pressure rise in an RFQ, possibly due to sparking, will cause the
PLC to remove the low level RF permit until the vacuum recovers. A
general vacuum fault, due to a pump tripping off or a leak, will be
reflected in the status from the turbo pump or the rise in pressure. The
PLC will close the appropriate valves to isolate the section. If the
pumping system status returns to normal or an operator resets the
system, the PLC will attempt to restart the pump-down sequence.

In the event of a target vacuum fault that causes the fast valve to
close, the PLC will issue a fast beam inhibit to the low level RF system
until the source isolation valve is closed. This reduces the amount of
10.5 MeV beam deposited on the fast valve. The PLC also stops the water
flow to the target diagnostics and closes all the vacuum valves in the
target and HEBT.

The entire vacuum logic uses 1562 words of the 15K available. Typical
scan times are 25 mSec.

### Power Supply and Water Skid PLC

#### Types of Equipment

Thermal switches (klixons) are on the eight water cooled magnets. The
IRM provides a beam inhibit signal. The beam permit switch comes in from
the control room. Seven status contacts come from the water skid. Power
supply status is reported from the IRM over the serial link.

#### Modules used

Two 16 bit 24 VDC input modules, one 16 bit 24 VDC output module and two
8 channel relay modules are used.

#### Communications

The power supply PLC has a 19,200 baud serial port connection with IRM
node 587. 16 words of digital status are read during each transfer.
Power supply status changes are written into the PLC by the IRM. A few
bits are exchanged with the vacuum PLC over a multiconductor cable.

#### PLC Logic

Ladder logic is used for the power supplies and water systems. The full
logic diagram is available on the PET Web site^4^. The entire power
supply and water skid logic uses 652 words of the 15K available. Typical
scan times are 17 mSec.

##### *Power Supply Operation*

The primary requirement for magnet power supply control is protection of
the magnets from over temperature. In addition, ground fault detection
and water flow protection are provided by the combination of the IRM and
the PLC. The IRM handles monitoring of the magnet current and ground
fault voltage. Fine resolution current regulation loops run in the IRM.
The PLC continuously monitors a heartbeat bit from the IRM to assure
that the IRM is operating correctly. The heartbeat bit is read from the
PLC serial port by the IRM and returned over a hardware connection. Loss
of the heartbeat for more than 30 seconds will cause the PLC to shut
down the power supplies. The PLC also provides a convenient method of
transferring overall power supply status to the control system.

##### *Water Systems*

Even though the water skid is capable of stand-alone operation, the
ON/OFF control and general status are routed through the power supply
PLC to provide a uniform interface to the control system. The PLCs also
monitor water flow in the RFQs and LCW conductivity and temperature. If
flow or vacuum are out of limits for a particular RFQ, the vacuum PLC
will inhibit the RF power amplifier. The vacuum PLC will inhibit
operation of the RF power amplifiers and the power supply PLC will
inhibit magnet power supplies if the LCW conductivity is out of
tolerance.

##### *Other Functions*

The power supply PLC provides a beam permit to the low level RF system.
The vacuum PLC can quickly inhibit the RF using this bit in cases of
poor vacuum in the RFQs or if the fast valve closes. This beam permit
can also be removed by any of the IRMs or by flipping a switch in the
control room.

## Operational Experience

The PLC systems were easy to install and connect. In general, cable
routing and terminal block crowding were the areas requiring the most
attention. The ribbon cable connections to the 32 bit I/O modules helped
alleviate some of the cabling congestion although with the added cost of
the transition terminal blocks. Indicators on the front of the digital
modules provides an easy means for monitoring input and output bit
status. No PLC hardware has failed.

PLC programming was easy to learn. The ladder logic and state machine
functions evolved during the installation and testing of the various
sections of the accelerator. The software allows real time monitoring of
the internal bit values which helps in debugging the PLC logic. The
ability to change programming without shutting down the PLC has been
extremely helpful in keeping the machine running smoothly. It is
interesting to note that the vacuum PLC logic requires over 40 pages to
print out but consumes only about 10% of the available PLC program
space.

## Summary

The equipment interlocks in the PET accelerator require reliable,
intelligent control beyond what can be provided by simple circuitry or
external high level computer monitoring. The use of inexpensive
commercially available programmable logic controllers provides a
flexible means of sequencing vacuum systems and monitoring power supply
and water parameters. The PLC modules are able to handle the variety of
signal types needed to control and monitor the accelerator systems.
Additional functionality is easily added without the need for rewiring
in hardware. The programming of the PLC is relatively simple and
provides for ladder logic and state machine representations.

## References

1\. PLC Direct, 3505 Hutchinson Road, Cumming, GA 30040. Phone :
800-633-0405 Web address : http://www.plcdirect.com

2\.
http://garlic.fnal.gov/LINAC/software/locsys/localapp/DirectNETPLCAccess.html

3\. http://garlic.fnal.gov/pet/papers/plc_stuff/VacuumLadder.pdf

4\. http://garlic.fnal.gov/pet/papers/plc_stuff/PowerLadder.pdf

**10. Radiation Safety & Shielding**

**Introduction and Background**

The original PET RFQ accelerator beam energy was 8 MeV. It was thought
that the 8 MeV PET RFQ accelerator would produce little radiation (BI,
1994)^1^, and thus radiation safety and shielding would not be major
issues. In fact, the original accelerator was to be located at BRF on
the second floor of a building with little shielding. Furthermore, the
desire was to have continuous occupancy in the room that contained the
accelerator. This would require minimal beam losses along the length of
the accelerator. Transmission efficiencies for this initial 8 MeV design
were predicted to be 97% and thus beam losses might be small enough.

When the PET RFQ Project was resumed at Fermilab, the collaboration
decided to increase the beam energy to 10.5 MeV. The issue of shielding
had to be revisited for this higher beam energy. A significant amount of
time was spent trying to determine what the beam on target and
accelerator loss radiation levels might be. The RFQ vanes were composed
of copper plated aluminum, and no published cross sections could be
found for 10 MeV ^3^He incident on copper or aluminum. Few published
cross sections existed in literature for low energy ^3^He incident on
any metals. At the same time transmission efficiencies were being
determined for the 10.5 MeV accelerator design. Indications were that
20% of the beam would be lost in each RFQ, a number significantly higher
than the initial design's predicted 97% transmission for the entire
accelerator. If losses were near 20% in the final RFQ, where the beam
energy was at least 8 MeV, dose equivalent rates due to neutron
production could be high (GR, 1997)^2^. Further study of the 10.5 MeV
PET RFQ accelerator (GR, 1997)^2^ showed that the target vault would
need to be sizable also.

Fortunately, Lab G, where the accelerator was to be located at Fermilab,
already had a significant amount of shielding in place. There was a
concrete wall at least 6\' thick between the accelerator location and
the Control Room. In addition, the accelerator location is surrounded by
at least 3\' of concrete on three sides and has a large steel magnet on
the fourth side, thus making shielding outside the accelerator enclosure
not much of an issue at Fermilab. Previously the Lab G accelerator area
had also been interlocked. If the dose equivalent rates exceeded 100
mrem/hr (FRCM, 1998)^5^, adding interlocks to the accelerator area would
not be too difficult. Due to lack of published cross sections for this
beam energy and type, it was decided that frequent measurements would
need to be made during commissioning to determine the extent of the
radiation levels due to both beam on the targets and beam losses in the
accelerator.

While the calculations of the accelerator and target radiation levels
were being performed and studied, the first RFQ (212 MHz) was being
commissioned with RF. Measurements of x-ray production from the RFQs at
SAIC had shown that 1/8" of lead was sufficient to reduce the rates
below 2.5 mrem/hr at one foot. At the time, it was not clear what the
exact parameters of the RF were when these measurements were taken, or
what instrument was used. It was clear that some shielding would be
needed at Fermilab if the collaboration wanted to have the accelerator
room be a continuous occupancy area. The x-ray radiation levels at
maximum RF power and duty cycle were found to be very high. Several
layers of lead blankets and interlocked detectors were used to ensure
personnel safety. It was decided that the RFQs would all be encased in
thick (\~1.3 inches) stainless steel vacuum vessels to keep the x-ray
levels below 1 mrem/hr at one foot.

Another issue that complicated the shielding and interlocks for Lab G
was that the radiochemists wanted to be able to enter the accelerator
enclosure with the beam hitting a molybdenum Faraday cup just upstream
of their target. Indications were that the dose rates from full beam on
molybdenum might be quite high. The high coulomb barrier of tantalum and
the fact that the Faraday cup could be made out of tantalum, made
tantalum seem a good alternative material. It was hoped that the
radiation levels on the tantalum would be below 20 mrem/hr at one foot,
the maximum dose equivalent rate allowed for beam on access at Fermilab.
Only the chemists would be allowed to enter under these conditions, and
interlocked detectors in the area would remove the beam if dose rates
became too high. The allowed beam on dose equivalent rate for personnel
access based on the LA Regulations (LAC, 1996)^4^ appeared to be 100
mrem/hr. Thus even if beam on access were not possible at Fermilab, it
could be at BRF.

The accelerator continued to be commissioned at Lab G. Preliminary
measurements were made of the x-ray energy spectrum (GRO, 1997)^3^ in
order to help understand both the shielding and the 212 MHz RFQ
acceleration. These measurements indicated a higher x-ray spectrum than
was initially expected, with the maximum x-ray energy being near 90 keV.
Once the RFQs were encased in the steel cans, the x-ray level dropped as
predicted. Beam was slowly commissioned and the first neutron radiation
was seen at 5 MeV, at the end of RFQ A. It was decided at that point to
add RFQ B and C at the same time, rather than commission 8 MeV beam with
RFQ B only, thus jumping from 5 MeV to 10.5 MeV. The neutron levels at 5
MeV were high enough to indicate that the area would most likely need to
be interlocked for 10.5 MeV beam. At that point the accelerator
enclosure interlocks and gates were installed. The three interlocked
radiation detectors around the accelerator, that were used during
commissioning to ensure personnel safety, were left in place to be used
as beam-on interlocked detectors. The three enclosure area interlocked
radiation detectors near the doors and fence would always be active,
those near the accelerator would only remove beam when personnel were on
access in the accelerator area.

10.5 MeV beam was commissioned and many measurements were made at low
beam currents (4 to 25 µA average) to determine if Lab G was
sufficiently shielded (mainly the Control Room) and how one would shield
the accelerator at BRF. The dose equivalent rates at the RFQ interfaces
were quite high. At the RFQ B & C interface, rates were generally 1
mrem/hr per µA. It was clear that the accelerator area could not be a
continuous occupancy area, even when beam was hitting the tantalum
faraday cup, unless a significant amount of localized shielding could be
placed around the RFQ interfaces. Measurements have also shown that a
significant amount of shielding is needed for the target vault, if it is
to be located at BRF. The following sections elaborate on these issues.
Section 10.2 describes the initial issues with x-ray radiation and the
eventual solution. Section 10.3 describes the shielding assessment
report (GR, 1997)^2^. The shielding requirements for BRF are based on
the LA Regulations (LAC, 1996)^4^ that state that the dose equivalent
rate to the public must be less than 50 mrem/year and less than 2 mrem
in any hour. The shielding requirements for Fermilab are based on a
maximum dose equivalent rate of 0.25 mrem/hr in generally occupied
Controlled Areas (the Lab G Control Room) and less than 5 mrem/hr (and
that no one exceeds 100 mrem/year) in minimally occupied Controlled
Areas. These requirements are based on the Fermilab Radiological Control
Manual (FRCM, 1998)^5^ which implements 10 CFR 835^6^.

Subsequent sections describe interlocks used at FNAL, extensive neutron
and gamma measurements made at FNAL, and suggestions for BRF shielding
based on measurements at FNAL. Many different instruments were used in
order to try to separate the neutron and gamma dose equivalent rates as
accurately as possible. The shielding needed to reduce the neutron dose
equivalent rates was found to be sufficient to also reduce the gamma
dose equivalent rates and thus the main thrust was on neutron
measurements. The general layout for the accelerator at BRF and the
concrete vault design is shown in Figure 10.1. Figure 10.2 shows the
layout at Fermilab\'s Lab G.

**Dose Rates Due to X-RAYS From the RF**

Indications were that 1/8" of lead would be sufficient to shield the
x-rays from the RFQs (based on measurements made at SAIC) down to below
2.5 mrem/hr at 1 foot. Extensive x-ray measurements were made when the
212 MHz RFQ was turned on (GRO, 1997) using a Smart Ion survey meter.
This instrument is 80% to 100% efficient for measuring x-rays with
energy down to 10 keV. As the RFQ conditioned, the radiation levels
decreased, and, as expected, the dose equivalent rate depended on the RF
duty cycle and power. After conditioning, dose equivalent rates up to 20
rem/hr were seen on contact with the RFQ. Three to four layers of lead
blankets were needed to reduce the dose equivalent rate to below 2.5
mrem/hr.

With steel shielding next to the RFQ, the dose equivalent rate was found
to vary linearly with duty cycle and as RFQ power5.4. Attenuation of the
x-rays in steel and measurements with a high purity germanium detector
indicated that the maximum x-ray energy was approximately 90 keV. Very
few x-rays had energies below 50 keV. The RFQs were going to be encased
in steel cans, and thus measurements were made to determine how much
steel would be needed to reduce the rate to 2.5 mrem/hr. (It was not
clear at that point that the Louisiana regulations required less than 2
mrem/hr.) Assuming a 2.3% duty cycle and 120 kW of RF power, it was
determined that \~1.3" of steel should reduce the dose equivalent rate
to \~1 mrem/hr at 1 foot. This provided a good margin of safety.

The 212 MHz RFQ was the first RFQ encased. The stainless steel vacuum
vessel was machined a final thickness of 1.312 inches to fit the RFQ.
Survey measurements showed localized higher radiation levels at the
downstream flange and at the turbo pump. The rate in these areas at full
power (120 kWatts) and duty cycle was projected to be 5 to 10 mrem/hr at
one foot. Along the RFQ body, the highest dose rate seen at one foot was
projected to be 2.5 mrem/hr at full power and maximum duty cycle. The
425 MHz RFQs were encased in 1.375" of steel, due to constraints with
the RFQ structure. At 100 kW, the highest dose equivalent rate at one
foot was seen at the interface between RFQ B and RFQ C. This rate was
0.6 mrem/hr, which projects to \~1.6 mrem/hr at full power and maximum
duty cycle.

All of these measurements indicated that (with the steel cans) the x-ray
dose equivalent rate would be less than 2.5 mrem/hr at one foot
everywhere. It was hoped that, with time, this level would decrease due
to the conditioning of the RFQs. Recent measurements have been made of
the x-ray radiation at 90-100 kW of power and show dose equivalent rates
comparable to background around the 425 MHz RFQs.

**Summary of the Shielding Assessment Report**

This section summarizes the report \"Shielding Assessment for the PET
RFQ Accelerator at Fermilab\" (GR, 1997)^2^ which discusses the
anticipated radiation shielding issues associated with a 10.5 MeV, 300
µAmps 3He PET RFQ accelerator.

![](media/image37.wmf){width="5.665972222222222in"
height="8.971527777777778in"}
![](media/image38.wmf){width="5.7347222222222225in" height="8.44375in"}

*Figure 10.2: Fermilab PET RFQ Accelerator and Vault Layout. (Dose
equivalent rates are predictions based on FNAL measurements.)*

*Dose Rates due to Beam Loss in the Accelerator*

Radiation levels due to accelerator losses were very difficult to
predict. Cross sections for neutron production on aluminum, molybdenum
and tantalum could not be found in published literature for 10 MeV (or
less) 3He. One had to extrapolate from the available data. Also
accelerator losses were not well known. One had to use beam optics
calculation predictions, rather than measurements. It was clear that as
the accelerator was commissioned, many radiation measurements would need
to be made to determine what shielding would be needed at BRF. Reference
(SAD, 1997)^7^ has a more complete discussion of these issues.

*Normal Operational Losses*

During normal operation, according to beam optics calculations, the
majority of the beam loss occurs within the first 20% of each RFQ. Most
of this beam loss should not produce neutron radiation since the beam
energy is too low relative to the Coulomb barrier of copper and
aluminum, the materials that line the RFQs. (The Coulomb barrier
energies for Cu and Al are 12.8 MeV and 7.0 MeV, respectively.) Most
likely, little neutron radiation would occur until the beam energy
reached 8 MeV. Thus losses in RFQ C and on the beam absorber were
considered the main issues.

Beam optics calculations estimated that 24% of the beam would be lost in
RFQ C. At 300 µA, this corresponds to 70 µAmps of 8 MeV 3He. The cross
section for neutron production from 8 MeV 3He on copper was found in
published literature. Though the coulomb barrier is 12.8 MeV for Cu, it
is not an absolute cut off and some neutron production occurs at 8 MeV.
The dose equivalent rate for copper was estimated to be about 20 mrem/hr
(at 300 µA) at a distance of one foot. For aluminum the dose equivalent
rate would most likely be much higher due to its lower coulomb barrier.
An interlocked detector would have to be placed in this area or the
entire area would need to be interlocked.

Initially the idea was to use a molybdenum beam absorber just upstream
of the target area. This would allow the radiochemists to enter the
vault without stopping the beam. It was estimated that the dose
equivalent rate from neutrons produced by 300 µAmps of 10.5 MeV 3He on
molybdenum was 9 rem/hr per millibarn. The cross section for neutron
production on molybdenum with 19 MeV 3He was published as 30
millibarn/sr at 0o. This suggests that the dose equivalent rate due to
beam on the molybdenum beam absorber (at 10.5 MeV) could be quite high,
not permitting beam on access by the radiochemists. The coulomb barrier
for tantalum is significantly higher than that of molybdenum. Thus it
was decided that a tantalum beam absorber would be used.

*Off - Normal Operational Losses*

For accident scenarios there are two failure modes that could cause
short term radiation problems. One scenario is full beam loss in RFQ C,
when the power to RFQ C is lost. The dose equivalent rate due to this
failure mode should be about three times that estimated for normal
operational losses in RFQ C. The second scenario is loss of all the beam
in the HEBT due to mis-steering. This is 10.5 MeV beam on a heavy
material such as tantalum and thus would be similar to the losses one
might expect from beam hitting the tantalum beam stop. To protect
against high radiation levels from these conditions, an interlocked
detector would have to be placed in this area or the entire area would
need to be interlocked.

*Shielding of Accelerator*

Due to the extensive amount of concrete shielding already in place at
Lab G, additional shielding for the accelerator would most likely not be
needed. It was decided that we would wait and measure the radiation
levels due to losses on the accelerator at Fermilab before trying to
estimate the amount of shielding that might be needed at BRF.

**Dose Rates due to Beam on the Target**

*Source Terms*

Unshielded gamma ray and neutron dose equivalent rates were measured by
K. Krohn and J. Link for various target materials and beam energies with
a 1 to 2 µAmps 3He beam. A summary of the results most relevant to the
discussions here are shown in Table 10.1, extrapolated to 300 µA. It is
assumed that the 10.5 MeV ^3^He beam loses 1 MeV in the target window.
The neutron dose equivalent rate at 90 degrees to the beam axis was seen
to be about 30% to 50% lower for the C, N2 and O2 targets and 40% lower
for the Be target. These results were used to estimate the target vault
shielding. C, N2 and O2 targets were anticipated to be used at Fermilab
and BRF. Be was being considered as a target at Fermilab, depending on
the anticipated amount of radiation produced and the difficulty with the
associated shielding.

  ---------------------------------------- ------- ------- ------ --------
  Radiation Type                           C       Be      N2     O2

  *Gamma-ray Dose Rate (rad/hr)*           1.83    6.06    0.77   0.93

  *Neutron Dose Equivalent Rate (rem/hr)*  10.1    103     6.54   6.33
  ---------------------------------------- ------- ------- ------ --------

*Table 10.1. Gamma-ray dose rates (rad/hr) and neutron dose equivalent
rates (rem/hr) for 300 microamps (electric) of 9.5 MeV 3He2+ ions on
various targets at a distance of 10 feet (3.05 m) along the beam axis
(axial).*

*Shielding Requirements*

Reference (GR, 1997) discusses both an initial concrete vault and a
lead, polyethylene vault. Only the concrete vaults for Fermilab and BRF
will be discussed here. Calculations were done for a carbon target
(considered the \"typical\" target) and a beryllium target (worst case
target), using the source terms in Table 10.1. The goal was to achieve
2.5 mrem/hr gamma and 2.5 mrem/hr neutron dose equivalent rates at 10
feet from the target. This was initially thought to be the requirement
in order to be able to locate the accelerator on the second floor at
BRF.

Based on the estimated concrete thickness needed, the available space at
BRF and Fermilab, and the available concrete block sizes, the vaults
shown in Figures 10.1 and 10.2 were designed for the BRF and Fermilab
sites, respectively. It was estimated that 42 (50) inches of concrete
would be needed at 0 degrees and 38 (46) inches of concrete at 90
degrees for a carbon (beryllium) target. Figure 10.3 shows the estimated
dose equivalent rates around the vault at 300 µAmps (the original goal
at BRF), based on the shielding assessment calculations. The BRF vault
is nearly the same design as the FNAL vault, except it is made with much
smaller blocks \~8" thick. The BRF vault is 48" thick on the north and
west sides (1 foot thicker than the FNAL vault). Both designs have an
18" thick roof and south side wall and use the same lead/polyethylene
wall around the HEBT.

![](media/image39.wmf){width="5.846527777777778in"
height="7.082638888888889in"}

*Figure 10.3: Fermilab PET RFQ Accelerator and Vault Layout with Dose
Equivalent Rates Based on Shielding Assessment Calculations. (Does not
include radiation due to accelerator losses.)*

Even in the concrete design, it was decided that a lead and poly wall
would be built around the HEBT region to reduce back streaming
radiation. Calculations for the carbon target suggested that about 16
inches of borated polyethylene and 7 inches of lead would be needed.
However, because of the space limitation between the end of the HEBT and
the entrance to the target, the final design had only 11.25 inches of
polyethylene and 5.5 inches of lead. It was clear that this region might
need more shielding at BRF. Measurements at FNAL would clarify this
issue.

The labyrinth door was to be built of polyethylene and lead.
Calculations for the carbon target estimated it should be made of 0.71
inches of borated polyethylene and 0.5 inches of lead. Due to the
difficulty of structurally supporting 0.5 inches of lead, and
recognizing that full beam current would not be possible for some time,
the entrance door was initially designed to be 0.25 inches of lead and 1
inch of borated polyethylene. It was to be designed to support an
additional 0.25 inches of lead. Calculations showed that extra shielding
would most likely not be needed for the 6 inch wide cable access hole
above the vault door.

In determining the amount of necessary shielding for the labyrinth door,
the neutrons were assumed to be mostly thermalized before reaching the
door. Actually, this is likely not the case because it is possible, in
the present shielding vault design, that some neutrons scatter only once
or twice before striking the door. These neutrons are energetic and
would require additional shielding (mainly polyethylene) to reduce the
dose rates to more acceptable levels. The consequences resulting from
the present design are discussed later.

Finally, the issue of the radiation back streaming through the
accelerator was addressed. Estimates suggested that this radiation might
reach 10 rem/hr at the alpha magnets with the beryllium target or a few
rem/hr with the carbon target. It was decided that an interlocked
radiation detector would be placed in this area to monitor these rates.

**Interlocks at FNAL**

When the high x-ray dose equivalent rates were seen, a chipmunk detector
was placed outside the lead blankets (at the highest dose equivalent
rate area) and set to trip off the RFQ power at 5 mrem/hr. When 1 MeV
beam was imminent, a second detector was placed at the end of the
accelerator and set to trip off the RFQ power at 5 mrem/hr. No beam
induced radiation was seen above background at 1 MeV. Initial
measurements with 5 MeV beam hitting a tantalum beam stop showed neutron
dose equivalent rates of 1.5 mrem/hr at an estimated beam current of 2.5
µAmps (2.5 mA out of the ion source). The FRCM (Fermilab Radiological
Control Manual) does not allow beam-on access if the dose equivalent
rate is above 20 mrem/hr. Thus it was determined that the accelerator
area would need to be interlocked before 8 or 10.5 MeV beam was allowed
since cross sections for neutron production could be considerably higher
at these energies.

The interlock system was designed to allow beam-on access. This was
initially a requirement for quick access to the vault for the
radiochemists and for the initial tuning of the accelerator. Due to this
need for beam-on access, two interlock systems were used. The enclosure
interlock system (see Figure 10.4, detectors 4, 5 & 6) consists of three
chipmunk detectors located near the highest accessible dose rate areas.
These detectors are set to trip (which means they cause the tantalum
faraday cup to be inserted, and, if necessary, the ion source gate
valve) at dose equivalent rates that correspond to 5 mrem/hr,
conservatively, at accessible locations outside the accelerator
enclosure. The beam-on access system consists of three detectors located
near the accelerator (see Figure 10.4, detectors 1, 2 & 3) that close
the ion source gate valve if the dose equivalent rate exceeds 20
mrem/hr. The interlock system does not allow beam-on access unless the
tantalum faraday cup (or equivalent) is in place. The beam-on access
detectors are active for tripping the ion source gate valve only when
someone has entered the enclosure.

When higher current levels were achieved, the dose rates due to losses
at the RFQ B/C interface were getting high on the west side of the
vault. The area behind the vault and magnet was fenced off and posted as
a radiation area. This allowed dose rates near the west side fence and
detector to exceed 5 mrem/hr.

![](media/image40.wmf){width="5.958333333333333in"
height="7.540972222222222in"}

*Figure 10.4: Fermilab PET RFQ Accelerator Interlocks and Detectors.*

**Neutron and Gamma Measurements at FNAL**

*Target Materials*

Aluminum and tantalum targets were used in order to determine how much
radiation was due to accelerator losses and how much was due to beam on
the target. Later a copper target was used to try to determine if the
loss of copper plating on the RFQs could be the reason the accelerator
dose equivalent rates at the RFQ B/C interface appeared to be increasing
with time.

Tantalum was naturally a good target as its high coulomb barrier should
cause it to produce little radiation when irradiated by the 10.5 MeV 3He
beam. This would allow one to determine losses along the RFQs with
little interference from radiation due to beam hitting the target. When
radiochemistry was not occurring, this would be a good beam absorber
because it would keep dose rates in the area low.

Aluminum was used because it was easily obtained and published
measurements at higher energies (19 MeV) suggest that the cross sections
for 3He on carbon and aluminum were approximately equal. Thus it would
give an indication of what the radiation levels might be when the
radiochemists started bombarding their target materials. Indications
were that, if anything, the dose equivalent rate from beam hitting
aluminum should be higher compared to beam striking carbon.
Unfortunately, due to the back streaming radiation from beam hitting the
aluminum target, surveys near the HEBT and RFQ C will contain components
from losses on the accelerator and on the target.

*Survey Instrumentation*

Several instruments were used for surveys. Each had its own response to
neutrons and gamma rays. The snoopy is an Anderson/Braun type moderated
BF3 counter, with thermalizing polyethylene and a boron loaded sleeve
(to improve energy response), that is only sensitive to neutrons. It
displays the results in mrem/hr with the correct quality factor. It was
estimated, using the Bonner sphere measurements, that the snoopy
over-responded by approximately a factor of 2 in the neutron energy
range appropriate for the PET RFQ measurements. Another detector used,
the chipmunk, is a tissue equivalent ion chamber, sensitive to both
neutrons and gamma rays (down to 100 keV). It measures in mrem/hr with a
quality factor that is preset by the user. Two other instruments, the
HPI 1010 and the Smart Ion, were also used. The HPI 1010 is a tissue
equivalent proportional chamber sensitive to both neutrons and photons
and measures in mrad. Based on previous studies (KR 1990)^8^, an 80%
response for neutrons was assumed and a 95% response for gamma rays. The
Smart Ion is an unsealed air equivalent ion chamber and thus it was
assumed to have a similar response to a Cutie Pie (see KR, 1990). For
the Smart Ion a 15% response for neutrons was assumed and a 100%
response for gamma rays.

A set of Bonner Spheres was used to determine both the neutron energy
spectrum and the quality factor. The quality factor is the conversion
factor from mrad/hr to mrem/hr. It is one for gamma rays, but energy
dependent for neutrons. The Fermilab Bonner sphere system uses eight
6LiI scintillators for detectors. One detector is bare while the other 7
are surrounded by polyethylene spheres of varying sizes, up to 12" in
diameter.

*Determination of Neutron and Gamma Dose Equivalent Rates*

Unless otherwise specified, the neutron dose equivalent rate was
determined from measurements obtained with the snoopy, dividing by a
factor of 2 in order to compensate for its over response. The gamma ray
dose equivalent rate is more difficult to determine. In most cases it
was found by subtracting the neutron rate from the chipmunk rate. This
requires one to know the quality factor of the neutrons. Initially it
was assumed to be 10 in order to be conservative. Later, Bonner sphere
measurements showed it to vary from 4 to 9, depending upon where the
measurement was taken. Results quoted here assume the quality factor
measured by the Bonner spheres at the most appropriate location (see
Tables 10.2 and 10.3, shown later).

In some cases measurements were made with the HPI 1010 and the Smart
Ion. In those cases, two equations with two unknowns (the unknowns being
the gamma ray and neutron dose equivalent rates) were solved using the
response functions mentioned above. Here, also, an educated guess was
made at the quality factor based on the Bonner sphere measurements.

Dose equivalent rates are usually extrapolated to 150 µA, a beam current
considered acceptable for reasonable radiochemistry production rates.
The neutron dose measurements are good to within a factor of 2. This
uncertainty should improve when measurements at higher current are
performed. Uncertainties on the gamma ray dose rates are quite large due
to the uncertainties in the neutron quality factor. Values quoted are
most likely over estimates. These uncertainties do not include the day
to day differences due to different operating conditions within the
accelerator. These variations were on the order of a factor of 2.

**Initial Measurements with 10.5 MeV Beam**

*Difficulties Encountered*

Initial measurements were very hard to quantify. The beam stability was
poor and many variables affected the radiation rates. Also, consistent
methods needed to be developed for determining the power in each RFQ and
the beam current. The RFQs sparked frequently and it was not clear how
much of an effect this would have on the survey measurements. Later it
was determined that this had little effect on the survey measurements,
as long as they were intermittent. It was clear that the radiation
levels next to RFQ B and C were much higher when the RFQs were not
running at optimum power. Measurements had to be made near 90 to 100 kW,
the calculated optimum power. Also, initial measurements were made at 4
to 20 µAmps and thus linear extrapolations to 100 µAmps introduced some
uncertainty.

*Linearity vs. Duty Cycle and Current*

The dose rate increased to within 20% of linearity when the current was
doubled (4 - 8 µAmps and 10 - 20 µA). Higher current dose equivalent
rates were generally less than expected due to a linear extrapolation.
The same was found to be true for dose equivalent rate versus duty
cycle. These measurements were done using the aluminum target and thus
do not clearly look at the losses along the 425 MHz RFQ chain.

*Initial Neutron and Gamma Dose Equivalent Rates*

The highest dose rate areas around the vault with the aluminum target
were found to be at the lead/polyethylene wall around the HEBT region
and just outside the vault door. This is what was expected from the
shielding assessment calculations. Surveys were also performed, with the
aluminum target, at the cracks in the vault wall, at the cable access
hole, and on all sides of the vault. It was found that the cracks in the
wall and the cable access hole were not problem areas. After the HEBT
lead/polyethylene wall and the vault door, behind the vault and the
vault roof were the next highest dose rate areas.

Surveys were also performed to see how the neutron dose equivalent rate
fell with distance from the vault. It was found to generally drop off as
1/r on all sides of the vault, if one assumed that the source was the
target. Measurements were not made beneath the vault as this area is
inaccessible at Fermilab. Three locations around the vault did not
follow a 1/r or 1/r2 rule, if one assumed the source was the target.
These locations were the vault door, the cable access hole above the
vault door, and behind the vault. The vault door showed that the dose
equivalent rate decreased by a factor of 4, four feet from the vault
door. Measurements made at the cable access hole showed that the dose
equivalent rate decreased by a factor of 2.4 in three feet. Behind the
vault the rate dropped by a factor of 6 in 3 feet.

Next the RFQs were surveyed, with beam hitting the tantalum target. The
neutron dose equivalent rates outside the HEBT lead/poly wall and vault
door were small, thus confirming that the radiation produced by the beam
hitting the tantalum target was significantly less than that from the
aluminum target. Figure 10.5 shows the results of these surveys.
Measurements inside the vault with the tantalum target, and later with
the tantalum faraday cup, showed neutron dose equivalent rates of 40 to
80 mrem/hr (extrapolated to 100 µA). The regulatory requirement for
Louisiana is no personnel access for areas with the dose equivalent rate
above 100 mrem/hr at one foot, therefore indications are that beam-on
access would be possible at these current levels. A detector would be
needed, though, to remove beam if the levels exceeded 100 mrem/hr. These
measurements were made with 10 to 30 µAmps of beam current. At present,
beam on access to the target vault is not allowed or necessary for the
current experimental program.

**Further Study: Accelerator Losses**

*Neutrons, Bonner Sphere Measurements*

One good set of Bonner sphere measurements was taken next to RFQ C with
about 10 µAmps of beam on the tantalum target. The average neutron
energy was found to be 1.36 MeV with the peak between 2 and 3 MeV. The
quality factor was found to be 9.3. Figure 10.6 shows the neutron
spectrum. The dose equivalent rate determined by this Bonner sphere
measurement agreed with the snoopy neutron dose equivalent rate if one
assumed an over response of two. Thus this is what lead to the use of
the factor of 2 correction to the snoopy neutron measurements.

![](media/image41.wmf){width="5.888194444444444in"
height="7.485416666666667in"}

*Figure 10.5: Neutron Dose Equivalent Rates Along the RFQ Chain (5/97).
Units in boxes are mrem/hour at one foot for the corresponding beam
current.*

![](media/image42.wmf){width="5.540972222222222in"
height="3.9027777777777777in"}

*Figure 10.6: Neutron Spectrum next to RFQ C.*

Later, Bonner sphere measurements were made with both the aluminum and
tantalum target at 6 locations. Unfortunately it was later discovered
that the beam was scraping in the HEBT region and one Bonner sphere was
damaged. Also the measurements with the aluminum target were not
completed. Despite this, the tantalum results were analyzed, correcting
for the damaged sphere. These results are summarized in Table 10.2. The
results for the spectrum at RFQ C differ from those measured earlier due
to the scraping in the higher energy HEBT region.

> **Table 10.2: Bonner Sphere Results with the Tantalum Target**

+-----------------+-------------+----------+--------+----------------+
| location        | > n         | >        | > Q    | > Average      |
|                 | eutrons/cm2 |  mrem/hr | uality | > Energy       |
|                 | > per       | >        | >      | >              |
|                 | > µcoulomb  | > (150   | Factor | > (MeV)        |
|                 |             | > µA)    |        |                |
+-----------------+-------------+----------+--------+----------------+
| outside vault   | > 3.3       | > 33     | > 8.5  | > 1.3          |
| door            |             |          |        |                |
+-----------------+-------------+----------+--------+----------------+
| inside vault    | > 1.26      | > 10     | > 8.8  | > 0.6          |
| door            |             |          |        |                |
+-----------------+-------------+----------+--------+----------------+
| HEBT            | > 4.0       | > 36     | > 8.5  | > 1.1          |
+-----------------+-------------+----------+--------+----------------+
| RFQ C           | > 13.2      | > 205    | > 7.4  | > 4.0          |
+-----------------+-------------+----------+--------+----------------+
| east side at 13 | > 2.0       | > 19     | > 9.4  | > 0.6          |
| feet            |             |          |        |                |
+-----------------+-------------+----------+--------+----------------+
| east side vault | > 0.7       | > 2      | > 6.7  | > 0.3          |
| wall            |             |          |        |                |
+-----------------+-------------+----------+--------+----------------+

*Neutron dose Equivalent Rates along the RFQs and HEBT*

Chipmunk dose equivalent rates per microCoulomb at the B/C interface
seemed to have increased with time. At times significant beam losses had
occurred in the HEBT and this chipmunk would see these losses also. In
order to isolate the radiation sources along the 425 MHz RFQ chain, a
polyethylene enclosure with one end exposed (doghouse) was made for the
snoopy and measurements were taken about every 6" along the accelerator
back to the MEBT (with the tantalum target in place). The results of
this survey are shown in Figure 10.7. These measurements showed that
radiation from the HEBT was insignificant. They also showed that the
dose equivalent rate at the B/C and A/B interfaces had significantly
increased since previous measurements (see Figure 10.5). After these
measurements were performed, inspection of the RFQ vanes showed some
copper plating loss on the RFQ vane tips.

An unshielded snoopy was used to determine the attenuation of the
neutron dose equivalent rates with distance from the accelerator. At 6
feet the attenuation varied greatly due to scattering of the neutrons
off the various structures near the RFQ. Measurements at 12 feet were
more consistent and the attenuation was 1/r at worst.

![](media/image43.wmf){width="5.506944444444445in"
height="3.7083333333333335in"}*Figure 10.7: Neutron Dose Equivalent
Rates Along the RFQ Chain (7/97).*

*Neutron Dose Equivalent Rate Dependence on Power in RFQ C*

Snoopy measurements were made at the B/C interface, using the
polyethylene doghouse, as the power in RFQ C was varied. The results are
shown in Figure 10.8. The three curves in Figure 10.8 are the losses in
RFQ C at the entrance end, center, and exit (downstream) end of the RFQ.
Losses in RFQ C increased as the RFQ power decreased below the optimum
region, as suspected earlier. During these measurements the beam current
into RFQ C was not changed.

*RFQ B/C Interface Neutron Attenuation in Polyethylene*

Sheets of polyethylene were placed in front of the snoopy in the
polyethylene doghouse at the B/C interface (tantalum target). Six inches
of polyethylene gave a factor of 10 attenuation. This same result was
obtained by applying polyethylene attenuation curves to the neutron
spectrum determined from the Bonner sphere measurements made along RFQ
C.

![](media/image44.wmf){width="5.502777777777778in"
height="4.4527777777777775in"}

*Figure 10.8: Neutron Dose Equivalent Rate versus Power along RFQ C.*

*Loss of Copper Plating and the Tungsten Insert*

The losses at the B/C and A/B interfaces appeared to be increasing with
time. Initial measurements (5/97-6/97) indicated approximately 1 mrem/hr
per microamp of beam. In July and early August dose equivalent rates
were relatively consistently at levels of 4 to 6 mrem/hr per microamp of
beam. This was thought to be due to the loss of copper plating at the
interface, thus beam was hitting more aluminum in the upstream end of
RFQ C, where most of the beam loss was expected to occur. More neutron
production was expected from aluminum than from copper, due to the lower
Coulomb barrier. Neutron measurements made outside the vault door with
an aluminum target and then a copper target indicated that aluminum
produces approximately four times more neutrons. A monitoring detector
in the vault gave the same results.

Inspection of RFQ C has shown some flaking of copper from the vane tips.
A tungsten aperture plate was designed to be inserted at the B/C
interface. It was hoped that this plate would intercept the beam halo
rather than allowing it to hit the aluminum in RFQ C. The high coulomb
barrier of tungsten, as compared to aluminum, should minimize neutron
production due to beam loss.

When the tungsten plate was inserted, radiation surveys were performed.
Measurements were made at 15 to 30 µAmps of beam current. Measurements
of gamma and neutron dose equivalent rates were made with detectors
positioned in the Control Room before and after the tungsten aperture
plate was installed. The neutron dose equivalent rate remained the same
and the gamma ray dose equivalent rate was a factor of two higher with
the tungsten plate. It should be understood that dose rates can vary by
a factor of two from one day to the next due to varying running
conditions.

Measurements of separate gamma and neutron dose rates had not been made
next to RFQ C since May. Neutron measurements were made in July.
Comparison of chipmunk dose equivalent rates (the chipmunk next to RFQ
C) suggested that the dose equivalent rate was the same or perhaps
higher with the tungsten plate. Before the tungsten plate was inserted,
that chipmunk read from 120 to 600 mrem/hr for 100 µA (the lower value
was from early July) and, after, the chipmunk read about 350 to 400
mrem/hr for 100 µA. If either the ratio of gamma to neutron radiation
has changed or the energy spectrum of the neutrons have changed
considerably, then a direct comparison of the chipmunk rates is not
valid.

Neutron and gamma measurements were made, with the tungsten plate in
place, using an HPI 1010 and a Smart Ion survey meter in a lead and
polyethylene doghouse. The results agree with the readings on the
chipmunk next to RFQ C. The neutron dose equivalent rate does not appear
any different than before the tungsten plate was installed. The gamma
ray dose rate is comparable to that measured in May of 1997 (within a
factor of 3). This agrees with what the direct before and after
measurements in the Control Room implied. Beam losses in RFQ C were
about a factor of two higher with the tungsten plate, and thus more beam
needed to pass into the RFQs in order to obtain the same current at the
faraday cup. This may be why the tungsten plate did not help the neutron
radiation problem.

The chipmunk detector next to the B/C interface is logged periodically
as is the beam current. Looking back over these logs for the last few
months showed that even in May and June of 1997, fluctuations were seen
up to 6 mrem/hr per microamp of beam. And, more interestingly, recent
measurements (8/29/97, 9/2/97 & 9/18/97) at 75 to 100 µAmps of beam
showed levels back at 1 mrem/hr per microamp of beam. To check this,
measurements need to be made with the snoopy at the B/C interface at
these higher currents.

*Gamma Rays*

Some measurements were made with a prototype ion detector which was
insensitive to neutrons. This detector was calibrated in a gamma ray
energy range similar to what was expected outside the vault. Assuming
100% response leads to agreement within a factor of 2 of gamma ray
results deduced as described previously. This lends validity to the
method of determining gamma and neutron dose equivalent rates from
chipmunk and snoopy data.

Measurements were also made, using a 60Co source, to determine the
attenuation of gamma rays in polyethylene. It was found that the dose
equivalent rate (DER):

DER = (0.995) 10 -0.06(inches of poly).

Thus 6 inches of polyethylene would attenuate the gamma ray dose
equivalent rate by at least a factor of two.

*Recent High Current (\~100 µA) Measurements with the Tantalum Target*

Measurements were made at higher currents at locations near the
interlocked enclosure in order to ensure that the interlocked detector
trip levels were set appropriately. Dose equivalent rates outside the
enclosure must remain below 5 mrem/hr and dose equivalent rates in the
control room should remain below 0.25 mrem/hr. Surveys showed that the
only locations that needed further measurements were at the magnetic
enclosure door and in the Control Room. Chipmunk and snoopy measurements
were made with 100 µAmps of beam, inside and outside the magnetic
enclosure door, in the Control Room, and in the Radiochemistry area.
These measurements showed that the Control Room and Radiochemistry area
dose equivalent rates were about 0.30 mrem/hr (0.15 mrem/hr neutron,
0.15 mrem/hr gamma rays) at 100 µA. Does equivalent rates at the door
were about 0.8 mrem/hr at 100 µA.

**Further Study: Beam on the Aluminum Target Measurements**

Some measurements were made (aluminum target) placing various widths of
polyethylene in front of the snoopy detector and placing the detector at
the vault door and the HEBT lead/poly wall. These measurements showed
that 6 inches of polyethylene transmitted 33% of the neutrons at the
HEBT. Four inches of polyethylene at the vault door transmitted 40% of
the neutrons. These measurements were made at low currents (less than 20
µAmps) without a "doghouse". They need to be redone at higher currents
with a doghouse if the accelerator is to be located at BRF.

Bonner sphere measurements were made with the aluminum target, but not
completed, in the same locations as shown in Table 10.2. A modified
analysis was used to correct for the missing Bonner sphere data. These
results give one a general idea of what the quality factor and average
neutron energy might be at the various locations (see Table 10.3). Since
data from two or more spheres are missing from each measurement, and the
beam was known to be scrapping in the HEBT when these measurements were
made, they should only be read as an indication of what the real
spectrum might be.

***Table 10.3: Bonner Sphere Results with the Aluminum Target***

+------------------+-------------+----------+---------+--------------+
| > location       | > n         | >        | >       | Average      |
|                  | eutrons/cm2 |  mrem/hr | Quality | Energy       |
|                  | > per       | >        | >       |              |
|                  | > µcoulomb  | > (150   |  Factor | (MeV)        |
|                  |             | > µA)    |         |              |
+------------------+-------------+----------+---------+--------------+
| > outside vault  | > 5.3       | > 60     | > 7.8   | 2.0          |
| > door           |             |          |         |              |
+------------------+-------------+----------+---------+--------------+
| > inside vault   | > 168       | > 470    | > 4.7   | 2.3          |
| > door           |             |          |         |              |
+------------------+-------------+----------+---------+--------------+
| > HEBT           | > 5.3       | > 100    | > 9.2   | 2.4          |
+------------------+-------------+----------+---------+--------------+
| > RFQ C          | > 8.5       | > 130    | > 7.2   | 3.8          |
+------------------+-------------+----------+---------+--------------+
| > east side at   | > 4.1       | > 20     | > 7.6   | 0.4          |
| > 13 feet        |             |          |         |              |
+------------------+-------------+----------+---------+--------------+
| > east side      | > 2.6       | > 3      | > 3.8   | 0.1          |
| > vault wall     |             |          |         |              |
+------------------+-------------+----------+---------+--------------+

Another issue is the back streaming radiation, which appeared minimal at
first (16 mrem/hr at the alpha magnets at 300 µA). Recent (September
1997) chipmunk detector rates at 30 µAmps indicate dose equivalent rates
of \~70 mrem/hr at 100 µAmps, back at the alpha magnets.

**Conclusions for FNAL**

Figure 10.2, shown earlier, gives the estimated gamma and neutron dose
equivalent rates at Fermilab. Values that are circled are calculated
from the accelerator loss dose equivalent rates (rates measured using a
tantalum target). Values at the B/C interface assume a worst case of 6
mrem/hr per µAmps at 1 foot, while values in parenthesis are for rates
of 1 mrem/hr per µAmps at 1 foot at the B/C interface. Values in boxes
are extrapolated from measurements made with the aluminum target. Linear
current extrapolations are used and 1/r distance extrapolations are
used. Uncertainties are a factor of 2-3 until high current (\~ 100µA)
measurements are made.

Eight inches of polyethylene (4\' by 4\' sheets) has been placed next to
the RFQ B/C interface to keep the rate below 0.25 mrem/hr in the Control
Room. This has decreased the dose equivalent rate in the Control Room by
a factor of 2. Enough polyethylene has been purchased to build a second
4\' by 4\' by 6 inch wall. This wall can be used next to the HEBT on the
east side, depending on how high the dose equivalent rates get due to
radiochemistry studies.

**Updated Shielding Design for BRF**

Figure 10.1, shown earlier, gives the expected dose equivalent rates at
BRF at 150 µA. Uncertainties are large until further measurements are
made at high currents using the snoopy and chipmunk detectors. It should
be noted that the concrete vault at BRF has an additional foot of
concrete on the south and the back walls as compared to the Fermilab
vault (this is corrected for in the dose equivalent rates shown in
Figure 10.1).

Interlocks would need to be used around the accelerator enclosure in a
similar fashion to those at Fermilab. The ceiling at BRF would also most
likely need to be interlocked. In order to decrease the amount of extra
shielding needed, the south room and north ramp (outside the building an
interlocked gate would need to be installed at the end of the ramp)
would also be interlocked. Additional shielding for the vault, door and
HEBT lead/poly wall may not be needed with such an interlock system**.**
The main problems appear to be radiation from the B/C and A/B interfaces
(due to accelerator beam losses) at the north wall, south wall and
ceiling, and possibly the back streaming radiation. Secondary issues are
the dose rates from the door and the HEBT lead/poly wall (due to beam on
target). Indications are that the shielding for the neutrons will be
sufficient to stop the gamma rays as well. The following shielding
design assumes one wants to obtain \~1-2 mrem/hr neutrons at locations
accessible to the public.

The \"worst case\" shielding design for the accelerator would consist of
polyethylene collars around the B/C and A/B interfaces and fixed and
rolling polyethylene walls. Depending on the results of higher current
measurements at these interfaces, either 6 or 8 inch thick polyethylene
collars could be machined to fit around both these interfaces. This
would decrease the dose equivalent rate by a factor of 10 (6\") or 20
(8\"). If the accelerator were moving to BRF, it would be best to make
and install these collars at Fermilab. This would allow measurements to
be made to determine if any additional shielding would be needed around
the accelerator. If further shielding is needed a polyethylene wall
system could be made relatively easily as shown in Figure 10.9. This
figure shows 6 to 8 inch thick walls in several locations. With the
polyethylene collars it is doubtful that this much additional shielding
would be needed. The rolling walls shown in Figure 10.9 can be used
wherever they are necessary. Whether additional shielding for the vault,
vault door, back streaming radiation, or HEBT lead/poly wall would be
needed (at BRF) could best be determined after the polyethylene collars
were installed at Fermilab.

If the door shielding is found to be a problem, up to 6" of polyethylene
(or borated polyethylene) could be added to the door before the weight
would become an issue. The hinge and opening mechanism would need to be
designed appropriately. To reduce the flux of neutrons which scatter
once or twice and exit the door, small amounts of concrete shielding
could be stacked strategically within the vault near the target region.
The blocks would need to be placed in a location to reduce any
interference with the radiochemist's work. If additional shielding is
needed around the lead/poly wall, a polyethylene wall on rollers could
be set at an angle off the northeast corner of the vault. If additional
shielding is needed at the MEBT due to back streaming radiation, a
rolling polyethylene wall could be placed there also. If extra shielding
is needed on the vault north and south sides, one row of the concrete
blocks (or part of) on the north and south sides (inside the cave) could
be replaced with 6 or 8\" of polyethylene.

Lead shielding does not appear to be needed anywhere (at least for
accelerator produced radiation) as the attenuation achieved with the
polyethylene should be sufficient.

**Summary**

The biggest shielding issues with this accelerator are a result of the x
rays from the RF and the neutron radiation emitted at the RFQ interfaces
due to beam losses. The x rays were \"easily\" mitigated with a steel
casing on the RFQs. The B/C losses might be mitigated by using vanes
made of copper. Additional measurements with a copper and aluminum
target would need to be made to verify this.

Figures 10.1 and 10.2 show the estimated dose equivalent rates due to
accelerator and beam on target radiation for Fermilab and BRF and Figure
10.9 shows a proposed possible solution for the BRF location. If the
accelerator were to move to BRF, additional measurements would need to
be made at Fermilab at higher currents with both a tantalum target and
aluminum target in order to determine how much of this shielding would
be needed and if it were sufficient. Until the determination to relocate
the accelerator is made, further radiation measurements have been put on
hold as they are unnecessary for operations at Fermilab.

![](media/image45.wmf){width="5.760416666666667in"
height="8.753472222222221in"}

*Figure 10.9: Proposed Accelerator Shielding for BRF Layout.*

**References**

1\. (BI, 1994) J. Bida, D. Finley, P. Young, K. Krohn, "Proposal For A
Pilot Program For Development of a 3He RFQ-Based System for Low Cost
Production of PET Tracer Isotopes in a Clinical Environment", March
1994.

2\. (GR, 1997) N. Grossman and D. Holslin, "Assessment for the PET RFQ
Accelerator at Fermilab", February 1997.

3\. (GRO, 1997) N. Grossman, K. Graden and F. Krueger, "RF, X Rays and
Dose Equivalent Rate for the 212 MHz RFQ", February 1997**.**

4\. (LAC, 1996) Louisiana Department of Environmental Quality,
Environmental Regulatory Code 33:XV

5\. (FRCM, 1998) Fermilab Radiological Control Manual, January 1998
edition. This document contains Fermilab\'s policies for assuring worker
safety and protection of the public and the environment from sources of
ionizing radiation. It is part of the Fermilab Environment, Safety, and
Health Manual (see FESHM, 1998).

6\. (10 CFR 835) Code of Federal Regulations, Title 10, Part 835,
\"Occupational Radiation Protection\".

7\. (SAD, 1997) "SAD Document for PET/RFQ Project at Fermilab", February
1997.

8\. (KR, 1990) F. Krueger, "Determining the Fast Neutron Energy Response
Characteristics of Fermilab Instrumentation", RP Note 86, June 1990.

**11. Magnet Power Supply Systems**

**System Description**

The power supply system for the PET accelerator consists of 22 current-
regulated DC power supplies that are used to drive various magnet loads.
Most of the units are commercial supplies. Some came with the project
and some were purchased to complete the installation. The power supply
circuits and their characteristics are listed in Fig.11.1.

LOAD SUPPLY TYPE RATING REGULATION

Alpha EMI ESS 80-125-2-10T-0806 80V/125A 0.1%

Alpht Lambda LLS-50-60 60V/1.4A 1%

MEQ3 "

MEQ1 EMI EMS 10-200-5-D-10T-0806 10V/200A 0.1%

MEQ2 "

HEQ1 EMI EMS 40-50-5-D-10T-0806 40V/50A 0.1%

HEMP1 EMI-EMS 80-60-2-D-10T-0806 80V/60A 0.1%

MEQ4 EMI EMS 15-300-2-D-10T-0806 15V/300A 0.1%

MEQ5 "

Q1TRM FERMI TRIM SUPPLY +/- 20V/6A 1%

Q2TRM "

D3VER "

D3HOR "

Q4TRM "

Q5TRM "

LES1 EMI BOSS 20/20/1 +/- 20V/20A 1%

LES2 "

HES1 "

HES2 "

SOLND EMHP 30-600-12211 30V/600A 0.1%

METQ1 EMI EMS 30-20 1D-10/T 30V/20A 1%

METQ2 "

*Figure 11.1. PET Power Supply Ratings*

The major engineering effort required was in integrating the power
supplies into the PET control system so that they could be controlled
and monitored. A variety of protection systems such as ground fault and
over temperature detection trips were also designed in. The regulation
stability of seven of the supplies was improved through the use of
external current transductors and computer feedback loops.

**General Approach to Control and Regulation.**

In order to minimize the additional circuitry and expense required to
integrate the power supplies into a complete system, it was decided to
take full advantage of two of the control systems that were being built
into PET: the PLC's (programmable logic controllers) and the IRM's (
Internet rack monitors).

The PLC's were already being used to interlock vacuum, RF power and
other systems. Power supply interlocks were added to their list of
functions.

The IRM's house the A/D and D/A capability of the PET control system.
IRM channels and programming are utilized to perform interlock functions
requiring analog monitoring. The IRM also performs slow P.I.D. feedback
adjustments to enhance the stability of some of the power supply
circuits.

Figure 11.2 is a block diagram of the control system for a typical power
supply. It shows the interlocks for over current, ground fault and load
over temperature as well as the circuit path for enhanced regulation
stability.

![](media/image46.wmf){width="5.704166666666667in"
height="3.5194444444444444in"}

*Figure 11.2. Typical Power Supply Controls*

**Enhanced Regulation Stability**

In the PET project, seven of the circuits required 0.1% stability to
meet their specifications. Commercial power supplies are generally
capable of maintaining 1% current regulation in the presence of normal
line, load and temperature variations. The most severe limitation in
these supplies is a temperature coefficient of 0.03%/degree C.

In a previous Fermilab project (the LINAC upgrade solenoid supplies),
true 0.1% regulation was specified and went out for bids. The end result
was specially modified units that had a long delivery time and cost
almost twice the price of the standard supplies. For the PET project, it
was decided to use the standard supplies and to monitor the load current
with an external transductor of adequate stability. The IRM computer
makes small changes in the power supply reference to keep the external
transductor signal within tolerance.

The external current monitor chosen was a LEM 205-S. This is a 200 amp
Hall device transductor with a temperature stability of 0.002%/degree C,
and a price of only \$55. The LEM's require a power source and precision
burden resistors to operate them, but these are also inexpensive items.
In the circuits requiring significantly less than 200 amps, multiple
turns were wound around the LEM monitoring head.

The IRM reads the LEM output at a repetition rate of 10 Hz. The MADC
built into the IRM is a 16 bit device with good enough stability to
easily handle the 0.1% regulation task. The IRM processor is set up with
standard P.I.D. loop algorithms. For the power supply loops it makes
"integral" corrections, changing the power supply reference by 10% of
the measured current error. The reference is produced by a 16 bit D/A
output channel. The controls group installed a special 8-channel 16 bit
D/A in the IRM to provide the resolution to smoothly make these
corrections.

**Over Current Trips**

The commercial power supplies are not supplied with over current trip
circuits. To protect the magnets from excess currents, the IRM system is
employed. The load currents are already being monitored for either
feedback or display purposes. The controls group has programmed the IRM
to also compare the monitored value with a maximum current, and send a
trip signal to the PLC if this level is crossed. The PLC latches the
trip and turns the power supply OFF.

To guard against an IRM crash and loss of current checking, a
"heartbeat" monitor has been installed. If the IRM fails to send a 10 Hz
pulse to the PLC, the PLC waits several seconds and then turns OFF all
supplies. The IRM is programmed to send out its heartbeat directly after
it has completed its power supply scan.

**Ground Fault Detection**

High power loads are protected with ground fault detector circuits. The
approach is to float the power supply output terminals and balance them
with respect to ground using two 1k Ohm resistors. (See Fig. 11.2. ).
The summing point of the resistors is held at ground through a 100 Ohm
monitoring resistor, and the voltage across the 100 Ohms is zero unless
there is some external current going to ground. The summing resistor
voltage in monitored by the IRM, which is programmed to trip the power
supply if the voltage exceeds a preset level.

The advantage of this type of ground fault detection is that it keeps
ground current below a milliamp even if the load has a dead short to
ground. This protects both the load and operations personnel. The
disadvantage of the approach is that it requires floating the power
supply output terminals. Unfortunately, commercial supplies reference
their remote signal inputs to one of the output terminals (usually the
negative). Since the IRM reference is single ended, isolation amplifiers
were added in the reference lines of these power supplies. The IRM
monitors the ground fault signals in the same way that it detects over
currents. The integrity of both checks is protected by the heartbeat
system.

**Over Temperature Protection**

The magnet loads are protected from over temperature conditions in a
redundant manner. A 70 degree C thermal switch detects the problem and
tells the PLC to turn OFF the power supply. As a backup, an 80 degree C
switch is placed directly in the power supply hardware OFF circuit . In
addition, the water-cooled loads are de-energized if the PLC detects
that the pump system is not operational.

**Safety**

All of the power supplies but one are powered from the same 480 VAC
feed. This feed can be LOTO'd at a safety switch, making the supplies
safe for maintenance. Alternatively, the supplies may be individually
locked out by putting a lock box on their power cords. The Solenoid
supply, which is fed from a separate 480 VAC source, has a safety switch
at which it may be locked and tagged . Hazardous terminals at the inputs
and outputs of the power supplies have been protected .

**Alpha Magnet Ramping**

The Alpha magnets are constructed with a solid steel core rather than
laminations. Due to the nature of the design, there was a concern that a
rapid change in current may generate eddy currents sufficiently high to
change the residual magnetic field at places in the magnet. To avoid
this possibility, an electronic slew-rate-limiter circuit which slowly
ramps the power supply reference to a newly entered value was installed.
The circuit also ramps the reference to zero before turning OFF the
supply in the event of a trip condition. This circuit cannot protect
against all situations, such as a power failure, but it prevents
inadvertent generation of high eddy currents.

*The Alpha Trim Supply*

The Alpha magnet system consists of two magnets that run at nearly the
same current (approximately 90 Amps). The power supply system consists
of a main supply to power both magnets in series, and a 1.4 Amp trim
supply which sits piggy-backed across one of the magnets. A diode in
series with the trim supply protects it from the main supply current.
Some problems were anticipated by stemming from two supplies regulating
parts of the same load, but the system functioned smoothly without any
buffer impedances or other de-coupling mechanisms.

*Spares*

Each power supply in the PET accelerator has a spare of equal or greater
capacity. One spare supply may be the backup for several different
units. Use of a non-identical spare might require some temporary jerry
rigging.

*Commissioning Issues*

The commissioning of the power supply system was relatively smooth,
given the number of people who worked hard at getting all the cabling
straight, the bits lined up and the algorithms checked out. The major
technical problem was an instability in some of the floating supplies
that used isolation amplifiers in line with their reference signals. It
was discovered that the high frequency common- mode noise of the power
supply was picked up and converted into erratic wobble by the isolation
amplifiers. The problem was solved by putting small chokes in the signal
and common lines of the cables between the iso-amps and the power
supplies.

**Summary**

The power supply system has run quite smoothly. Regulation stability has
been adequate for stable machine operation and reliability has been
good. A number of ground fault trips have been experienced due to the
added noise of the operating accelerator. These have been mitigated by
increasing the trip levels with no compromise in system protection.

**12. Mechanical and Vacuum Systems**

**General Mechanical Performance Issues**

*Accelerator Footprint /Physical Layout*

The PET ^3^He++ accelerator physical layout is arranged to facilitate
aisle access to all components, maximize support equipment packaging
density, and minimize cable lead lengths. To accommodate such
requirements, an accelerator support tree is used to locate beamline
components in a cantilever fashion, provide a centralized overhead
equipment bay location for vacuum support hardware and power supplies,
and provide equipment power service and cable tray instrumentation line
routing (Figure 12-1).

The general accelerator component alignment criteria dictates that the
ion source, RFQs, alpha magnets, and HEBT positional tolerance is
maintained to within 0.010\" of true position. Additional requirements
included provisions for easily changing solenoid position during beam on
conditions to maximize LEBT transmission and the ability to easily
change and monitor the location of the MEBT quadrupoles Q1-Q5. When
changing the position of the MEBT quads, dial indicators are used to
monitor positional changes in increments as small as 0.001\".

To meet alignment requirements, a rigid accelerator support system is
necessary. The accelerator tree is constructed of 6061-T6 Aluminum
channel shapes. The main horizontal channels are of sufficient
cross-section to minimize vertical deflection in any single location to
less that 0.005\" with full accelerator component loading. Individual
beamline component stand legs and stands are fabricated primarily using
mild steel box tubing. The accelerator was constructed by first
installing the support tree and overhead equipment bays and performing a
survey to within 0.030\". Components were placed on their associated
stands starting with the ion source, conducting an optical survey to
within 0.010\", and working down the beamline. The support structure is
proven to be rigid and functional, and components are easily placed well
within alignment tolerance.

The overall accelerator footprint is designed and constructed in the
Fermilab Lab G location with a footprint that is compatible with the
area prepared at the Shreveport BRF facility.

**Ion Source**

The original ion source consists of a duoplasmatron ion source, electron
trap, ground electrode, current toroid, and an x-y steering magnet
mounted in an aluminum vacuum housing. The original hardware
configuration existed from the previous PET accelerator project.

Efforts during the current phase of the project focus on reducing the
longitudinal distance from the ion source plasma electrode to the exit
of the housing while maintaining an adequate vacuum conductance path for
mounting the internal source related components.

A new stainless steel vacuum housing was designed and constructed to
accommodate the more compact longitudinal geometry. The new housing
shortens the distance from the ion source plasma electrode to the exit
of the housing by slightly more than 3 1/2 inches. Additionally, the
conductance of the vacuum pump connection port is slightly improved to
increase the effective pumping speed at the chamber. The new housing
also allows for a compact connection to the LEBT (low energy beam
transport) section.

![](media/image47.wmf){width="6.388194444444444in"
height="6.624305555555556in"}

*Figure 12-1: PET Accelerator Beamline Component Layout and Accelerator
Tree Support Structure*

**LEBT**

The LEBT section consists of a solenoid magnet, a vacuum beam pipe and
diagnostic collectors located at the exit of the beam pipe. All of these
components are newly designed for this project. The design of the LEBT
section is a collaborative effort between the program's beam optics
experts, Fermilab's Technical Support Magnet Manufacturing Facility, and
the mechanical design team.

The LEBT beam pipe interfaces with the ion source vacuum housing on the
upstream end and with the 212.5 MHz RFQ on the downstream end.
Functional requirements include the ability to easily adjust the radial
and longitudinal position of the solenoid to facilitate beamline tuning
to maximize beam transmission and match to the 212.5 MHz RFQ. In
addition, beam position diagnostics are required at the exit of the beam
pipe.

The solenoid magnet is designed with a 5.5 inch ID bore, a 16 inch OD
housing, and an overall length of 8.8 inches. The type 304 stainless
steel vacuum beam pipe flanges interface with adjacent components using
O-ring seals. The original vacuum beam pipe allowed for nominal solenoid
adjustments of ±1.6 inches longitudinally and 0.06 inches radially. The
exit beam position diagnostics consist of four copper collectors that
essentially form a segmented Faraday aperture.

Upon completing fabrication of the solenoid, tests indicated that the
magnetic center was displaced from the physical center by as much as
0.25 inches. This discrepancy prevented proper alignment of the solenoid
magnetic center with the beamline axis due to interference with the
vacuum beam pipe. To minimize schedule impact, a decision was made to
fabricate an entirely new LEBT beam pipe with a smaller outside
diameter. The new LEBT vacuum beam pipe allows a nominal radial
clearance of 0.5 inches.

**212.5 MHz and 425 MHz RFQs**

The accelerator system has four RFQ structures, a single RFQ operating
at 212.5 MHz and three RFQs operating at 425 MHz. The 212.5 MHz RFQ
structure accelerates the singly charge He3+ beam from 22 keV to 1.0
MeV. It is physically connected at the upstream end to the LEBT beam
pipe and to the charge stripper on the downstream end. The 425 RFQ chain
accelerates the doubly charged He3++ beam from 1.0 MeV to 10.54 MeV.
Three separate RFQ cavities, referred to as 425 RFQs A, B, and C, are
connected in series. The 425 RFQ A interfaces with the MEBT (medium
energy beam transport) on the upstream end and the 425 RFQ C interfaces
with the HEBT (high energy beam transport) on the output end.

The original 212.5 MHz RFQ and two 425 MHz RFQs A and B existed from the
1992 project. The additional 425 MHz RFQ C was newly designed and
constructed for this project. Each RFQ cavity assembly is made up of the
following major components: a vane housing, four vanes, and 2 cavity end
walls. Each of the RFQ structures is enclosed in its own vacuum housing.
A series of separate events led ultimately to the replacement of the
entire 212.5 MHz RFQ cavity assembly and the 425 MHz RFQ A and B vanes.

In addition, radiation shielding studies during initial testing
indicated that the original vacuum housings provided inadequate
radiation shielding during RF operation. The following discussion covers
the above issues separately.

*Replacement of Vacuum Housings*

In December of 1995, 1 MeV test beam efforts were conducted using the
original 212.5 MHz RFQ. Additional off line (i.e., no beam loading) RF
testing was performed using the original 425 MHz RFQ A from the 1992
project. The test results indicated an unacceptable level of x-ray
radiation produced by the cavity during operation with RF power levels
sufficient to accelerate beam. The requirement to maintain acceptable
radiation levels adjacent to the accelerator for personnel occupancy
dictated that some form of shielding would be necessary for all RFQs.

Since a more functional form of shielding is desirable (i.e., as opposed
to lead blankets hanging over the accelerator structure), new type 321
stainless steel vacuum housings for all RFQ structures were fabricated
incorporating a nominal wall thickness of 1.38\" to replace the original
aluminum vacuum housings featuring a 3/8\" nominal wall thickness.
Subsequent operation of all RFQs indicate that the thick wall vessels
provide adequate x-ray shielding during RF operation. Further discussion
of neutron shielding for beam loading conditions are found in the
radiation section of this report.

*Replacement of 425 RFQ A Vanes*

In June of 1996 it was discovered that the vane tips of 425 RFQ A
exhibited damage in the form of melted/pitted sections. Investigation
led to the conclusion that the damage most likely occurred during the RF
testing performed in December of 1995. Replacement vanes were procured.

*Replacement of 425 RFQ B Vanes*

The existing RFQ cavities are designed such that the vanes are separate
from the tubular housing and when assembled they form the quadrupole
cavity (reference Figure 12-2). The physical geometry of the 7075-T6
aluminum vanes feature small ears that run along the length of the vane
base to form a line contact with the inside diameter of the vane housing
achieving an RF contact joint. Threaded fasteners are used to hold the
vanes in place and allow for adjustment of the cavity fields. The forces
created by the adjustment of these fasteners cause the ears of the vane
base to act as a cantilever beam, thereby applying the necessary RF
joint contact force.

In July of 1996, cracks were discovered in the spring ears of the 425
MHz RFQ B vanes. During the 1992 project the 425 MHz RFQ A vanes
exhibited the same failure mode and were replaced. Finite element
analysis conducted by the 1992 project team indicated that the stress in
the vane spring ears was excessive, leading them to redesign the spring
ears and replace the 425 MHz RFQ A vanes. The original 425 MHz RFQ B
vanes did not exhibit similar failure at that time and were not
replaced. The current project incorporates the redesign changes made on
the 425 RFQ A vane spring ears in the new 425 RFQ B vanes. Similar
procurement specifications were applied in the fabrication of the new
425 MHz RFQ C vanes.

![](media/image48.wmf){width="4.402777777777778in"
height="4.069444444444445in"}

*Figure 12-2: Typical RFQ Cross-Section*

*Replacement of 212.5 MHz RFQ Vanes*

In September of 1996 vacuum leaks were detected in the 212.5 MHz RFQ
vane cooling water passages. Thorough examination of the problem
revealed that the cooling passages of the 7075-T6 aluminum vanes had
corroded through the nominal wall thickness of 0.085 inch. Since the
procurement cycle to obtain new vanes is several months, a short term
solution was required to continue operation and commissioning of the low
energy section of the accelerator. The short term fix consisted of
pressure impregnation of the individual vanes to seal the leaking water
to vacuum passages. This procedure was performed at Casting Impregnators
located in Skokie, IL, and featured a 90 psi impregnation using Locktite
RTC anaerobic material.

The pressure impregnated 212.5 MHz RFQ vanes have subsequently operated
successfully for over 1 year, allowing accelerator commissioning to
continue during fabrication of the new assembly with only minor schedule
impact. While the repaired 212.5 MHz vane/housing structure is still
considered a functional assembly, long term concerns exist regarding the
robustness of the corroded water passages. Considering scheduling and
reliability issues, the decision was made to replace the entire 212.5
MHz RFQ vane and housing structure.

A mild redesign of the 212.5 MHz RFQ vane/housing structure was
conducted to replace the existing 212.5 MHz RFQ cavity with a new RFQ
cavity that fits inside the original RFQs vacuum housing. Several design
improvements are incorporated.

The vane contour is modified from the original RFQ to provide an
improved optics match with the entire accelerator system.

The location of the gun-drilled holes which provide for cooling passages
within the body of the vanes are slightly relocated to improve
manufacturability.

The hardware configuration for fastening the vanes to the vane housing
is simplified which results in a more economic design and an improved
tuning adjustment.

Water fittings are designed and fabricated which allow for an
all-stainless steel RFQ vane to housing cooling circuit connection. The
original RFQ required the use of stainless steel fittings and soft
copper tubing in this region, resulting in dissolved copper ions in the
water system exacerbating the corrosion of the 7075-T6 vane water
passages.

The vane housing cooling passages are redesigned and simplified so that
all of the water connections to the vanes could be completed at the vane
housing end flange. The original 212.5 MHz RFQ design incorporated
flange port feed throughs in the vacuum housing to deliver water to the
vanes. As an additional benefit, the manufacturability of the housing
was improved as compared to the original design.

Prior to final assembly of the new 212.5 MHz vane/housing structure, He
leak checks indicated small leaks on two of the four vanes. The leak
rates were found to be on the order of 10-7 atm-cc/sec. The leaks were
traced to water passage plug weld regions used in the fabrication of the
vanes. Since the vanes were copper plated with all general machining and
welding complete, the only option outside of fabricating new vanes was
to seal the leaks using the Locktite RTC impregnation procedure
delineated above.

*RFQ Vane Corrosion Considerations*

An important point to note is that the 212.5 MHz RFQ vanes were replaced
primarily due to corrosion of the water passages leading to a leakage
path into the vacuum system. Prior to fabrication of the new vanes,
discussions focused on various configurations of redesigning the 212.5
MHz RFQ vanes whereby different materials less susceptible to corrosion
could be used for the body of the vane and other materials with an
appropriate modulus of elasticity could be used for the base of the
vanes where the spring ears are located. It was determined that a major
redesign effort of the RFQ vanes was not feasible and that replacement
vanes of similar construction to the current design would be procured.

The 7075 aluminum alloy used for the RFQ vanes contains copper,
magnesium, and zinc. While this alloy has the ideal structural
mechanical properties for the vane design, it unfortunately is not
stable in water. Mixing yellow metal (copper or brass) and aluminum in a
cooling system often results in local cell corrosion. Copper ions that
settle onto the aluminum water passage surface act as local cathodes
resulting in a plating out of surrounding aluminum. Pitting develops
which increases the surface area and permits formation of localized
acidic environments. The process catalyzes itself. With copper already
present in the 7075 alloy, the corrosion cycle can take place even
without other sources of copper in the cooling loop. There is also
evidence of zinc plating out of the alloy in a corrosion pattern known
as white rust.

Several options are implemented to address the corrosion problem. The
long term solution includes the application of bonded Teflon to the
water passage surfaces in all 16 of the newly fabricated RFQ vanes prior
to assembly and operation. The Teflon provides an electrically neutral
barrier to protect the water passage surfaces. The application of bonded
Teflon involves a flow-through chemical process whereby approximately a
0.0001 inch thick layer of Teflon chemically bonds with the internal
water passage surface. Bonded Teflon, as opposed to typical commercial
application of Teflon film, is not attacked by solvents such as MEK, and
requires machining of the surface for removal. A water passage test
billet fabricated from 7075-T6 with a bonded Teflon passage is installed
in the PET accelerator water system to monitor the efficacy of this
barrier layer relative to corrosion. Results indicate that this solution
is effective in controlling corrosion of the RFQ vane water passages.

Additional monitoring is achieved by the installation and regular
inspection of 1 micron filters in the RFQ water system. The filter pores
are small enough to trap aluminum particles and give a good indication
of the amount of corrosion occurring. Results show a major improvement
with the bonded Teflon application but still some loss of aluminum. It
is believed that part of the filtered particulate comes from the 6061-T6
housings. This alloy is comparatively much more stable in water than
7075-T6. Since most of the RFQ housings that came with project were
previously used and did not have virgin machined water passage surfaces,
the decision was made not to Teflon coat the housing passages.
Additional experiments were conducted by mounting wafers consisting of
different aluminum alloys in a flow chamber and monitoring weight loss.
The test results yield data on what to expect under various conditions.
Unprotected 7075 aluminum in our system was corroding at an average rate
of 30 to 40 mils per year. Thus a water passage with an 1/8 inch nominal
wall thickness would last approximately three years.

The other major part of corrosion control is water chemistry. A program
has been implemented with a local vendor to add corrosion inhibitors and
monitor water chemistry . Sodium silicate is used to lower the corrosion
potential of aluminum, tolytriazole to stabilize yellow metal, and
carbonates to maintain a slightly basic pH. Aluminum cooling loops need
to be maintained at a pH of 8.5 to 9. 7075-T6 aluminum wafers (not
Teflon coated) are mounted in each of the Neslab chillers. The wafers
are weighed before insertion. The wafers are inspected at regular
intervals to indicate how efficiently the water chemistry program
protects uncoated aluminum. It is believed that proper water chemistry
coupled with the application of bonded Teflon will protect the aluminum
for the duration of the project.

*425 RFQ C Vane Housing leak*

After installing the new 425 MHz RFQ C in the beamline a leak was
discovered in the vane housing at a water passage weld joint. It is
believed that the cause of the leak was somehow related to the plating
process, since the housing had been thoroughly leak checked prior to
plating. The housing consists of 6061-T6 aluminum alloy, which produces
sound weldments using the Tungsten Inert Gas (TIG) welding process.
Aluminum parts with leak tight weld joints have in the past been noted
to fail a leak check after plating. Although the plating process does
involve slight cleaning and etching of the aluminum base metal, it is
not very aggressive but is still absolutely necessary.

The leak was isolated to a vacuum weld located at the downstream end
face of the vane housing. This face of the vane housing mates to the
cavity end wall where an RF joint is formed (reference Figure 12-3).
This functional requirement complicates the fabrication process because
it requires the surface to be machined after welding. Re-welding this
joint was not possible since the RFQ had already been plated, assembled,
tuned, and installed into the beamline. A flat viton gasket was designed
and fabricated to fit between a groove machined in the cavity end wall
and the face of the vane housing, while still allowing room to
accomplish the required RF joint. The fasteners used to assemble the
cavity end wall to the vane housing provide the clamping force for the
seal and RF joint. The 425C RFQ has operated successfully with no
deleterious effects as a result of this in-situ repair.

![](media/image49.wmf){width="5.638888888888889in"
height="3.6666666666666665in"}

*Figure 12-3: RFQ 425 C Housing End Repair*

*Copper plating difficulty*

During fabrication of all RFQ cavity components the largest difficulty
was related to the copper plating, bright acid copper on aluminum base
metal. This plating process involves a cyanide copper strike followed by
electroless nickel and finished with 1 to 3 mils of bright acid copper.
The supplier that had originally provided the plating services for the
1992 project team no longer has the size capability for all of the
processes involved. During fabrication of the 425 RFQ A and B vanes this
supplier subcontracted out those processes which they could not do
in-house. Due to the control and time sensitivity of the steps involved
in the plating process this subcontracting led to recurring adhesion
problems. The plating schedule effectively suffered a several month time
delay due to such problems.

A new plating supplier was required for the new 212.5 MHz RFQ
components. The first challenge was to find a supplier that had large
enough tanks ( almost 4 feet long) for all steps of the plating process.
An exhaustive search consisting of contacting dozens of suppliers
throughout the nation was conducted. A plating shop located in Chicago
was identified. The high degree of control required by the processes as
well as the difficulty associated with these unusually large parts made
the task a difficult one. After several weeks of working with the vendor
and providing input to accelerate the learning curve, the 212.5 MHz
vanes and housing were plated successfully.

**Charge Stripper**

The beamline optics design allows a longitudinal space of approximately
11 inches for charge stripper installation. Considering the adjacent
212.5 MHz RFQ vacuum requirements of 10-6 Torr for acceptable RF
operation, stripping efficiency requirements exceeding 70%, and the
respective gas load required to achieve acceptable gas molecule
cross-section, a pulsed jet design is chosen. The previous 1992 project
used a Lasertechniques piezoelectric valve to control gas flow to a
static cell charge stripper. Analysis and investigation of the previous
charge stripper design indicates that a static cell would not allow for
optimal stripping efficiency and acceptable pressure in adjacent
beamline elements. In addition, the piezoelectric valve exhibits
unacceptable leak rates exceeding of 10-5 atm-cc/sec as well as
insufficient throughput to achieve proper gas cross-section for electron
stripping. Several automotive type fuel injectors were bench tested by
using a piezoelectric pressure transducer to measure gas throughput and
pressure rise time.

Additional efforts focused on refinement of the injector drive circuit.
A Nissan fuel injector P/N 1660-40F00 is found to provide acceptable
rise time on the order of 200 µsec as well as exhibiting leak rates less
than 10-7 atm-cc/sec. The injector is found to be compatible with
operation in vacuum.

The production charge stripper utilizes a single injector design. Figure
12-4 shows a cross-section of the physical layout of the charge
stripper. Gas is delivered from the fuel injector at a stagnation
pressure of 0.3 MPa to a converging/diverging nozzle featuring a 0.020
inch throat diameter. The gas is expanded across the beam path to
supersonic velocity . The pressure in the strip region is calculated to
be approximately 0.5 Torr over an interaction length of about 0.42 cm to
achieve a stripping cross-section of approximately 3E16 molecules/cm2.
The high velocity gas is directed through an exit aperture into a
scavenge section of the vacuum housing. The scavenge section evacuates
the gas molecules and segments the pressure from the vessel top
(beamline) section, thereby preventing gas molecules from migrating
toward the 212.5 MHz RFQ. The average gas load is on the order of 13
Torr-l/sec, with a peak load of about 150 Torr-l/sec during the 240 µsec
pulse width.

Multiple pumps are required to evacuate such a large gas load with the
constraint that the assembly must physically fit into the available
beamline space. The scavenge section accommodates two Balzars 1500 l/sec
turbomolecular drag pumps. The top portion of the vacuum housing
features conductance limits to prevent the migration of stray gas in the
scavenge section toward the upstream 212.5 MHz RFQ. To assist in further
evacuating stray gas in the top portion of the housing, two 500 l/s
Balzars turbomolecular drag pumps are used. A more detailed discussion
delineating charge stripper and adjacent beamline element vacuum
characteristics is found in the vacuum portion of this report.

The charge stripper has exhibited overall performance consistent with
design analysis. The fuel injectors survive in excess of 108 pulses and
operate satisfactorily at 360 Hz operation (we have yet to fail a fuel
injector due to excessive gas leakage or open circuit of solenoid
windings after 1-1/2 year operation). It is noted that during 360 Hz
operation utilizing argon as a stripping gas, the scavenge section 1500
l/s pumps exhibit excessive heating as a result of the rather large 13
Torr-l/sec average gas load and resultant 5 mTorr scavenge pressure. The
stripping gas was changed to nitrogen at the expense of a few percent
stripping efficiency. The nitrogen gas molecule is lighter than the
argon molecule and has better heat transfer characteristics, allowing
for greater gas throughput than Argon while maintaining acceptable pump
temperatures. The switch to nitrogen as a stripping gas is intended to
increase the lifetime of the turbo pumps.

![](media/image50.wmf){width="5.707638888888889in"
height="3.3604166666666666in"}

*Figure 12-4: Production Charge Stripper Cross-Sectional View*

**MEBT Design Considerations**

The MEBT section of the accelerator interfaces with the charge stripper
at the upstream end and the 425 RFQ A at the downstream end. This
section of the accelerator consists of the many magnets, diagnostic
elements and vacuum equipment which are shown in figure 12-5. The goal
of the design team was to integrate all of the necessary diagnostic and
pumping elements with the magnet beam pipe structures. Each magnet beam
pipe must be non-magnetic and must not obstruct the beam. This is
complicated by the very large size transition of the beam as it travels
from the entrance to the exit of the MEBT. Additionally, the beam pipes
must allow for sufficient adjustment of the magnets during the
installation and commissioning stages of the project.

The design takes advantage of the drift space between the first and
second alpha magnets to integrate most of the pumping and diagnostic
elements into a major piece of the MEBT beamline called the MEBT
Diagnostic Box. The MEBT Diagnostic box utilizes the underside of the
top cover and a removable false floor to mount most of the diagnostic
elements which allows for bench-top assembly and servicing. The floor of
the box is used to hang the two MEBT vacuum pumps (reference vacuum
section). The large cross-sectional property of the MEBT box allows for
a significant vacuum conductance path to facilitate pumping in the
region adjacent to the small cross sectional alpha beam pipes.

The nonmagnetic requirement for the alpha beam pipe and wall thickness
constraints dictates stainless steel construction. Precautions were
taken to address the magnetic issue before fabrication. However, testing
showed that the areas of the welds were indeed found to be slightly
magnetic. This occurs due to the formation of precipitates and grain
structure alignment during the resolidification of the molten metal. A
solution heat treating process was performed on the finished welded
assemblies which restored the material to an acceptable condition.

![](media/image51.wmf){width="6.374305555555556in"
height="4.263194444444444in"}

*Figure 12-5: MEBT Plan View*

**HEBT**

The HEBT (high energy beam transport) is the interface for targets to
the accelerator. Its design goals included the following:

a\) Transport beam to the target with a uniform beam density over a 2 x
10 cm target zone

b\) Provide for rapid exchange of targets by users

c\) Control radiation in the vault so users could enter this area with
beam on

d\) Provide users with a means of shaping beam size and density

e\) Protect the accelerator components from target implosions.

Figure 12-6 depicts an elevation view of the HEBT, which consists
primarily of two parts: a beam transport section and a target
diagnostics section.

![](media/image52.wmf){width="3.0131944444444443in"
height="1.3881944444444445in"}

*Figure 12-6: HEBT Elevation View*

The HEBT beam transport design was based on various optics studies
prepared by Dr.\'s Schulze and Larson. The quad and multipole focusing
magnets were inherited from the 1992 phase of the project. The x-y
steering magnet, consisting of pairs of wound flat plates, are similar
to the steering magnet used in the MEBT. A Pearson current toroid is
located at the HEBT beamline focal point to monitor current into the
target region. An aperture plate is placed just upstream of the toroid
to protect it from incident beam. A debris baffle to deflect particulate
in the event of a window failure is located downstream of the toroid.
The large 6 way cross in the center of the HEBT beamline provides a
platform for the vacuum pump, a lodging area for target debris, and
access for cleanout.

The target diagnostic area includes a pair of horizontal collimators, a
pair of vertical collimators, an electron suppresser shield, the Faraday
cup, an isolation gate valve, a spool piece with a target roughing pump
port, an isolation flange with a target electron suppresser shield
mounted in it, and an NW160 vacuum interface for targets. The
collimators are mounted on stepper motor driven vacuum feed throughs.
They are electrically isolated, biased, and water cooled. The Faraday
cup is pneumatically driven. It is also electrically isolated, biased,
and water cooled.

The collimator and Faraday cup incident beam surfaces are comprised of
tantalum. A high Z material is selected to control neutron radiation in
the vault. These devices have copper water cooling jackets on the
backside. Attaching copper to tantalum was a significant design effort.

Solid tantalum plates forming the Faraday cup beam surface are furnace
brazed to a copper back plate using a platinum silver foil at a braze
temperature of about 1200 C. Operational experience indicates that this
design appears to function well.

The collimators consist of a .012 inch thick tantalum layer explosion
bonded to an 1/8 inch copper back plate. The original fabrication scheme
called for furnace brazing the explosion bonded material to a copper
back plate. Due to differential rates of thermal expansion, the strips
curled (i.e., acting like a bimetallic strip) and the braze joint
failed. This approach could probably be made to work with a lengthy test
effort and more elaborate clamping fixtures. However, due to schedule
constraints, soldering the copper backed tantalum to the cooling plates
with a 320 C silver solder foil in an inert gas chamber was selected.
While this approach successfully joined the explosion bonded material to
the copper cooling plates, it leaves the collimators vulnerable to
damage if they overheat. One solder joint failure has already occurred,
resulting in a water leak into the vacuum system.

Enough experience with the HEBT operation exists to evaluate its
performance in terms of the design goals.

a\) Transport uniform beam to the target: The optics studies using
TURTLE and TRACE 3D do indeed correctly predict beam behavior in the
HEBT. The beamline and internal aperture plates and baffles also
successfully fit the beam path. The performance spec projecting +/- 25%
beam uniformity has not yet been met. Beam uniformity to this degree
appears only possible if we throw away 40 to 50% of the beam. The HEBT
quad and multipole do not appear to be up to this performance level. Our
best efforts to date would allow for about 50 µAmps of actual +/- 25%
uniform beam, or about 80 µAmps of +/- 40% uniformity. Ongoing studies
continue to address the beam uniformity specification.

b\) Rapid target exchange by users: The mechanical interface seems to be
working well. The radiochemists can install their targets quickly and
commence pump down with a roughing pump switch in the vault.
Difficulties have arisen with outgassing from the targets. The fast
valve system requires a local pressure less than the high 10-5 Torr
range. Outgassing from the targets can generate transients as high as
the 10-2 Torr range. This effect has caused a number of complications in
getting the HEBT vacuum under control quickly after a target exchange.
One solution implemented is the addition of a cold trap in series with
the target roughing pump to increase pumping speeds below 10-3 Torr
before opening the target isolation gate valve. Also under consideration
is the ability to maintain the radiochemistry targets and foils under
vacuum prior to use to minimize outgassing and assist in controlling the
high HEBT pressures noted during initial radiochemistry.

c\) Control radiation in the vault to permit beam on access:
Considerable design effort went in to making sure the beam entering the
HEBT would strike only the tantalum surfaces on the collimators or
Faraday cup. Off axis beam would be intercepted outside the vault. This
effort has been successful. The choice of tantalum greatly assists in
meeting such requirements. While radiation data in the vault are
incomplete, it appears that one may expect 40 to 60 mrem/hr with the
Faraday cup intercepting 100 µAmps of beam. Such radiation levels are
within the radiochemist\'s realm of experience.

The Faraday cup operates as designed. One failure was noted when water
cooling was left off briefly while beam was incident on the Faraday cup
surface. Upon restarting water flow, an initial surge of pressure in the
HEBT was noted. The leak was traced to a Swagelock fitting and quickly
repaired.

Ironically, the levels of radiation outside the vault (due primarily to
neutron production during beam loading conditions in the 425 MHz RFQs)
are typically higher than those found in the vault. The original design
goal of vault access during beam on conditions may no longer be
applicable due to the high levels of radiation adjacent to the 425 MHz B
and C RFQs.

d\) Provide users control over beam size and density: The collimators
have proven successful at shaping beam assuming the incident power
levels are maintained at reasonably low levels. At higher power levels
(e.g., more than 100 watts or so per blade) the collimator overheats.
This leads to major failure in the form of a failed solder joint and
subsequent water leaks into the vacuum system.

The collimator system is packaged in an extremely small space, thereby
restricting the use of more sizable cooling lines, fittings, and water
channels. The radiochemistry requirements called for keeping the
collimators as close as possible to the target. Additional design
options become available if more space is provided for the collimators.
The system would become more reliable and more easily serviced if the
collimators could be located in short, rectangular beam tubes with
bellows to isolate water and electrical feed throughs from the vacuum
system.

The current method of scanning to measure beam density patterns involves
moving the collimators across the beam in a pixel by pixel pattern. This
technique has worked but is limited to lower power beams to avoid
overheating. Due to space limitations, the collimators are mounted with
a single bearing at each end. The resulting mechanical play in the
system limits the spatial resolution. Wobble in the blades results in at
least +/- .5 mm variation. While this is not a major problem, it would
be desirable to improve the resolution.

The software designed for this system and the diagnostic electronics
have operated quite well. Some jitter exists in the position readback
requiring the software to iterate to position the collimators at the
correct location. Again, while not a major problem, it would be
desirable to improve the resolution.

e\) Protect the accelerator from target window failure: The fast valve
system is designed to isolate the accelerator (in particular the RFQs)
in the event of target failure. Current operation calls for disarming
the fast valve trigger system due to high outgassing rates from
radiochemistry targets. The fast valve system currently installed in the
HEBT uses a cold cathode vacuum sensor that is limited to pressures in
the 10-5 Torr range or lower. As a result of the large radiochemistry
target outgassing loads, pressures in the 10-3 Torr regime have been
noted. The end result is that disarming the fast valve defeats the
protection the system provides to the RFQs. A new fast valve sensor
system is on order that will tolerate pressures as high as 10-2 Torr
without tripping. The new desensitized system should be installed prior
to the next radiochemistry session. A request by the radiochemists to
allow for operation of the fast valve with some sort of \"knob\" control
over sensitivity is under investigation.

Additional measures are implemented to prevent target debris from
migrating into the RFQ. Prior HEBT bench tests indicate that if
turbulence could be created in the flow pattern as debris moved
downstream after a window rupture, debris would move off axis. The HEBT
beamline transitions rapidly at several points from small to large
volumes.

Also implemented are strategically placed baffles and apertures. This
system has worked well to prevent migration of fractured window
particulate into the RFQ cavities. Roughly 75% of the debris settles out
in the target diagnostics chamber, with the remaining material stopped
by upstream baffles or settling out in the HEBT cross. Clean out
apertures in the HEBT beamline also have proven to provide an adequate
means for clean up of window debris. Clean up techniques are improving
as more operational experience is gained.

In summary, HEBT performance is mixed. Radiation control and debris
control have largely been successful. However, specifications for beam
uniformity on target have yet to be met and work is ongoing to improve
the collimator system.

**Vacuum System Performance Issues**

*General Layout*

The vacuum system design is driven by four main requirements:

a\) maintain acceptable pressure to minimize sparking in the RFQ
cavities during operation

b)provide adequate gas cross-section to strip electrons from He3+ to
form He3++ in the charge strip region

c\) control gas flow in the ion source and evacuating excess He3
molecules to prevent excessive LEBT/212.5 MHz RFQ pressure

d\) ensure the vacuum system operates reliably to maintain clean RF
surfaces with minimal oil back streaming

The requirements for items (a) through (c) are addressed by analyzing
gas loads in the system, providing adequate pumping to maintain
acceptable pressures and evacuate gas molecules efficiently, and
providing a thorough mechanical cleaning prior to assembly. Item (d) is
addressed by utilizing an industrial type PLC controller to monitor
pressure and control isolation valves during pump down cycles, normal
running operation and fault conditions, and vent cycles.

The lowest pressure requirement is found in the RFQs, where it is
desirable to maintain pressures below about 2x10-6 Torr. With such
requirements, an unbaked system primarily utilizing O-ring seals for
ease of assembly and maintenance is acceptable. Turbomolecular pumps are
selected for this pressure regime since they evacuate larger gas loads
from the system efficiently (as opposed to gas capture pumps such as
cryopumps which may require frequent regeneration) and allow for clean
operation with little risk of oil back streaming into the system (e.g.,
as compared to diffusion pumps). Several Balzars horizontal
turbomolecular pumps were inherited from the previous phase of the
project and are used in the 425 MHz RFQ cavities, ion source, topside of
the 212.5 MHz RFQ, and a portion of the MEBT. New Balzars turbomolecular
drag pumps were purchased for the lower pumping port on the 212.5 MHz
RFQ, charge stripper, MEBT, and HEBT. The drag style pumps are selected
for their greatly reduced foreline requirements as compared to
conventional style turbos. This results in a cost and space savings,
allowing the purchase of smaller throughput roughing pumps to maintain
foreline pressures on the order of 1 Torr for drag style pumps as
compared to the 50 mTorr requirements of conventional style turbos.

Since the accelerator space for packaging support instrumentation (such
as vacuum gauge controllers) is limited, Balzars PKR 250 Full Range
Gauges are selected. These gauges feature a Pirani gauge coupled with a
cold cathode gauge. All gauge electronics are packaged in the gauge
head. These gauges indicate pressure from 1000 mbar down to 5x10-9 mbar.
They read back a linear voltage signal vs. log pressure over the noted
range, and are easily interfaced with the PLC.

The PLC minimizes roughing pump back streaming by monitoring beamline
pressures and closing foreline valves in the event of pump or pressure
alarms. The hardware is configured for normally closed operation to
ensure closing valves in the event of power outages. The accelerator has
been subjected to spurious site-wide power outages during operation and
the vacuum system has exhibited safe shutdown and restart in all cases.

A general schematic of the vacuum system is found in Figure 12-7.

*Comparison of Measured Pressures with Design/Analysis Pressures*

Since the largest accelerator gas load of 13 Torr-l/sec in the charge
stripper occurs adjacent to the 212.5 MHz RFQ, efforts focused on
modeling this area and specifying appropriate pumping capacity and
conductance limits to meet the requirements of stripping cross-section
and acceptable RFQ operational pressures.

To simplify such calculations, the charge stripper and adjacent beamline
elements were modeled using analogous systems and the commercially
available finite element code ANSYS®. For most vacuum systems in
molecular flow the following equation applies:

q=Sp + V dp/dt where q= gas source, Torr-l/sec

S= effective pump speed, l/sec

p= pressure, Torr

V= system volume liters

![](media/image53.wmf){width="6.054861111111111in"
height="4.665972222222222in"}

*Figure 12-7: PET Accelerator Vacuum Schematic*

Gas flow in the molecular flow regime between two points at steady state
is given as:

q=C(p2 - p1)

where C= conductance, l/sec. A steady state one-dimensional thermal
conduction network satisfying Fourier\'s law of Q=- (kA) dT/dx where Q
is heat, k is thermal conductivity, A is cross-sectional area, T is
temperature, and x is linear path length (L), is as follows :

![](media/image54.wmf){width="3.7354166666666666in"
height="0.9298611111111111in"}

A thermal nodal network such as depicted above is easily modeled using
finite element methods. An analogous molecular flow vacuum system may be
similarly modeled using the following relations:

[Thermal Network]{.underline} [Analogous Gas flow Network]{.underline}

Q=∆T/Rt q=C∆p

Q (heat flow) q (gas throughput)

∆ T (temperature) ∆ p (pressure)

Thermal Resistance (L/kA) 1/C (set C=k; L,A=1)

Pumps are effectively modeled by setting the element value of
conductance equal to the pump speed and the terminating node boundary
condition temperature equal to 0. The system has been modeled for both a
prototype charge stripper used in the 1 MeV test beam and the production
charge stripper. The theoretical and measured values at 4 locations
(212.5 MHz RFQ, charge stripper scavenge, charge stripper top sections,
and MEBT entrance) are presented as follows in Figure 12-8, where an
enlarged view of the gas jet nozzle and stripping region are depicted.
The numbered points in Figure 12-8 represent node numbers used in the
ANSYS® model. Note that the 1 MeV tests were conducted at a rep rate of
60 Hz. Pressures at similar points in the system resulting from 360 Hz
operation of the production charge stripper are found in Table 12-9.

Table 12-9 lists the design pressure goal in various portions of the
accelerator as well as the associated gas loads from both surface
outgassing and point gas loads introduced into the vacuum enclosure. The
nominal operational pressure values are typical ranges and are dependent
upon accelerator rep rate (higher pressures are associated with 360 Hz
operation) and source and charge stripper gas loading conditions.

Examination of Table 12-9 indicates that the majority of gas load
results from charge stripper operation at 360 Hz rep rate (total gas
into system is 13 Torr-l/sec average, 150 T-l/s peak). Of the 13
Torr-l/sec that enters the charge stripper, over 95% of the gas is
directed and removed by the scavenge section of the cell. The nominal
operational pressures are within the general bounds of design/analysis
values. The 360 Hz operational pressures slightly exceed design goals
(i.e., ion source and charge stripper top section) but have not proven
to be detrimental to accelerator operations.

**Review of Vacuum System Problems**

The only examples of hardware failure relative to vacuum have been
associated with the full range gauges and the HEBT turbomolecular pump.
An additional problem arose in the 425 MHz RFQ coupling loop vacuum seal
when first commissioning the accelerator at 360 Hz rep rate.

Five examples of gauge failure have occurred: one associated with the
HEBT as a result of radiochemistry window failure leading to gauge
contamination from Havar window debris, two on the 425 MHz A RFQ, and
two as a result of operating in the higher pressure region of the charge
stripper. In all cases, the remedy was a simple field cleaning of the
cold cathode portion of the gauge (a 30 minute operation) and returning
the gauge to service. Gauge replacement in the event of a window failure
may prove to be a standard operational condition. In such instances, a
spare gauge may be installed at the expense of approximately 5 minutes
time. The gauge associated with the window failure could then be cleaned
off line.

![](media/image55.wmf){width="6.75in" height="6.638888888888889in"}

*Figure 12-8: Enlarged View of Charge Stripper Interaction Region and
Theoretical vs. Measured Pressure*

*Table 12-9: Comparison of Design vs. Operational Pressure*

+-----------------+-----------------+-----------------+-----------------+
| [Accelerator    | Design Pressure | Nominal         | Gas Loading     |
| Locati          |                 | Operational     |                 |
| on]{.underline} | [(Tor           | Pressure        | [(Torr-l/se     |
|                 | r)]{.underline} |                 | c)]{.underline} |
|                 |                 | [(Tor           |                 |
|                 |                 | r)]{.underline} |                 |
+-----------------+-----------------+-----------------+-----------------+
| Ion Source/LEBT | 1x10-6          | 1x10-6 to       | 2.5x10-2        |
|                 |                 | 5x10-6          |                 |
+-----------------+-----------------+-----------------+-----------------+
| 212.5 MHz RFQ   | \< 2x10-6       | 7x10-7 to       | 6.55x10-4       |
|                 |                 | 1x10-6          |                 |
+-----------------+-----------------+-----------------+-----------------+
| Charge Stripper | 5.5x 10-3       | 3x10-3 to       | 12.9            |
| Scavenge        | (max.)          | 5.5x10-3        |                 |
+-----------------+-----------------+-----------------+-----------------+
| Charge Stripper | 1x10-5          | 5x10-5 to       | 0.065           |
| Top             |                 | 1x10-4          |                 |
+-----------------+-----------------+-----------------+-----------------+
| MEBT            | 1x10-4 (max.)   | 2x10-5 max.     | 2.88x10-3       |
+-----------------+-----------------+-----------------+-----------------+
| 425 RFQs        | \< 2x10-6       | 7x10-7 to       | 4.76x10-5       |
|                 |                 | 1x10-6          |                 |
+-----------------+-----------------+-----------------+-----------------+
| HEBT            | 10-5(max.) -    | 3x10-6 (No gas  | 2.63x10-5       |
|                 | 10-6            | loading         |                 |
|                 |                 | conditions from |                 |
|                 |                 | radiochemistry  |                 |
|                 |                 | targets)        |                 |
+-----------------+-----------------+-----------------+-----------------+

The HEBT turbomolecular pump failure occurred most likely as a result of
contact with fractured Havar foil window chards. The pump is topside
mounted and is protected with a fine splinter shield and a gate valve
that closes in the event of fault conditions. This may prove to be
inadequate protection in the event of window failure. More operational
experience is required to confirm or deny this issue. A possible
solution to recurring turbomolecular pump failure in the HEBT would be
to use a small cryo pump as an alternative to the turbo pump. An
additional benefit to installing a cryo pump to replace the turbo is
that a larger pump could be selected to control the higher HEBT
pressures associated with real radiochemistry targets.

The original RF coupling loops which provide input power to each of the
RFQs were designed by the 1992 project team. They utilize a disc-like
rexolite window and viton o-rings to provide the vacuum isolation. The
original windows seal on the center conductor of the loop using a piston
seal design with the o-ring groove machined in the window. The window
seals the outer conductor utilizing a crush type seal. Operation using
the original drive loops was satisfactory at the lower duty cycles
(e.g., 120 Hz or less) run during the early commissioning phases of this
project. However, once the RFQs were operated at 360 Hz vacuum leak
failures were occurring at the inner conductor seal. Modeling showed
that the RF power deposited in the viton o-ring was significant enough
to cause the seal to fail thermally. Alternate seal material with lower
loss tangent properties such as silicone were investigated for an
immediate solution. Analysis indicates that the dominant factor in the
power deposited in the o-rings is the location of the seal. Modeling
indicates that by transferring the seal gland from the rexolite window
to the inner conductor, the RF power deposited in the seal is greatly
reduced. Figure 12-10 shows the original design and the new coupling
loop configuration. New center conductors and rexolite windows were
fabricated and installed. Operations at 360 Hz have been conducted for
months with no o-ring failures or evidence of excessive heating.

![](media/image56.wmf){width="5.94375in" height="4.138194444444444in"}

*Figure 12-10: RF Coupling Loop Redesign*

## 13. Pet Water Systems

## Introduction

The cooling of the PET accelerator components is primarily accomplished
with a water system composed of three major components:

1\) Low Conductivity Water (LCW) at 90 F provided by the LCW skid.

2\) Clean filtered water at 25 C provided by individual NESLAB portable
chillers.

3\) Glycol-water mixture at 45 F provided by chilled water skid and
air-cooled chiller.

![](media/image57.wmf){width="5.991666666666666in"
height="4.595138888888889in"}

*Figure 13.1 Basic layout of the various systems with respect to the
accelerator.*

![](media/image58.wmf){width="6.027777777777778in"
height="7.875in"}*Figure 13.2 Schematic of the LCW and chilled water
systems.*

![](media/image59.wmf){width="5.875in" height="6.166666666666667in"}

## Figure 13.3 Water system specifications

All three of the components are closed loop systems with plate heat
exchangers supplying the interface where the heat transfer occurs. The
LCW is used by the ion source, solenoid, alpha magnets, water cooled
quads (Q1, Q2, Q4, Q5), HEBT magnets and diagnostics, and RF power
amplifiers. Four individual NESLAB portable chiller units supply cooling
to each of the RFQs. The chilled water is a 50/50 glycol-water closed
loop system used by the LCW and NESLAB systems to remove the heat to an
outdoor air-cooled chiller. Figure 13.3 shows the heat load on the
cooling system from the accelerator components.

Each of these components will be further described in the following
sections.

## LCW Skid

The LCW skid provides cooling water to the accelerator systems mentioned
above. Components are mounted to a rigid frame such that a minimum
amount of space is occupied and the skid can be relocated via a fork
lift truck when necessary.

A 20 hp pump is used to pump approximately 60 gallons per minute (gpm)
at a supply pressure of 110 pound per square inch (psi) and a return
pressure of 10 psi. Flow is measured with a paddle wheel type flow meter
on the return side of the piping system. Pressure gages, thermocouples,
and resistivity probes are installed at key locations throughout the
system to monitor the performance of the system.

The conductivity is maintained at 5 Mohm-cm. This is accomplished via a
normally closed solenoid valve on a polishing loop with a portable
deionizer bottle and filter. When the conductivity of the water leaving
the LCW skid reaches a level below 5 megaohm-cm, the solenoid valve
opens and allows approximately 5 gpm of the system to flow through the
polishing loop. The valve closes once the conductivity reaches
approximately 5.25 megaohm-cm. A second conductivity probe is installed
in the polishing loop to periodically check the condition of the
deionizer bottle. The conductivity meter has the capability to switch
readings from one probe to another.

The temperature of the supply water is maintained by use of a three way
valve with an electric actuator. A temperature controller monitors a
thermocouple readback at the supply header leaving the skid. The three
way valve is proportionally adjusted to direct flow either through or to
by-pass the heat exchanger based on this temperature. The desired
temperature is set at 90 F on the controller.

An eighty gallon expansion tank is situated on the low pressure side of
the pump to provide a constant suction head to the pump, allow for
thermal expansion of the water, and act as a reserve supply of water in
case of leaks. A level indicator is mounted on the side of the tank with
switches. One switch is used for high level indication and one is used
for low level indication. These two switches are used in the interlock
system. An intermediate switch is used to activate an audible alarm if
the level of the tank falls below the set level. Interlocks are not
affected by this intermediate switch.

## RFQ Water System

Individual NESLAB portable chillers are used to cool the RFQs.
Approximately 4 gpm is supplied at a 60 psi supply pressure. Individual
flows for the vanes and housing of each RFQ are monitored via rotameter
type flow meters. A throttle valve is available to adjust the flows.

Distilled water is initially used and strainers along with string wound
filters keep the circulating water clean. The temperature of the supply
water is set to 25 C via the control panel on the front of the chiller
units.

## Chilled Water System

The chilled water system provides the cooling water for both the LCW
skid and the four NESLAB units. A 50/50 glycol-water mixture is used to
prevent freezing in the pipes running to the outdoor chiller. All
components are mounted to a rigid frame separate from the LCW skid.

A 7.5 hp pump is used to pump approximately 100 gpm at a supply pressure
of 60 psi and with a return pressure of 5 psi. Flow is measured with a
paddle wheel type flow meter on the discharge side of the pump. Pressure
gauges and thermocouples are installed at key locations throughout the
system.

After the water/glycol is pumped to the LCW skid and to the NESLAB
chillers for heat removal via their heat exchangers, it is routed to an
outdoor air-cooled chiller. This chiller is capable of handling 45 tons
of refrigeration. The controllers within the outdoor chiller maintain
the water at approximately 55 F.

An eighty gallon expansion tank is situated on the low pressure side of
the pump and provides the same function and interlock capability as the
LCW system described earlier.

## Water System Protection Interlocks

*Shuts down LCW skid:*

LCW flow too high or low - major leak may have occurred

Chilled water flow too low - insufficient cooling will be available

LCW supply temperature too high - problems with chilled water or control
system

LCW expansion tank level too low - leaks in system

LCW expansion tank level too high - overfill of system

*Shuts down Chiller skid*

Chilled water flow too high or low - major leak may have occurred

Chilled water temperature too high - problems with air cooled chiller or
control system

Chilled water expansion tank level too low - leaks in system

Chilled water expansion tank too high - overfill of system

## Performance Characteristics and Concerns

The PET water system has performed fairly well once a few early
operational problems were fixed. It is not expected to have any future
problems except for the usual maintenance issues concerning components
that simply wear out. The problems encountered and the fixes
incorporated to date are as follows:

*Problem:* Temperature controller not regulating the temperature of the
LCW correctly. This would cause the temperature of the water to rise
beyond the high level trip point for the interlocks and the system would
shut down. The auto tune feature of the controller was not setting the
parameters correctly and when the accelerator was operational, the
controller did not respond fast enough to cool the water sufficiently.

*Solution:* Parameters for controller were set manually.

*Problem:* Chilled water flow meter broke. This was probably due to a
defective shaft on the paddle wheel. The flow meter had been in use for
approximately 6 months. The flow meter is also positioned at the
discharge of the pump where considerable turbulence is expected. The
original design had the flow meter situated on the return side, but due
to space limitations, the paddle sensor was installed on the discharge
side.

*Solution:* Flow meter was replaced with spare. Relocation of flow meter
will be considered if another failure occurs in near future.

*Problem:* NESLAB portable chillers' pump shafts wore out and little or
no flow was experienced. Three of the units in operation were part of
the original SDIO project and had a considerable number of hours of
operation. This problem was due to typical wear and tear on the units.

*Solution:* Chillers were refurbished with new pumps, motors, and
shafts.

# 14. Control System Hardware

# Overview

The control system for the PET project consists of a networked group of
Internet Rack Monitors (IRMs), connected to an Ethernet network.
Consoles attached to the network can request data and make settings to
parameters that are read and controlled by the IRMs. Because the PET
accelerator was intended to be relocated to Shreveport, Louisiana, the
control system does not rely on the traditional Fermilab ACNET system.
Software in the IRM causes it to read input data, maintain a data base
of current values, return data requested by remote requesters via the
network, monitor the analog and digital input data so as to alarm on
out-of-tolerance analog values and digital data found to be in the wrong
state. Messages are issued on the network to report alarm conditions.
Because IRMs are Internet nodes, it is possible to access them from
anywhere on Internet.

# Internet Rack Monitors

The IRM is a chassis that serves as a small stand-alone control system.
Each IRM contains a Motorola MVME-162 computer card and the interface
for 64 analog input channels, 8 bytes of digital I/O, 8 analog output
channels. The I/O capability may be expanded by adding IndustryPack
Modules and VMEbus interface cards. A complete description of IRMs is
given in *www-linac.fnal.gov/irm/.* IRMs for this project were purchased
from Bi-Ra Corp. of Albuquerque, NM.

Nine IRMs are used for the PET project; one for each of the four RF
systems and one IRM each for power supplies, the ion source, vacuum/PLC,
Low Level RF, and Target Diagnostics. One VMEbus crate containing the
same MVME-162 CPU running the same software, controls the accelerator
Repetition Rate Generator, the Tevatron-style event clock generator, and
several delay timers.

# IRM Software system

Software in the PET IRMs is the same as that described in
*www-linac.fnal.gov/irm/.* For the PET project, Local Applications were
added to communicate with PLCs supplied by *PLC-Direct ,* and to operate
PID control loops.

# Network

All front end computers of the PET control system are connected to an
Ethernet network. Locally, each node connects to a high speed switch
that in turn connects via fiber optic cable to an Ethernet switch in the
Cross Gallery. This arrangement provides connectivity to the PET control
system from Internet.

# Consoles and Applications

A variety of operator consoles are used to operate the PET accelerator.
These include Macintoshes, PCs, and Sun workstations. Application
programs written for PET are described in the control software chapter
of this PET Report.

**Goals and Performance**

The goal of the control system was to provide a system that assists the
operators in commissioning and operating the PET facility. We believe
the goal has been met. The system outlined here, combined with the
control system application software described elsewhere, has been in
daily use and has operated reliably for the past several months. This is
partly due to the fact that front end hardware and software is the same
as that used in other parts of the Lab such as the Linac, DZero
detector, and the Booster high level RF, and the A0 Photoinjector.

**15. Control Software**

**Software Development Issues**

The control software for the 10.5 MeV PET isotope accelerator must
facilitate the following aspects of operating the machine:

> • The reliable operation of the system within equipment
> specifications,
>
> • The day-to-day operation of the machine by the SAIC operations
> personnel and
>
> • The day-to-day experiments conducted by experimenters.

To accomplish these goals, software meeting the unique needs of this
machine has been written or adapted at all levels of the control system.
Imbedded systems software to ensure that equipment operates within its
specifications, user-level application software to automate various
operating procedures, display software to tune, observe and diagnose the
operation of the machine, software to continually collect data , and
alarms both for archiving and for diagnostics are part of the package.
Finally, a World Wide Web interface to the documentation and selected
machine data has been constructed.

Minimizing the amount of newly developed software was an important
consideration in the selection of a control system. Use of off-the-shelf
hardware and software was essential to provide a serviceable control
system within time and budget constraints. To this end the control
system is based upon commercially available hardware and off-the-shelf
software components. Newly developed software is written using efficient
high level programming tools including TCL/TK and LabVIEW.

The commercial control system hardware consists of nine distributed
Internet Rack Monitors (IRM) from Bi-Ra Systems, two Programmable Logic
Controllers (PLC) from PLC Direct, a SPARC-5 workstation from Sun
Microsystems and a Macintosh desktop computer from Apple Computer. The
IRMs provide robust real-time analog and digital data acquisition and
local control system functionality at the equipment level. All controls
information is gathered through queries of the IRMs. The PLCs are slaved
to IRMs and mimic conventional hard-wired interlock logic for operating
vacuum, water, DC power and conventional interlock systems. The Sun
workstation supports high level non real-time control system functions
and the Macintosh provides the primary control console for the machine.
The control system hardware architecture is described in detail in the
Control System Hardware chapter 14.

The off-the-shelf control system software consists of programs for the
IRM, Sun and Macintosh computers that was previously developed for the
Fermilab Linac control system. The Linac software provides a fully
operational data acquisition and low level control system. Thus
subsystem development and commissioning is supported essentially from
the moment the control system hardware is available. The Linac control
system software is described in the World Wide Web based documentation
located at
http://www-linac.fnal.gov/linac_controls/software/software.html

Software for the control system is generally classified as
\"application\" or as \"embedded\" level code. Application software runs
on the Sun and Apple computers and adds high level functionality to the
basic IRM based system. Embedded software runs in the IRMs to extend
their low level real-time control capabilities or to meet special
machine requirements. Newly developed application and embedded software
is described in the sections below; the PLC ladder logic software is
described in the Interlocks chapter 9.

**Application Software**

The off-the-shelf control system application software consists of
various maintenance and utility programs and run-time libraries which
run on the Sun workstation, and a \"Parameter Page\" and plotting
program which runs on the Macintosh. These programs provide basic
control system maintenance tools and a human interface which supports
direct access to control system functions and machine data and control
points.

Higher level control system functions such as alarm display, data
logging and archiving and parameter save/restore were developed for the
project. A human interface program was developed to supplement the
parameter page data display by presenting machine status and control
point information in a more intuitive graphical manner. Additional
software was developed to access logged machine data, operational status
and project documentation via the World Wide Web.

*Alarm Display*

A pair of very simple programs on the Sun workstation are used to
monitor the alarms information for the PET control system. One program
listens to the Internet protocol multicast channel that contains the
alarm information generated by the IRMs. It writes files on the local
disk representing all devices currently in alarm state. The second
program simply reads this file at a regular interval and displays the
alarm information. The first program is written in the C, the second is
written in the TCL/TK scripting language.

The IRMs determine that an alarm state exists as follows. For each
device that has alarms enabled the present reading is checked against
that device's nominal and tolerance. If it is outside of the range
"nominal ± tolerance", then an alarm message is generated. No further
alarm messages are generated for this device until the reading goes
within the range \"nominal ± 0.5 \* tolerance\". At this point an "alarm
has cleared" message is generated. A tolerance of zero is valid, but
only occasionally useful.

*Data Logging and Archiving*

There are two web documents that describe the data logging software. The
most detailed is
http://www-linac.fnal.gov/\~mccrory/memos/PET_datalogging/datalogging.html.

Another document containing slides for an introductory data logger
presentation is at
http://www-linac.fnal.gov/\~mccrory/memos/datalog-talk.

Briefly, the data logger system is a suite of programs and UNIX system
directives that collects and maintains archival information on the
devices in the IRM-based control system. The suite is composed of
programs that control data collection, data storage, data display and
system integrity checks. The logging system is kept in the petcon2 file
directory /home/pet/data-logging. The logged data are stored in the
subdirectory /data, the programs are stored in the subdirectory /scripts
and database files that index the logged data are stored in the main
data logging directory.

Data are collected in multiple, parallel UNIX processes. There are two
basic schemes for logging. The first scheme collects and archives data
on all the devices in the control system at a very slow rate: once per
two hours. It takes five separate processes to collect data from all
devices for this archive. In the second scheme the user specifies any
number of devices to be logged at any rate up to and including 1 Hz. The
user can also specify the data collection period making
experiment-specific logging possible. A unique process is created for
each specified set of devices and one UNIX file contains the data for
each logging process, be it an archive process or a user-defined
process.

A pair of databases is maintained that allows access to the data for a
specified device. The first database uses the device name as a key and
returns the number of files containing data for the specified device.
The second database uses the device name and the index number obtained
from the first database as keys (e.g., DEVICE_0 for the first place
where data are stored for DEVICE). A database access returns the full
filename of the file and the column number within the file that contains
the requested data. Many programs exist that manipulate these databases.
The databases are regenerated twice per day to ensure that all data are
readily available to the user. The data files are backed up to tape on a
regular schedule, currently, twice a week.

Logged data can be displayed on an X terminal screen using a TCL/TK
program. Among other things, the user can specify the frequency of the
data that s/he wants plotted. All data for the device(s) is read,
including archive data, and the points closest to the specified
frequency are plotted. Up to eight devices may be plotted
simultaneously.

UNIX jobs which check the integrity and timeliness of data storage are
started at regular intervals via the UNIX cron command to ensure that
the data collection is proceeding normally. These "cron jobs\" can
restart a user's data log collection process if it is determined that
collection has stopped prematurely. Another cron job is used to make
sure that the data in each binary data file is readable.

*Save/Restore*

A program is provided on the Sun workstation that allows the user to
save the set points for devices in the control system. These set points
are:

> • the analog device setting (D/A value, if appropriate),
>
> • the nominal and tolerance values for the alarms on this device and
>
> • the alarms flags.

This information is stored in a file on the local disk in ASCII format.
The user may specify a list of devices to be saved (e.g., the MEBT
magnets), or may request that all devices be saved. The user may at any
time restore the set points for a list of devices from any file.

The main interactive program is a TCL/TK script which launches one of
several C++ programs to perform the requested task. As mentioned above,
there are two types of files maintained here: the list of devices that
the user has made for her/his own purposes and the saved set points.
These files are kept in separate directories in the petcon2 directory
/home/pet/save-restore: devices and savefiles.

**Graphical Human Interface**

A family of graphical display programs developed with LabVIEW implements
the primary machine console interface. LabVIEW was chosen as the
programming language because it supports the required graphical display
types and it is available on the Macintosh and Sun computers thereby
allowing the software to run on either computer without modification.
The graphical programs provide an intuitive display of machine status
and easy to use control fields for adjustable parameters. The human
interface is based upon a model where machine elements are displayed on
the screen in a schematic or a block diagram format, and data points are
displayed using graphical indicators such as lights, switches and
meters. For adjustable parameters these graphical indicators may be
manipulated by the operator using the mouse and keyboard.

The human interface software consists of several \"Subsystem\" programs
each addressing a particular support subsystem of the machine, a
\"Status and Control\" program which displays machine status broken down
by accelerator stage and several special purpose \"Application\"
programs which support machine commissioning.

The \"Subsystem\" programs are oriented toward the requirements of the
engineering staff and display status of the various machine subsystems.
Seven programs cover:

• Control System,

• DC Power System,

• Instrumentation System,

• RF Systems:

\- High Level RF

\- Low Level RF

\- Resonance/Frequency Control,

• Timing System,

• Vacuum System and

• Water System.

A typical subsystem program (Fig. 15.1) provides a bird\'s-eye view of
the status of an entire machine subsystem.

**Vacuum Subsystem Program**

![](media/image60.wmf){width="5.416666666666667in"
height="4.277777777777778in"}

*Figure 15.1 Vacuum subsystem LabView Display.*

The \"Status and Control\" program is hierarchical with each level
displaying more detailed views of the status of the various accelerator
stages. The highest level (Fig. 15.2) displays the overall machine
status using simple numeric, digital and status summary indicators.
Clicking on a status summary indicator causes a detailed view of the
associated machine stage to pop-up. The ten machine stages which can be
displayed include:

• Ion Source,

• LEBT,

• 212.5 RFQ,

• Stripper,

• MEBT,

• 425-A RFQ,

• 425-B RFQ,

• 425-C RFQ,

• HEBT and

• Target.

**Status & Control Program**

![](media/image61.wmf){width="6.388888888888889in"
height="3.4305555555555554in"}

*Figure 15.2 Status and Control LabView Display.*

A typical stage pop-up (Figure 15.3) contains detailed status for all
related critical machine data and may optionally allow control of
related parameters.

An additional feature of the stage pop-ups is that they may contain
links to the various subsystem programs so that the operator can trace
an error indication directly to the related support subsystem.

The \"Application\" programs perform specialized functions in support of
machine commissioning or operations. These programs typically automate
some measurement or process which would otherwise be tedious or time
consuming. Four such programs have been developed including:

• PLC Diagnostics,

• Collimator Control,

• Automated Parameter Sweep and

• Radiochemistry Status and Control.

A typical application program (Fig. 15.4) has the visual appearance of
the front panel of an instrument that one could imagine designing to
make the related measurement or to control the related process.

**212.5 RFQ Stage Status Display**

![](media/image62.wmf){width="4.388888888888889in" height="2.94375in"}

*Figure 15.3 RFQ subsystem LabView Display.*

**WWW Interface**

There are two relevant web home pages available on the PET project:
http://www-linac.fnal.gov/pet and http://petcon2.fnal.gov. The first
provides access to PET project resources, including a description of the
project, detailed project documentation, links to collaborators' web
pages, links to other relevant information and an executive summary of
the PET web pages. The second URL points to the Sun/UNIX computer
situated in the PET control room and provides access to selected on-line
data.

Data logger data are available on-line at
http://petcon2.fnal.gov/datalog.html and
http://petcon2.fnal.gov/dump_datafile.html. These two web pages,
respectively, allow one to see archived data for a single device for a
specified time period, or to see all of the data from a single data
logger data file.

A list of all the analog devices defined in the PET control system is
maintained and available on the web. The lists are updated several times
per day, so the list is almost always up to date. They are organized by
the IRM "node number." These lists are accessed from the web page:
http://petcon2.fnal.gov.

**Embedded Software**

The IRMs execute off-the-shelf generic data acquisition, alarm and
control scans and make the associated data available to clients such as
the Sun and Apple computers through the local area network. They also
support customized \"local application\" programs which may be used to
perform feedback control of the equipment. Five such local applications
were developed or adapted from Linac software for the project.

**Collimator Control**
![](media/image63.wmf){width="5.930555555555555in"
height="5.652777777777778in"}

*Figure 15.4 Collimator control subsystem LabView Display.*

*LLRF Interface Control*

The DSP based RF equipment described in the \'Low Level RF\' chapter
resides in the LLRF IRM. The DSP performs all LLRF functions
autonomously in real-time but is configured by two IRM local
applications programs which make DSP data and parameters available to
the basic control system. Local application LOOPLODR downloads the DSP
software into the VXI modules while LOOPLLRF manages the transfer of
LLRF data and parameters between the control system and the DSP. Two
copies of each program, one for each LLRF module, are continuously
running in the IRM.

At power-up LOOPLODR downloads the DSP program into its associated LLRF
module, sends a signal to LOOPLLRF indicating the download has completed
successfully and then loops waiting for another download request. In
this way a download of new DSP code may be accomplished without
disturbing the operation of the host IRM node. Copies of the latest DSP
object code files are stored within the IRM local file system. Updating
the DSP program involves transferring the object code from the DSP
development environment into the IRM\'s file system and then restarting
LOOPLODR to load the code into the DSP\'s memory.

LOOPLLRF waits for LOOPLODR's message indicating that the DSP has been
loaded successfully. At this point LOOPLLRF begins regular 10 Hz
execution of a parameter update routine, a data read routine, a LLRF
amplitude control routine and an error detection routine. The parameter
update routine transfers LLRF parameter settings from IRM channels into
DSP control structures. In the process of this transfer values are
converted from the IRM scaled integer type to real values in engineering
units and, if necessary, compared against reasonable bounds. The data
read routine transfers DSP generated RF values from the DSP structures
into IRM channels and into waveform memory where it can be accessed by
the control system. In the process of this transfer values are converted
from real engineering units into the IRM scaled integer type. The
amplitude control routine monitors the LLRF spark detection data and
adjusts the RF amplitude as sparking in the RFQs increases or decreases.
If excessive sparking occurs the RF level is set to zero and an error is
signaled. Finally the error detection routine checks for RF system
errors and causes a shutdown if required. System status conditions that
are monitored include:

• LLRF On/Off control,

• RFQ Excessive Sparking,

• High Level RF Operational,

• RFQ Tuning System Operational,

• Vacuum Below Threshold,

• Software Errors and

• Communications Errors.

Any error can cause an LLRF shutdown. Configuration masking bits are
provided to individually enable/disable each status condition from the
scan.

*PLC Status And Control Communications*

Communication of PLC data for PET is via an RS-232 connection running
from the IRM to the PLC. Two nodes of the nine have such interfaces to
PLC controllers. An area of memory in the PLC, about 50 words in size,
is interrogated by the IRM every two or three 10 Hz cycles, using the
standard DirectNET protocol. This data represents a data pool of analog
and digital data words that are mapped into the IRM data pool for access
by host computers. When a control action is to be performed, whether
analog or digital, a part of this memory is modified; the PLC acts on
the change to control the appropriate hardware device. Only a single
control action may be performed between successive data acquisitions;
any additional control actions are queued. The communication logic is
contained within the local application named LOOPDNET that runs in the
IRM.

*Power Supply Control*

While the PLC monitors the digital logic of the power supplies, watching
for interlock trips and other digital status, it can monitor neither
over currents nor ground faults. Support software for the power supplies
provides monitoring for over currents and ground faults, because the PLC
does not have analog interfaces to the power supplies. The digitized
readings of the power supply currents and ground fault currents,
however, are interfaced directly to the IRM via its own A/D controller.
The LOOPPETP local application performs the necessary checks every 10 Hz
cycle on over currents for 22 supplies, and it checks on ground faults
for 8 supplies. Based upon what it finds, it maintains a status bit for
each of the 30 tests that is passed to the PLC controller, thus enabling
the PLC to be aware of it. Node 587 operates this logic, including the
heartbeat protection described in the next paragraph. The time required
for all of the analog checking logic is about 1.5 ms each 10 Hz cycle.

A heartbeat scheme is used to assure the PLC that the analog checking
described above is operating so that the PLC can rely upon its digital
status output. The PLC controls a bit that is part of the digital data
periodically collected by the IRM, toggling its state every half second.
The heartbeat logic watches these bit transitions and reflects the
current state to a control line that is wired directly to a PLC digital
input. As long as this line is seen by the PLC to be toggling regularly,
the PLC can be assured that the IRM is working, that the LOOPDNET data
acquisition is operational, and that the LOOPPETP analog checking logic
described above is being performed. This heartbeat logic is used in node
587. Two additional heartbeats are monitored in node 586, although there
is no associated analog checking being done in that node, so that the
PLC in that case need only be assured that the IRM is working and that
LOOPDNET communications is operational.

*Power Supply Regulation*

A new piece of imbedded system software is the PID control loops, which
are used to regulate the MEBT magnets and the RF system frequency and
temperature. This software is described and documented on the web page
at
http://www-linac.fnal.gov/\~McCrory/memos/PID_description/PID_description.html.
Briefly, PID stands for "Proportional, Integral and Differential." These
quantities are calculated from an "error signal," in the appropriate
way. A PID loop is a feedback loop, the output of the feedback loop is
proportional to expert-defined coefficients times each of the three
pieces, P, I and D.

The critical magnets in the HEBT are regulated by the IRM using the PID
control loop called LOOPPIDM. The magnets being regulated with this PID
loop are:

> 1\. ALPHA, the Alpha magnet pair
>
> 2\. MEQ1, the first quad in the MEBT
>
> 3\. MEQ2, the second quad in the MEBT
>
> 4\. MEQ4, the fourth quad in the MEBT
>
> 5\. MEQ5, the fifth quad in the MEBT
>
> 6\. HEQ1, the first quad in the HEBT
>
> 7\. HEMP, the multipole magnet in the HEBT.

The effect of this loop is to increase the overall regulation of these
magnets to close to 0.1%. Mostly, this loop removes long-term drifts in
the current through the magnet.

*RF / Temperature Control*

All four of the RF cavities are controlled in the steady state and
during transients by a suite of five PID loops called LOOPPIDC. These
loops are variants of the PID loops described in the Power Supply
Regulation section above. Additionally, there is a control loop running
in the IRM that orchestrates when each loop runs and whether or not the
RF drive comes from the line or from a local voltage-controlled
oscillator; this loop is called LOOPOVCO. These five PID loops are
described in the \'RF Tuning Systems\' chapter and are referred to as
the:

> 1\. Average power loop,
>
> 2\. VCO Resonance control loop,
>
> 3\. VCO control loop,
>
> 4\. Temperature control loop and the
>
> 5\. Master resonance control loop.
>
> ![](media/image64.wmf){width="0.5833333333333334in"
> height="0.5138888888888888in"}FERMILAB

August 30, 1996

3He++ RFQ PET Isotope Production Project

BRF, Fermilab, SAIC, University of Washington

Phase 1 Accelerator Performance Parameters

> **Introduction**
>
> This document defines performance parameters of the 3He++ accelerator
> which are to be met and demonstrated at Fermilab before shipment of
> the machine to the Biomedical Research Institute (BRI) in Shreveport,
> Louisiana.
>
> **Beam Current**
>
> The accelerator shall be capable of delivering, through the HEBT, l50
> microamperes of l0.5 MeV 3He++ beam current (10 second, time averaged
> electrical current). The machine control system shall provide for
> adjustment of this average beam current in steps of 5% or smaller. It
> is expected that the nominal pulse current is fixed and that the
> average current is controlled by pulse gating. Pulse-to-pulse current
> fluctuations over a 30 minute interval shall not result in pulses at a
> current greater than 12O% of the nominal pulse current chosen for that
> run.
>
> **Beam Parameters Available at Chemistry Target**
>
> A beam size of 2.5 cm (horizontal) by 14 cm (vertical) shall be
> available at the target window. At that location, the beam current
> distribution over a 2 cm by 10 cm area shall be uniform to within
> +/-25% of the mean value in all 2 mm by 2 mm regions as determined by
> foil activation experiments. Neighboring regions within this area
> shall be uniform to +/-10%. Measurements shall be performed to
> demonstrate that the required uniformity is repeatable from run to
> run.
>
> **Beam Availability and Reliability**
>
> Beam availability and machine reliability shall be demonstrated by
> operation during 5 pre-scheduled 8 hour running periods within a two
> week time interval. operation shall be at a nominal l50 microampere
> electrical beam current on target or on Faraday Cup with less than 25%
> downtime (target/window failures excluded). During these periods the
> machine shall be operated by SAIC personnel who are expected to
> perform this function at BRI. Machine turn-on time of less than one
> hour from cold start (systems under vacuum; RF filaments hot; ion
> source, RFQ cavity water systems, and all magnet power supplies off)
> to stable beam on Faraday Cup shall be demonstrated. Availability of
> stable beam on the Faraday Cup in less than 30 minutes following a
> normal target vault access shall be demonstrated.
>
> revision date: 8/30/96
>
> revision: 2 printed: 9/4/96
>
> PAGE 1 oF 2

**Target Studies**

> Target and related studies shall be performed to confirm beam
> parameters at the target location and to assure that the HEBT, target,
> and shielding designs are acceptable for machine operation at BRI.
> Specific targetry goals at Fermilab include: 1) learn how to tune beam
> onto and extract heat from the target while maintaining target window
> integrity with gas cooling and with water cooling, 2) measure
> radiation dose rates, both beta/gamma and neutron, and evaluate
> shielding performance, 3) learn what isotope quantities will be
> produced by the beam, and 4) learn in what radiochemical form these
> isotopes will be produced and extracted from the targets.
>
> **Radiation Safety and Radiation Shielding Measurements**
>
> The goal is to operate the machine safely at Fermilab and to ensure
> that the machine, as delivered to and assembled at BRI, can be
> operated there in a manner consistent with all applicable regulations
> and safety codes. All necessary measures shall be taken to conform to
> Department of Energy (DOE) and Fermilab radiation shielding, exposure,
> and monitoring regulations while the machine operates at Fermilab. All
> necessary measures shall be taken to assure that the machine and
> target station design facilitates compliance with radiation safety
> regulations applicable at BRI. In particular, radiation dose and
> radiation shielding effectiveness measurements shall be performed at
> Fermilab to a degree sufficient to determine design modifications,
> administrative measures, and operational procedures necessary to
> satisfy requirements at BRI.
>
> **Electrical and Conventional Safety**
>
> Passive safety devices and active safety interlocks shall meet the
> requirements of applicable DOE and Fermilab standards and of safety
> codes applicable at BRI.
>
> Jerry Bida, Biomedical Research Foundation Ken Krohn, University of
> Washington

Ralph Pasquinelli, Fermilab Phil Young, SAIC

revision date: 8/3O/96 revision: 2

> PAGE 2 oF 2 panted: 9/4/96