#  April 10, 2000

TO: Phil Martin

FROM: John Marriner

SUBJECT: Charge to the Proton Driver Internal Technical Review Committee

Over the past year a team in the Beams Division has been working on the
design of a proton driver for Fermilab. The proton driver is an
accelerator complex intended to provide the demanding proton beam
capabilities required by physics programs based on intense cooled muon
beams, for example a neutrino factory based on a muon storage ring
and/or a muon collider. The system would also serve as a complete
functional replacement for the Fermilab Booster, providing upgraded
capabilities in the future for the programs that the Booster would
otherwise have served. New physics programs based on the stand-alone
capabilities of the proton driver, as an intense source of proton beams
would also be enabled.

For the past several months, design work has focused on an intense
16-GeV proton synchrotron, which in Phase I may be used to feed a
neutrino factory based on a muon storage ring. The design effort for
that Phase I synchrotron has reached the point where a first formal
technical review is deemed appropriate by Fermilab management. Decisions
have been taken about major parameters and approaches, and significant
progress has been made on the technical design of major subsystems, yet
there is still time for expert advice to generate \"midcourse
corrections\" and thereby have a significant positive effect on the
final design.

Therefore, the Technical Review Committee is requested to evaluate the
overall design, and in particular: to examine the appropriateness of the
major parameters and design choices that have been made; to assess the
progress of the design, identifying areas most in need of further design
work; to judge feasibility, identifying areas of highest technical risk
and assessing any countermeasures that are proposed to address those
risks; to advise on the content and scope of the R & D plan that will be
presented; and to comment on the organization of the collaboration that
has been formed to carry out the design work.

While undertaking this task, the focus of the committee should be on
technical design matters, with budgetary and scheduling issues expected
to arise only insofar as they may impact the technical design.

The committee is asked to please provide a written report of the results
of the review to the Fermilab Beams Division Head by May 31, 2000.

## **Review of the Proton Driver**

####### April 17-19, 2000

Reviewers: P. Martin (Chair), R. Webber, D. Finley, G. Krafczyk - FNAL;

## H. Schönauer - CERN, Y. Mori - KEK, A. Thiessen - LANL

#### General Overview

The Proton Driver presented at the review consists of a high-intensity,
16 GeV proton synchrotron. Injection is at 400 MeV from the present
Fermilab Linac, but the Cockcroft-Walton and the first tank of the Linac
are to be replaced by a new ion source, a pair of RFQs and a new Tank 1
with injection at 2.23 MeV.

The intensity requirements for the Proton Driver are high; initially
(Phase I) 3 x 10^13^ protons per cycle at 15 Hz. This is to be later
upgraded (Phase II) to 10^14^ protons per cycle; this review was limited
to the Phase I requirements, however. These high beam power levels will
require careful attention to control of beam loss. Bunch length at
extraction is also important, requiring careful control of the
longitudinal emittance.

The Proton Driver is intended to feed both the Main Injector and a
future target station to be used for a muon-storage ring neutrino
factory and/or a muon-collider. The Proton Driver's multiple roles are
not all clearly defined, especially with regard to its use as the proton
source for the Main Injector. For future reviews, the roles and
requirements for each role should be more clearly defined, with regard
to desired intensity and bunch structures in particular.

The Proton Driver design parameters are still in a state of flux. Very
recently the decision was made to increase the circumference to 711 m
(1.5 times the Booster), corresponding to h=18 for the 7.5 MHz rf
system. Neither lattice presented matched this requirement, although
both should be straightforward to modify to meet this criteria. This
change in circumference, was dictated by the choice of energy, 16 GeV,
combined with the limitation of 1.5 Tesla for the maximum magnetic
field. The larger circumference increases the voltage requirements for
the rf system(s). Another parameter chosen, which was not discussed in
the review, was the cycle time for the machine. Quite naturally, 15 Hz
was chosen, based on the current Linac and Booster. These choices have
made the demands on the rf system and voltage control at capture time in
particular, very challenging. In light of this, the committee recommends
further evaluation of the total cost of the major systems and an
estimate of the technical difficulty of achieving the desired goals vs.
all of these parameters: peak energy, (consider 12 to 15 GeV for
example) repetition rate (consider 12 Hz and 10 Hz, for example, even if
the physics output is reduced as a consequence), and circumference
(consider the range of harmonic numbers from 16 to 20, perhaps). This
evaluation does not need to be terribly detailed at the outset; a fairly
simple spreadsheet might be adequate. In addition to evaluating initial
construction costs for the various options, it might also be instructive
to look at the expected operating costs as well.

The proponents suggested an R&D program on ion sources and RFQs, as
discussed in more detail below. The committee notes that this R&D could
lead to improvements to the present Booster performance, and in turn,
improved physics capability for the Tevatron and for fixed target
experiments at 8 and 120 GeV. Fermilab management should keep the lab's
long-range schedule in mind when considering what level of resources
should be devoted to this R&D effort.

The committee notes that many of the participants in the Proton Driver
design effort are not assigned full time to this effort. In spite of
this limitation, a very significant amount has been accomplished, and
the committee congratulates the design team for their efforts. However,
the next step in the design will require a substantial increase in the
level of effort. Are the resources going to be available to carry out a
full conceptual design?

**Proton Driver Ion Source/Linac Requirements and Design**

*Proposal Summary* The proposal calls for using the existing 400 MeV
Linac in its present location. 4.5 x 10^14^ protons (H-) per second will
be delivered by accelerating beam at 15 Hz on the RF pulses already
available. The beam pulse length is stretched to 90 microseconds from
the present HEP pulse length of approximately 25 microseconds and the
peak beam pulse current is nearly doubled from 45 mA to 86 mA. The
existing RF systems are said to be able to accommodate the longer beam
pulse length without high power system modifications. The beam current
increase depends on two factors: higher ion source current and reduced
beam loss up to 2 MeV. An Ion Source development program is called upon
to raise the source current to 115 mA from the present 75 mA. The
reduction of beam loss is to be achieved by replacing the
Cockcroft-Walton, 750 keV transport line, and front end of Tank 1 with
an electrostatic low energy transport line feeding a two stage 2.23MeV
RFQ system that injects into a shortened Tank 1. The proposed RFQ system
includes a 1 MeV \"Larson\" beam transport line between two RFQ cavities
and allows the possibility of a redundant source/low energy RFQ
combination to assure beam availability. A beam chopper upstream of Tank
1 shapes the time structure of the beam to suit the required
distribution at injection in the Booster. This chopping lowers the
average beam macropulse current seen by the Linac RF systems to about 60
mA, two-thirds of the 86 mA peak current.

*Review Comments* The committee was presented no motivation for the
selection of 400 MeV as the final Linac beam energy. No technical data
was provided to support this choice from a 16 GeV Proton Driver system
design viewpoint. Certainly this information must be required in any
subsequent review of the project. This committee assumes that the choice
was driven by: a) the availability of the existing machine, b) a
perceived economic advantage, and c) a belief that the synchrotron could
be designed and built to handle this beam. Therefore, our comments on
the design are based on the premise that the Linac energy of 400 MeV is
a given.

Subject to the comments below, the consensus of the review committee is
that the design and parameters as presented for the Ion Source/Linac of
the Proton Driver are sensible, very economical from the Phase I
viewpoint, and have reasonable potential of being realized. There are
broader concerns based on two factors. First is the apparent lack of
technical contingency available to meet the 400 MeV proton requirements;
each part of the proposal pushes the envelope and all must work. There
is little wiggle room to meet delivery of the required beam rate. The
pulse length is limited by otherwise expensive RF modifications,
significant ion source improvements are already called for, and
radiation shielding issues are likely to prove more troublesome than
anticipated. Second is that this Phase I proposal pushes the existing
Linac to the limits without major equipment expenditures. There is no
apparent path to Phase II from here without starting over: new tunnel,
new accelerating structures, new RF power systems, etc.

Areas clearly in need of further work include the ion source improvement
R&D plan, the RFQ/transport system simulations, RFQ/transport R&D plan
and design, emittance propagation simulations from RFQ through 400 MeV,
and beam loss reduction plans.

The ion source presents an area of significant technical risk to the
project. The proposed design requires a combination of beam current,
pulse length, and emittance well beyond the state-of-the-art. The
presentations acknowledged the need for a significant R&D program to
address this demand, but there was no plan. The committee suggests that
this should be an area of highest R&D priority if the project is to be
pursued. Without an ion source, the entire Linac proposal falls flat.
Additionally, the achievable ion source parameters may impact RFQ system
design in a significant way.

While the committee questioned the emphasis on ion source redundancy and
the decision to use two RFQs with an intervening Larson transport vs. a
single 2.23 MeV RFQ, it was generally agreed that the design as proposed
was acceptable and even attractive to pursue. A risk is that performance
of the Larson transport design is unproven for H- beams and for 100 mA
beam currents. Further simulation and design work needs to be done in
order to model the acceleration and transport system from source through
matching into the modified Tank 1 and also to better define R&D
requirements and goals. An estimate of H- stripping in the Larson
transport should be made and compared to that from a pair of RFQs in a
straight configuration. A preliminary R&D plan was proposed to assemble
an ion source of the present design, LEBT, RFQ, and Larson transport
using an existing 750 keV RFQ and surplus components from the Fermilab
PET project. That plan calls for 1.5 physicists, 1 engineer, and 2
technicians for 18 months. The committee recommends that additional
simulation and design work be completed before this R&D begins. Then a
cost/benefit determination should be made to consider whether the R&D is
best pursued with the surplus components at 750 keV or with newly
procured components believed suitable for the \'real\' 1 MeV design. The
committee found this R&D to be attractive for reasons beyond the Proton
Driver project. If the suggested performance of the design is
demonstrated, it could have applications for numerous projects. This R&D
setup could benefit ion source development by providing a source testing
environment free of the constraints imposed by the Cockcroft-Walton. The
R&D could provide important information regarding potential cesium
contamination of the LEBT and even of the first RFQ. Nevertheless, in
the scope of the Proton Driver project, it is believed that the ion
source R&D should be placed at higher priority than the
acceleration/transport R&D.

It was reported that radiation safety issues presently limit Linac
accelerated beam to about 2 x 10^14^ per second with normal losses. This
is a factor of two less than the Proton Driver requirement. The proposed
solution to this problem depends on reduced beam loss made possible by a
smaller emittance beam. The new RFQ and modified Tank 1 front end are
expected to produce an emittance of 2.8π mm-mrad at 86 mA compared to
5.2π mm-mrad at 50 mA presently out of Tank 1. The committee is not
convinced that this front-end emittance will actually be achieved and
then preserved down the Linac. Present Linac performance numbers
identified an emittance growth from 5.2 out of Tank 1 to 7.8 at 400 MeV;
yet the emittance budget for the project design allowed for only minimal
growth (2.3 to 3.0) through that region. The committee questions if the
expected emittance can be achieved, and simulations of beam transport
through the Linac should be done to support the design assumptions. It
was also noted that \~2 x 10^13^ particles per second would be dumped at
400 MeV by chopping the lower quality front end of each Linac pulse
before transport to the synchrotron. The potential losses due to this
chopping should be considered and beam dump requirements for the chopped
beam should be reviewed. (The yearly limit for the two Linac dumps
combined is 1.28 x 10^21^; the chopping discussed above is about
one-half of the yearly limit, so things are reasonable from that
standpoint. Losses and other beam power issues still need to be
considered. In addition, the Safety Assessment Document hourly limit for
the 400 MeV dumps is 3.54 x 10^17^; the expected intensity for the
chopping under Phase I will be 7.2 x 10^16^, about 20% of the SAD
limit.)

The advisability of banking a critical part of a new project, still
several years down the road, on equipment already 30 years old (e.g.
7835 and other power tube concerns) should be considered seriously.

#### Proton Driver Lattice

The general features of the proton driver lattice are predominantly
driven by the intent to avoid crossing transition during the operation
of the synchrotron. While transition gamma jump systems are employed in
other accelerators to mitigate the effects of mismatch at transition,
these systems are generally complex and difficult to tune. The committee
concurs with the decision to avoid transition, and the complication of
these systems, due to the high intensities being sought with the Proton
Driver. It was noted, however, that the final choice of the transition
gamma must give rise to a slip factor which is compatible with the
longitudinal matching requirements between the proton driver and other
Fermilab machines such as the Main Injector and the Antiproton Source.

Two potential lattice designs were presented at the review. The first
one, designated the \"Ritson Lattice,\" is essentially a FODO lattice
with missing magnets which forces the dispersion function to be small
(and/or negative) within the dipole magnets thus making the momentum
compaction factor small or even negative. The second design, the
\"Johnstone Lattice,\" uses modules with low-beta inserts generating
rapid phase advance and performs the same feat. From the information
provided at the review it was difficult to see the relative merits of
the two systems. The Johnstone lattice appears to have the ability to
better compact the bending. Since both lattice designs are working to
the same circumference, the Johnstone lattice may have more useful
straight section space. It was also indicated that this lattice may have
better chromatic properties due to the placement of the sextupole
circuits.

Many questions remain to be answered concerning the Proton Driver
lattice design and the work cannot proceed until the top level
parameters are settled and a design philosophy is agreed upon. The
circumference of the accelerator appears to be in a state of flux. It
was recently decided that the circumference should be 50% larger than
the existing Booster circumference, due to B-field limitations, though
it was not clear as to the reasoning for this choice vs. a larger
circumference. Additionally, it could be questioned whether the choice
of final energy \-- 16 GeV \-- is optimal. The committee felt that this
is indicative of the fact that the project requires clearer direction
from the laboratory management as to what the functions of this
accelerator should be and what programs it should be designed to
support.

Once a general lattice is decided upon, it must be studied and iterated;
the work has not reached this stage yet. The lengths and uses of
straight sections must be decided upon early on. In particular, the
injection and extraction regions must be carefully designed especially
for these purposes. Space must be allotted in the lattice to allow for
the proper diagnostic equipment required for this high intensity machine
where beam quality, and beam losses, will be major issues.

The periodicity of the synchrotron must be carefully studied, especially
in the context of resonance strength reduction at injection where space
charge effects are strong. The number of magnet types and magnet
circuits must be minimized and correction circuits and tuning scenarios
must be developed.

Various standard sensitivity analyses must be performed, such as: 1)
orbit distortion / alignment tolerances 2) tune control / beta mismatch
(+/- 0.5 units of tune control, for instance) 3) chromaticity control /
chromatic properties (beta mismatch vs. dp/p; compaction factor vs.
dp/p; etc.) It was also noted that the complete acceleration cycle of
this accelerator \-- approximately 10^4^ revolutions \-- can be
simulated with a particle tracking code, including the effects of space
charge. The committee strongly recommends that this be pursued.

Fractional tunes of 0.25/0.3 or 0.25/0.8 have provided best loss
management in high-current machines; the tuning range should cover these
regions without degradation of lattice quality. Beyond the specified
'static' tune control of ±0.5 units, fast 'dynamic' tune variation of
\~0.2 units must be possible within the 15 ms of early acceleration.
Dynamic working point programming is essential in ISIS and in CERN
machines. With the lesser tune shift (once reduced) the benefit may be
less dramatic, but nevertheless it may be necessary to quickly remove
the beam from a perturbing stopband.

Overall, the layout and lattice design of the proton driver is not yet
at a mature stage. This situation would be helped by the settling of
high level system parameters and project scope.

**High Intensity Issues:**

Amongst the 'classical' high-intensity issues, space-charge effects and
limits to instabilities, only space charge seems to be of concern for
the phase I proton driver.

*Space Charge* The quoted tune shift values, between -0.36 and -0.42
(referring to Gaussian beams), are too high for a machine of this
intensity, even assuming major stopbands being compensated: Resonances
of order ≤ 3 should be avoided at all costs. Possible cures are:

\(i\) An efficient painting strategy improving the transverse
distribution. If the foreseen injection kicker pulse shapes can be
implemented, a certain reduction of the tune shift can be expected.

\(ii\) About 25% reduction in linear density and thus in tune shift can
be obtained with a second-harmonic RF system. As this is needed only in
the early phase of the acceleration cycle, a few tens of kV should
suffice and are even for free for the four-bunch 1.7 MHz system: some of
the Finemet cavities can be excited at twice the frequency. The
flattened bucket shape also facilitates a lossless RF capture, which is
one of the critical issues in all fast-cycling synchrotrons. In fact, a
scenario which cannot be made loss-free in ESME- or comparable
simulations should not be accepted.

\(iii\) Raising the injection energy; 400 MeV has been chosen as the
baseline design, although 1 GeV was presented as one possibility for
Phase II. Perhaps a higher injection energy should be considered further
even for Phase I.

###### Instabilities The number of 3 x 10^13^ protons in the ring is comparable to existing machines like AGS or the CERN PS which have the stability of their beams under control, although they cycle at much slower frequencies. No faster instabilities have been identified. Not addressed was the microwave stability prior to the final bunch rotation where the RF voltage is reduced. The stability against coupled-bunch oscillations for the 53 MHz and 7.5 MHz systems needs to be verified too; note that the latter presents the broadband impedance of 100 Finemet cavities.

An impressive amount of work has been done for the collimation system,
including the detailed structure of the utility insertion, and
impressive is also the predicted collimation efficiency of \>99%.
However, tolerances seem tight (some secondary collimators are only 0.5
mm recessed) and the performance for more realistic closed orbits up to
10 mm peak-to-peak amplitude, created by random perturbations, remains
to be assessed. In this context it should be noted that the ingeniously
managed coexistence of injection, extraction and collimation in one
straight section is prone to considerable operational and maintenance
problems. The committee strongly suggests a redesign of this area - one
more argument for the design of a new lattice. Also, the Phase-2
scenario (pre-booster or not, if not: which injection energy) ought to
be defined before the design of the injection area.

It may turn out that the circumference of 1.5 times that of the Booster
is insufficient; in this case a glance at the RAL design for a 15 GeV
proton driver of twice the Booster size is recommended. In their
scenario, a γ~t~ \> 20 allows to produce bunches of natural length of
1 ns rms with an RF system of 7 MHz, a simple approach that could be
useful also for this proton driver. Their lattice has a continuous
doublet focusing structure, less halo-generating than e.g. the
Johnstone-Lattice with low-beta sections.

# **Injection, and Extraction**

*Injection* The committee is concerned about the results of imposing on
the injection insert a quad pattern that is natural for the circulating
beam. This was the case for both of the lattices that were presented.
The committee strongly suggests designing an injection insert specific
to the needs of multi-turn H- injection. Also, the committee is
concerned about the "reverse bend in series with the main bus" shown in
the injection insert, because it results in a periodicity 1 lattice.

The current vs. time profiles of the injection kicker currents needed
during the approximately 30 turns required for transverse phase space
painting are recognized as a first pass solution. The committee suggests
that a better balance be achieved between the needs of a uniform beam
distribution and a more realistic power supply design for providing the
current vs. time profiles.

The committee has several concerns on the scheme presented for stripping
the H- beam, but expects that these concerns can be addressed by further
work. The thickness of the foil has probably not been optimized. The
stripped electrons will need to be collected carefully and the scheme as
presented did not appear to do this. The behaviors of the neutral beams
(H^0^ and its excited states H^0\*^) need to be understood and their
destinies need to be well controlled. The committee points out that
realistic schemes for doing these things would be far easier if the
optics in the injection insert were designed specifically for the
injection insert. As a final comment on stripping H- beams, the
committee suggests that collaboration with LANL (PSR) and BNL (SNS)
could be very informative.

The committee was told that eventually an upgraded beam will likely
replace the 400 MeV H- beam. However, it is apparent that the present
plans do not allow for such upgrades in any obvious way. The upgrade
path should be identified because it certainly affects at least the
injection system layout. For example, one upgrade path included a 1 GeV
H- beam which would impose a constraint on the maximum magnetic rigidity
of the beam transfer line magnets (and consequently the curvatures of
the beam enclosures) to avoid field stripping of the H- beam. On the
other hand, another upgrade path included a 3 GeV proton beam from a
pre-Booster. Assuming the upgraded beam transfer line and injection
insert occupy the beam enclosures initially constructed for the project,
they would require an injection system with substantially higher fields
than allowed for an H- beam. Although designing for all three might be
instructive, providing for all three in fact would likely cost more than
choosing one upgrade path.

# *Extraction* The presentation on the design of the extraction sub-system was well done. It is not clear what the final energy of the synchrotron will be. The baseline was consistently presented as 16 GeV. However, an energy as low as 6 to 8 GeV was mentioned as perhaps best for the muon production needs of a neutrino factory. And it was mentioned that an energy higher than 16 GeV might serve the Main Injector best for higher intensities. The details of the extraction system will depend on the final energy, but the concepts that were presented and the comments given below should serve in any case.

The committee is skeptical that the Ritson lattice that was presented
can accommodate the extraction sub-system that was presented. In
particular, the combination of kicker rise time and phase advances
between the kicker and the septum magnet did not appear to give an
adequate extraction sub-system. The application to the Johnstone lattice
did appear to be adequate.

The committee strongly suggests designing an insert specific to the
requirements of extraction, and pay particular attention to reducing
beam losses. A solution with no quads, or perhaps a solution with quads
incorporated to the needs of beam extraction and beam transfer, should
be developed.

Any beam in the extraction kickers during their rise time will be cause
for concern because such systematically errant beam likely results in
residual radiation somewhere. The committee recommends a very careful
analysis of the origins and handling of such errant beam. One example is
beam that is not properly handled during bunch rotation. There are
undoubtedly other examples.

# *Collimation* The committee recognizes that beam loss control will be crucial for reliable operation and realistic maintenance, and that this machine pushes beam power to a new regime for Fermilab. The presentation on using collimators to help provide this beam loss control was clear and presented a well-considered sub-system.

However, the committee observes that further integration into the
machine design will likely significantly improve beam loss control. For
example, the phase advances between the primary collimators and the
secondary collimators are not ideal. A moderate iteration with the
lattice design is expected to result in a superb collimation design.

The committee observes that a realistic scheme must be developed for
controlling the beam as it passes through the collimators. This control
obviously includes beam position. But the committee also suggests that
changes in beam size at the collimators due to tune changes should be
investigated to determine if these effects are significant, and if so,
how to accommodate such changes. The committee believes this control
must include beam position at both the primary and secondary collimators
throughout the entire beam cycle. Finally, control of the beam angle at
the secondary collimators must be investigated to determine how well it
also needs to be controlled.

##### Technical Components

*Magnets* The committee saw preliminary designs of both dipole and
quadrupole magnets. The magnets as presented were quite large: dipoles -
132 cm by 96 cm by 5.1 m long, and something like 25 tons, quadrupoles -
50" by 50" by 1.5 m long (no weight given). Since the details about the
design were sketchy at best, it is hard to comment. One problem noted by
the presenters indicated that eddy currents in the copper windings were
recognized to be a problem with the proposed winding configuration.

The peak magnet voltage will be supported by the inter-lamination
insulation. The laminations are large and the peak field is
approximately twice the present Booster. A simple calculation indicates
that the voltage will approach 0.2 volts per lamination; this should be
examined more carefully.

Fermilab should get to the point of developing a prototype magnet as
soon as practical. This effort will allow real life measurements of the
magnets at a 15 Hz rep rate with all harmonics that will be present in
the power supply system.

As the final lattice becomes available, finalize magnet parameters such
that corrector magnets can be designed and power supplies proposed.

###### Power Supplies The committee was presented work that has taken place over the last 18 months. Three drive scenarios were presented. In all three the peak current was 7,200 Amps, the minimum current was 400 Amps and the rep rate was 15 Hz.

The first case was the single resonant system at 15 Hz. This system is
similar to the present Fermilab Booster and although it is the simplest,
it requires the most RF power during acceleration. The second case was
also a resonant system adding about 12.5% second harmonic to the current
wave shape and thus linearizing the magnet field rise with the effect of
decreasing the amount of RF needed. The third case was similar to the
present Main Injector magnet power supplies that use ramped power
supplies to supply the voltage necessary for R*i* + L*di/dt*.

As presented the single resonant system was rejected because of the
excess RF needed. And the third scenario (MI style) was rejected because
of the large amount of installed MVA power required. (MI's maximum is
120 MVA, MR was 300 MVA @ 500 GeV and the Proton Driver was projected to
be 600 MVA) Although costs were not addressed or presented, they were
characterized as about twice the dual resonant solution.

In each of the scenarios the dipole and quadrupole magnets were assumed
to be on a common bus. A trim coil system was presented to provide
tuning that would account for magnet saturation differences and lattice
tune control.

The committee encourages the Proton Driver group to keep an open mind
when it comes to the particular scenario that is the final choice for
the main bus drive. The MI system has many intangible benefits that the
others are lacking including minimizing the equipment installed in the
tunnel. The MI system has the maximum ramp control even to the point of
matching the rep rate to the program requirements. This solution
minimizes the RF requirements. The best (cost effective) solution may
have been selected but don't lose sight of the total solution.

Whatever the final drive solution is the committee strongly recommends
the construction of a development system capable of driving at least one
magnet (perhaps one cell) at the design rep rate and peak field. This
system can be used in development of the magnet measurements and beam
tube design. This development system will drive home the tunnel and
service building requirements for the final solution.

The trim coil solution is nontrivial. Firm up the trim requirements (the
committee saw quad to dipole ratio differences of 1.0 to 1.5%, which
must be corrected to the level of a tenth of one percent; this is a
fairly substantial correction at high field). Once the lattice is
stable, look at the correction magnet requirements. These will take
power supplies and occupy free space in the machine. Will the corrector
power supplies be able to make corrections throughout the ramp?

*Vacuum* The committee was presented with a fairly complete vacuum
design. The presentation was well thought out and highlighted many
unknowns. As beam pipe design is completed the unknowns can be
eliminated.

*Beam Pipe* The committee was presented with what seemed to be a very
aggressive R&D project. The physical size of the beam tube that is
required is an ellipse of order 9" by 5". Eddy current losses were
estimated any where between 200 W/m to 8.5K W/m depending on tube
thickness. This will require water cooling. The committee feels that
there are numerous risks associated with use of epoxies or other organic
materials in conjunction with the cooling, and recommends that only
proven, robust systems be explored. Stress calculations were presented
but no realistic solutions were presented. Continue with the R&D effort.
Keep in mind that a fall back position may require NO beam tube in the
15 Hz magnet field. Beam impedance may have to be attacked in a
different way.

# **Beam Losses**

The presentations on beam loss issues demonstrated much progress on
these issues. Passive shielding (\~30') will be used to provide
personnel protection. The need to consider air activation was
acknowledged; the committee believes this will indeed be a serious
issue. The numerous penetrations that will be required for the rf
systems in particular will make sealing the tunnel difficult.

The high beam power has implications for component damage and for the
ability to do hands-on maintenance. The committee recommends that
further analysis be done to examine the consequences of both
single-pulse and repetitive beam loss at a single point. An abort system
or some other means of quickly dumping (but not necessarily in a single
turn) the beam in a controlled fashion at a location capable of
receiving the full beam power may be required. The stated goal of
limiting component activation to 100 mrem/hr at 30 cm should be reduced,
in the spirit of ALARA, for those areas that require routine maintenance
in order to avoid the likelihood of cumulative dose to maintenance
personnel exceeding the Fermilab administrative goal for annual exposure
of \<1.5 rem. In addition, remote handling capabilities may need to be
incorporated into design of some areas; while this review did not
concern itself with any cost or schedule issues, the committee feels it
appropriate to at least highlight the cost implications of remote
handling facilities.

The problem of groundwater activation restricts the allowable beam loss
to about 0.6 W/m unless additional measures are taken. These measures
(thicker walls and/or water-excluding liners around the enclosure) will
certainly be required in the vicinity of the injection area and the
collimators. The committee notes that the calculations are being done
with a simplistic model of a magnet in the center of a circular
enclosure, and suggests that some calculations be done with a more
realistic geometry of a magnet or other component near the wall.

# *Additional Activation Issues* The committee was given information from which it is appears that there will be several sub-systems requiring radioactive water (RAW) systems. These include the collimator cooling water, the injection and extraction magnet cooling water, probably the cooling water for the lattice magnets immediately downstream of the collimator stations, and perhaps the cooling water for the beam pipes in the rest of the lattice. The committee suggests that attention be given to these RAW systems early enough in the design stage to avoid potentially expensive solutions later. In addition, the ES&H cost implications of providing low conductivity water that is also radioactive should be understood at the appropriate time. One such implication would likely require provision for secondary containment of all RAW systems.

The committee notes that several active beam components (magnets in
particular) are encapsulated in the shielding required for the
collimator stations. The shielding is required to keep the radiation
field below 100 mrem/hr @ 30 cm outside the shielding. Presumably, the
radiation fields from the encapsulated beam line components are much
higher, and will very likely require additional equipment and procedures
for maintenance and repair in order to responsibly apply ALARA
principles. The committee strongly suggests that further integration of
the collimator sub-systems into the lattice might be one way to minimize
the encapsulation of active beam line components under heavy shielding.

The committee was not presented any details of the beam enclosure and,
at this point in the very early conceptual design, it may not yet be
appropriate. However, it is clear that the details of the requirements
of an enclosure containing a beam with 1 MW of power need to be
incorporated into the conceptual design earlier rather than later. One
example of these needs is provided by the very heavy and substantial
shielding required by the collimation sub-system. The weight and volume
are not of fundamental concern; rather, plans for maintenance of the
beam line equipment should be developed, and the civil construction must
accommodate these plans. Cranes and other special handling equipment
(perhaps remote handling in some cases) may be required. Also, in
keeping with ALARA principles, perhaps the tunnel aisles should be made
wider than would otherwise be the case in order to provide workers with
additional distance between themselves and the areas outside the
collimator shielding where the fields are expected to reach 100 mrem/hr
@ 30 cm. A second example is the total volume of secondary containment
required by RAW systems that will need to be included in the design of
the civil construction of the beam enclosure. Other examples, which were
presented to the committee and are accommodated at beam powers more
familiar at Fermilab, include above ground shielding and ground water
protection.

### Longitudinal Beam Dynamics/RF for the Proton Driver

We thank the Proton Driver RF team for their clear presentations. We
note that there were several handicaps including unclear goals in
preparing a design for the rf system of this difficult machine. We
believe that the team has done a good job of concentrating on the most
important objectives. Indeed, they have produced a concept that comes
very close to meeting the basic needs of the machine and delivering the
desired longitudinal beam properties.

We saw three fundamental problems described. First, there was the
process of capturing the Linac beam into a "bucket" with suitably small
longitudinal emittance and with no beam loss. The second fundamental
problem is the bunch rotation at 16 GeV that is needed to meet the
requirement of a narrow time width pulse at extraction from the driver.
Along the way, it appears that beam loading is a fundamental problem
that requires attention, especially during bunch rotation and also
during beam capture.

The rf capture process looks to be the least well understood. It was
reported that the requirement is for a bunch of 2 eV-sec. The best
"lossless" bunch obtained in simulations was \~3 eV-sec. One
collaborator reported a result of 1.7 eV sec, but did not show details
or losses. The committee sees that there is very little time to form the
bunch. This is made worse by the magnet current cycle, which lowers the
acceleration rate at the expense of shortening the "flatbottom". (The
argument given for rejecting a switching power supply that might create
a "flatbottom", namely, ripple in the magnet current at injection time,
appeared to be incorrect because the calculated ripple current was much
smaller than the injected beam momentum spread.) We note also the need
to pay attention to maximizing the bunching factor during capture and at
the start of acceleration, because the specified Laslett tune shift of
0.36 is rather high.

The question of the desired/required longitudinal phase space of the
beam deserves more attention. Although the specified phase space of 2
eV-sec appeared reasonable, there are tradeoffs to be considered.
Smaller area would simplify the rf manipulation required to achieve the
final goal of 3 ns rms bunch width and would reduce the momentum
acceptance required for the lattice. Smaller momentum spread would lead
to a smaller beam stability margin, but this margin presently appears to
be generous. The effect of a smaller phase space goal on the
injection/rf capture process is unknown. The committee recommends that a
more systematic approach be taken and that the project would benefit
from a clearer understanding of the longitudinal phase space requirement
for the proton driver.

We think that the rf team should take a fresh look at what is required
to create a bunch of the desired longitudinal area without beam loss.
Something along the lines of comparing capturing the beam from rf off to
filling a pre-existing bucket, either with second harmonic or a barrier
bucket. A study of what it takes to match the longest possible Linac
bunch into such a bucket would be appropriate. The rf simulation should
include both capture and the start of the acceleration cycle. After an
understanding is reached, it may be possible to make the real world
compromises to use the existing Linac (momentum spread, macropulse
length, charge per pulse) and a pre-defined magnet current cycle. It may
turn out that achieving the bunch area in a routine way with no losses
will require a major change in one or more of these constraints. The
problem with the treatment presented to the committee is that it is not
clear which constraint, or even if only one constraint, is causing the
difficulty in meeting the initial bunch requirement.

The second fundamental problem is the bunching at 16 GeV, which had
difficulty meeting the bunch-length requirement. The dependence of the
momentum compaction factor on beam energy seems to cause a wider bunch
than would be given by a simple linear model. The committee would like
to see a study of a lattice change (minimizing the dependence of
momentum compaction on momentum spread, or a change in circumference, or
choosing transition gamma near the extraction energy), non-linear RF,
and/or self-bunching by inductors to see if one or more of these would
provide flexibility to provide bunching in a range from the desired
bunch width towards zero bunch width.

Beam loading appears difficult for Phase I, and is very problematical
for Phase II. The proposed solution for Phase I is a two-tube power
amplifier. To the best of the committee's knowledge, the proposed
two-tube power amplifier with "feed-forward" has never been applied in
practice on any other accelerator. If such a solution is to be adopted,
then a hardware test, preferably with beam, is suggested. We note in
passing that one of the team mentioned that cathode followers are one
way to provide feedback, but that there were problems in implementing
cathode followers at Los Alamos. The problems with cathode followers at
Los Alamos have now been solved by the installation of a better
"driver", and we suggest that the team contact either Andy Browman or
John Lyles for details.

One issue that was fairly unclear to the committee is the
RF-compatibility of the Proton Driver with the Main Injector. The issues
are two-fold, (a) the 53 MHz systems in the Proton Driver, and (b) the
ability of the Main Injector 53 MHz systems to accept beam from the
Proton Driver bunched either at 53 MHz or at lower frequencies. With
respect to the first, the committee learned there is an improvement
program for the present Booster 53 MHz cavities, which would be used
presumably for part of Phase I operation (Main Injector only). But they
were not discussed in conjunction with the other part of Phase I, the
neutrino factory. The issues related to operations with the 53 MHz
cavities were not addressed. For example, there was no discussion of
capture into 53 MHz buckets, of a 53 MHz kicker gap, and of what it
takes to match between the Proton Driver and the Main Injector.

On the second issue, there was limited discussion, as perhaps was
appropriate for a review of the Proton Driver, but a bit more would have
been useful. The only issue presented was that of transition crossing,
and it appears that the MI can accelerate both 53 MHz and
7.5 MHz-bunches, provided a gamma-t jump system is installed. An
inductive insert may also be required; clearly more study is called for.
Presumably the four 1.7 MHz bunches exceed the capability of the Main
Injector. Whether there are other issues that must be confronted remains
an open question.

It was not completely clear that the Ritson lattice contained sufficient
space for the rf system. In addition, it was not clear whether providing
beams for the neutrino factory program might need to be interlaced with
injection into the Main Injector. If so, two rf systems in the Proton
Driver might be required simultaneously, leading to a demand for more
space. Such modes should be investigated and a clear decision to include
these or not should be made soon, since the consequences for the lattice
design might be significant.

The committee liked the FineMet cavities proposed. However, we were
impressed with the large number of units needed. Perhaps there is a way
to provide more drive power and better cooling so that fewer units are
required.

*Inductive insert* The need for inductors for longitudinal space charge
compensation was unclear, at least for implementation in Phase I. It is
at present not clear whether such a device is needed or useful. Note
that the space charge Z/jn is \~400 ohms at 400 MeV and \~2-3 ohms at 16
GeV. The insert must obviously be variable over this range! Perhaps
simulations that address this issue directly would show whether there is
a clear economic reason to apply inductor technology in Phase I.
(Warning: In ESME simulations, the g-factor seems to be a fixed quantity
-- not suitable for simulation of a complete acceleration cycle.)

Several other issues were discussed. Continued work is required. But if
there are limited manpower resources available, most of the attention of
the team should be devoted to the fundamental issues raised above.