The Thirteenth KEKB Accelerator Review Committee Report
November 29 – December 5, 2007
Introduction
The Thirteenth KEKB Accelerator Review Committee meeting was held on November 29 –December 5, 2007. Warren Funk and Flemming Pedersen were unable to attend this meeting,and Wang Shuhong was only able to attend for one day. Heino Henke has resigned from theCommittee. Appendix A shows the present membership of the Committee.
The meeting followed a new format, with two separate parts. The first part of the meeting
was spent in the Control Room with the Committee members observing the machine
maintenance recovery and downloading and testing of a new machine optics. The Committee
members followed all of the steps leading up to colliding beams of high luminosity, and wereable to interact directly with the KEKB staff who were actually performing the tuning
operations, pose questions and obtain a good understanding of the processes. The second partof the meeting consisted of oral presentations by the KEKB staff members and discussion by
the Committee members. The Agenda for the meeting is shown in Appendix B.
The Committee was as always impressed by the high standard of the talks, both the technicalcontent and the presentations themselves. The KEKB staff showed great confidence in theirability to demonstrate tuning techniques on the machine in real time, and the Committee wasimpressed with the ingenuity and complexity of the tuning algorithms. The recommendationsof the Committee were presented to the KEKB staff members before the close of the meeting.
The Committee wrote a draft report during the meeting that was then improved and finalizedby e-mail among the Committee members.
Contents
Executive Summary
1) Foreword
2) Recommendations
Findings and Recommendations
Section 1: Control Room Observations
a. Daily commissioning meetings
b. Recovery from maintenance and installation of new optics
c. Performance optimizationd. Use of diagnostic systems
e. Crab Cavity Voltage Scan
f. Luminosity Operation
Section 2: Presentations
1) Commissioning Progress before July2) Commissioning Progress since October
3) Crab Cavity
4) Belle Status5) Knob Tuning Simulation and More Theories on Crab
6) Fuji Test Beam Line
7) KEK Roadmap and KEKB’s future8) BPM Displacement Monitor
9) Tune/Orbit Measurements
Page 1 of 2210) Wiggler Antechamber/Collimator R&D

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Executive Summary
Foreword
In the past nine months, all attention has been focused on commissioning KEKB with the crab
cavities and increasing the luminosity for BELLE. The world record for luminosity is still
1.71 x 1034 cm-2s-1 established in November 2006 without the crab cavities. The initialdedicated machine studies of collisions using the crab cavities started in February 2007 andfocused on few-bunch operation. These studies resulted in the highest specific luminosity
ever observed in KEKB. Higher current running was then tried and the design luminosity wasreached again on June 22, 2007. High current operation resumed in October after warming up
the crab cavities to outgas them. The luminosity has climbed rapidly since then, and BELLE
data taking has co-existed with machine optimization. The peak luminosity that has been
attained with crab crossings is 1.47 x 1034 cm-2s-1, which is not as high as the previous record
but a major accomplishment for commissioning such a major new operating mode.
The Belle detector reached an integrated luminosity of ¾ ab-1 on November 25, 2007, a major
milestone. The Committee congratulates the entire KEKB project staff on this new record.
The BELLE detector has published, or submitted for publication, a total of 242 papers inrefereed journals (206 at the last meeting).
The meeting was devoted to the present status of KEKB, the accelerator physics limitations to
the luminosity and, in particular, progress with the crab cavity. There was also one talk on thevacuum system R&D for upgrading KEKB to a luminosity of 6-8 x 1035 cm-2s-1. The KEK
Director General, Prof. Atsuto Suzuki, presented the Roadmap for the future of KEK, which
he is proposing to the funding agencies. The Committee was pleased to see that SuperKEKB
was an integral part of this plan, with construction starting in 2009, immediately following theclosure of KEKB, foreseen when BELLE has accumulated an integrated luminosity of 1 ab-1.
The Committee understands that fiscal constraints may not allow all elements of the Roadmap
to be implemented, but strongly supports the inclusion of SuperKEKB in the planning.
The new (and longer) format of the Review allowed the Committee to observe first-hand the
dedication of the KEKB staff supporting machine operations and the high quality of theirwork, which has led to the impressive results that have been achieved.
Recommendations
Last year, the Committee made many recommendations, and the KEKB staff hasaddressed all of them. Some of these have not been completed and are reiterated below,
together with recommendations for continuing the work that has already been started.
1. The Committee endorses a major luminosity upgrade (SuperKEKB) aimed at a luminosityof 8 x 1035 cm-2s-1 as part of the KEK Roadmap.
2. The Committee strongly endorses the plan for simultaneous injection in KEKB rings,
currently foreseen for October 2008.
3. The Committee recommends continuing the extended alternating high and low beam
current studies to improve the chances of success with crab cavities and to make sure there
is sufficient beam study time to obtain satisfactory results and complete understanding.
BELLE could take data during the high current running.
4. The Committee recommends continuing the studies of the accelerator physics issues anddevelopment of the specialized components needed for SuperKEKB.

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The Committee has made new recommendations throughout the different sections below.
Highlights of these recommendations are summarized here.
5. The Committee feels that the KEKB accelerator team should take some time tocontemplate results and possibilities without daily distractions. Therefore we recommend
holding a 2-3 day retreat away from the lab during winter shutdown to brainstorm ideas
for luminosity improvement. Computing facilities and access to data should be available.
6. The BELLE collaboration should work with the KEKB team to make a strategy to
optimize the integrated luminosity between now and the end date of March 2009 and todevelop plans, if any, to run on other Upsilon resonances.
7. The Committee recommends that every effort is made to speed up the present luminosity
optimization algorithms, to improve the probability of convergence.
8. The Committee recommends finding a parameter (luminosity) optimization algorithm
which works rapidly at the peak luminosity to improve parameters. This algorithm should
be combined with trying, from time to time, very different starting conditions to avoid
local minima.
9. Clarify the bunch-by-bunch orbit displacement effect on the luminosity and study its
relevance to the luminosity limitation.
10. Beam-beam simulation will be critical to the success of KEKB. The Committee
recommends integrating the simulation efforts with the machine operation as a guide to
improving luminosity.
11. Since the feature of a sharply-peaked optimum luminosity peak on a broad shoulder is so
critical in guiding the machine luminosity optimization, it should be confirmed withanother established beam-beam simulation code.
12. Provide the full set of strong-strong collision simulation results relevant to the gatedbunch spectra measurement in order to improve their interpretation. Consider the potential
application of the gated tune measurement technique for the luminosity optimizations inoperation.
13. The search for the gas source in the LER crab cavity should continue. The starting pointshould be a repetition of a global leak check of the cavity assembly followed by an in
depth search of all subcomponents.

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Findings and Recommendations
Section 1: Control Room Activities
a. Daily Commissioning Meetings
A commissioning meeting is held every day to go over the progress in the preceding 24 hours.
Members of the Committee attended six of these meetings. Exceptionally, the meetings were
held in English for the benefit of the Committee, and this was extremely helpful. The
meetings were detailed and focused on the problems as well as any new insights that had been
gained. Overall, the meetings were very impressive.
b. Recovery from maintenance and installation of new optics
A well organized procedure efficiently brings the KEK-B machine up from the bi-weekly
maintenance shutdown. Preliminary steps include a review of activities during the shutdown
to identify any specific components requiring special attention during tuning and runningmagnets through a standardization cycle. Once the electron beam is established to the end of
the linac, the beam profile monitors are swept to document emittance and optics. The linac
cavity parameters are tuned to set the beam energy and minimize energy spread, and then theorbit feedback may be enabled. This procedure is repeated for two-bunch positron operation.
Once the beams reach the end of the transfer line the energy spread is once again optimized.
This whole procedure proceeded very efficiently while Committee members were in thecontrol room.
Minimal tuning was necessary to accomplish injection into the storage rings, whereupon beam
position monitor (BPM) calibration proceeded with 30 mA total beam current in each ringindividually. BPM cables up to 240 m in length introduce long term drift in the effective gain
for individual pickup buttons, causing changes in gain and non-linear characteristics of thesystem. Four dipole correction elements in each ring (0° and 90° phases for vertical andhorizontal planes) are stepped through a rectangular grid pattern and all BPMs acquire data
for each point. While the physical space swept out varies at each detector, the nonlinearity ismeasurable to sufficient accuracy. The reduction in BPM errors is significant.
Comment: Recording betatron tunes at each corrector setting can provide useful information
on orbit-dependent tunes, specifically checking proper sextupole operation.
The BPM gain is calibrated by comparing the off-set beam position evaluated from the signalsof three of the four individual buttons to calculate the position in four different ways. The
consistency check is done by putting an upper limit on the standard deviation of the four
results. Those BPMs exceeding the limit are rejected in further orbit analysis.
Trim coils on quadrupoles are switched to a few trim power supplies permitting measurementof the BPM electronic offset with respect to quadrupole magnetic centers. A local closed
orbit perturbation samples the quadrupole field at several positions while the kick from asmall change in quadrupole field is measured by fitting the global closed orbit change.
Because of long time constants to change beam position (for compatibility with the global
closed orbit feedback system) the calibration rate is 3 minutes per quadrupole. Therefore
BPMs are calibrated only when the need arises.
New optics are installed directly, controlling tunes with the feedback system, and are checkedwith an orbit response method. Typically, several sextupole candidate distributions arecomputed. Each is evaluated by the “bungee jump” technique – taking a 30 mA stored beam

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across the 2Qx + Qs = n resonance, selecting for minimum beam loss. This is normally only
done for the LER.
38 fast position monitor circuits are installed in each ring to provide injected bunch position
measurements and betatron phase advance data to check against the ORM optics
measurements. The two optics measurement methods agree at the ±12% level in the LER and±10% in the HER. Simulations suggest that closed orbit errors may be responsible for someof these differences. While Committee members were in the control room, a single gradienterror was introduced using the trim windings on a quadrupole. The change in phase advancemeasured with the fast BPMs agreed well with the lattice model.
c. Performance optimization
Optimization of luminosity relies primarily on two techniques, scanning individual parametersand multi-parameter optimization using the simplex method.
Single parameter scans are performed for several critical interaction point parameters: 4
coupling parameters, vertical dispersion and dispersion slope, vertical waist position, andhorizontal dispersion. Coupling and vertical dispersion control is realized through verticalbeam positions at 16 sextupoles. Quadrupole current changes control the vertical waist andhorizontal dispersion. On occasion the tilt (coupling) of the crab kick, vertical dispersion at
the crab cavities, the crab angle and the coupling parameters R2,4, may be scanned.
Some of the tuning parameters (horizontal and vertical collision offsets and angles) are in acontinuous feedback loop, so the scan is actually performed by scanning the feedback target
values. The scan speed is limited by luminosity monitor statistics and the need to measureeach point at similar machine conditions – i.e., at the same currents. Thus only one point per
injection cycle is possible, or about one every 5 minutes.
The simplex scanning technique dates back to 1965 and is an extremely robust algorithm
which is, however, slow to converge. In the application on KEKB, 12 parameters are
optimized, 6 for each ring: R1, R2, R3, R4, ηy, and ηy’. The method initially involves taking
thirteen scans which takes about one hour. This is slow because the colliding beam currents
are kept constant, so both rings must be topped up between each measurement. The algorithm
then attempts to improve the parameter set which gives the lowest luminosity (which iscounter-intuitive) and after about a shift the luminosity is maximized. Alternatively, the beam
size of each beam is minimized at the collision point; this is somewhat faster but is less direct.
Comment: In all cases, the optimization starts from a condition with the horizontal position
feedback maintaining the two beams offset from one another. This centers the colliding
beams in the aperture, determined by observing beam loss as the offset is varied. It is difficult
to understand why the aperture is not centered on zero offset, but it is not necessarily true thatthe luminosity peak is similarly offset. It would be worth trying to center the offset and retrying the simplex optimization. This may result in finding a better operating parameter set
for luminosity and may or may not also improve the aperture available for beam loss.
Comment: The simplex routine requires that a scale be applied to each variable. The criteria
used to select this scale is the range over which the parameter can be changed without
significant beam loss. The implicit assumption is that this is related to the luminosity
optimization. It would be better to use information from the simulation to determine the scalefor each variable. Starting from the optimum luminosity condition found in the simulations,vary each parameter individually and identify the “knee” in the luminosity curve. Use theparameter offset at the knee as the “luminosity” scale factor. This should do a better job ofbalancing the relative step size during the Simplex process.
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Comment: Simulation of the KEKB machine optics with errors should be performed toestimate the relative size of the twelve tuning knobs. Comparing these values to theluminosity scale factor would allow ranking the tuning parameters by sensitivity. Single scanluminosity optimization should then use the tuning knobs in order of decreasing sensitivity.
Any optimizing algorithm depends on knowing that the conditions are stable over the time ittakes to perform the optimization. If this is not the case, the optimization may never converge.
The simplex algorithm in KEKB takes about a shift to converge and it is not at all clear thatthe machine conditions are sufficiently stable over this period. Optimizing the “green ratio”
as the beam currents decay would significantly speed up the process, and the reduction in driftin other parameters would outweigh any effects from the lesser accuracy.
Recommendation: The Committee recommends that every effort is made to speed up thealgorithm, to improve the probability of convergence on the best machine conditions.
During production running these tuning methods are employed continually, the exact
sequence and parameters used are left to the discretion of the shift leader. Generally thisresults in a slow but positive increase in performance in any 8-12 hour period if machineconditions are not disturbed (e.g., beam aborts, equipment failure, earthquakes, or
unexplained “glitches”).
Comment: As the performance profile becomes sharper minimizing perturbations to thisoptimization process and uncontrolled parameters becomes more critical. It is vitally
important to assure that the procedures and control of parameters is sufficient to satisfy thiscondition.
Recommendation: Consider dedicated tuning periods with 100 bunches and slightly lower
bunch current where parameter and simplex scans can be completed more quickly, andfocusing efforts on finding the narrow peak in parameter space. High rate luminosity monitor
would be useful.
d. Use of diagnostic systems
Several highly developed diagnostic systems are available, including wide angle Bhabha andbunch-by-bunch radiative Bhabha luminosity monitors, beam position monitors, gatedtransverse beam spectrum, imaging streak cameras providing x, s profiles for both rings,
vertical and horizontal profile monitors (interferometers for high resolution), and beam loss
monitors. We will not review these in detail here but will make a few suggestions for their use.
Comments:
Electron cloud activity may usefully be monitored occasionally by recording tune shifts along
the bunch train – both rise time and saturated levels – as a function of beam current.
Installing a small number of fast counters by collimators would quickly give information on
particle loss rates and bunch dependence. E.g., is poor lifetime in a given condition primarily
because of loss of a few lead bunches.
Some enhancements to the gated tune spectrum monitor – averaging and flexible zoom
features – may provide useful information on beam size at the interaction point, useful in
confirming relative beam-beam parameters and understanding discrepancy between beam size
measurement and luminosity. This might even provide a fast luminosity tuning monitor.
Improving statistics on luminosity monitoring would be useful. Perhaps one might find thatsome parameters could be tuned without disturbing the calibration factor of the fast radiative
Bhabha monitor.
Page 7 of 22e) Crab Cavity Voltage Scan
The Committee was invited to observe the procedure for scanning the crab cavity voltages tofind their optimum values for luminosity. The procedure, explained by our colleagues in thecontrol room, is as follows.
First the crab voltages are set to their nominal values. The phase of the HER cavity is
increased by 2°, followed by the same value for the LER cavity. This is repeated until 6° isreached. A difference in phase of 2° leads to a difference of -30µm in the beam-beam kick. If
a 6° offset on both leaves the kick unchanged the cavity ratio is correct. If a change is seen
then the ratio must be adjusted and the process repeated. This took about 20 minutes for one
cycle. The order and sign of the changes is important. A wrong move leads to beam loss
(HER orbit negative giving beam loss and radiation dump) and a time lost of ~ 45 minutes.
With the correct voltage ratio the voltages are scanned keeping the ratio constant. The
observable is the total luminosity which can be acquired every 10 seconds. A number ofmeasurements (twelve) for reasonable statistics are made at each voltage pair. Changing thevoltages is the most time consuming part as only small incremental changes, to prevent cavity
trips, are allowed at the present time. The total scan took about 60 minutes
It was impressive how easily and quickly the data was recorded and then presented ingraphical form with curve fitting.
The result of the scan, carried out with 1.55 A in the LER and 0.81 A in the HER, gave a veryclear optimum voltage pair 0.796/1.32 MV, very close to the previously found value of
0.784/1.30 MV.
f) Luminosity Operation
The KEKB accelerator is operated to produce B mesons for particle physics studies. The
operation of KEKB has been exceptional for many years. The total delivered integrated
luminosity to BELLE is the largest particle physics data sample in the world at over 750 fb-1.
This is a very significant achievement.
Recently, the control room efforts have been centered on delivering as much integrated
luminosity to the BELLE detector while doing as much as possible to advance the efficiencyof the crab cavity system along with the beam-beam effects to produce higher luminosity. At
the present, KEKB has produced a peak luminosity of over 1.4x1034/cm2/s with the BELLE
detector and crab cavities operational. The near term luminosity goal is to get back to theformer peak luminosity of 1.7x1034/cm2/s and in the longer term to exceed this value by up toa factor of two.
The present operational mode is to produce data for BELLE but adjusting parameters slowly
and parasitically to optimize luminosity and to find new better accelerator parameters. A
simplex optimization code is used to find the best combination of 12 accelerator parameters.
Six parameters are used in each of the Low Energy Ring (LER) and six in the High Energy
Ring (HER). In each ring there are four interaction region coupling variables and two
dispersion knobs. These efforts have kept the peak luminosity near the recent maximum.
More extensive accelerator studies have been tried periodically to increase the peakluminosity, with the recent concentration on changing the horizontal beta function at theinteraction region. Definite luminosity changes and improvements have been seen as a resultof these studies.
Page 8 of 22

The simplex optimization code is designed to be very robust and not get stuck in local minima
but, as a result, does not converge rapidly. Other algorithms exist which work constantly at
the peak of the luminosity trying to find better condition although they may not be able to
avoid local minima. The tuning and filling cycles of the KEKB accelerator make the iteration
times long for the present optimization code and a more rapid code may well be better.
Recommendation: The ARC committee suggests finding a parameter (luminosity)
optimization algorithm which works rapidly at the peak luminosity to improve parameters.
This algorithm should be combined with trying, from time to time, very different starting
conditions to avoid local minima.
Recommendation: The ARC committee suggests finding methods to reduce the time needed
to refill the KEKB rings after a beam abort. A reduced time will speed the luminosity
optimization as the optimum accelerator parameters likely drift with time during refilling.
Recommendation: The beam-beam simulation code should be upgraded to include theactual crab cavity and transport lattice to and from the interaction region with chromaticeffects. These code improvements will likely offer new handles on the errors which can affectpeak luminosity and the beam-beam parameters.
Recommendation: Vertical jitter has been shown to affect peak luminosity. A renewed
effort should be made to understand all the sources of vertical jitter in both rings and possible
reductions.
Recommendation: The beam-beam simulations show that smaller vertical emittances are
better for beam-beam effects. The present vertical emittances are about 1% of the horizontal.
The committee recommends trying to reduce the vertical emittance to about 0.5 % as thebeam-beam simulation suggests about a 25% luminosity improvement with this reduction.
Recommendation: The luminosity responds to the tuning knobs. The knobs should beorthogonal as possible since non-orthogonal knobs makes it difficult to control errors in otherparameters, which are sometimes very sensitive to small changes. The committee
recommends checking the orthogonality of the IP tuning knobs and fixing them if needed. If
the knobs are only partially orthogonal, then the very sensitive knobs should be scanned more
often than the less sensitive ones.
Recommendation: Information from the position monitors should be fully exploited tounderstand transient and long term effects, as well as in the routine correction of BPM data.

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Section 2: Presentations
1) Commissioning Progress before JulyAs reported by Koiso-san in the March, 2007 meeting of this Committee, the crab cavities
were commissioned successfully, checking optics, crabbing angles, and beam stability. Evenat this early stage (less than one month experience with beam) the vertical beam-beam
parameter in the HER had increased by more than 50%. The work was performed with alimited number of bunches.
During the period from March 20 to June 30 covered in this section the beam currents wereincreased to production values, the horizontal emittance was varied, and the luminosity passed
10 /nb/sec. Crab cavity hardware development is covered in detail in another section of thisreport. During this period the voltage limits on the crab cavities were 1.0 MV for the LER
and >1.7 MV for the HER cavity.
With an achieved luminosity greater than 10 /nb/sec the concept of crab compensation ofcrossing angle was validated. Beam-beam parameters were better balanced, with all being at
or above 0.09. Beam currents when optimized were found to be closer to the energy
transparency condition than with the crossing angle and the optimum vertical betatron tunes
increased as predicted by simulation.
Specific luminosity performance quickly exceeded that with crossing angle, indicating thatthe geometric penalty was fully compensated. While the lower current specific luminosity
was further improved, reaching a higher vertical beam-beam parameter ,the change of specificluminosity with current2 was steeper than with crossing angle, intercepting the latter at a value
around 1 mA2. Strong-strong beam-beam simulations in fact indicated that the high predicted
luminosity was easily compromised by small errors in any of several parameters.(e.g., 1 mradcrossing angle)
Higher LER and HER emittances were briefly explored; however, specific luminosity was
unchanged and current limits only slightly changed. The original (zero current) emittances of
18/24 nm (LER/HER) were finally selected.
Because of the importance of the 2Qx+Qs resonance in the tune plane, and to reduce the bunch
length, a negative momentum compaction optics was tried. This was successful in that theHER could be operated with horizontal tune below this resonance and the bunch length was
reduced from 6 to 4.5 mm. However, overall luminosity was not improved and this optics
was abandoned when a strong longitudinal oscillation was seen in the LER.
During this period an asymmetry in lifetime with respect to horizontal offset scans wasobserved. This phenomenon persists unexplained today.
An important early test result was the confirmation of the ability to store beam currents
similar to before crab cavity installation if the cavities are detuned. This result reducesconsiderably the likelihood of having to remove a ring cavity from the ring due to poor
luminosity performance.
Additional issues studied during this period included the reduction in dynamic aperture due tothe beam-beam interaction (88 minute HER lifetime predicted at 1.2 mA/bunch) and thedependence of luminosity on the initial vertical emittance.
Comment: The Committee congratulates the KEKB staff and management on the excellent
work in installing, commissioning, and understanding the properties of the crab cavities andPage 10 of 22

the crab compensation process. During this period many aspects of this mode of operation
were studied and the groundwork was laid for the developments that followed.
2) Commissioning Progress since October
KEKB restarted fullintensity
operationwith crab cavities in
October
after thesummershut-down.
During this period,
bunch spacing waschanged from 3.5 to
3.06, and ßx* changed
from 68 cm to 90 cm.
The present status issummarized in the
figure above.
More specifically:a. Simulation and thespecific luminosity(Lsp) data agree forthe case without
crabbing.
b. Simulation predicts a gain of Lsp by a factor of ~2 with crabbing, yielding a maximum
tune shift of 0.15.
c. With crabbing, Lsp agrees with simulation around IbunchLER * IbunchHER = 0.25 mA2. Abovethese beam currents, it falls off rapidly as beam currents are increased, while below thesebeam currents, there is no evidence of agreement between data and simulations.
d. The beam lifetime deteriorates when the tune shift reaches ~0.08. The red data points arewhen beam lifetime reaches ~100 minutes.
e. Simulations show that Lsp is extremely
sensitive to setting parameters. In addition to
the list of many standard parameters such as
beta*’s, betatron tunes, chromaticities, andbeam collision orbit displacements, Lsp isfound to also depend sensitively on crossing
angle, orbit noise, and the 12 Knob-1
parameters. The parameters space is therefore
huge, perhaps 24-dimensional in all.
f. Simulations indicate (already during theprevious Review) that one needs to be very close to the optimum in order to find the
optimum. Otherwise, a broad shoulder of Lsp is reached in the 24-dimensional parameters
space, falling short of the factor of ~2 from the sharply peaked optimal value. Careful but
lengthy optimization procedures have been designed and followed. So far no extreme
sensitivity has been found, indicating we are not close to the optimum. Assuming that thisPage 11 of 22

picture is valid, the conclusion is that, with all the efforts invested, the red data do not
represent the optimum values but rather the broad shoulder values.
g. Example of sensitivity (i): Lsp drops sharply when the crabbing angle is not exactly 11
mrad. Tolerance is ~0.1 mrad. Note that the crabbing angle measurements using streak
camera has an error bar >> 0.1 mrad limited by the insufficient light from synchrotron
radiation.
h. Example of sensitivity (ii): Simulation indicates a sensitivity to orbit noise, which seems
to be consistent with the observation that Lsp improves when the orbit feedback gain is
reduced.
i. The present effort concentrates on pushing for higher beam currents along the red curve.
As mentioned, it is possible that we are just staying on the shoulder values, not the
optimum values, of Lsp.
j. An apparent asymmetry is observed in beam lifetime when the horizontal collision orbit isscanned. The cause of this asymmetry is so far unidentified.
Recommendations
Instead of concentrating on pushing for higher beam currents along the red curve, it might bemore critical to search for the sharply peaked optimum at fixed beam currents. At present, the“red ceiling” stands as the real critical path limiting the crab cavity performance.
Investigations should emphasize how to break this ceiling at fixed beam currents instead of
pushing for higher currents. Long bunch spacing (therefore low total currents) can be used inthese investigations.
The above investigation could include: (a) finding the best optimization algorithm, (b)identifying the necessary diagnostics tools, (c) simulation including measurement errors andcorrector setting errors, (d) simulation with “missing knobs”, (e) understanding why Simplex
scan so far did not yield the Lsp predicted by simulation.
o(a) finding the best algorithm: A simplex algorithm has been developed. This algorithm
deals with all scanned parameters on an equal footing in contrast to a more traditional
approach which deals with each parameter in single-parameter scans. Simulations indicatethat both approaches give basically the same optimal Lsp. In an environment of a multidimensional shoulder with a sharply peaked optimum, however, the effectiveness ofdifferent algorithms might be very different. It is suggested that a combination of Simplex
and traditional algorithms be tested on a wide range of parameters to look for much
shortened turn-around times.

o(c) and (d) more simulations: To understand why simulations seem to produce muchbetter optimum than actual operation so far, it is suggested that measurement errors andcorrector setting errors be included in the simulation. Also, since the parameter set isnever 100% complete, it is suggested simulating some cases with “missing knobs”. If
some of the scanned parameters are left out, can the optimum still be found in thesimulation?

o(c) and (d) more simulations: Since the picture of a sharply-peaked-optimum-on-a-broadshoulder (SPOOABS) is somewhat unexpected, and is so critical in guiding the next
efforts, it is suggested that this feature be tested with another established beam-beamsimulation code for benchmarking. Confidence in this picture will be substantiallyenhanced if another code predicts similar results.
Page 12 of 22

o (e) Understanding the mechanism: So far, there is no experimental confirmation ofsimulation results except that they agree at the point IbunchLER * IbunchHER = 0.25 mA2. To
establish confidence of the simulation results, it is urgently needed to explore the region
when beam currents are < 0.25 mA2. The expectation is that data and simulations would
indeed agree in this region, but this has yet to be established. Once confirmed, this study
will also give a sharply defined beam-beam limit in KEKB.
o More on (e): If after some studies, the SPOOABS picture turns out not applicable, thenext efforts should not be looking for the sharp optimum. An alternative approach would
be to see if Lsp can be increased by relaxing on the beam lifetime requirement. In thispicture, one assumes the red ceiling is simply due to some beam tails generated by beambeam, followed by some aperture limitation. A test is suggested to relax the beam lifetime
requirement to see if that would break the red ceiling. Along the same line of reasoning,
intentional orbit deviation and/or scraper settings can be tried to study the aperture
situation as needed. On the simulation front, it is suggested that a systematic investigationof beam-beam tails and beam lifetime be carried out, which so far has understandably not
been the emphasis.
o Still more on (e): Considering the breaking of the “red ceiling” to be the critical path, it issuggested to push for higher Lsp fixed beam currents at 49-buch spacing. The result so far
when the “green ratio” reached 110% is particularly encouraging in the sense that asubstantial further increase by any means would be considered breaking the ceiling, andthus of great significance. Ways to further increase beyond 110% could include reducing
β*x from 100 cm to 80 cm.
o Because of the operation near half-integer tunes, there are well-defined synchrobetatronresonances very near the working point, particularly 2Qx + Qs and 2Qx + 2Qs resonances.
It is believed that these resonances are not driven by the crab cavities but by sextupoles
and other nonlinearities in the lattice. It is suggested that the source of these resonances be
first identified and calculated, yielding two strength coefficients: one “cosine harmonic”
and one “sine harmonic”. Two control knobs (a “cosine knob” and a “sine knob”) could
then be designed consisting of stringing up some lattice magnets (quadrupoles and/or
sextupoles). These knobs can then be tweaked during operation to compensate for thedriving terms of the resonance. Since these resonances are located close to the working
point, their compensation might help to improve the beam lifetime.
o Concerning the apparent asymmetry of lifetime observed when collision orbits are varied,
if it persists, it is suggested that effort be made to understand its mechanism. In casesimulation is used to do this, it should emphasize understanding the mechanism rather
than only getting the results to agree. One possible mechanism, for example, could be an
aperture limit induced by the beam-beam displacement and/or kick.
o At present, the simulation runs are time-consuming. It is suggested to try to speed up thesimulation procedure either by streamlining the code or by running a smaller well-selected
set of cases. When a more efficient simulation code becomes available, it is further
suggested that the simulations be more integrated into the actual machine operation. In
particular, it could be used to guide the optimization procedures, or even to eventually
become part of the control software.
3) Crab Cavity Operation

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The crab cavity is now demonstrably a fully operational device working in the presence ofhigh beam currents in both the LER and HER rings. Full congratulations must go to the teamthat has produced this excellent result.
At low beam currents 1.6/1.7 MV been achieved in the LER/HER cavities respectively,
beyond the design values. During a high-current physics run in June this year 1.0/1.7 MV wasachieved in the presence of beam currents of 1.35/0.8 A, currents that were not limited by the
crab cavities themselves! Typical actual operating voltages for physics are now 0.8/1.3 MV. It
is important to note that no single beam instabilities, attributable to the crab impedances, havebeen observed. The dampers work as predicted; the HOM + LOM power level is within
specification taking into account for the LER the power absorbed by the external SiC damper,and the spectrum of beam-induced parasitic modes is as expected. The calibration of the
crabbing voltage and phase using the beam has confirmed test bench measurements, though aclosed orbit bump is needed to place the beam in the electrical centre of the cavity andminimize the power requirement.
With a world “first” that comprises sophisticated cavities never before seen in a machine, it isclear that there will be some issues to contend with initially. The most important, from theCommittee’s point of view, are as follows.
Trip rates: The changing trip rates of the LER and HER cavities can perhaps best be
understood by following their operational history. The voltage capability of the LER cavity
dropped during the first few months from 1.6 MV to 1.3 MV and then, following aparticularly strong quench in March, went to 1.1 MV which is now the maximum value
obtainable even after conditioning. This suggests that some damage has occurred to thesurface of the cavity. A measurement of the unloaded cavity Q might lend some weight to this
supposition, but this is a difficult measurement to do in the ring. Perhaps a method is tomeasure the Helium consumption at high cavity voltage. Eventually it will be necessary to
remove the crab cavity from the ring to examine the surface and possibly re-treat it. For thepresent the cavity runs routinely at a lower voltage of 0.88 MV and trip rates are low. A
voltage higher than the design value can still be achieved in the HER crab cavity. However
the trip rate at the operational voltage has risen to 2-3 trips per day. Frequent shortconditioning periods of the cavity and the coaxial line (the latter by displacing the line
horizontally) have not improved the trip frequency, but the reason is not clear. It may bepossible to improve the situation by a much longer conditioning period pushing the voltage at
all frequencies within the range of the tuning system.
The turn around time following an abort is about one half hour so these trip rates, whileannoying, are deemed acceptable in the short term. Pollution of the cavities from the“external” warm vacuum system must be minimized and in this context the installation ofsome trap, maybe superconducting, on either side of the cavities could be envisaged.
The tuning system. The excessive LER motor driven tuner backlash and the demise of thepiezo-tuners leading to drifts of up to ± 15° in phase, is compensated to a large extent by the
low-level beam control loops. This is not entirely satisfactory, especially with the possibilityof the coupled beam instability (see below). We agree with the RF group wish to address andsolve the problem of backlash and develop reliable piezo-tuners soon.
Coupled beams instability. With high currents in both beams a large oscillation in crab
amplitude and phase at 540 Hz was observed. A model to explain this has been proposed
involving coupling of the two crab RF systems by the beam-beam kick and the associatedbeam displacement at each crab cavity. In this case the observed frequency should depend on
the low-level loop parameters. A cure has been found experimentally by shifting the crabbing
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phase and de-tuning the cavities. The resulting offsets in closed orbit beam position are
corrected by bumps. This model should be confirmed by theoretical analysis, simulation andbeam studies. This instability is important for other colliders wishing to use crab cavities.
Crab vacuum: LER cavity shows a high total pressure reading of ~7x10-6 Pa on the cold
cathode gauge (CCG) near the coaxial line. This gauge is located at the entrance of a 500 l/s
ion pump. A rough estimate of the gas load, neglecting the pumping through the annular spacearound the coaxial pipe is 3.5 10-3 Pa l/s. The residual gas composition from a mass
spectrometer is consistent with an air leak. However, the committee has been informed that a
localization of this leak from the outside of the system has been unsuccessful.
Under this assumption, the possibility can be investigated whether a significant quantity of air
has remained trapped in a ‘dead’ volume inside the system. With the help of detailed
drawings, at least one possible candidate could be identified. The closed-off volume could beat the bottom of an M8 tapped hole for the support structure of the coaxial line, which has no
specific provision to be evacuated. There are three mounting bolts in each cavity. The
combination of a stainless steel bolt in a soft copper block could be the equivalent of a leaky
vacuum seal. Assuming a trapped volume of 3 ml initially filled with atmospheric air would
amount to 3 102 Pa l. The ratio of the two numbers provides an order of magnitude estimate
for the time constant of the pump down ~ 105 s. This time constant is quite long, but still shortcompared to the total time the LER crab cavity has been operated and thus may not be theprimary reason for the observed vacuum degradation.
Recommendation: The search for the gas source should continue. The starting point should
be a repetition of a global leak check of the cavity assembly followed by an in depth search of
all subcomponents.
Recommendation: Independently, as an additional means to obtain information about gas
trapped by the cold cavity surface, the integral gas load released during each warm-up to
room temperature should be measured, e.g. it can reveal differences between LER and HER
cavities.
4) BELLE Status
BELLE is the particle physics detector installed on the KEKB accelerator. The BELLE
collaboration has about 400 members from 14 countries. This collaboration is very active.
The collaboration of the BELLE detector is very appreciative of all the data given them over
the years by the KEKB team. This data has given rise to many physics analyses leading tomany publications and discoveries. The recent measurement by BELLE of B  DTauNu wasvery challenging and a significant worldwide result. BELLE has also recently discovered
several new exotic states which will expand the world’s understanding of the B-meson system.
They have also taken data on other Upsilon resonances namely the Y(5S). The BELLE
collaboration may wish to run more on other resonances before the end date of March 2009.
Recommendation: The BELLE collaboration should work with the KEKB team to make astrategy to optimize the integrated luminosity between now and the end date of March 2009.
Recommendation: The BELLE collaboration should work with the KEKB team to develop
plans, if any, to run on other Upsilon resonances before the end of the run in March 2009.
5) Knob Tuning Simulation and More Theories on CrabIn line with recommendations of the previous Review, extensive work has been carried out on
simulation of the crab collision including the realistic modeling of the tuning procedure, andPage 15 of 22

including various imperfections present in real beam-beam operation: transverse wakefield
effects, noise of the orbit position, beam halo etc.
At present the achieved specific luminosity at bunch currents usually operated before, withcrossing collision, is 1.5-2 times lower than the simulation prediction for perfect collisionconditions. It is vitally important to prove that the presently used collision optimization
procedures are efficient in raising the luminosity. In simulation, seriously detuned initial
settings of Knob1 (12 parameters including the coupling parameters R1-4, vertical dispersion,and its slope in both rings) resulted in a reduction of the luminosity by a factor 3. The
parameters were iteratively scanned and corrected one-by-one to maximize the luminosity.
The 3rd iteration already recovered the “design” luminosity.
This conclusion might be too optimistic, because i) during the first iteration cycle, theluminosity maxima vs scanned parameters were so flat that the measurement error (notrepresented in simulation) could easily fake the maximum position which at the very least
would result in slowing down the convergence; ii) in reality the parameter set in eachparticular optimization is not complete, e.g. the Knob1 set does not include waist position, x-y
coupling at the crab cavity, other parameters combined in Knob2 and Knob3, collision phaseerror, etc; and the incomplete parameter set has not yet been simulated; iii) it is not clear
whether the degenerate action of certain knobs on luminosity can always be resolved; iv)
some level of random change in parameters from hardware drift/instability will be present in
the real machine.
It is important to continue the realistic tuning simulation with the feedback from the control
room tuning procedures where the scans are sometimes limited to a narrow range due to beam
lifetime limits, which means that the complicated profile of the luminosity maxima may be
misinterpreted.
A considerable effort was invested in weak-strong simulation of the beam halo with the aim to
explain the mysterious observation that with high bunch currents in collision, the (weaker)
beam lifetime is asymmetrically dependent on the horizontal collision offset. Although thevertical beam halo was clearly dependent on the vertical size of the strong beam as well as thehorizontal collision offset, the latter dependence was always symmetric in simulations.
Further work is recommended on the weak-strong lifetime simulation including the machine
nonlinearities; this could reveal asymmetries in the dynamic aperture vs horizontal collisionoffset.
Effects of the wakefield on the crabbed bunch were also evaluated with the suspicion that
additional bunch crabbing can appear or the effective vertical emittance may suffer frombending the longitudinal bunch axis in a head-tail type motion. From simulation, this effectdoes not appear to be serious for the KEKB parameters,.
Finally, the realistic evaluation of the Touschek lifetime with the dynamic aperture reducedby the beam-beam interaction is reportedly under way. This work is important because thiseffect can put a lower limit on the minimum vertical beam sizes at collision.
Recommendation: Continue efforts on realistic simulation of the luminosity tuning,
optimization of the beam lifetime, simulating interplay of the machine imperfections, such as
nonlinearities and noises, with the beam-beam effects.
Recommendation: Augment the simulation of luminosity optimization with additionalparameter errors not included in the scans or simplex, realistic noise in measurement ofluminosity or beam size, and some random change in parameters representing drift in machine
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conditions. Work with KEKB staff to determine limits for these effects that would prevent
reaching an optimum tuning condition in machine operations.
6) Fuji Test Beam LineThe Fuji Test Beam Line (FTBL) was
proposed to provide a GeV class test
beam following the shutdown of the
KEK 12 GeV PS. The facility is basedon the use of bremsstrahlung photons
created by scattering 8 GeV electrons
off residual gas in the HER Fuji straightsection. The photons are converted toe-/e+ pairs in a target creating electrons
with a sharp forward peak, sufficient
for a test beam of a few GeV/c.
The beamline was designed to re-utilize
as many components as possible, inparticular spare magnets from KEKB,
recycled magnets from Tristan, and power supplies from the PS beamlines. However, a new,
purpose-designed vacuum chamber was required. A peculiarity of the line is that the magnets
are rolled up to 30° so the layout and magnet supports had to be designed carefully.
Construction began in 2006 and was completed in September 2007.
The first test beam was observed on October 12th, soon after the KEKB operation started in
October 2007. The FTBL magnet leakage field was observed to affect the KEKB operation so
the leakage field shields were improved. The FTBL has started operation with test beams and
FTBL commissioning will be continued.
The Committee congratulates the FTBL team for an extremely successful project. The Fuji
Test Beam will be an important facility for developing new detectors.
7) KEK Roadmap and KEKB’s future
The Director General of KEK presented the preliminary Roadmap for KEK. The boundary
conditions to be considered included the end of construction of J-PARC, foreseen for 2009;
continued operation of KEKB to produce an integrated luminosity of 1 ab-1, also foreseen for
2009; and continued operation of the photon factory. Outside of KEK, results from the LHC
and progress in the engineering phase of the ILC will affect the future of KEK. Starting fromthis scenario, it is proposed to continue J-PARC operations through 2020, including upgrades
to the facility. Construction of SuperKEKB would begin as soon as KEKB has reached theluminosity goal with operation expected to begin around 2012. A compact ERL would bebuilt in parallel to SuperKEKB as a test bed for a larger ERL light source.
This ambitious plan is now being presented to the government funding agencies and istherefore subject to change. The Committee strongly supports the proposed construction of
the SuperKEKB and endorses the Director’s plan.
8) BPM Displacement Monitors
Movement of the BPMs in sextupoles is a critical operational consideration. 100 µm errors
can introduce significant coupling and optics distortion. Following on preliminary work done
last year, many sextupole to BPM displacement monitors were installed in both rings during
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the summer shutdown, bringing the total number to 104 in the HER and 108 in the LER. In
addition a few BPM displacement monitors have been installed to sample quad motion andbeam duct motion relative to the floor.
The temperature coefficients of the position measurements, primarily caused by the linear
coefficient of thermal expansion of the materials, were carefully studied. The largest was in
the expansion of the target, which is heated more directly by beam current (10° C change).
This introduces about 10 µm error from no beam to full beam current.
Data were collected from all position sensors during an initial filling of the rings. The shifts
in position in the LER sextupoles followed an exponential-like approach to final values withan rms shift of 15-20 µm and a few transient motions of ~120 µm. The HER sextupoles
exhibited a similar shift but of 20-25 µm rms plus an initial transient as large as 500 µm onseveral individual magnets. Corrections for temperature changes were not made. Duringcontinuous top-up filling operation the changes were generally less than 10 µm. The motions
from initial filling were used in a SAD simulation to determine the effect on the optics.
Following the magnitude of position changes, the HER showed generally larger changes in
the optics parameters – β*H,V changing by 3%, 4% respectively and coupling parameterschanging by 1 to 6 tuning units. For the LER the β*V changed by 4% and coupling
parameters by 0.2 to 2.7 tuning units.
The magnitude of these changes is expected to affect luminosity performance, though, to date,a clear correlation has not been established. Data from the position monitors are used tocorrect BPM data to the sextupole centers while recording orbits and performing global orbit
feedback.
Comments: The Committee applauds the excellent work accomplished on the BPM
displacement monitoring system. The data provided will be essential to controlling andoptimizing machine tuning and optics.
The large transients seen in the HER during initial filling suggest a source of change in
conditions observed after a beam abort where the transients may be significantly larger.
Motions this violent may push hardware beyond elastic limits.
Recommendation: Information from the position monitors should be fully exploited tounderstand transient and long term effects, as well as in the routine correction of BPM data.
9) Tune/Orbit Measurements
The gated BPM technique developed at KEKB makes it possible to perform bunch-by-bunch
position and oscillation measurement, which provides a lot of information important for
multi-bunch operation. Bunch-by-bunch betatron tune measurements proved to be useful inprevious electron cloud studies including the electron cloud wake measurement.
This time three different studies have been performed:
1) An attempt to get the horizontal bunch size from the profile of the coherent beam-beamkick measured as a function of the horizontal collision offset was done. The gated BPMs inboth LER and HER are located at a betatron phase advance from the IP that is sensitive to thehorizontal beam-beam kick while scanning the horizontal collision offset. Account has beentaken of variation of the dynamic betas with the opposite bunch current. The measurements
done with the crossing collision are compared with new ones taken in crab collision. While
the former are in quantitative agreement with the expected size reduction at higher bunch
currents, the latter indicate smaller sizes due to stronger beam-beam effect with the head-on
collision and some quench in the profile curve at about 100 microns collision offset, if the
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currents are high. This measurement may be interpreted as signature of the serious blowup of
the horizontal bunch size caused by the collision separation by about 1 sigma. A correlation
between the size reduction seen below this blowup and the nominal horizontal emittance (i.e.,with the initial size) is also experimentally observed.
2) Bunch-by bunch gated betatron tune measurements during collision provide the response
spectra with gated excitation under many different conditions. These spectra can be comparedwith the response of the pilot bunch as well as with data from an arbitrary bunch chosen
within the train and missing the partner bunch in the opposite bunch train, i.e., with an
ordinary bunch out of collision. While the full analysis of these data is complicated by thenonlinear nature of the beam-beam interaction, some features are already understood and thebeam-beam parameter is evaluated from the measured spectra. The vertical tune shows anapparent saturation at low currents at a level of about 0.04, which is less than half the
maximum attainable value as estimated from the luminosity. This discrepancy may signify
complex current-dependent changes in the vertical tune spectrum, which are misinterpreted
when using it for evaluating the vertical beam-beam tuneshift. On the other hand, thehorizontal beam-beam parameter is measured up to about 0.1 and shows no saturation withcurrents. Some disagreement with the expected value is probably due to using a constantemittance assumption in processing the measured data instead of the actual dynamic
emittance. Many of the features of this technique, particularly, the dependence of the spectraon the excitation amplitude (no surprise in a highly nonlinear system) may be understood bysystematic comparison of the measured spectra with those obtained in strong-strong
simulation with adequate conditions.
Recommendation: Provide the full set of strong-strong collision simulation results relevantto the gated bunch spectra measurement in order to improve their interpretation. Consider the
potential application of the gated tune measurement technique for the luminosity
optimizations in operation.
3) Recently, bunch-by-bunch orbit displacements have been measured with the gated BPM.
While the vertical displacements are not systematic with the bunch number and generally
small (except for the LER bunch train head), the horizontal displacements show a clear trend
from the bunch train head to tail amounting to about 110 microns in LER and 300 microns in
HER, at the gated BPM position. The observed trend is in correlation with the 3 degree
synchronous phase shift over the bunch train due to beam loading modulation of the RF
resulting from the abort gap in the beam filling pattern. The systematic bunch-by-bunchhorizontal orbit displacement may be caused by this systematic trend in the bunch arrival
phase at the crab cavities resulting in the (linearly) different orbit kicks seen by the bunches.
In terms of the IP orbit, the horizontal collision offset is estimated as the HER-to-LER
difference of 40–19 = 21 microns at 1500/800 mA beam currents. As was shown, the detailed
bunch-by-bunch signal has a correlation with the employed 3334… filling pattern, possibly
coming from the wakefield signal. If the result is confirmed by further measurements of thebunch-by-bunch orbit displacements, the observed effect may become critical for raising thebeam currents in collision. Presently, for better lifetime at collision with high bunch currents
the HER bunch should be set at a 10-20 microns outward horizontal offset; this requirement
cannot be met for all bunches in the train with the existing hardware.
Recommendation: Work on further clarification of the bunch-by-bunch orbit displacement
effect and study its relevance to the luminosity limitation.
10) Wiggler, antechamber and collimator R&D
Chamber elements
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A detailed update of the R&D work for the KEKB vacuum system was presented as acontinuation of the work shown during the last Review. The production and first installationof several Cu beam ducts with antechamber design was completed during the summer. The
new components represent a total length of about 30 m in the NIKKO wiggler section.
The vacuum chambers contain beam ducts for dipole and quadrupole magnets, bellows withcomb-type RF shields and BPM units. Enlarged solenoids have been wound around the new
drift chambers. Vertical steering magnets with larger aperture have been prepared and fit
around the antechamber profile. The inner surface of the beam channel, exposed to the beam,
has been coated with a 200 nm thick TiN layer to achieve a low secondary electron yield.
After beam scrubbing it is expected to reach a δmax value of 0.84. The coating is done at KEKwith a dedicated system, which accommodates up to 4 m long vacuum pipes. Further design
work will be necessary to optimize the beam duct shape for Super KEKB for impedance,
coherent synchrotron radiation and deformation of the vacuum chamber due to heating.
A first attempt has been made to produce curved beam pipes as required for dipole magnets.
Bending magnet vacuum chambers will contain a linear NEG pump assembly on the innerside of the antechamber to increase the pumping efficiency in the arcs. In view of the highthermal power load, it is proposed to replace the stainless steel end-flanges by special copper
alloy flanges, to take advantage of the better thermal conductance. This should reduce the riskof leaks due to thermal gradients in case a flange is exposed asymmetrically to synchrotron
radiation. This aspect may be particularly relevant for the arc chambers where synchrotron
radiation preferentially strikes along the outer walls of the chamber.
From a vacuum point of view the new wiggler section has performed very well. The dynamic
pressure graph presented to the Committee shows a decrease with beam dose consistent withexpectation. The new comb-type bellows installed in the ring have performed without
problem. The unfortunate fact that a vacuum leak appeared during this first running-in period,
however, illustrates the increased importance of the vertical alignment of such wide vacuum
chambers with respect to the synchrotron radiation fan. The Committee encourages thevacuum team to continue this development work and to install vacuum pipes with
antechambers including the new RF bellows over a representative length of bending magnets,
to study vacuum performance, mechanical movements and the need for any additional
supports.
Movable mask
The ongoing R&D work for a movable mask was presented to the Committee. The previous
version has been based on a copper coated head mounted on a Ti-coated Al2O3 support.
During beam tests in LER, this model was destroyed due to evaporation of the copper coating
and subsequent melting of the alumina head. The failure is attributed to several effects: thechange of the dielectric coefficient with temperature, degraded heat conduction and a lower
than assumed emission coefficient as the mask heats up. A new proposal was presented to theCommittee, which aims to overcome the deficiencies of the old design. To improve thetemperature capability of the head and to increase the heat conduction of the support post, it is
proposed to make the support out of artificial diamond, which has more than twice the heat
conductivity of copper. The temperature of the diamond head is limited to 1500°C; above thistemperature graphitization occurs. At this moment, it remains to be shown that artificial
diamond can be obtained in the required size and shape. The mask head for the new version
will be made of graphite, which in principle is capable of withstanding temperatures of up to3000°C. ANSYS calculations have been made to estimate the performance of this design and
it is planned to validate the concept with a prototype mask when it becomes available.

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The Committee acknowledges, that the mask remains a very challenging machine component
in Super KEKB. The new mask version 6.1 is promising, but the Committee is concerned thatweak elements of this design could be the joint between the graphite head and the diamondsupport, as well as the thermal contact between the support and the water cooled heat sink.
These basic aspects should be validated independently before a prototype is constructed.
Clearing electrode test for dipole magnet chamber.
Clearing fields for removing electrons from an electron cloud have been proposed as apossible remedy for many high current machines. A solenoid field, which can be appliedalong magnet free sections does not work inside dipoles or quadrupoles. Depending on the
beam parameters, a relatively weak field should be sufficient to extract the electrons along themagnetic field lines. The purpose of the proposed test system for the LER is to study theelectric clearing mechanism and to confirm that the electron cloud can be locally suppressed.
The proposal is to install a clearing electrode and an electron detector in a LER wiggler
magnet. To obtain more detailed information about the cloud distribution, an array of 7
detector electrodes has been proposed. As compared to other experimental set-ups, e.g. the
LHC test system at CERN, the bunch length in the LER is very short. Up to an estimated 150
W could be dissipated in the test chamber. The system is now under construction and planned
for installation early next year.
Since the e-cloud effect is likely to remain a performance limitation for the LER also in the
future, the proposed program is strongly supported by the Committee.

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Appendix A
KEKB Accelerator Review Members
Andrew Hutton

JLab (Chairman)

Alexander Chao

SLAC

Warren FunkJLab – unable to attend

Oswald Gröbner

CERN (retired)

Trevor Linnecar

CERN

Won Namkung

POSTECH

Flemming Pedersen

CERN – unable to attend

Eugene A. Perevedentsev

BINP, Novosibirsk

David Rice

CornellJohn Seeman

SLAC

Wang Shuhong

IHEP, Beijing – unable to attend

Katsunobu OideKEK Secretary, Accelerator

Shin-ichi Kurokawa

KEK Secretary, Accelerator

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