# Report of the SNS Beam Diagnostics Advisory Committee

# Review of BNL Beam Diagnostics for SNS Ring

# July 23-25, 2002

Committee members in attendance: Jim Crisp, Ron Johnson, Tom Powers,
Marc Ross, and Bob Webber

This review covered instrumentation, mostly for the SNS Ring, being
provided to SNS by BNL. The committee was welcomed by Jie Wei, leader of
the BNL design team for the SNS Ring, and was given its charge and
introductory remarks by Tom Shea.

The review agenda was organized by instrument or instrumentation system.
The Beam Position Monitor, Beam Loss Monitor, and Beam Current Monitor
Systems were presented as Final Design Reviews. A range of beam profile
measurement instruments including carbon scanning wires, IPMs,
luminescence monitors, and stripping foil monitors were presented as
preliminary designs. Transverse tune measurement schemes and beam-in-gap
measurement methods were also presented as preliminary designs. In
addition, the review committee was presented with an overview of
possible electron cloud effects in the Ring and RTBT, electron detection
methods, and an update on the status of laser wire profile monitors for
the SNS Linac. The committee recognizes the considerable effort that was
put into preparing and presenting the information for our review.

## Charge to committee

-   Review the designs

    -   Are requirements clear?

    -   Are system concepts sound?

    -   Are technologies appropriate?

-   Categorize findings

    -   Identify critical findings (potential show stoppers in the
        designs)

    -   Provide relevant observations and recommendations

    -   Makes suggestions for performance and/or cost improvements

-   Consider key observations from audience participants

-   Provide verbal out-briefing at conclusion of review

-   Provide written report

## General Remarks and Comments Relevant to the Charge

The BNL team presented the review committee with comprehensive
instrumentation design and status information. It was particularly
helpful to the review committee that concerns raised in the December
2000 preliminary review report were specifically identified in this
review as actions that have been/are being taken to address those items
were described.

The committee appreciates correction where our observations may be off
the mark and offers suggestions and recommendations, not criticisms, as
hopefully constructive ideas or approaches toward a successful and on
schedule suite of beam diagnostics for SNS.

The committee thanks the presenters for their efforts in creating clear
and informative presentations and for their patience in answering our
questions.

## Committee Findings, Observations, and Recommendations

**Critical Finding**

-   **In the context of the Beam Current Monitor presentation, it was
    observed that the collaborating laboratories supplying beam
    diagnostics components to the project are unaware of or do not
    generally understand the SNS "spares" strategy and requirement. The
    lack of understanding on this point is pervasive among all
    diagnostics systems, not just current monitor system. It is
    important that the project clearly defines, advertises, and explains
    the "spares" requirements and strategy to each of its "suppliers"
    and identifies adequate funding.**

### BPM System \-\--

##### Mechanical and Pick-up Electrodes

**Critical Findings**

-   **BNL staff voiced a concern about the long-term reliability of the
    pressure contact used between the BPM electrode and the vacuum
    feed-through connection. We suggest BNL and SNS staff resolve the
    concerns about the long-term reliability of the pressure contact
    ASAP through physical testing and/or detailed reviews of the
    mechanical design.**

**Committee Observation** -- The BPM pick-up electrodes are already
designed, machined, and mostly assembled in order to meet installation
schedules. The committee believes that the BPM pickup designs are
adequate to provide the signals necessary to meet baseline system
requirements.

**Committee Observation** -- Acceptance criteria for BPM pick-up
assemblies when they arrive at ORNL have yet to be defined and
documented by SNS personnel.

**Recommendation** -- Establish acceptance criteria and acceptance tests
as soon as possible.

**Committee Observation** -- There is some controversy concerning proper
design of BPM electrodes for matching sum mode versus difference mode
impedances of the four-stripline pickup. The review committee believes
this controversy is a red herring for any baseline application of the
BPMs. It may matter only if the pickups should be required to provide
absolute bunch length or absolute energy (phase) measurement that rely
on the response of the pickups at very high frequencies.

**Recommendation** -- The pickups are required for mechanical
installation very soon. Quickly determine if there is any significance
of this impedance design parameter for baseline measurements and move
on.

##### Electronics

**Critical Findings**

-   The BPM electronics system is far from Final Design Review stage.
    Open issues remain in requirements, cable system design,
    signal-processing electronics design and specifications, calibration
    system, and data acquisition. We recommend a dedicated review of the
    Ring and Transfer Line BPM systems when design is at a more suitably
    advanced stage or when the design team feels that detailed committee
    input might be useful. There may be schedule critical issues here,
    especially in design of the tunnel-to-electronics cabling system.

-   Specifications for the analog front-end electronics are incomplete.
    Input impedance match, channel-to-channel gain match, calibration
    signal injection levels, channel-to-channel cross talk as function
    of gain, and the peak power/peak voltage and frequency spectrum of
    signals that the front end has to handle all need to be addressed.

**Committee Observation** -- The committee was provided with the
"specification" provided by BNL to the BPM front-end electronics vendor
and found numerous shortcomings. The committee offers to review and
comment on improvements to this document.

**Committee Observation** -- The requirements for the BPM calibration
and on-line test system seem ill defined. An electronic calibration
system and a beam based alignment (really BPM center offset finding)
scheme were both presented, each with merit. However, the contribution
of each scheme to meeting a clear set of requirements was not clearly
demonstrated.

**Recommendation** -- Review and/or specify the BPM calibration system
requirements in order to assure that the goals are quantitatively
understood.

**Committee Observation** -- We are concerned whether the described
design of the electronic calibration system is sufficient for
operations. The current proposal injects signals into the electronics
beyond the front end matching system. It seems desirable to also use the
calibration system for cable and/or electrode centering checks, etc. The
pickup electrode-to-electrode coupling at 5 MHz is on the order of
--46 dB. It may not be possible to achieve sufficient calibration source
to signal chain isolation to calibrate the electrical center of the BPM.
If cables are sufficiently long a TDR technique might be implemented.
For instance, it may be possible to inject a calibration impulse signal
and look at the returned signal after it is spread out by the input
filters.

**Recommendation** -- Revisit the electronic calibration scheme and see
if it can be adapted to correct for differences in cable losses and VSWR
errors. This may require interaction with the front-end electronics
vendor.

**Committee Observation** -- There was little detail provided of the BPM
signal cabling system. We have special concern for the last few feet of
the cables connecting to the BPM electrode connectors. It is not
apparent that the scheduling and installation aspects of this part of
the BPM system are understood.

**Recommendation** -- The cabling system design must be taken as
seriously as the electronics design, especially as it will have the
first schedule requirements. The transition between the BPM pickup
connectors and the large diameter heliax cable needs to be defined. It
should include strain relief and possibly a transition cable of smaller
diameter.

**Committee Observation** -- If nonlinear correction factors for beam
positions not close to the center need to be used to meet accuracy
requirements, then those quantities must be measured and included in the
software specifications.

**Committee Observation** -- There appears to still be some confusion
and/or lack of consensus about the beam position and beam current ranges
over which the accuracy requirements apply.

**Committee Observation** -- How the BPM gain setting/read back will be
addressed was not presented, nor was how many samples are lost during
the gain switching process.

**Recommendation** -- Dead time due to gain switching should be
quantified and passed on to AP personnel.

**Committee Observation** -- There was discussion about implementing a
dual analog front-end scheme. It is not clear if the dual analog
front-end is effective or efficient for the limited number of channels
that require the wide bandwidth.

**Committee Observation** -- Electron cloud effects may have a big
impact on the operation of the BPMs in the ring and the RTBT as there is
a potential for electron interaction with the electrodes.

**Recommendation** -- Work towards understanding the impact of electron
currents on signal integrity and overall reliability of the electronics.

**Committee Observation** -- There continues to be talk of using BPM
electrodes to generate electron-clearing fields. Other than the
insulated pickups electrode design, there is no provision for this in
the electronics system design.

**Recommendation** -- Determine if there might be simple considerations
made in the front-end electronics design at this time to facilitate
future use of the electrodes to supporting clearing fields.

### BLM System \-\-- 

### Critical Findings

-   None

**Committee Observation** -- The committee believes that the design of
the Beam-Loss Monitor System for SNS is in good shape.

**Committee Observation** -- There were several choices described for
the integrator design in the analog front-end electronics. Each choice
has advantages and disadvantages.

**Recommendation** -- This is simply a design choice, but we recommend
that the decision be made soon. Avoid putting timing on board; it adds
complication, expense, and a possible failure mode.

**Committee Observation** -- There is a choice between an old and new
design of the ion chamber. The new design has several advantages that
the committee recognizes. The only proven disadvantage is that it
somewhat more expensive.

**Recommendation** -- From a technical basis, we highly recommend
choosing the new ion chamber design.

**Committee Observation** -- It appears that the requirement that a
minimum of half the BLMs always be operational, even for example when
half are undergoing maintenance or repair, was not recognized in the
proposed layout.

**Recommendation** -- Revise the layout to meet this requirement.

**Committee Observation** -- The concern about BLM ion chamber response
to x-rays from RF cavities was noted. This might contaminate the real
beam loss signal.

**Recommendation** -- Move forward with the tests of this problem. We
suggest that tests be made at Jefferson Laboratory.

**Committee Observation** -- The electronics has a jumper that controls
beam-line mode or ring mode. There needs to be a method to insure that
the electronics is in the proper mode.

**Recommendation** -- Make this jumper readable by the control system.

**Committee Observation** -- It appears that BLMs may be the only beam
diagnostics inputs to the machine protection system.

**Recommendation** -- This situation needs to be recognized and reviewed
to determine that this is sufficient to meet machine protection goals
(see \"critical finding\" on the BCM System). Also, the detailed
elements and features that will be necessary in the BLM System to meet
these goals should be determined and implemented. These may include:
software and hardware checks of the system, a minimum number of
operating BLMs, critical BLMs, etc.

**Recommendation** -- It is advisable to set hardware upper limits on
any programmable MPS trip thresholds.

**Committee Observation** -- Precise measurement of very high losses is
not possible, but we believe that it is also not necessary.

**Recommendation** -- Modify requirements to reflect what is possible
and necessary.

**Committee Observation** -- Requirements for the "fast beam loss
monitors" seem to be loosely defined at best. Nevertheless, the
committee believes that the system as presented with provide valuable
information on beam losses that happen on short time scales.

**Recommendation** -- Contact ORNL/SNS safety organization to determine
what complications (e.g. mixed hazardous liquid waste handling and
disposal) you may be buying into by using liquid scintillation devices.

**Committee Observation** -- Fast BLM cabling plant and equipment layout
was not presented.

### Beam Current Monitoring System \-\-- 

**Critical Findings**

-   There appeared to the committee to still be confusion about
    connections between beam current monitor systems and the Machine
    Protection System. Apparently the present plan is to divorce beam
    current inputs to the MPS from "this" BCM system, i.e. the one being
    reviewed. The committee believes this to be a serious mistake. Great
    effort is going into this system to provide reliable and accurate
    beam current measurements; to connect anything less to MPS would be
    negligent and to develop a parallel system would be duplication of
    expenses. We believe beam current input requirements to MPS should
    be clearly defined ASAP and incorporated into THIS/THE beam current
    monitoring system.

**Committee Observation** -- The Beam Current Monitoring system appears
to be in good shape. The data obtained recently at LBNL looks great even
though some problems were encountered with cables and connectors.

**Recommendation** -- We suggest examining those cable/connector
problems from a "lessons learned" perspective to insure reliability in
future installations.

**Committee Observation** -- The PCI data acquisition card (also used
for the BPMs) is not yet finalized. It was suggested that the timing
module might be moved onto this board and a newer faster PCI standard
might be employed.

**Committee Observation** -- The calibrate module is not yet finalized.

**Recommendation** -- It should produce pulses of sufficient amplitude
to evaluate all of the BCM ranges. However, care should be taken to
prevent damage to the test winding or transformer in the event of
failure.

**Recommendation** -- A bandwidth-limiting filter should be placed in
front of the BCM amplifiers to avoid problems caused by out-of-band high
frequency signals.

**Committee Observation** -- There is apparently evidence of a 600 to
700 MHz resonance in the BCM shroud.

**Recommendation** -- Consider placing distributed series RC elements
across the ceramic gap to shunt high frequency beam image currents.

**Committee Observation** -- In the SNS ring, the shroud must carry the
full beam image current that can approach 50 Amps.

**Recommendation** -- The shroud and its connections to the beam tube
should be designed and tested to handle this current.

**Committee Observation** -- The Ring BCM requires ranging with the gain
settings controlled through a FPGA.

**Recommendation** -- The FPGA should be programmed to not allow
nonsense settings.

**Recommendation** -- Software should verify the integrity and
sensibility of measurements, e.g. check for saturation that may result
from incorrect gain settings.

**Committee Observation** -- User requirements for beam current
monitoring information seem to still be lacking. For example, we were
not clearly presented with a requirement to provide total integrated
accelerated charge values that will be required for reporting to DOE and
will be useful for tracking weekly/monthly machine operational
performance.

**Recommendation** -- The beam physics and commissioning requirements
for beam current monitoring have been well covered for BCM as well as
most other diagnostics systems. Requirements from the machine operations
organization (which may or may not yet exist) must be clearly defined as
well. It is time to move away from the "digitize it now and worry what
to do with it later way" of thinking and develop an understanding of
what data presentation, "post-processing", or even additional front-end
processing will be necessary.

### Beam Profile Monitors \-\-- 

### - Carbon Wire Scanners 

**Critical Findings**

-   None

**Committee Observation** -- **Wire breakage** is a primary design
concern. Wire survival limits beyond 1225º C are not known. Carbon wires
should not be planned for operation beyond this limit until more tests
are done.

**Committee Observation** -- The advertised scanner resolution appears
to be close to or **beyond the state-of-the-art**.

**Recommendation** - It should be understood to what extent the machine
operation and related studies **would rely on this very good
performance**.

**Committee Observation** -- The carbon wire scanner system for the Ring
and RTBT appears to be in **very good shape**.

### - Ionization Profile Monitors 

**Critical Findings**

-   Requirements call for three IPMs, horizontal, vertical, and 45º.
    There is apparently no space in the Ring for 3 devices and the plan
    is to drop the requirement for the 45º device. Accelerator Physics
    group and Diagnostics Group need to reach consensus and either
    update requirements or identify space.

-   Accelerator Physics needs to provide specifications for the maximum
    allowable residual dipole kick and higher order field components
    from the IPM magnets. Individual magnet strengths are the order of
    100 gauss-meters; this is potential death for an 800-1000 MeV beam.

**Committee Observation** -- At high intensity, the IPM operation and
results may be strongly altered by "electron cloud" effects.

**Recommendation** -- Modeling and possibly beam tests at PSR should be
considered in attempting to understand the electron cloud implications
on IPM operation and performance.

**Recommendation** -- Design modifications, including segmented sweeping
electrodes and collector electrodes, should be considered in order to
ameliorate electron cloud related effects.

**Recommendation** -- Use of the IPM for study of the electron cloud
should not be overlooked.

**Committee Observation** -- Asynchronous 10MHz sampling of IPM data was
presented. This is inconsistent with all other SNS beam diagnostic
systems.

**Recommendation** -- Attempt to make IPM data sampling consistent with
that of other beam diagnostics systems.

### - Luminescence Monitor 

**Critical Findings**

-   The luminescence (gas fluorescence) monitor was presented as one
    idea to satisfy an ill-defined non-invasive profile-monitoring
    requirement. The justification for this device and the requirement
    that it fulfills should be more clearly identified.

**Committee Observation** -- From the information presented, it would
appear that such a device **might require introduction of a pressure
bump** for detectable light to be produced, even at high intensities.
This might not be acceptable to Accelerator Physics.

**Recommendation** -- If this device is necessary, **beamline space**
should be identified and reserved.

### - Video Foil Monitor 

**Critical Findings**

-   The video foil monitor was presented as one idea to satisfy an
    ill-defined profile-monitoring requirement. The justification for
    this device should be more clearly identified. Device requirements,
    including timing and spatial resolution, display characteristics,
    update rates, spectral response, etc. should to be defined and
    quantified.

**Recommendation** -- Consider different locations for the camera and
the cost savings associated with going from a RAD hard camera to a CCD
camera at a lower cost that has the additional feature of near-IR
sensitivity.

**Recommendation** -- Because this is a low count system making use of a
technology that is constantly changing, the hardware decisions should
not be made to early just to meet review schedules.

### Transverse Tune Monitor 

**Critical Findings**

-   The tune monitoring presentation provided a nice overview of the
    envisioned methods, capabilities, and possible uses of tune
    measurement in the SNS ring. It is important at this time that the
    accelerator physics group and the diagnostics group refine and
    specify the required measurements and methods that are expected on
    "day one".

**Committee Observation** -- Tune can be measured using the turn-by-turn
mode already included the BPM system. Driving the beam with the
beam-in-gap kicker can augment the available signal.

**Committee Observation** -- [Dedicated]{.underline} tune or schottky
detectors, as planned, are a good idea.

**Committee Observation** -- Numerous tune measurement methods were
described, but implementation details were not, i.e. what goes between
the detector and the machine operator was not discussed.

**Committee Observation** -- Some of the injection gymnastics described
as possibly necessary to measure incoherent tune require large
pulse-to-pulse current differences that might be inconsistent with the
committee's understanding of "agreements" to limit stress on the neutron
production target.

**Recommendation** -- Zero in on areas that have early decision impacts
and establish the schedule by which those decisions are required, e.g.
reservation of beamline space, construction of one-of-a-kind devices or
electronics, etc.

### Electrons 

**Committee Observation** -- The present beam physics understanding of
electron cloud effects anticipated in the SNS was described.
Implications may be significant for beam stability and for beam
diagnostics.

**Recommendation** -- Close communication between beam physics and
diagnostics people needs to continue through design, commissioning, and
operation so that deleterious electron cloud effects, both on the beam
and on diagnostics capabilities, might be unambiguously identified and
quantified.

**Committee Observation** -- In particular, it was noted that electron
effects might confound BPM and IPM performance and signal
interpretation.

**Committee Observation** -- Electron collection panels on IPMs and
"Argonne style" electron monitors were noted as possible means of
measuring electron presence in SNS, but requirements seem to be ill
defined.

**Recommendation** -- Beam physics and diagnostics groups should
determine clear requirements for electron detection devices.

### Beam-in-Gap Monitor 

**Critical Findings**

-   There was a disconnect between the diagnostics team and beam physics
    people concerning the magnitude of the cleaning kick. This must be
    resolved.

-   The number of kicker modules has not been determined.

**Committee Observation** -- The gap cleaning kickers will be stripline
type devices.

**Recommendation** -- The kicker striplines should be designed for 50
ohm driving point impedance in the differential mode, i.e. when opposing
plates are driven with equal and opposite polarity inputs.

**Recommendation** -- Pay attention to effective risetime of kicker
system as multiple kicker modules are powered electrically in series.

**Committee Observation** -- An absolute measure of the charge in the
kicker gap to 20% accuracy is apparently the requirement. This is a very
tough requirement to fulfill in a regular operational sense; however,
observing beam loss during the "gap-cleaning" process will may offer a
suitable relative measurement. Even to obtain good relative
measurements, a gated phototube system will probably be required to
differentiate between losses due to the large circulating beam and
losses due to "beam-in-gap".

**Committee Observation** -- We have some concern about the achievable
absolute accuracy of "beam-in-gap" calibration using the method of
kicking out a known quantity of beam not in the gap.

**Committee Observation** -- Overall, the plans and ideas presented to
the committee look like a suitable start with a good chance of success.

**Recommendation** -- The design team should continue to pursue and
detail instrumentation methods along the lines of those presented.

### Laser Wire Monitors for Linac 

**Critical Findings**

-   A comprehensive and realistic schedule for the laser wire system
    needs to be developed and integrated into the project schedule so
    that the timing of critical decision points and installation tasks
    are understood.

**Committee Observation** -- It was learned that the decision has been
taken that laser wires will serve in place of carbon wire scanners in
the SCL; there are to be eight laser wires and no carbon wires in the
superconducting section of the Linac. The committee applauds this rather
courageous decision. The chairman and one other member of this committee
have provided written comments on this decision to SNS management in
response to individual requests.

**Committee Observation** -- Excellent progress has been made on the
development and application of the laser wire concept. Very good results
were obtained with the low energy beam during the Ion Source-RFQ
front-end test at LBL and promising results were obtained with 200MeV
beam at BNL.

**Committee Observation** -- Considerable detailed engineering is
necessary to design a complete laser wire system for the SNS Linac and
the schedule is tight for some aspects, e.g. JLAB expects delivery of
the "beam box" vacuum assemblies in February 2003. There are serious
questions concerning the achievable stability of the 200-meter laser
beam transport system.

**Committee Observation** -- There was continued talk in the
presentation about laser wire testing in the Fermilab 400MeV H- beam. It
is noted that no test proposal details have yet been communicated from
SNS to Fermilab. Proposed laser wire activities at ORNL, BNL, and FNAL
tests may be duplicative and inefficient in terms of short personnel
resources.

**Recommendation** -- A realistic, comprehensive laser wire R&D plan
should be laid out that will accomplish necessary development work while
not short-changing critical SNS-specific engineering activities that
have project schedule impacts.

**Committee Observation** -- Plans apparently include utilizing
collection of stripped electrons as a source of signal.

**Recommendation** -- We recommend an early test of the viability of
using the electron collection signal.

**Recommendation** -- Continue to develop and improve detection methods
using current transformer and BPM electrode signals; don't rely solely
on the electron signal at this early stage.

**Recommendation** -- Operational impacts, if any, of the unavailability
of laser wires due to failures etc. should be assessed and addressed.
These might drive reliability aspects of system design.