The Eleventh KEKB Accelerator Review Committee Report

Introduction
The Eleventh KEKB Accelerator Review Committee meeting was held on March 20-21, 2006.
Since John Seeman from PEP-II at SLAC was unable to attend, Yunhai Cai, an accelerator
physicist at PEP-II, replaced him for this meeting. Dave Rice, Flemming Pedersen, and Shinichi Kurokawa were also unable to attend this meeting. Appendix A shows the present
membership of the Committee.
The Eleventh Committee meeting followed the usual format 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 first day concentrated on KEKB performance and, inparticular, the progress on the crab cavity, and this was continued on the second day. The
Committee was again impressed by the high standard of the talks, both the technical content
and the presentations themselves. The recommendations of the Committee were presented to
the KEKB staff members before the close of the meeting. The Committee wrote a draft reportduring the meeting that was then improved and finalized by e-mail among the Committee
members.
Contents
Executive Summary
A. Foreword
B. Summary
C. Recommendations
Findings and Recommendations
1) KEKB performance and near term plans
2) Progress since the last ARC
3) Physics by Belle4) Optics
5) Vacuum components R&D
6) Electron Cloud Instability measurement
7) Beam measurements
8) Solenoids in Quads
9) New QCS
10) Injector Upgrade11) Crab Cavity Overview
12) RF characteristics of crab cavity
13) RF couplers of crab cavity
14) Cryostat, jacket, etc.
15) Peripheral devices16) HOM absorbers17) Horizontal test of crab cavity18) RF system for crab cavity19) Installation of Crab cavity
20) Beam Commissioning Plan with Crab Cavity
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Executive SummaryA) Foreword
KEKB continued to make excellent progress since the last Committee meeting. The peakluminosity has increased to a new world record of 1.627 x 1034 cm-2 s-1, 62% above the design
luminosity of 1034 cm-2s-1 (at this time last year it was 1.52 x 1034 cm-2s-1 and the year before it
was 1.16 x 1034 cm-2 s-1). The Committee congratulates the entire KEKB project staff on thisnew record. The BELLE detector has accumulated 563 fb-1, another world record (365 fb-1 at
the last meeting) and has published, or submitted for publication, a total of 163 papers inrefereed journals (120 at the last meeting). KEKB was recently evaluated by the Council for
Science and Technology Policy and was awarded the S-Equivalent rating – the highest
possible. This rating was awarded not only for the Physics output of KEKB, but also
explicitly for the machine performance. In addition, the Thomson Scientific Company has
selected 6 BELLE publications among the twenty “hot papers.” These are extremely
impressive recognitions of the quality of the work being carried out at KEKB.
The meeting was devoted to the present status of KEKB and, in particular, the accelerator
physics limitations to the luminosity and progress with the crab cavity. Unlike last year, there
was little news on the plans for upgrading KEKB to a luminosity of 2-4 x 1035 cm-2s-1. This isdue to the fact that the proposal for SuperKEKB has not been actively pursued by KEK. The
Committee continues to endorse the proposal for SuperKEKB, particularly in light of thenumber of High Energy Facilities that are planned to close in the next two to three years(HERA, PEP-II, and Tevatron).
The KEKB machine group has done an enormous amount of excellent work, and is to becongratulated on the impressive results that have been achieved.
B) Summary
KEKB continued to make excellent progress over the last year, and holds all of the worldwide
luminosity records: highest peak luminosity: 1.627 x 1034 cm-2 s-1, highest daily integrated
luminosity: 1.183 fb-1, highest seven-day integrated luminosity: 7.36 fb-1, and highest 30-day
luminosity: 29.02 fb-1. As was expected, the luminosity has continued to slowly increaseduring the year, but no large factors of improvement seem likely until the crab cavities can beinstalled. The crab cavities suffered a minor set-back during assembly, so the installation has
been put off until summer 2006.
The Belle detector has now accumulated 563 fb-1 and of these 198 fb-1 were obtained since thelast Review. The “Oide scenario” made several years ago was that 550 fb-1 would beaccumulated by this time – an amazingly accurate forecast. At the present rate, 1 ab-1 would
be accumulated by summer 2008. The crab cavities would advance this date by about a year.
Virtually all of the running has been at the Upsilon 4S state, but this year the accelerator was
tuned to the Upsilon 5S and 3S states for the first time. In the three-day 5S engineering run,
BELLE obtained more than four times the total integrated luminosity of CLEO, obtained
during 15 days running. This is an exciting new Physics opportunity for BELLE.
The Belle detector is working well and has had only minor difficulties with the dataacquisition system and the SVD detectors. Backgrounds are low, and losses during beam
injection are small. Continuous injection has been acceptable to the detector but has
necessitated suppressing 3.5 ms of data taking during each injection cycle. Overall, the Bellecollaboration is extremely pleased with the quality and quantity of the beam being provided
by the accelerator.
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C) Recommendations
Last year, the Committee made many recommendations, and the KEKB staff hasaddressed all of them. Some of these related to the priorities of the program and arereiterated below, together with recommendations for continuing the work that has
already been started.
1. The Committee strongly endorses the plan to assemble, test, install and commission one
crab cavity in each ring in the summer shut down of 2006.
2. The Committee endorses the studies to design a major luminosity upgrade (SuperKEKB)
aimed at a luminosity of 2-4 x 1035 cm-2s-1 and recommends that they be continued.
3. The Committee strongly endorses the plans for simultaneous injection in all four rings.
4. The Committee recommends continuing the studies of a movable mask which is‘electrically invisible’ (rod with ceramic support) and has a much lower loss factor. A
significant amount of R&D remains to demonstrate a reliable, operational movable maskwith a reduced loss factor.
5. The Committee feels that HOMs is one of the limiting factors in increasing the current.
Detailed studies of the vacuum components including proper damping and component
cooling should be continued.
The Committee has made new recommendations throughout the different sections below.
Highlights of these recommendations are summarized here.
6. The Committee supports the present approach to assembly of the coaxial coupler into the
crab cavity, and recommends that continued efforts be made to improve the alignment
fixtures, rather than searching for a new assembly approach.
7. The Committee recommends that the crab cavities should be installed in both rings at theearliest possible time, and commissioned, so that the benefits and limitations associatedwith them can be determined.
8. The Committee recommends that both cavities should be cryogenically tested with thecomplete RF system before any installation is started.
9. While the Committee believes that the crab cavity program will be successful, it requests
that the KEK management create a road map for KEKB operations under two different
scenarios:a. The crab cavity is successful and KEKB operations continue at ~400 fb-1 per year.
b. If after considerable effort, the crab cavity is unsuccessful, KEKB operations continue
at ~200 fb-1 per year.
It would also be extremely helpful for planning and optimizing operations if the KEK
management could provide an indication of the kind of information that would influence a
decision on SuperKEKB (e.g. new physics found at Upsilon 3S or 5S).
10. The Committee recommends studying carefully the tolerances of the new crab lattices tocheck if they are more susceptible to errors, such as orbit drift, and identify the causes ofthe luminosity fluctuation.
11. The Committee recommends measuring the nonlinear dispersion as an additional check of
the optics model.

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12. The Committee recommends that a close follow-up of the failures of the standard vacuum
components should be made to establish critical operating parameters, and to put limits onthem.
13. The Committee recommends studying the effect of C-type magnets mounted on theantechamber, adding a solenoid field, and reducing the secondary electron yield (SEY)(e.g. by coating or surface treatment) in simulations of the electron cloud densities on the
beam axis.
14. The Committee expresses strong support for further studies of low SEY coatings and fortesting these coatings under machine conditions.
15. The Committee recommends that an effort be made to understand the mechanism of the
sidebands, their role of being a signature of the electron cloud instability onset, and theirapparent correlation to the luminosity.
16. The Committee supports the proposal by the KEKB team to directly measure the photoelectrons in the quadrupoles; it may give clearer information on the electron cloud in thequadrupoles. Refined simulations of this effect should also be continued.
17. The Committee recommends that efforts should be made to estimate the power depositedat the joints between different parts of the coaxial coupler using the presently understoodmechanical tolerances.

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B) Findings and Recommendations
1)

KEKB performance and near term plans

KEKB operated very stably until the summer shutdown in 2005 when the machine optics waschanged to be compatible with the crab cavities. The start-up was not easy and it took manyweeks to recover the previous performance level. A new record luminosity of 1.627 x 1034
cm-2s-1 was established on December 19, 2005 indicating that the machine performance had
been fully restored.
The daily integrated luminosity took longer to recover, exceeding 1 fb-1 per day on January 22,
2006, and later reaching a new world record value of 1.183 fb-1 per day. The daily luminosity
has shown a much greater variability than before the summer shutdown, so the weekly andmonthly integrated luminosity records set in March 2005 have still not been exceeded.
Some fraction of the slower increase in long-term integrated luminosity is due to the overall
percentage of scheduled time that the complex was delivering physics, which decreased from
85.6% in 2004 to 78.8% in 2005 (it should be noted that these rates are extremely high for amachine of this complexity, which is being pushed to the limit). The increase in accelerator
downtime was confined to September and October 2005 and can be attributed to difficulties in
restoring the accelerator following the summer shutdown (the downtime in September 2005
alone exceeded the total accelerator downtime in 2004). Since the accelerator downtime has
returned to the historically low values, accelerator downtime is not considered to be a longterm risk to the physics program. The Belle downtime increased from 0.6% of scheduled timein 2004 to 8% of scheduled time in 2005. This increase was due to burning of the power
connectors to the Silicon Vertex Detector (SVD). The BELLE downtime has already returnedto the historically low values.
More importantly, the peak luminosity that is attainable is still fluctuating from day to day.
Problems with day-night variations in the beam position monitor readings were tracked down
to thermal problems with the cables exposed to the elements, and this was corrected by adding
insulation. There is now a suspicion (but no evidence yet) that the remaining variability may
be due to physical movement of the beam position monitors themselves due to warping of thevacuum chambers due to synchrotron radiation heating. This should be investigated further.
The KEKB Team are concerned that the data indicate that the possible luminosity is now
reaching a plateau. The Committee agrees that there is unlikely to be any major breakthroughin luminosity, with the exception of the crab cavity program. Pursuing the installation of the
crab cavities is therefore the highest priority. However, everyone should be aware that there
will undoubtedly be a learning period of several months, during which the peak and integrated
luminosities will be significantly below the recent record values.
2)

Physics by BELLE

Belle continued to accumulate data at a high rate in the past year. The integrated luminosity
was 159 fb-1 in 2004 and 189 fb-1 in 2005. In 2005, a peak luminosity of 1.627 x 1034 cm-2 s-1
was reached. So far, Belle has accumulated a total over of 550 fb-1 (compared with 300 fb-1 at
BaBar) and 565 million BBbar events. During the last year it has published 43 journal papers.
However, there seems to be a leveling off of the weekly and monthly integrated luminosity in
2006.

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One of the thrusts of Belle experiments has been the measurement of the parameters of theunitary triangle. With 386 million BBbar events analyzed, the value of sin2φ1 has beenmeasured to be 0.69 with 5% accuracy. This is consistent with the Standard Model, andserves as a solid anchor for explorations beyond the Standard Model. The B0 → ρ+ρ - isospin
analysis gives the most precise measurement of φ2 as 96º with 13% accuracy. The B→DK
Dalitz mode gives some handle on φ3. The b→d γ penguin or b→d gluon penguin modes
give a measure of Vtd/Vts ratio.
A second thrust of Belle is to search for new physics beyond the Standard Model. The B0→φ
K0 penguin is potentially sensitive to new physics. With input from Belle, a simple averageover all modes that specify sin2φ1eff is found to be 2.6 standard deviations from sin2φ1. Thismight be a tantalizing hint of new physics.
A number of new resonances have also been discovered with the available data. In addition, a
few engineering runs were made at Upsilon (3S) and (5S) energies during the past year. They
give integrated luminosities of 1.86 fb-1 on Upsilon (5S) and 3 fb-1 at Upsilon (3S).
There is some difficulty in the data acquisition system not being stable. In addition, there
seems to be some sensitivity of the luminosity monitor to changes in accelerator operation.
These problems are being handled by the Belle group.
3)

OpticsThe lattices suitable for the crab cavities were implemented in two stages, first in the HER
and then in the LER, after the summer shutdown 2005. After the implementation of the crab
lattices, the luminosity recovery following the shutdown took much longer than usual. Afterseveral months, both lattices were modified in such a way that the changes were isolated to beinside the Nikko straight section, retaining the phase advances across the section, and thisresulted in a new peak luminosity record. However, the fluctuation of the luminosity is (still)larger than the historical data prior to the summer shutdown. We recommend studying thetolerances of the new lattices carefully to check if they are more susceptible to errors, such as
orbit drift, and to identify the causes of the luminosity fluctuation. A possible solution to
reduce the fluctuation is to optimize the chromatic compensation in the lattices.
The Committee is pleased to see that efforts were made to understand the non-linear
properties of the machine, in particular the study of the nonlinear chromatic optics. It was
found that there were significant differences between the optical model and the actual
measurement of the chromatic beta beating. The difference was mostly attributed to strength
errors in the sextupole magnets at the level of a few percent. It is encouraging to seeing theconvergence of the fudge factor as the correction was iterated. However, the residual
remained nearly one percent. A possible cause of the residual is that the underlying model
itself may not be accurate enough. The Committee recommends measuring the nonlinear
dispersion as an additional check of the optics model.
4)

Vacuum components R&D

The Committee is impressed by the large amount of progress, which has been made over thelast year with the design of improved vacuum components.
a) Problems with ‘standard’ components
There is a concern that some ‘standard’ components have started to fail during the recent
running period; most notable are the standard vacuum bellows with finger contacts and thePage 6 of 21

finger contacts in vacuum gate valves. These component failures have a direct implication on
the overall efficiency of the machine. Photographs were shown indicating that these vacuum
problems start to limit the increase of integrated luminosity. The Committee recommends
that a close follow-up should be made to establish critical operating parameters, and to put
limits on them. Observing the outside temperature of vacuum bellows or of the other vacuum
components may not be sufficiently rapid to be a machine interlock. Detecting vacuum bursts
could be a better ‘early warning system’. The occurrence of such vacuum bursts has beenreported on several occasions.
b) New copper beam pipes
New versions of copper beam ducts with an antechamber have been developed to gain
experience with the manufacturing process. These chambers have antechambers on one orboth sides for trapping the synchrotron radiation and for a linear NEG pump. Symmetriccooling channels are provided for uniform cooling of the chamber to reduce temperature
gradients. A trial section has been manufactured and tested successfully in a wiggler section
with beam currents up to 1.7A. The photoelectron current was found to be reduced by several
orders of magnitude. Nevertheless, multipacting can still occur at higher currents due tosecondary electrons in the central beam duct. To further suppress and trap photoelectrons,
one could consider small C-type magnets around the antechambers. Solenoid fields would
still be needed for the central beam pipe to suppress multipacting at higher currents. Coating
the central beam pipe with low secondary electron yield material is a very promising solution
which should also be investigated.
The Committee recommends studying the effect of C-type magnets mounted on theantechamber, adding a solenoid field, and reducing the secondary electron yield (SEY) (e.g.
by coating or surface treatment) in simulations of the electron cloud densities on the beamaxis.
c) Comb-type bellows
Three versions of the proposed new comb-type bellows have been tested and the results were
presented in this meeting. Version 2, which includes an outer back shield for the low
frequency and DC components as well as a set of small combs without contact fingers, gave
the best overall performance. A test model installed in the machine showed acceptable
heating of ~90º C and no pressure bursts at currents up to 1.7 A. This RF bridge concept canbe applied to gate valves as well, and the Committee encourages the team to test a new valveof this design in the machine as soon as it becomes available from the company.
Since the operating principle of the comb-type RF bridge has been demonstrated successfullyin the machine, it would be good to complement the final design with impedance calculations,and to optimize the geometry of the comb fingers, e.g. fewer, but wider fingers, maximum
gap and length of fingers, ease of assembly and mounting in the machine, etc. The goal
would be to have better control of the separation between the teeth.
d) Vacuum flanges
Conventional Con-Flat vacuum flanges are not suitable for machines with very short bunches.
The design of the new MO- flanges eliminates any step or gap between flange pairs. Verypromising test results have been obtained and were shown to the Committee with flanges
made out of stainless steel and with copper gaskets. Future work will include real-life testingof flanges made out of Cu-alloy. Since some of these flanges will have to be very large andattached to heavy vacuum chambers, it will be good to follow established assembly andPage 7 of 21

mounting procedures which take account of the actual situation in the machine. Sufficient
statistics and operating experience must be gained to assess the reliability of the new MOflanges.
e) Movable masks
A first design concept of a low impedance movable mask was presented at the 10th
Accelerator Review Committee, and an update of the ongoing R&D work was presented at
this meeting. The main features are a head made of carbon on a ceramic support stem.
Several basic design parameters and impedance limits are not sufficiently defined, so a model
was built for testing. Since detailed specifications are missing, the Vacuum Group has tried
to find solutions for sub-parts of the movable mask design e.g., how to make the carbon head,
how to make the support with a given electrical conductivity to avoid charging up and how tosolve related electromagnetic problems.
f) Low secondary electron yield coating
Following the presentation in the last Accelerator Review Committee, the Committee had
expressed strong support for further studies of low SEY coatings and for testing thesecoatings under machine conditions. Since the issue of the Electron Cloud Instability remains
a central issue for the luminosity upgrade, this recommendation is maintained.
5)

Electron cloud instability measurement

Detection of the betatron sideband in the bunch-by-bunch spectra has substantiated the singlebunch head-tail mechanism of the LER vertical beam size blowup caused by the Electron
Cloud Instability (ECI).
In line with recommendations of the previous Committee meeting, a new comprehensive
series of experimental studies of the sideband position, threshold and intensity has been
performed in LER, both in single-beam mode and in colliding-beam mode.
With a single beam, the observed dependence on the RF voltage and chromaticity is
consistent with the strong head-tail theory. The initial beam size has no effect on the
instability threshold, being much smaller than the effective cloud size contributing to thewake, as was predicted by simulation of the cloud response to the bunch displacements.
Recently, sideband studies were carried out on bunch trains with 3-bucket and 2-bucket
spacing. The interpretation of the spectra where the sideband splits-off from the main
betatron tune is challenging. The measured sideband pattern with different settings of theinitial betatron tune indicated that the sidebands vanish with the operating conditions that givethe best beam-beam performance. This is an intriguing observation, and more studies onthese correlations are needed.
Sideband spectra were observed in LER in colliding-beam mode with the test bunch placed in
an off-collision bucket to avoid smearing of the colliding bunch spectra. A localized 2-bucket
spacing was arranged in a bunch train with a 3.5-bucket spacing by injecting an extra bunchahead of the test bunch. The sideband thresholds were measured either at constant clouddensity (topping-up the extra bunch while the test-bunch intensity decayed) or at constant testbunch intensity (while the extra bunch ahead was decaying). Both measurements showed thatthe thresholds, both in the cloud density and in terms of the bunch current, are in agreementwith previous measurements in the single-beam mode. But the point of these measurements
was to correlate those thresholds with the degradation of the specific luminosity that isPage 8 of 21

characteristic of the 2-bucket spacing collisions. Indeed this correlation was observed,
thereby confirming that ECI in collision can cause the onset of beam-beam blowup even
before the single-beam size blowup threshold is reached. Thus, the KEKB luminosity
performance is shown to be limited by ECI.
An important study was carried out to explain the reason for the non-zero horizontal collisionoffset needed for the best luminosity. The maximum luminosity corresponds to the minimum
sideband intensity in the bunch spectrum. So, ECI manifests itself on the scale of a single
bunch, causing the oscillations of the bunch tail. The specific luminosity is improved when
the HER bunch is shifted horizontally so that it collides mostly with the LER bunch head,rather than colliding with the bunch tail, which is diluted by ECI.
The Committee recommends that an effort be made to understand the mechanism of the
sidebands, their role of being a signature of the ECI onset, and their apparent correlation to
the luminosity. The study should include the use of sideband measurements as a monitor for
optimizing the luminosity performance of KEKB. To this end, more studies are encouraged
to confirm the reproducibility of the sideband spectra.
An experiment was performed that indicated a correlation between bunch luminosity and thelocation of the bunch in the bunch train. It was found that a bunch following a 3-bucket space
has a consistently higher luminosity than a bunch following a 4-bucket space. This is aconvincing indication of the ECI having an effect on the luminosity.
6)

Beam measurements

A set of beam measurements were presented that investigate effects near the half-integer tune.
These are important measurements since the horizontal tune of both rings is very close to thehalf integer, and theoretical analysis, numerical simulation, and luminosity scans all show thatsuch a working point is desirable. However, the half integer must be approached very
carefully, since the beta beat and the emittances can become large when the half integer stopband is approached.
During these measurements it was shown that the Proton Synchrotron (PS) magnet cycle
caused fluctuations of some quadrupole currents. This modulated the tune, and it was foundthat this modulation was worse close to the half integer, potentially making it significantly
harder to approach half integer tunes. However, since PS operation has recently been
discontinued, a further analysis of this effect is not necessary. Some of the measurements
presented might have been made while the PS was in operation and this makes theirinterpretation somewhat problematic.
For example it was shown that the spectrum of the LER horizontal tune of a pilot bunch
develops a peak at the half-integer. This peak is stronger when the tune is closer to the halfinteger and it also tends to increase with increasing chromaticity. There is no satisfactory
explanation for this effect at the moment. Candidates are (a) asymmetrically populatedislands of even order (even though it is not clear why the slow tune excitation procedure
should populate islands asymmetrically), or (b) a perturbation due to the PS (but since the PS
no longer operates it will be hard to further analyze this effect). Since operation close to thehalf-integer is essential, we strongly recommend that either it is possible to determine that theold measurements were in fact taken while the PS was off, or that these measurements berepeated without the PS. Furthermore, the simulation effort should be continued, and should
include errors in the accelerator model as well as off-momentum particles.

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Beam size measurements for a parasitic bunch close to the half integer were performed with agated CCD camera, and with the interference device. Both lead to similar results for the small
increase of the beam size and the width of the stop band. A much stronger increase of thebeam size was observed for bunches under collision, mostly caused by the dynamic beta beat.
However, the theoretical understanding of the beam size change as a function of tune does notseem to be complete. While it was observed that the measured beam size as a function of Hmode frequency agrees well with the simulated beam size as a function of unperturbed tune, itwould be good to have a better understanding of this correlation.
A second set of measurements was presented which concerned tune-shifts that provide
information about wake fields.
The most important effect that was considered is the wake of the electron cloud. For thesemeasurements a pilot bunch with a variable distance after the bunch train was used. It wasverified that the tune shift decreases with distance after the bunch train, as the electron clouddensity decreases. It is interesting to see that this decrease is smaller when all the solenoid
magnets are on. This indicates that much of the tune shift is due to the electron cloud wake,
which dies away more slowly when the solenoids trap the electron cloud at larger amplitudes.
An analysis of the tune shift as a function of the charge of the pilot bunch was also presented.
This effect seems to be nearly completely eliminated by the solenoids, at least in thehorizontal plane. This shows that the solenoids are also very effective in eliminating thehead-tail effect of the electron cloud. However, the data is not very accurate and werecommend that these studies be continued experimentally and theoretically.
7)

Solenoids in quads

Beam blow up induced by the Electron Cloud Instability (ECI) is still observed when thebunch spacing is reduced to 3.27 RF buckets or shorter. Recently the ECI effect caused byelectrons trapped in the quadrupoles has been investigated both experimentally and by
simulation. The experimental results show that solenoids with a field of 17 gauss, installed in
88 quads (25% of the total quads) in the LER have no clear positive influence on the ECI.
These experiments include interesting measurements of the size of the sideband peaks, theseparation between sideband-betatron peaks, the vertical beam size, and the effects on theluminosity with and without the solenoid fields in the quads. The simulation results show thatthe threshold electron density which causes ECI in the LER is about 7.4×1012 m-3, while theexperiments were done at an estimated density of 1.2×1012 m-3. It therefore appears that theexperimental result, while not inconsistent with the simulation, does not provide confirmationof the effect.
The Committee recommends that the KEKB team continue to study these effects, bothexperimentally and by simulations, since no clear physics explanation of the results has beenfound. The possibility still remains that the magnetic field (solenoid or otherwise) required toclear the electrons will be of the same order of magnitude as the pole-tip field in thequadrupole, which would be excessively high. If this is the case, electrons may accumulate in
the quads, perhaps even reaching the threshold level. The Committee supports the proposalby the KEKB team to directly measure the photo-electrons in the quadrupoles; it may give
clearer information on the electron cloud in the quads. If a significant electron density isobserved, a cure needs to be found. This study may have important implications for reaching
higher luminosity.

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8)

New QCS

The design and construction of a new QCS R&D magnet for SuperKEKB is going well. This
new QCS is designed according to the basic requirements for an Interaction Region magnet.
A design with three double-layer pancake coils was selected, and calculations show that boththe two-dimensional multipole components and the integral multipole components meet therequirements. The construction process of this new QCS is also well organized, and includes
coil winding, curing, end spacers, and several hundred construction tools. The measured coils
had some geometrical errors which, according to calculations, cause acceptable field
deformations. However, further improvements of the dummy coil blocks for curing the coils
and some tooling for tuning the axial coil length should be considered to improve theperformance of this R&D magnet. Magnet testing is planned for June 2006.
9)

Injector Upgrade

The injector linac has to supply four different beams to the various accelerators. The otheraccelerators would also prefer to have continuously injected beams for topping up when
KEKB is in continuous-injection mode operation. The only way to meet these demands is toutilize electronic control of the klystron RF phases for beam energy adjustment in eachsection of the transport lattice. The Injector Group has developed a three-phase plan forsimultaneous injections to the four machines. They installed a new beam transport line to the
Photon Factory in the summer shutdown in 2005 as the first phase of this work. It bypassed
the ECS system, and halved the time required for switching beams between KEKB and the
Photon Factory. This year, they are going to install the fast beam-mode switch for improved
injection to the KEKB and PF electron beams as the second phase of the plan. In the final
stage next year, it will fully operate the fast beam-mode switches for injection into all fourmachines.
They also confirmed that the transport efficiency is about 90% for the electron beams throughthe bypass hole on the positron target. It would require more machine studies to confirm thesimulations of the beam parameters. The Committee extends its congratulations to the
Injector Group for their excellent ideas and the success in last year’s work. We believe theproject will be carried out successfully with careful fine adjustments of the phase controls andwith fast-switching magnets.
10) Crab Cavity Overview
Implementation of crab cavities is the largest single opportunity remaining to the KEKB
accelerator team to substantially increase the luminosity for BELLE. The quality andintensity of the effort being applied to the realization of the crab cavities is first class andentirely appropriate to their importance. The Committee was provided with an excellent
overview of the status of development and construction activities, which was followed by anumber of talks to address details of the program.
Three crab cavity systems are being constructed: one will be retained for test anddevelopment while the other two will be installed, one in each of the rings. Construction iswell advanced and, for the most part, has gone well, including vertical testing of the cavities
themselves, first cool-down, and low-power testing of the prototype in a horizontal
configuration. Successful development of RF power couplers, helium jacket, cryostat, tuners
and other peripheral devices, higher order mode dampers and RF power systems were covered
in separate talks which are discussed later in this report.
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A leak developed during the first stages of cryogenic cavity testing, but it was rapidly
diagnosed and repaired. All cavities have been processed and vertically tested, withacceptable results, after dealing with multipacting and field emission issues. The mostsignificant challenge presently facing the team is the in-situ assembly of the coaxial coupler.
This component serves to tune the crab cavity and provides a conduit to conduct any HOM
power deposited in the cavity by the beam to an absorbing load.
The inner conductor of the coaxial coupler is a very long component which, in the present
design, is fabricated in three segments and assembled in place, within the beam pipe. The
connection mechanism between segments is similar in concept to the bayonet connector used
to fasten a lens to an SLR camera body. Successful connection of these two components
requires very careful alignment of the two parts, insertion of the end of the outboard segment
into a receiving socket in the end of the niobium segment, followed by a 30˚ rotation to lockthe two segments together. Up to now, after about three weeks of work, the two innersegments are assembled, but connection to the outermost segment has been unsuccessful,apparently because of an alignment problem.
The Committee was asked to address the following questions:
1. How to assemble the coaxial coupler:a. Assemble the three pieces vertically?
b. Welding? If so, how and when?
c. Silver coating?
The experiences of the cavity assembly team to this point seem to indicate that thefundamental problem is, in fact, one of alignment. Insertion of the male part of the connector
into the female part proceeds smoothly, but rotation is difficult from the beginning, becoming
steadily more difficult until all motion stops after about 15˚ of rotation.
After reviewing the mechanisms in use, it seems that conversion to a completely different
assembly technique, as suggested by the question, would not be the best course of action.
Instead, attempts to improve alignment should be continued. The fixture holding the very
heavy outer segment should be evaluated for bending, and if necessary, stiffened. The loadon the fixture might be reduced by removing the notch filter prior to assembly.
2. How to ensure the RF / beam shielding:a. Does silver coating also work for that purpose?
b. Welding? If so, how and when?
Welding would not seem to be a practical solution at this point, since it would probably notwork unless the cavity/cryostat system were completely disassembled, and then reassembled
around the complete coaxial center conductor. Silver plating might be helpful, if it can bedone without significant generation of particles that might migrate into the cavity to cause
field emission. But the key is likely to be the design of a support system that removes or
mitigates transverse loads on the connecting mechanism through improved alignment. As analternative solution, assembly with self-fixing screws and RF gaskets should be considerednow, to be ready if the primary solution cannot be made to work.
3. When should the improved coaxial coupler be installed? After the beam test of the current
design?

Page 12 of 21

The Committee is agreed that the cavities should be installed in the rings at the earliest
possible time, and commissioned, so that the benefits and limitations associated with themcan be determined. Improvements of various kinds will almost certainly be indicated, and theimproved center conductor can be installed as part of a general upgrade project. Certainly,
installation of the cavities should not be delayed while the improved coupler is designed,
approved and constructed.
11) Tapered Coaxial Coupler
The RF experts have identified parasitic modes inside the coaxial coupler which could
generate high impedances at the open end leading to beam instability. In particular, there is adipole mode with a λ/4 resonance at 595 MHz (Q of ~106), defined by the stub support
position. Various modifications are foreseen to allow propagation of this mode to the HOM
damper, and thus damp this resonance. The first involves tapering the coaxial line, with thediameter increasing along the length, to lower the cut-off frequency to below 600 MHz. The
second involves machining the outer diameter of the inner conductor “tip cutting” to raise theλ/4 mode frequency and lower the R/Q. The last involves a modification to the notch filter“cut slots” to split the TE mode stop-band, so that the λ/4 mode is not prevented fromreaching the HOM damper. Calculations of the resulting λ/4 mode impedance suggest that for
low intensity operation, tip cutting is unnecessary; the other two modifications providing asufficiently low impedance of ~ 30 kΩ. For high intensities, tip cutting is necessary to lower
the impedance by a further factor of five to ~ 6 kΩ. The impedances of the other modes havebeen calculated and all now seem to be sufficiently low. It was also claimed that tip cutting
lowered the peak electric field on the inner conductor slightly but caused a detuning of thecavity, which necessitating a 5 mm shift of the conductor to restore the resonant frequency.
The Committee congratulates the RF engineers, both for finding the problem and for the
proposed solution. However success depends on the accurate placement of the cut-off andnotch filter frequencies, which so far rely mainly on calculation. The Committee recommends
that all these parameters be measured on the bench before installation, using the final coaxial
structure and cavity.
12) RF couplers of crab cavity
The RF input coupler of the crab cavity needs to provide 100kW. It is a coaxial, capacitive
coupler with an RF window outside of the cryostat and a T-support structure. It wasassembled after rinsing and drying and was baked at 80°C. Diagnostic devices are an electron
sensor, an arc detector and a vacuum gauge.
The coupler has been in a test stand which allowed for standing and travelling wave tests.
The stand includes a series circuit with two couplers, an accelerating cavity coupler, and thecrab cavity coupler, and a phase shifter. The conditioning went very smoothly, typicallywithin eight to 15 hours, and power levels of 250KW for travelling wave operation and
100KW for standing wave operation were achieved without major problems. In the case ofstanding wave conditioning, the phase shifter was adjusted such that power nodes on the
coupler were shifted in steps of 5cm. Trips originated from electron loading wereaccompanied by vacuum breakdown, but were rapidly recovered. No major multipacting
levels were observed and thus no countermeasures were necessary.
In conclusion, the crab cavity RF input coupler seems to perform well and no further
measures are necessary.
Page 13 of 21

13) Cryostat, jacket, etc.
The design of the cryostat for the crab cavity presented to the Committee is straightforwardand effective. It features magnetic shielding assembled directly onto the cavity for maximum
effectiveness, although the consequent need for high permeability at low temperatures forces
the use of more expensive materials. In this application, where initially only two cavities arebeing built, the additional material cost is modest, and the shielding improvement issignificant.
It also features a limited volume of liquid helium through the use of separate helium vessels,
or jackets. One of these is for the cavity itself, and one provides coolant flow to the niobiumtipped coaxial center conductor, which couples HOMs to an external load, as well as being the
primary tuner for the cavity. The main cavity helium vessel incorporates a heating element tomaintain a constant load for the helium refrigerator. The cryostat utilizes the standard 80 K,
liquid nitrogen cooled thermal shield to reduce the heat load to the 4.2 K helium circuit.
Heating of the niobium center conductor will be a maximum near the peak, where thestructure is exposed to the maximum RF magnetic fields. It is therefore important to assure
adequate liquid helium flow to the tip from the support stubs where the coolant enters. The
center conductor itself has an annular structure with elements to channel the flow to the tip,and the exhaust may be sub-atmospheric to ensure both adequate flow and potentially lower
resistivity.
14) Peripheral devices
The Committee was given an overall view of the status of the cavity peripheral equipment; allhave made good progress. The vacuum design for the crab cavity is complete. An RF shield
has been designed to isolate the vacuum pumps from the coaxial beam line. It should beverified that the regular pattern of holes does not allow resonant coupling of the highfrequencies into the pump. This would create a high beam impedance and deposit HOM
power in the pump. The HOM damper, notch-filter, coaxial coupler and input coupler haveall been manufactured and are ready for assembly. The electrical ‘filter’ characteristics of theensemble of the first three should be measured after assembly on the cavity. The complex
tuner design has been slightly modified to facilitate manual pre-tuning; a wise decision. The
tuner is being put together on a special test stand. This can be used to test reliability,
determine possible back-lash and setting accuracy, and also to measure the tuning speed of thepiezo/mechanical combination as a function of the stroke amplitude. Cryogenic transfer lines
have been designed and are under construction. The design of these lines is simple but
elegant, and with the 80˚K shield should produce low heat loss. The installation of these lines
on the D10 test stand will allow the isolation efficiency to be measured.
15) HOM absorbers
The Committee was presented with a description of the processes by which the higher ordermode dampers were fabricated, processed and tested. The ferrite dampers benefit from a highdegree of similarity to those already in use on the superconducting cavities. Two dampers,
one for the large beam pipe and one for the coaxial coupler, are required for each of the two
crab cavities. The production of dust from the porous material has not been observed and has
not caused problems during operation. Processing included extensive outgassing, primarily
for water, to reduce the possibility that evaporated gases from the dampers could affect theperformance of the cavity when they condense and freeze on the cryogenic surfaces.

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The use of organic O-rings for the bake out is questioned and this production step may be
simplified by providing a vacuum oven where several absorbers could be outgassed
simultaneously. During the final vacuum test, which can only be done once the end flanges
have been put on, it would be desirable to measure the residual gas composition. Oncecompleted, the maximum allowed temperature is given as 60°C, which appears to leave only asmall temperature margin as compared to the operating temperature. A limited set of highpower tests demonstrated that the absorbers are probably sufficiently robust to performreliably with beam for long periods.
However, the ferrite in one of the absorbers cracked during such a test. While it was foundthat further testing does not increase the length of the cracks, the Committee would like topoint out the danger of relying on a cracked ferrite, even as a spare. During operation,
particulates from the crack could find their way into the superconducting cavity.
Low-power tests with a copper cavity demonstrated that the coaxial coupler and ferrite
damper were very effective in damping cavity higher order modes up to 1.5 GHz. The only
exception was the TE111 mode around 875 MHz, which does not couple well to the coaxial
line, but which is not expected to be a problem. Damping of the crab mode is prevented by anotch filter that reflects energy at that frequency back to the cavity. The damper was also
tested in a beam-pipe transmission configuration. Attenuation of RF energy was relatively
low, with a maximum of 4 dB at 1 GHz, falling to 2 dB at 500 MHz and 2 GHz. Thus HOM
power above 2 GHz, which is certainly expected from the short bunches, will propagate out of
the cavity, preferentially through the large beam pipe, but will not be effectively absorbed by
the damper installed just outside the cryostat for that purpose. Since a similar setup is used
for the accelerating cavities, problems are not expected, but it would probably be wise to bealert for them.
16) Horizontal test of crab cavity
The horizontal test that is proposed will test the assembly of the cryostat without the coaxial
beam pipe. It will test that the previously observed leak does not reopen under cold/warm
cycles; it will check that no significant motion occurred under transport, and it will verify thatthe static loss corresponds to the predictions. Also the coupler can be tested in its cold state.
We believe that all these things are important activities.
The cool-down test was very successful. So far the static heat load was measured to be about
30W, as expected, and it was shown that the vacuum leak does not reopen during cool down.,
Several vacuum bursts were observed during heat-up, and it was not clear whether the amountof released gas was consistent with the vacuum level before cool-down, or whether, in fact, aleak should be suspected. A closer analysis seems necessary. The cool-down is evaluatedwith 14 monitors, and there were no unusual observations; however it was however not
mentioned how these data correspond to thermal calculations.
A frequency shift of 0.6 MHz during cool-down to 4.2K was measured, which reportedly
corresponded to expectations. Somewhat less expected was the change of the loaded Q,
which suddenly increased at low temperature. It is suspected that the cooling of the innerconductor of the coupler changed its length, and therefore the coupling coefficient. If this is
the case, the observed loaded Q of 1.3 x 106 might not be representative of an operational
value, since the inner conductor will be at a different, presumably higher temperature during
operation with power. However, one should also investigate alternative explanations which
are more relevant to the crab cavity operation, since this effect is not observed in theaccelerating cavities.
Page 15 of 21

17) RF system for crab cavity
The RF system of the crab cavity was presented to the Committee, the presentation
concentrating on beam-loading issues and phase error tolerances, with a summary of thepresent construction status and a brief overview of the beam-control system.
The required RF power depends on the kick voltage and the induced beam-loading voltage.
For a 1 mm difference between cavity center and beam position, the power increases by ~ 20
kW for the range of QL (loaded Q) values considered (1-3 x 105), and for the nominal beam
current. This is acceptable, since the total power (<100kW) is well within the RF coupler
capability of 200 kW. However the “beam loading” power increases as the square of thebeam-offset, hence it is important to keep the total beam offset below 1 mm. For the QL
values chosen, it is possible to operate the cavity at up to 30º off-tune before coupled bunch
instability growth rates exceed the radiation damping rates; this should an easy target.
However, it will be necessary to control the beam using bunch-by-bunch feedback in parked
mode, even though the cavities can be de-tuned by up to 100 kHz.
Phase errors lead to a horizontal offset at the IP and affect the beam-beam performance; themagnitude of the effect depending on the phase noise correlation time. The tolerances arequite tight, 0.27º at the crabbing frequency. Slow phase errors can be readily corrected, but
fast changes which depend on the quality of the beam-control system are more difficult topredict. It would be very interesting to measure the noise on the complete system (beamcontrol loops, high power and cavity) before operation with beam. In principle, the noise
should be defined by the low-level beam control system. The effects of transient beamloading on the phase are largely self compensating if the beam–abort gaps pass the cavities
simultaneously. At high beam currents, it may be necessary to optimize the intensities in
HER and LER.
The RF power system, which is based on existing 1 MW klystrons, has been installed and thewaveguides already head down towards the ring. Since the power in the couplers is limited to
200 kW, klystron operation should be reliable; we note that any RF power loss will lead to abeam abort. The beam control system is fairly conventional and based on the existing wellproven KEKB superconducting cavity system. It includes RF feedback, klystron correction
loops, cavity voltage phase and amplitude loops and tuner loops. In addition, the horizontal
position of the coaxial beam pipe can be adjusted. As mentioned above, all these can andshould be tested with the rest of the system in the laboratory if time allows.
Globally the RF system has been very well and exhaustively studied and is well on the road tocompletion. While the crab cavity remains a challenging project, all the necessary RF tools
seem to be in place.
18) Installation of crab cavity
A very detailed plan of the installation of both crab cavities was presented. The plan for
installation techniques was supported by a set of convincing photographs and drawings, andthe timeline that was presented to the Committee after the talks showed detailed planning forwork, both in industry and at KEK. While it is hard to verify the soundness of these timelines
in the time available, it became clear that planning has been taken very seriously.
The actual installation has been planned carefully; the tunnel geometry, pass-ways, etc. havebeen taken into account. The neighboring beam line is very close to the cryostat vessel andthe installation therefore has to be performed very carefully; this has been also taken into
account in the planning. The masses that have to be moved over very crucial equipment lead
Page 16 of 21

us to suggest that this activity should be practiced with a wooden model of the correct
dimensions.
The timing of the crab cavity installation has not been fully specified yet. Four possibilities
were presented.
After significant discussion, the Committee makes the following
recommendation:
Finish one crab cavity and start processing and testing it with RF power outside the tunnel,
while building the second cryostat in parallel. Both cavities should be tested with RF before
any installation is started. Both cryostats should then be installed, if possible during thesummer shutdown, and should be commissioned simultaneously. We are not convinced thatcommissioning of one ring with a crab cavity, and one without, will lead to information thatcan be used to significantly improve the second cavity in a reasonable time. We rather thinkthat this could lead to a significant loss of luminosity during the time in which only one crab
cavity is installed.
19) Beam commissioning with crab cavity
A crab lattice was prepared and implemented in summer 2005. It should first be establishedthat this lattice is as robust as the previous lattice for production runs. More attention should
be given to the accumulation of experience in actual running and tuning before the installation
time. A slower start-up after a long shut-down, for example, could be interpreted as a sign of
non-robustness of the crab lattice, and that possibility should be minimized by the installation
time. The phase advances between the crab cavity and both the IP and the streak camera havebeen optimized.
A well-considered plan has been presented for the commissioning of the crab cavity runs.
This plan is to start immediately following the installation of the crab cavity. The Committee
suggests adding an engineering commissioning period after the installation but before beamcommissioning with the purpose of debugging the system as a whole. A detailed plan of thezero-stage operation with beam may also be useful: i) with the crab cavity in passive mode,
finding its electrical beam center so its effective electrical axis can be aligned with the normal
beam orbit; ii) with the crab RF on, finding the zero phasing where the orbit shift in the crab
cavity itself is zero; iii) calibrating the crab RF amplitude against the crabbing angle observedwith the streak-camera; iv) finding the working point range limitation from synchro-betatronresonances caused by the crab cavity with the non-zero chromaticity needed in KEKB; v)
checking the noise sources and their magnitudes of the main and crab RF systems.
The goal of this commissioning session is to establish that the crab crossing scheme increases
the specific luminosity. The Committee wishes to re-iterate the fact that using a single crab
cavity without the compensating one is a new concept, and, in addition, crabbing has neverbeen done before. The Committee wishes to support the early start of this commissioning
plan by the KEKB team.
Many issues involving the crab cavity are likely to need attention during the commissioning
stage. For example, the λ/4 dipole mode (barely above cut-off) propagates in the coaxial
coupler and is not absorbed well by HOM damper. This can be solved by tip cutting. In thecurrent design, with a tapered coaxial coupler without tip cutting, the beam current is limited
to approximately 700 mA, above which the instability growth rate exceeds the feedback
capability. The Committee recommends operating initially with the present design, and thiswill limit the beam current for higher luminosity operation. Another possible issue might bemultipacting in the coaxial coupler. Previous test runs indicate that a one-hour conditioning
can overcome this concern.
Page 17 of 21

Still other issues may involve orbit feedback to better than 0.5 mm at the crab cavity, thewider beam orbits during injection, and potential sensitivity to fast phase jitter. When thecavity is parked off-resonance, the beam stability will have to be assured by a feedback
system. Considerations of ways to relieve this requirement might be useful. Finally, trips ofthe crab cavities with the necessary beam aborts may be an operational concern.
The proposed focal point of the commissioning, after the initial phase of streak camera
measurement for the crabbing angle, has been given to the achievement of a high beam-beamtune-shift limit of ξ>0.1. This can be achieved with a moderate single-bunch current by
restricting the total number of bunches. The Committee concurs with this focus, and wishes
to emphasize the great importance of achieving this goal as early as possible during thecommissioning period.
In this sense, one beam-beam related concern caused by the crab cavity is that the abort gap
transient causes an x-orbit separation of the two beams of ±7 µm the IP (to be compared not
only with the rms beam size of 100 µm but also with the ~20 µm horizontal sensitivity of thehigh-performance collision). The Committee recommends that effect of this separation on thespecific luminosity be evaluated.
The Committee expects that beam commissioning will take about 30 days, but the previous
luminosity levels may not be reached for several months after that.

Page 18 of 21

Appendix A
KEKB Accelerator Review Members
Andrew Hutton

JLab (Chairman)

Yunhai Cai

SLAC (replacing John Seeman for this meeting)Alexander Chao

SLAC

Warren FunkJLabOswald Gröbner

CERN (retired)

Heino Henke

Technical University, Berlin

Georg Hoffstaetter

Cornell University

Trevor Linnecar

CERN

Won Namkung

POSTECH

Flemming Pedersen

CERN – unable to attend

Eugene A. Perevedentsev

BINP, Novosibirsk

David Rice

Cornell – unable to attend

John Seeman

SLAC – unable to attend

Wang Shuhong

IHEP, Beijing

Katsunobu OideKEK Secretary, Accelerator

Shin-ichi Kurokawa

KEK Secretary, Accelerator – unable to attend

Page 19 of 21

Appendix B

Agenda of the Eleventh KEKB Accelerator Review Committee
March 21-23, 2006 in the Meeting room on the first floor of Building No.3, KEK

Mar. 21 (Tue.)
8:30- 9:00

Executive Session

9:00- 9:10

KEKB Overview

Katsunobu Oide

9:10-9:50

Progress since the last ARC

Yoshihiro Funakoshi

9:50-10:20

Physics by Belle

Yoshihide Sakai

14:35-15:50

Break

10:40-11:00

OpticsAkio Morita

11:00-11:25

Vacuum components R&D

Yusuke Suetsugu

11:25-11:50

Electron cloud measurement

John Flanagan

11:50-12:15

Beam Measurements

Takao Ieiri

12:15-13:15

Lunch

13:15-13:40

Solenoids in Quads

Hitoshi Fukuma

13:40-14:05

New QCS

Norihito Ohuchi

14:05-14:35

Injector Upgrade

Masanori Satoh

14:35-15:50

Break

14:50-15:30

Crab Cavity overview

Kenji Hosoyama

15:30-15:55

RF characteristics of crab cavity

Yoshiyuki Morita

15:55-16:20

RF couplers of crab cavity

Kota Nakanishi

16:20-16:45

Cryostat, jacket, etc.

Hirotaka Nakai

16:45-17:10

Peripheral devices

Hirotaka Nakai

17:10-17:30

Executive Session

17:30-

Reception

Mar. 22 (Wed.)
8:30- 9:00

Executive Session

Page 20 of 21

9:00- 9:20

HOM absorbers

Yoshiyuki Morita

9:20- 9:50

Horizontal test of crab cavity

Yasuchika Yamamoto

9:50-10:20

RF system for crab cavity

Kazunori Akai

10:20-10:35

Break

10:35-11:00

Installation of Crab cavity

Masaaki Ono

11:00-11:20

Beam Commissioning Plan with Crab Cavity

Haruyo Koiso

11:20-11:50

PEP-II Status

Yunhai Cai

11:50-13:00

Lunch

13:00-16:00

Tour for Crab cavity

16:00-20:00

Executive Session with working dinner

Assembly Hall, Nikko, Nikko Tunnel

Mar. 23 (Thu.)
8:30-11:00

Executive Session

11:00-12:00

Close out with KEKB members

12:00-12:10

Adjourn

Page 21 of 21