The Tenth KEKB Accelerator Review Committee Report
Introduction
The Tenth KEKB Accelerator Review Committee meeting was held on February 21-23, 2005.
The Committee welcomed two new members: Trevor Linnecar from CERN, an accelerator
RF system expert and Heino Henke from Berlin Technical University, an RF, impedance andinstability expert. In addition, since John Seeman from PEP-II at SLAC was unable to attend,
Nadine Kurita, the PEP-II chief engineer, replaced him for this meeting. Oswald Gröbner andGeorg Hoffstaetter were unable to attend this meeting and Masanori Kobayashi has retired.
Appendix A shows the present membership of the Committee.
The tenth 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 isshown in Appendix B. The first day concentrated on KEKB performance and, in particular,
the progress on the crab cavity, and on the second day, the talks addressed the R&D towards
SuperKEKB. The Committee was again impressed by the high standard of the talks, both thetechnical 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 report during the meeting that was then improved and finalized by e-mail among
the Committee members.
Contents
Executive Summary
1) Forward2) Summary
3) Recommendations
Findings and Recommendations
1) KEKB performance and near term plans
2) BELLE status and plans
3) BPM issues4) Beam observations
5) Beam-beam experiments
6) ARES status7) Injector Upgrade
8) Crab cavity overview
9) HOM Damper10) Input/ coaxial coupler11) Cryostat12) Refrigeration system
13) RF control
14) Crab optics and dynamics
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15) Magnets, installation, orbit control
16) Super KEKB overview update17) Beam-beam update18) Vacuum upgrade19) ARES upgrade
20) SCC upgrade
21) Lattice and Dynamic aperture22) IR overview
23) IR vacuum chamber
24) Secondary electron yields from vacuum materials and in situ surface
characterization
25) QCS
26) Background27) Impedance28) Coherent radiation
29) C-band R&D status for SuperKEKB30) Tungsten mono-crystalline target for a high-intensity positron source31) KEKB controls: Status and upgrade plan

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A) Executive Summary
1) Foreword
KEKB continued to make excellent progress since the last Committee meeting. The peakluminosity has increased to a new world record of 1.52 x 1034 cm-2 s-1, 50% above the design
luminosity of 1034 cm-2s-1 (at this time last year it was 1.16 x 1034 cm-2s-1). The Committee
congratulates the entire KEKB project staff on this new record. Continuous injection has
been in regular operation since January 2004, and has helped to increase the daily integrated
luminosity by almost a factor of two since last year. The BELLE detector has accumulated
365 fb-1, another world record (197.5fb-1 last year) and has published, or submitted for
publication, a total of 120 papers in refereed journals (83 last year). Immediately before themeeting a new record was broken – accumulating 1000pb-1 in a single day. This is anindication of how well all facets of the operation are functioning. The first day of the meeting
was devoted to the present status of KEKB and the progress with the crab cavity, while thesecond day covered the plans for upgrading KEKB to a luminosity of 1-5 x 1035 cm-2 s-1. The
KEKB machine group has done an enormous amount of excellent work, and is to becongratulated on the impressive results that have been achieved.

2) Summary
KEKB continued to make excellent progress over the last year, increasing all of theworldwide luminosity records: highest peak luminosity: 1.516 x 1034 cm-2s-1, highest daily
integrated luminosity: 1.083 fb-1, highest weekly integrated luminosity: 6.242 fb-1, and highest
30-day luminosity: 24 fb-1. A bellows in the LER near the IP was damaged on November 3.
The current had to be limited to below 900 mA (it was 1650 mA before the incident) reducing
the luminosity until a repair could be carried out during the winter shutdown. It is expectedthat the luminosity will continue to slowly increase during the year, but no large factors ofimprovement seem likely until the crab cavities can be installed. The KEKB team has mademany hardware and tuning improvements in order to reach the peak luminosity including
precision tune feedback and detection and compensation of BPM motion relative to the
sextupole magnets near the interaction point. Continuous injection is now the standard
operating procedure and this has helped improve the integrated luminosity.
The Belle detector has now accumulated 365fb-1, and of these 168fb-1 were obtained this year.
The Belle detector is working extremely well and has had no major difficulties. 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 Belle collaboration is extremely pleased with the quality and quantity ofthe beam being provided by the accelerator.

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3) Recommendations
Last year, the Committee made several 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 believes that improving the luminosity of KEKB is the highest priority,and experimental and theoretical studies to improve luminosity should be given
precedence.
2. The Committee strongly endorses the proposal to construct, test, and install one crab
cavity in each ring with the highest priority. In the meantime, high priority should begiven to continuing detailed studies of the beam dynamics of the ring with a single crab
cavity, as well as continuing technical developments.
3. The Committee would like to encourage studies leading to increased luminosity in the
short term; e.g. a study of shorter bunch spacing, fast ion instabilities, working point,
improving linear and non-linear optics, etc.
4. The Committee recommends that Belle implement an azimuthally localized veto similar
to that of BaBar to avoid the 3% loss in luminosity caused by the detector trigger vetoduring injection.
5. The Committee endorses the direction of the present studies to design a major luminosity
upgrade (SuperKEKB) aimed at a luminosity of 1–5 x 1035 cm-2s-1 and recommends thatthey be continued.
6. The Committee recommends continued evaluation of all of the effects that would impact
the choice of which beam goes in which ring.
The Committee has made new recommendations throughout the different sections below.
Highlights of these recommendations are summarized here.
7. Coherent synchrotron radiation has the potential to have such a profound impact on thedesign of SuperKEKB that the Committee strongly recommends carrying out moredetailed studies at KEK, and comparing the results with those of other studies at SLAC
and elsewhere.
•

Prior to making any decision, the contributions from all other longitudinal
impedances present in the LER should be combined with the CSR wake and thepotential for partial compensation should be investigated.

•

The Committee also recommends studying the transverse effects of the CSR
impedance.

•

One of the consequences of the transient shielding is a radiated field ahead of thebunch, so the effect on multi-bunch stability should also be addressed.

8. In order to clarify the reasons for the reduction of the specific luminosity with increasing
numbers of bunches at the collision point, and to provide insight on the electron cloud
effect on the beam as well as diagnostics relevant to the cloud density, the Committee
recommends performing an experimental program for the 3-bucket spacing collisions.
This would be useful for KEKB as well as for SuperKEKB.

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9. The Committee strongly endorses the plans for simultaneous injection in all four rings,
including the crystalline tungsten positron target.
10.The Committee notes the problems encountered in achieving high gradients in the C-bandstructures and recommends studying alternative solutions in parallel with the present
solution.
11. A new conceptual design for a movable mask was presented which is ‘electricallyinvisible’ (rod with ceramic support) and has a much lower loss factor, but a significant
amount of R&D remains to demonstrate a reliable, operational movable mask with areduced loss factor. The Committee recommends pursuing this R&D work andsubsequent beam tests with high priority.
12. The Committee feels that HOMs will be one of the limiting factors in increasing thecurrent. Detailed studies of the vacuum components including proper damping andcomponent cooling should be carried out.

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B) Findings and Recommendations
1)

KEKB performance and near term plans

KEKB has progressed very well over the past year and more than reached its goals. The
luminosity has reached 1.5 x 1034 cm-2 s-1, a new world’s record. The accelerator continues to
set integrated luminosity records, for example, 1.08fb-1 per day and 24fb-1 in 30 days. The
total integrated luminosity over the life of KEKB has reached 363fb-1.
The horizontal tune of the Low Energy Ring (LER) operates very close to the half integer
tune (0.505) which, with the use of IP tuning knobs, gives higher luminosity. The change thisyear was to apply a more precise feedback on the bunch tunes, which has greatly stabilized
colliding beam operation.
KEKB now operates with about 1,200 bunches but with beam currents near the design values.
This high bunch charge is well tolerated by the beam-beam interaction and, as a result, gives
more luminosity. The design number of bunch (5,000) would allow for more current andluminosity in the future. However, the number of bunches is presently limited in the LER by
the Electron Cloud Instability (ECI). The KEKB staff is looking at measures to increase thecurrent and specific luminosity.
The high bunch currents and short bunch lengths have stressed several vacuum chambers withhigh temperatures and forces. Several vacuum leaks have occurred, and these chambers havebeen replaced and upgraded. The Committee suggests continued vigilance watching for the“next” chamber to have problems.
The number of beam aborts has averaged about four per day in each ring since May 2002.
Since continuous injection has been implemented, the beam aborts have had a noticeable
impact on the average luminosity (~5%). The causes have been carefully documented andassigned to different categories. It would be interesting to study these beam aborts in moredetail to try and eliminate them. This would be good for the present KEKB and, more
importantly, provide useful information for SuperKEKB.
2)BELLE status and plans

The Belle detector has made full use of the 363fb-1 delivered by KEKB, with 166 in the past
year. The Committee congratulates the Belle collaboration on the effective use of detector
and storage ring. From this nearly doubled number of BBbar pairs 60 contributed papers and24 talks were presented at ICHEP04 alone.
The Belle data have now confirmed direct CP violation with a 3.9 σ signal, improving to 5 σ
when combined with BaBar data. This result provides convincing support for the KobayashiMaskawa mechanism. CPV isospin analysis has confirmed Belle’s previous result
establishing an unexpected large branching ratio of 2.32x10-6 for B0 → π0π0 with a 6 σ signal.
Time dependent CPV analysis has reduced the error on sin 2ϕ1 to 5%.
Hints of new physics come from B0 → ϕKs and B0 → ϕK0 and other decays showing
significant deviations from Standard Model predictions. Earlier significant differences from
BaBar on B0 → π+π - results have been reduced significantly, but still there remains a 2.3 σdiscrepancy. Several new decay modes have been found with some candidate explanations
proposed. We look forward to confirmation with an increased data set.
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The next doubling of Belle’s data set will likely require at least two years. We expect arefining of the results presented in the mean time, and encourage exploring all avenues ofanalysis. These results will provide the physics case, which will be needed to justify
SuperKEKB.
3)

BPM issues

A novel and very clever scheme to check good transmissions from all 4 buttons of a BPM waspresented. Horizontal and vertical positions can be derived from all 4 electrodes, but it has
been discovered that in fact any combination of 3 out of the 4 electrodes is sufficient tocalculate a position estimate. If all 4-button signal transmissions are perfect, the four 3-button
position estimates should be identical. The consistency is defined as the standard deviation of
the four different position estimates and is usually better than 0.1 mm for a ‘good’ BPM.
Various BPMs have had degraded consistencies due to bad connectors, imperfect contact withconnectors, and thermal drift in cable impedance, broken cables. Bad BPM have thus been
identified, and bad connections identified using TDR and fixed.
Correct and stable BPMs are essential for KEKB operation due to the global orbit correction
scheme. Daily variations in consistency have also been identified as caused by sunshine
heating BPM cables and reduced by thermal insulation. Publication of this simple and cleverscheme is encouraged.
Thermal motion of BPM positions following an abort is a serious problem. For 21 BPMs
downstream of twin bends in the HER, this thermal motion has been reduced by an additional
support.
Movement of BPMs at LER local correction sextupoles cause tune changes due to thermal
motion. Electrostatic position sensor with 1 µm resolution shows a good correlation with theresulting tune changes, and correction of BPM data with position sensors has dramaticallyreduced the tune excursions caused by thermal motion after an abort. This improvement may
be necessary for all sextupoles in the future.
4)

Beam observations

Bunch-by-bunch spectra of vertical oscillations of the positron beam have been measured
using the feedback BPM and the Beam Oscillation Recorder. In the bunch train with 4-bucket
bunch spacing, above the threshold current of the vertical beam blowup, an upper sideband of
the vertical betatron tune is detected in the spectra of the bunches with numbers >5; thesideband peak becomes more pronounced in the following bunches through the tail of thebunch train. While it has been suspected for a long time that the emittance blow-up in the
LER above a certain threshold was caused by a coherent instability associated with the strong
and very fast short range electron cloud wake, this is the first time a coherent signal associatedwith this instability has been recorded.
The betatron tune was made visible by lowering the feedback gain down to the stability
margin. The vertical tune and the sideband peak both increase in tune along train, saturatingnear the 40th bunch. The difference between the tune and the peak also increases, thensaturates near the 40th bunch. The sideband can be reduced or eliminated by turning thesolenoid field on. The sideband appearing in correlation with the vertical beam size blowup
due to the electron cloud in LER can be interpreted as a signature of the strong head-tail (orPage 7 of 26

transverse mode coupling) instability causing the blowup, and caused by the strong, fast andshort range wake of the electron cloud. The presence of the sideband and its intensity may
serve as a diagnostic tool for the electron cloud density on the beam axis.
Similar spectra are observed by the optical diagnostics of coherent oscillations using the PMT
setup. The latter technique also provides the vertical beam size data. Their correlation withthe burst-like coherent oscillation signal reveals the relaxation behavior of the strong head-tailinstability.
Important information can be obtained by measuring these spectra as a function of thechromaticity, as the next step of the study. A considerable effort should be invested in thedetailed quantitative analysis of the measured spectra.
The frequency offset and calculations with a realistic short-range electron cloud wakesuggests that the mode frequencies merging at threshold are the m = 1 and m = 2 head tailmodes. While a reactive feedback system was effective in PEPII to raise the mode coupling
threshold caused by the broadband transverse impedance wake (raising the merging thresholdof the m = 0 and m = 1 modes by shifting the m = 0 mode frequency), it seems difficult toraise the mode coupling threshold much for the electron cloud wake due to the highfrequencies involved.
The Committee encourages further activities providing insight in the electron cloud effect on
the beam, as well as diagnostics relevant to the cloud density. In particular, the samemeasurement is recommended with the 3-bucket separation in the train as a way tounderstanding the reasons limiting the number of bunches at present.
5)

Beam-beam experiments

A comprehensive set of experiments relevant to the beam-beam effects was performed during
the past year. These include measurements of the beam-beam kick during a scan of thehorizontal offset at the collision point, the coherent beam-beam tune shifts, and the dynamic
beta. In everyday performance the KEKB team achieved very high values of the beam-beam
parameter, and routine tuning of the collision has become a state-of-the-art activity.
One of the goals of this experiment was to understand why the optimum luminosity is reachedwith a certain horizontal offset of the beams at the interaction point. The offset is believed toresult from a combination of the beam-beam and electron cloud effects. Accurate
measurements of the “egure” picture provide a solid experimental basis for future simulationand theoretical studies.
The dynamic beta and emittance can be reliably computed as a function of the beam-beam
parameter by an optics code with appropriate model optics of the machine. Therefore,
measurement of these parameters provides a diagnostics for the beam-beam parameter that iscomplementary to the coherent beam-beam tune shift measurement.
However, in the presented experiments the approach was different. The dynamical change ofthe horizontal beam size at IP was measured from the beam-beam kick measurements. These
measurements are made with pairs of detectors providing resolution of 10 mm in position and0.05 degrees in phase. Hence, the relation between the horizontal beam-beam parameter andhorizontal emittance was obtained. The latter is consistent with the measured tune shift value.

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In order to clarify the reasons for the reduction of the specific luminosity with increasing
numbers of bunches at the collision point, the Committee recommends performing a similar
experimental program for the 3-bucket spacing collisions.
6)

ARES status

The ARES concept is a very convincing approach to increase the stored energy in order toreduce the cavity detuning under beam loading, which is necessary to suppress thelongitudinal coupled bunch instability. Three cavities, accelerating, coupling and storage
cavity, are driven in the pi/2-mode. This assures a high stored energy for a given accelerating
voltage, while the losses are low because the coupling cell is only weakly excited. The twoother modes, 0- and pi-mode, are damped in the coupling cell. The pi/2-mode is also the moststable against detuning of the accelerating cavity by beam loading.
20 cavities are installed in the LER ring and 12 in the HER ring, together with 8
superconducting cavities. So far, operation has been very stable with only 1 trip per cavity
every 3 months. In the LER ring the beam current is up to 1.8A with a cavity voltage of
0.4MV, an input power of about 300KW and HOM power of 5KW. In the HER ring thebeam current is around 1.2A with a cavity voltage of 0.34MV. Most of the initial problems
have been overcome. In 2 cavities, multipacting was observed in the coaxial part of the RF
input coupler. After replacing 2 couplers one worked fine, the other still showed multipacting.
The reason why some couplers show multipacting is not known. Studies are going on todetermine whether to coat the coupler or to modify the shape. We believe that this problem
will be solved.
In the meantime, numerical multipacting studies have been performed and operational regions
found with no multipacting. An RF phase feedback has been installed which provides rapid
crossing of the multipacting regions and maintains the cavity in a safe zone during operation
with beam.
This is an elegant solution that nevertheless can give operational restrictions. We encourage
the continuation of studies on coupler shaping and coating to remove this problem at thesource. In conclusion, the ARES seems to be in a very good condition and we congratulate
the RF people for this nice idea and the solid realization.
7)

Injector Upgrade

Providing different beams to four rings simultaneously is an ambitious goal. Top-up injectionof both electrons and positrons is currently adopted for KEKB operations. In other words, theinjector linac is fully occupied for KEKB operation. However, it must also provide electron
beams of 2.5 GeV to the Photon Factory and 3.0 GeV to the Photon Factory Advanced Ring a
few times per day. In the near future, these rings will also require top-up operation.
In the first phase, electron beams of 8.0 GeV for KEKB and 2.5 GeV for the Photon Factory
will be provided using a DC bending magnet to switch between the two beams, until a pulsed
magnet is ready. The injector group obtained 2.5 GeV beams by decelerating 5.7 GeV beams
by electronic control of the RF phases. This is really an excellent and effective way ofoperating the 8 GeV linac. In order to have necessary space, the last accelerating section will
be removed, so a C-band pulse compressor will be used to compensate for the lost
acceleration.

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This Committee praises this excellent idea and the initial experimental results. For electronic
control of the positron production, a crystalline tungsten target with a hole will be installed
this summer. When electron beams are required, the primary electron beam is deflected
through the hole in the target. Even though it would be a complicated operation scenario, theCommittee encourages the proposed scheme of operations.
8)

Crab cavity overview

Considerable additional progress has been recorded in the evolution of the crab cavity concept
into detailed engineering design. Specific locations in “Nikko” with existing cryogen supply
have been identified and cavities have been ordered. Indeed, the half-cells have recently been
deep-drawn. The order for cryostats will soon be placed, and a schedule has been establishedthat leads to cavity installation in February 2006.
The cavity design, which originated in 1991-2 as a part of a KEK-Cornell collaboration,
retains the original concept, but has benefited from numerous refinements in the intervening
period, including transfer of the crab mode rejection filter to the warm beam pipe, provisionof transverse support stubs and the implementation of a cavity tuner based on the variable
insertion of the coaxial coupler. The tuner includes a capability for differential movement tocorrect transverse misalignment. Engineering design is well advanced and appropriate for
procurement of components. A stress analysis of critical components shows acceptablestresses and strains. Further analysis to update the understanding of the present design in theareas of cavity multipactor performance, allowable HOM Q's and anticipated dynamic losses
would be beneficial.
A development program for a niobium-coated copper coaxial coupler was only partiallysuccessful, with issues of inadequate niobium coating of the end of the coupler that there wasinsufficient time to address. The fallback position of fabricating the coupler entirely from
niobium has been adopted. There are no problems foreseen with this approach, although it
will be somewhat more expensive.
A somewhat unexpected issue has arisen with the need to obtain government certification of
the pressure-vessel aspects of the design. Interactions with the licensing Review Committee
have been very positive, with members of the Committee offering valuable suggestions fordesign improvements.
Although the schedule for completion of this activity is aggressive, it is being vigorously
pursued and the Committee looks forward to hearing about a successful installation on its next
visit.
9)

Crab Cavity HOM Damper

There are two HOM dampers, one on the large beam pipe and the other in the coaxial line
formed by the beam pipe and the coaxial coupler. These absorb unwanted modes generated in
the crab cavity by the beams. A systematic analysis to optimize the damping of the different
modes has been carried out (HFSS) and a set of design values for the different elements havebeen found such that the beam remains stable transversely and longitudinally. The ferrites
used as the absorbing material in the dampers are formed in an isostatic press and electron
beam welded to the beam and coaxial line tubes. The HOM power with the calculated

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impedances is estimated to be 18 kW, below, but close to, the limits of the capabilities of theferrites as extrapolated from measurements. This should be clarified.
10) Input/ coaxial coupler
The input coupler for the crab cavity, with its T-stub support structure, needs to provide
100kW. It has successfully been tested at all RF phases of reflection up to 200 kW incident
power. We support the initiative to complete the test set-up so that the couplers can be tested
at even higher power, for all phases, during a long period. Most of the components are ready,but a 90° bend has still to be built.
The coaxial coupler inside the beam tube to the right of the crab cavity serves as a means forseparating out the higher order modes to be damped while maintaining a high quality factor
for the crab mode. It also serves as the tuning element for the cavity, being moveable axially.
The original idea to use Niobium film on copper has been rejected for the moment in favor ofa solid Niobium solution. This was due to a lack of time for the R&D necessary for the
resolution of issues concerned with producing an even layer of Niobium. Both the design ofthe stub support in the coaxial coupler (which has to reject frequencies to beyond 950 MHz)and the design of the notch filter (which reflects energy in the line at the crab mode frequency
while allowing all other modes to pass to the ferrite HOM) have been fully studied andoptimized dimensions have been found. One estimation that should be revisited and shouldbe measured on the prototype is that concerning the amount of power that could be coupleddown the lines in the event of the ~ 1m long coaxial line tilting off-axis. The bellows thatallow the coaxial line to be moved longitudinally should also be checked for their HOM
impedance and the resulting power dissipation.
11) Cryostat for the Crab Cavity
The present state of the design of the crab cavity cryostat and its components was presented.
Simulations of the effect of stress on the cavity and the He jacket and the resulting stresscontours and buckling modes were shown. The cavity/jacket ensemble appears mechanicallystable under normal operating conditions.
The whole design is now nearly complete, including the Helium jacket, vacuum vessel andmagnetic shield. The cavity has a unique tuning system using a moveable inner coaxial
coupler, cooled by liquid He at its tip, and supported by a cooled stub. One issue, that has
been mentioned elsewhere, concerns the allowed horizontal deflection of the central tube,
which may occur over time or when moving the tuner. The excess power that would then beabsorbed in the 80 K copper bellows or the 300 K mode damper should be estimated, thenverified and measured on the prototype.
Significant progress has been made on the manufacture of many components of the cryostat,
and a prototype is under construction. Static losses in the cryostat have been calculated indetail. Dynamic losses due to the RF in the cavity are estimated at 50W, but these, and therequired HOM damping, seem to be based on the original design published in 1992. The
estimates should be updated to reflect improvements that have since been made to the design.
Particular attention may need to be paid to the TM010 mode, which depends for it’s damping
on sufficient insertion of the coaxial coupler, even though the latter’s position will be definedby the crab mode frequency. A list of the required damping for all cavity HOMs, as definedby the beam dynamics requirements, should be provided for comparison with the measured
values.
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Losses due to the presence of the beam in the cavity, whether it is on or off-axis, should also
be estimated. The cavity itself will be tested horizontally in a specially prepared pit, but using
an existing test bench. This is a good milestone towards the final construction.
12) Refrigeration system for the Crab Cavity
The locations in “Nikko” selected for the crab cavities are in close proximity to, and will besupported by, the liquid helium refrigerator originally installed for the TRISTAN
cryomodules. With an installed capacity of 8 kW at 4.4 K, the refrigerator is presently
supplying liquid helium to 8 superconducting cavities that are using less than 50% of its
capacity. The remaining capacity will be more than enough for the two crab cavities and for acrab cavity test stand also to be located in this area.
13) RF control for the Crab cavity
The beam loading aspects of a crab cavity (K. Akai et al, EPAC 96) was reviewed. The
influence of the loaded Q and the horizontal position error ∆x in the crab cavity has animportant influence on the required power.
As a crab cavity phase error will produce a horizontal displacement at the IP, the required
phase tolerance and phase jitter is determined by the acceptable horizontal displacement at theIP, which has been determined from beam-beam simulations. It turns out that the correlation
time of the jitter is important: <1% of σx is required for a 1-turn correlation while <5% of σx
is required for a 10-turn correlation. The bunch position shifts caused by the accelerating
cavity transients due to the 5% abort gap also translate into horizontal shifts at IP but are
mostly compensated as it has the same direction for the two rings.
With the available power in existing 500 MHz klystrons, a target QL of 2 x105 has been
adopted. This appears reasonable and results in a power requirement of typically 100 kW.
The feedback system is similar to that used for the superconducting accelerating cavities:
tuning loop, direct RF feedback, phase lock and gain loop for klystron, and amplitude andphase loop for Vc.
Slow phase drift is compensated through the continuous closed orbit correction system (CCC).
In addition, a local orbit feedback is planned for the crab cavity insertion to minimize
horizontal orbit excursions in the crab cavity. This local feedback loop may possibly receive
position reference information from the crab cavity power level.
The coupled bunch growth rates due to the fundamental mode were calculated by Akai et al.
in 1996 for a QL of 1 x106, and depend strongly on tuning error. For the lower QL currently
proposed, and with the addition of direct RF feedback, the acceptable tuning range is evensmaller, but should easily be maintained by the tuning loop.
Since the direct RF feedback loop has a significant influence on the effective transverse
coupling impedance of the crabbing mode, correct and precise phasing of the direct RF
feedback loop is also very important.
The crab cavity amplitude tolerance is fairly relaxed and easily achievable.
In brief, the tolerances looks achievable, the crab cavity RF control system looks reasonable,
and the Committee is looking forward to successful operation of the crab cavities with beamsome time in 2006, without too many bad surprises.
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14) Crab optics and dynamics
Since the previous Committee, changes were adopted in locations of the two crab cavities,
both in LER and HER. The appropriate modification of the crab optics was done.
Following the previous Committee recommendations, a study of the crab cavity effects on thecollective motion was performed. The conclusion is that the head-tail instability is not a
problem, provided that high beta values are avoided in the high-impedance elements.
The effects of imperfect optics on the dynamic aperture have been simulated. The results do
not reveal any serious reduction of the dynamic aperture as long as the parasitic horizontal
dispersion at the crab cavity is within 0.2 m, and the existing optics correction technique iscapable of keeping the parasitic dispersion below 0.05 m. No contribution to the 2νx + νs =integer resonance was found from the crab cavity.
A side effect of rotational misalignment of the crab cavity or of x-y coupling might be a
vertical crabbing angle at the interaction point. Simulations showed that the former could be
cured with the existing iBump correction, while the latter can be compensated by the “r1”coupling coefficient knob.
The Committee recommends continuing the studies of collective effects, including thecurrent-dependent contribution to the crabbing angle, and simulation of the electron cloud
effect on bunches that are crabbing all around the ring. We also suggest continued
examination of single beam stability issues related to the crab cavity, including transient beam
motion in the cavity and operating with the cavity detuned.
15) Magnets, installation, orbit control
An upgrade of the magnet system has been designed to maintain the horizontal beam orbit at
the crab cavities, including the installation of four new magnets, exchanging the positions ofsome steering magnets and realignment of the chicane magnets. The relevant power supply
modifications are also been carefully evaluated.
It is planned to control the horizontal orbit to within ±1 mm in the crab cavities to avoid lossof control of the crabbing mode field due to beam loading. A crab orbit feedback similar to
the iBump, the EPICS based collision feedback system, will be employed, using 4 horizontal
steering magnets in each ring to make an offset bump, and by using 4 BPMs around the crab
cavity to monitor the orbit offset.
All of the upgrade issues have been addressed and scheduled. A test of the control system isplanned this spring, and magnet installation will be done during summer 2005. This provides
a good opportunity to test the feedback system in the fall without crab cavities but with “craboptics”.
16) Super KEKB overview update
A concerted effort by the KEKB team gathered much momentum last year culminating in a
Letter of Intent for SuperKEKB. This letter has been published and was distributed to theCommittee before this meeting.
The Committee welcomes this effort and considers it a demonstration of technical confidence
as well as a determination to produce the maximum output of B-physics research at KEK. At
this point, the Letter of Intent includes a substantial amount of useful information, indicating a
range of luminosity potentially achievable by Super KEKB, i.e. 1-5 x 1034. The Committee
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concurs with the great potential of the Super KEKB as proposed by the KEKB team, andfurther agrees with the estimated range of luminosity achievable. A tentative cost estimate of
465 oku-yen was mentioned. A timetable has also been suggested that ends with SuperKEKB
starting Physics in 2011.
The KEKB team identified several technical items as requiring further R&D, both in terms ofaccelerator physics studies and hardware design and development. These items, as well as theCommittee’s views on them, will be discussed in the following sections. The highlights ofthese discussions have been given in the recommendations section. The uncertainty in theoutcome of these R&D items has been reflected in the range of achievable luminosity given
above. The Committee recommends strongly that R&D of these items be carried out in atimely fashion, and hopes that appropriate and strong support from the KEK laboratory isforthcoming.
17) Beam-beam update
Beam-beam limit is studied using a 3D PIC simulation code. Following the recommendationsof the 2004 MAC, an extensive survey of various lattice error tolerances has been done. It
was found that near the beam-beam limit of ξ=0.1, the beam-beam interaction accompanied
by either of the 4 components of the linear coupling in the arc can significantly perturb
particle motion, and it was suggested that the underlying mechanism could be due to Arnold
diffusion. The Committee finds this study to the point, but is not yet convinced of themechanism suggested, and encourages continuation of the study.
The PIC simulation also showed that the parasitic collisions have only minimal effect on the
beam-beam limit provided that the working point is optimized, even for a bunch pattern withevery bucket filled. Therefore, the choice of very large crossing angle of ±15mrad is justified.
The Committee finds this result and conclusion encouraging.
18) Vacuum upgrade
The vacuum system is very challenging with HER and LER vacuum currents of 4.1A at 8
GeV and 9.4 A at 3.5 GeV respectively. The design is further constrained by impedance andHOM issues due to the high bunch current and short bunch lengths. Although there have beensignificant studies of secondary electron emissions, the real impact of the electron cloud could
still be a limiting effect.
The conceptual design of the copper antechamber addresses several of the key issues for thevacuum system. It significantly reduces the broadband impedance because the beam does notsee synchrotron radiation masks and pumping holes. It also reduces the electron cloud effectby keeping the secondary electron production further away from the positron beam. Also, asshown by the secondary electron yield studies, the copper material can create a carbon layerthat protects itself from multipacting, however the results presented by S. Kato do not agreewith the data in this presentation. Collaboration is recommended here to determine how thesecondary electron yield studies performed by Kato can be applied to the vacuum system.
Also the vacuum group should consider last year’s comment on a design that uses a clearing
electrode.
The thermal analysis of the vacuum chambers including synchrotron radiation heating was notpresented during the Review. The chamber sees 28 W/mm on the back wall of theantechamber, which could create unacceptable peak temperatures and stresses. Copper data
indicates that the re-crystallization temperature could be less than 200ºC depending on the
Page 14 of 26

cold work of the material. Optimization of the cooling scheme will also be required. Twosmaller cooling channels would probably be much more effective than 1 large channel and,with proper placement, can reduce deformation of the vacuum chamber during temperature
excursions. The thermal motion should be considered in order to achieve the required BPM
stability. The ability of the comb style bellows to manage the operational travel requirements
and alignment of the system should be evaluated.
Since a synchrotron radiation ray trace study was not presented, it was unclear whether thecritical components such as bellows and flanges are masked from a direct synchrotron
radiation strike. The molybdenum flange on the wiggler chamber has a temperature rating of
150ºC and a synchrotron radiation strike could cause leaks in the vacuum system. Also, since
the vacuum sealing surface is at the inner wall of the vacuum chambers, it is possible that the
HOM thermal losses could create a large enough thermal gradient to open the flanges during
operation.
Mis-steering should be considered when designing the antechamber-style chambers. The
vertical position and angle steering could be limited by the height of the slot and synchrotron
radiation may be able to hit the entrance of the slot. Thermal-stress calculations should bedone to determine if cooling is needed here. Orbit interlocks could be applied at higher
currents if needed.
No pressure profile for the new vacuum system was presented. The chamber is conductancelimited by the screen and therefore optimization of the pumping screen should be addressed.
The cost impact of drilling more holes should be compared with the improved vacuum
performance. Also, the diameter (4 mm) and depth of the holes should be reviewed for
HOMs to prevent heating of the NEG strips. The issue of the capacity or frequency ofregeneration of the NEG was not discussed.
NEG material can produce dust and it is therefore possible that NEG particles may fall to thebottom of the chamber and migrate into the main beam channel. Dust particles have alreadybeen mentioned as a potential cause for beam instabilities in KEKB. The pump screens in the
wiggler style chambers that have a pump port opposite the synchrotron radiation mask could
be angled to prevent secondary scatter from striking the NEG strips and producing dust.
The beam stay clear or magnet clearance of the vacuum chambers was not presented.
Sufficient space should be allowed for manufacturing tolerances and alignment of these longvacuum chambers. Also the size of the chambers is dependent on many parameters. The
injection acceptance of the beam chamber requires a specific vertical height. The single
bunch instability is reduced with a very small vertical height and the tune shift is reduced witha round aperture. The Committee recommends that the aperture should be agreed upon by all
disciplines so that the vacuum group can proceed with a design.
If a 28 mm radius chamber is adopted, it is unclear if the fields produced by the beam are still
unaffected by the 18mm high slot. If the chamber diameter is greatly reduced, theantechamber might affect the single bunch instability or tune shift.
The comb prototype was installed and found to reduce the HOM power at the convolutions.
This is an innovative solution that successfully reduces the HOM power into the cavity spacebehind the shield. However, the test results still showed that some power leaks past the comb,
and this power will increase with increased bunch current and shorter bunch lengths. The
small bunch length keeps driving the size of the slots down to prevent transverse (TE) modeleakage into the bellows cavity, but the use of absorbing tiles and fans may still be necessary.

Page 15 of 26

An acceptable impedance budget for the vacuum system should be specified including
acceptable tapers and steps.
The designs for specialty equipment such as SR light monitors, transverse and longitudinal
feedback kickers and collimators should be started. The Committee strongly recommends
that a prototype of the moveable mask be installed in KEKB.
The Committee encourages continued prototyping of the vacuum chambers. A copper
extrusion with a welded antechamber may be less expensive and provide a dimensionallyreliable alternative.
Given the complexity of the technical issues influencing the vacuum component design, theCommittee recommends a formal procedure for technical approval, including a review ofsynchrotron radiation power handling and impedance characteristics (including frequency
dependence, resistive wall, HOM, and coherent synchrotron radiation).
19) ARES upgrade
With the increased current and the resulting increase in beam loading, the cavity detuning
should increase from –20KHz for KEKB to –200KHz for SuperKEKB. A larger ratio of thestored energies in the storage and accelerating cavity of 15 will bring the detuning back down.,
and to do this the opening of the coupling iris between the coupling and accelerating cavity
will be enlarged.
At the same time the input power and the HOM load increase.
The input coupler, which shows several multipacting zones at 150-190KW, has to operatereliably at 800KW. A 50nm thin coating with TiN is foreseen. In addition, we recommend
studying active graphitization and a longitudinal shaping of the outer coaxial line. The
coupler will be tested in an upgraded test stand with a 1MW klystron, two couplers, a storage
cavity and a water load.
The copper plating of the storage cavity is going to be improved using the J-PARC facility.
The first results are excellent with no degradation of the theoretical Q-value.
The increased HOM load requires multiple changes. The monopole and vertical dipole modesin the accelerating cavity are coupled out via four waveguide dampers and damped by bulletshaped SiC absorbers. SiC tiles in the grooved beam pipe absorb the horizontal dipole modes.
If the waveguide absorbers cannot stand the estimated 80KW losses, their number will be
increased. The grooved waveguide may be replaced, if necessary, by a winged chamber withdirectly water-cooled tiles able to absorb 20KW. Tests will be performed on a newly
established L-band test stand with a 1.2MW klystron. In summary, although many changes of
the actual ARES will be required, work is well under way and qualified solutions have beenfound.
20) SCC upgrade
Experience with operation of the SCC systems in KEKB has been very encouraging,significantly exceeding design requirements, specifically in RF power transferred to the beam
and in HOM power extracted to the beam-line ferrite absorbers.
The RF power that must be transferred to the beam of SuperKEKB increases to an average of
460 kW per cavity. Operation of the input coupler at 500 kW has been demonstrated intraveling wave conditions in a test stand, which is a good basis for confidence that the
Page 16 of 26

requirements of SuperKEKB can be achieved. The increase in beam current also requires a
reduction of Qext. A 30% reduction has been achieved by mounting the input coupler with athinner gasket, which causes the coupler to protrude further into the beam pipe for higher
coupling. Further reductions, if needed, could be achieved without further modification of thecoupler by the installation of an external waveguide matching network (iris).
A more challenging requirement is the increase in HOM power to be removed from 16 kW to60 kW. A test stand is being established to investigate the limits of existing coupler designs
and to evaluate proposed improvements. To date, dampers have successfully handled theequivalent of 43 kW without damage. One change being considered is a reduction in the
thickness of the ferrite layer to 2-3 mm, to decrease the thermal impedance from the heatedferrite through the copper wall to the cooling water. In addition, damper fabrication processes
that determine the integrity of the joint between the copper jacket and either the ferrite orsilicon carbide material are being reviewed. Prospects for a successful resolution of this issue
seem good.
21) Lattice and Dynamic aperture
The present successful performance of KEKB is due, in large part, to the merits of its optics.
In KEKB the arc lattice design, with a 5π/2 phase advance per cell and non-interleaved
sextupole pairs connected by I΄ transformations, provides independent tuning of the
momentum compaction and horizontal emittance. The sextupoles are mounted on x- and ymovers; their pair wise displacement (either ++ or +–) is used for correcting beta-beat,horizontal dispersion (by x-displacements) and also vertical dispersion and coupling (ydisplacements). The chromaticity correction is not accompanied by a nonlinear kick limiting
the dynamic aperture.
For these reasons, the present arc optics is adopted for the SuperKEKB. However, the IR
lattice had to be extensively modified to meet the SuperKEKB design goals.
With the new optics and a random model of magnet alignment and steering errors, the ability
to correct betas, dispersions and xy-coupling has been successfully checked. However, theapplication of local vertical bumps in SuperKEKB arcs is now limited by the need to keep thesynchrotron radiation fans within the antechamber.
A dynamic aperture study has been done with the new optics, showing only a small reduction
compared with the present KEKB optics.
The Touschek lifetime in the LER is estimated to be 50 minutes, with the momentum aperture
calculated in the dynamic aperture study. Although the dynamic aperture can be consideredreasonable for injection purpose, its effect on the Touschek lifetime is serious. It is suggested
that ways of improving the momentum aperture be pursued by first determining theunderlying mechanism presently limiting the observed momentum aperture.
The dynamic aperture study should continue as more mechanisms and error effects are added.
In particular, synchro-betatron effects in the presence of crab cavities, IR magnet multipole
errors, and parasitic collisions are among the next effects to be included in the simulation.
In simulating the effect of the beam-beam kick on the dynamic aperture, the Committee
suggests using the form of the beam-beam kick resulting from a non-Gaussian transverse
distribution shape such as the one obtained in the strong-strong beam-beam simulations, andto take into account the final bunch length.

Page 17 of 26

22) Interaction Region (IR) overview
The Super KEKB IR parameters have been chosen aggressively, but carefully, to optimize theperformance of the machine. The vertical and horizontal interaction point beta functions are 3
and 200 mm respectively. Dynamic and physical apertures are maintained by building new
QCS magnets with superimposed compensating solenoids placed close to the interaction point.
A larger crossing angle (increased from ±11mr to ±15mr) provides space for the necessary
focusing elements.
The design of the interaction region is complicated by the large dynamic beam-beam effects
that more than double the horizontal emittance and reduce β*x to keep the beam size at theinteraction point approximately the same. The vacuum chambers are designed to have no
synchrotron radiation striking within 5m (LER) or 8m (HER) downstream of the interaction
point, although the effect of vertical deflections due to the solenoid field have not yet been
considered. The QCS quadrupoles alone generate nearly 250 kW of synchrotron radiation.
At least as challenging is the disposition of more than 100 kW of higher order mode powerthat are expected to be deposited near the interaction region. Experiment backgrounds will be
more than an order of magnitude larger than at present.
The interaction region is the most difficult design challenges of a collider. The Super KEKB
interaction region now has a basic design covering the critical areas. However, there is muchwork to be done before one can be certain that all of the desired parameters can be realized.
As noted above, the synchrotron radiation fans must be completely calculated, including
vertical extent and the effects of allowed orbit bumps (and a means of limiting them).
Radiation impinging on the interaction region from the incoming beam should be rechecked.
The HOM generation has to date only been approximately scaled from KEKB estimates, andthe dissipation has not been addressed yet. Vacuum pumping and instrumentation needs to belaid out. A beam-stay-clear map should be made available. Magnet design and analysis is not
yet complete (particularly realistic multi-poles).
The KEK staff has made good progress on the IR design, but there is still much work to be
done. We strongly encourage a well coordinated and focused effort on the tasks listed above.
23) IR vacuum chamber
The preliminary design of the vacuum system is underway.
The interaction region vacuum is one of the most challenging systems. The Committee
commends the KEKB staff for extrapolating their past experience to help define the designtasks and improvements. The design presented is preliminary, so the following comments do
not address specific issues.
A 3-D graphical and analytical solution for the synchrotron radiation (SR) fans should bedone as soon as possible and should include the vertical deflection from the solenoid field. It
is critical to determine if the SR fans can strike the 1 cm proposed vertex chamber and thewalls of the other vacuum chambers. If SR power strikes the vacuum chambers close to theinteraction point, the power densities could be difficult to manage and could impact thebackgrounds. The vacuum group needs to work closely with the background group to
determine the placement and acceptable slopes for the masks. The design of the crotch mask
should be started soon to determine if the SR power is reasonable. A mis-steering envelope
should be defined to ensure that the vacuum chamber does not intercept SR power that could
damage the chambers.
Page 18 of 26

Another important issue is the HOM heating in this area. Analysis needs to be done here to
determine the amount of HOM heating expected. The staff needs to consider where thepower could go (bellows, resistive wall components, etc.) when designing this area. Also, thecomb bellows may still require absorbing tiles or convective cooling on the bellows
corrugations due to the increased HOM power from the short bunch lengths and high bunch
currents.
Since the IR geometry creates a trapped volume and the crotch mask is a HOM generator, thestaff should consider designing a HOM damper near the crotch so the modes can be absorbed
in a location that is designed for high power. Understanding the trapping mechanism for
HOM power generated in the space between the crotch and the central beam pipe is urgent.
Absorbing this power safely could impact the design of other components in this region.
Thermal analysis on the vertex chamber from resistive and HOM power should be performed.
A magnet prototype is underway, but the conceptual design of the vacuum system has only
just started. Caution is advised not to allow the magnet group to lock the vacuum system into
an insufficient space. It is essential to provide enough space between the beam stay clear andmagnet stay clear for the vacuum chamber, cooling and pumping. With the smaller βy* in theinteraction point the beam stay clear could get large outboard of the interaction point andcould lead to background problems. Beam stay clear ellipses need to be mapped in this area
using defined values for σx and σy.
A preliminary BPM design is needed to determine if it can be installed near the interaction
point in the QSR magnet. Presently, it appears that the space is too limited. It will be
important to decide how critical these BPMs are for steering the beams into collision. Twosets of BPMs separated by a fraction of the BPM processing wavelength might be considered
so that both beams can be independently measured.
Finally, pressure specifications for different distances from the interaction point should be
provided to the vacuum group based on the background effects. The interaction region area
will have space limitations for pumping, so instrumentation in this area is also critical to help
understand the backgrounds.
24) Secondary electron yields from vacuum materials and in situ surface
characterization
Studies by S. Kato et al. on the mechanism for the conditioning of copper to lower secondary
electron yields (SEY) have shown that carbon, initially dissolved in the bulk material, plays acritical role. The Committee was highly impressed by this work, both by its thoroughness,and by its relevance to a phenomenon critical to the operation of high intensity colliders aswell as other applications.
This team has found that carbon in its graphite form has a very low SEY without any
conditioning. By careful examination of copper surfaces using XPS spectra and measurementof SEY, carbon from the bulk material was observed to migrate to the surface under electron
bombardment. This process also served to drive off oxides, water, and other surface-boundsubstances. The carbon formed into a “rough” graphite structure with the low SEY properties.
Other metals, nickel, stainless steel, as well as Titanium Nitride and Chromium Oxide, were
measured though only the extensive study of copper was reported.
The researchers propose a conditioning procedure that avoids coating with Titanium Nitride
or other materials. The process starts with carbon enrichment if necessary, then surface
Page 19 of 26

roughening and electron irradiation (< 1 keV, 10C/cm2). The resulting surface has low SEY
which is maintained during venting.
The Committee enthusiastically applauds this work and strongly encourages continued studyof the properties of surfaces in accelerator environments and the development of surface
preparation and conditioning techniques.
25) QCS
The QCS design takes into account the Belle experience on required shielding and wiring
space. It adopts a non-magnetic metal, tungsten alloy, for shielding and eight rods to support
the cryostat. The rods would be adequate for weight, EMF, and heat losses. The Committee
would like to remind the design team of the safety margin of the ES fields and the multipole
components due to conductor displacement errors.
26) BackgroundA comprehensive review of background sources in the Belle detector was presented to theCommittee. These included synchrotron radiation, beam-gas scattered beam particles,
Touschek scattered particles, and neutrons and electrons from radiative Bhabha events. Theease with which the detector can be damaged was emphasized by an event in summer, 1999
when a mis-steered beam caused fatal damage to the vertex detector. Backscattered radiation
originating from the incoming HER beam in the QCS now accounts for 1/3 of SVD and CDC
background, though the dominant background in the detector comes from beam-gas or
Touschek scattered particles (the small beam emittance in all 3 dimensions aggravates
Touschek scattering). The azimuthal distribution of radiation on the SVD has beenreasonably well predicted by simulation.
All background levels will be higher for Super KEKB. More synchrotron radiation ispredicted to increase backscattered sources by a factor 6. Radiative Bhabha electrons will bea larger source of background (a factor 19) with the more closely spaced QCS quadrupoles
planned for Super KEKB, though polyethylene shielding could lower this. With the smaller
beam emittance the Touschek scattering rate will increase by a factor 97.
Shielding, both heavy metal and polyethylene, might limit this factor to 10~20. However, if
the central beam pipe is reduced in radius from 1.5 to 1 cm, the SVD rates could increase
dramatically. A statement was made that the detector will be upgraded to work with 20 to 30
times the background rates. If this and the predictions above are true, then the increases inbackground in Super KEKB may be manageable.
There was no presentation of backgrounds during injection. We recommend study andoptimization of detector backgrounds and minimization of detector dead time duringcontinual injection. We also recommend continued refinement and calculation of backgroundcalculations using parameters confirmed by machine designers.
27) Impedance
The shorter bunches (σz = 4 mm) and higher currents (ILER = 9.4 A) result in a dramaticincrease in HOM power dissipation in SuperKEKB. A substantial effort has been devoted tocategorize and minimize the HOM heating for various vacuum chamber components. As an
indication of the severity of the problem, it is noted that after this round of minimization, theresistive wall is one of the strongest heating sources (1 MW).
Page 20 of 26

Loss factors for a number of new vacuum components designed for SuperKEKB have been
estimated by the MAFIA T3 simulation code: pumping ports, photon masks, connection
flanges, RF shields for bellows. All these components occur in large numbers in SuperKEKB.
These components have all been redesigned to have a much smaller loss factor than thecurrent KEKB components: photon masks and pumping ports are now located in theantechamber, helicoflex gaskets are replaced by smooth MO-type OFC flanges, and finger
type bellow shields are replaced by smooth nested comb teeth shields (reducing HOM heatingby a factor of 3 to 0.7 MW).
There is a reasonably good agreement between estimated loss fact for the current KEKB LER
and the measurements (LER 2000, Ieri-san), which gives confidence in the calculations.
The 16 movable masks in each ring of the current design (which so far have gone throughmany versions!) have a large loss factor, currently estimated to be k (3 mm) = 16 * 2 [V/pC] =32 [V/pC] which extrapolates to PHOM = 5.7 MW for SuperKEKB LER, a very large number,even larger than the total RF system HOM power loss.
A new conceptual design with an ‘electrically invisible’ movable mask (a rod with ceramic
support) having a much lower loss factor was presented, but a significant amount of R&D
remains to demonstrate a reliable operational movable mask with a reduced loss factor. The
Committee recommends pursuing this R&D work and subsequent beam tests with highpriority.
The loss factors of RF cavities (ARES and SCC) are well known, power measurements agree
well with estimated loss factors, and R&D is in progress to address the higher power levels
anticipated in SuperKEKB. Tests on the ferrite HOM dampers of SCC so far did not damage
them, but have caused outgassing.
The HOM power in the interaction region is estimated to be a few 100 kW with most of theheating coming from the recombination chambers. It is expected that a large number ofabsorbers will be needed in the interaction region to absorb these HOMs. An effective
solution will require knowing where the HOM power will be deposited. This is a difficult but
unfortunately also unavoidable task. The Committee recommends dealing with this problem
systematically at the earliest possible time.
While the vacuum components mentioned above look very promising, there are still a number
other components (BPMs, IR chamber, IR mask, recombination chamber, high current crab
cavity, longitudinal and transverse feedback kickers, accidental trapped modes, etc) that need
to be addressed in terms of loss factors and HOM power handling capacity.
Furthermore, in addition to the HOM study, a study of the more general problem of singleand multi-bunch collective instabilities should be carried out using the impedances found.
28) Coherent synchrotron radiation
The near field of the Coherent Synchrotron Radiation (CSR) was computed with a code
available at KEK. Transient shielding due to a perfectly conducting beam pipe with a squarecross section was assumed. To evaluate the resulting longitudinal wake in the LER, the ring
was represented by a simplified model with about 100 straight and bent periods. The squarecross-section of the beam pipe was kept uniform around the orbit. In the next step, it isplanned to include wiggler sections.
The resulting wake was used in a longitudinal tracking code with radiation damping andquantum excitation. The bunch distribution stays stable with increasing bunch current, while
Page 21 of 26

the bunch length grows slowly. But above a certain threshold, both the length and the energy
spread of the bunch start to grow fast, ending up in a saw-tooth oscillation. The thresholdclearly increases as the size of the shielding pipe is reduced.
The conclusion of this study is that to prevent serious lengthening (<10%) of a 3 mm bunch,as specified for SuperKEKB, the beam pipe cross-section should not be larger than 56x56
mm2 compared with the present 94x94 mm2. The implication of the reduced beam pipe sizeon the other impedances has not yet been considered.
However, there is a common understanding among the Committee members that, prior to
making any decision, the contributions from all other longitudinal impedances present in the
LER should be combined with the CSR wake and the potential for partial compensation
should be investigated.
The Committee also recommends studying the transverse effects of the CSR impedance. Oneof the consequences of the transient shielding is a radiated field ahead of the bunch, and its
effect on the multi-bunch stability should also be addressed.
This effect has the potential to have such a profound impact on the design of SuperKEKB that
the Committee strongly recommends carrying out more detailed studies at KEK, andcomparing the results with those of other studies at SLAC and elsewhere.
29) C-band R&D status for SuperKEKB
A C-band test module was installed last year. It consists of a booster klystron, a modulator, a43MW, 2µs klystron with pulse compressor and an RF window, an accelerating cavity, and aload. After some initial breakdown and heating problems, the klystron and modulator operatewith no trouble. The RF window was tested up to 300MW in a resonant ring and has a
VSWR below 1.05 with 4% bandwidth. A SLED-type pulse compressor, operating in the
TE_038 mode, provided 200MW output power in a test stand and had a power multiplication
factor of 4.7. A 1m long, half-scale S-band accelerating structure was powered and showed agradient of 42MV/m at 56MW input power. Breakdown occurred at the input coupler, andsevere damage of the coupler iris and the first structure iris were found.
In order to make efficient use of the available power, it is planned to change the accelerating
structure. It will be a 2m long, constant gradient structure with the group velocity varying
from 3 to 1% and a single-sided coupler. An input power of 80MW will create a gradient of
42MV/m. Since this is at the limit of what we consider to be reliable due to breakdown andpulsed heating, we strongly recommend preparing backup solutions, e.g. a single input, twosided coupler and a 1m long structure with lower group velocity.
30) Tungsten mono-crystalline target for a high-intensity positron source
A preliminary study of a new type of the positron source using a mono-crystalline target has
been carried out at the KEKB injector linac. This source is expected to become one of thebright, new schemes for a high intensity positron source.
More experimental data are expected to clearly understand the physical interaction processesof the Channeling Radiation (CR) and the Coherent Bremsstrahlung (CB), since a full
simulation taking into account both CR and CB has not yet been carried out.
The experimental setup was installed at the 3rd switchyard in the linac beam line, including
the target system, the primary electron beam monitor system and a simple positron
spectrometer. Some preliminary experimental results have been obtained with incident
Page 22 of 26

electron energies of 4 and 8 GeV and with different thickness of the target. It is reported thatthe absolute positron yields with the crystal target were enhanced by about 26% for a positronenergy of 20MeV/c and by 15% and 18% for electron energies of 4 and 8 GeV respectively,
averaging over the positron momentum range of 5 – 20 MeV/c, compared with the maximume+ yield with the amorphous target.
The Committee recommends that this experiment be continued to improve the understandingof the positron yield, and to obtain a precise alignment of the crystal axis, since it is expected
to be used in the linac upgrade plan for simultaneous injection scheme to the four rings.
31) KEKB controls: Status and upgrade plan
The KEKB control system is working well and there have been few problems. However, likemost control systems, it is based on equipment that is either obsolete, or is expected to
become obsolete in the near future. Plans are under way to evaluate all the components anddevelop plans for upgrading those that are the most vulnerable. All the upgrades willcertainly be needed if it is decided to go ahead with SuperKEKB. If KEKB continues in it’spresent configuration, it can be expected that some fraction of the components will be
upgraded each year as part of a continuous improvement effort.
There are currently no plans to make major changes in the software, other than those that are
driven by hardware upgrades. In the high-level applications there will also continue to be
new algorithms required to support the new accelerator physics initiatives.
Overall, the Control system seems to be in good shape.

Page 23 of 26

Appendix A
KEKB Accelerator Review Members
Andrew Hutton

JLabChairmanAlexander Chao

SLAC

Warren FunkJLabOswald Gröbner

CERN (retired) – unable to attend

Heino Henke

Technical University, Berlin

Nadine Kurita

SLAC (replacing John Seeman for this meeting)

Trevor Linnecar

CERN

Won Namkung

POSTECH

Flemming Pedersen

CERN

Eugene A. Perevedentsev

BINP, Novosibirsk

David Rice

CornellJohn Seeman

SLAC – unable to attend

Wang Shuhong

IHEP, Beijing

Katsunobu OideKEK Secretary, Accelerator

Shin-ichi Kurokawa

KEK Secretary, Accelerator

Page 24 of 26

Agenda of the Tenth KEKB Accelerator Review Committee
Feb. 21(Mon.)
8:30- 9:00
Executive session
9:00- 9:05
WelcomeY. Totsuka

Status report
9:05- 9:20
9:20-10:00
10:00-10:30

Overview of KEKB project
KEKB performance & near term plans
Belle status and plans

K. OideH. KoisoY.Sakai

10:30-10:50
10:50-11:10
11:10-11:30
11:30-11:50
11:50-12:15

Break
BPM issues
Beam observations
Beam-beam experiment
ARES status

M. Tejima
J. Flanagan
T. Ieiri
T. Abe12:15-13:30
13:30-13:55

Lunch
Injector upgrade

M. Satoh

13:55-14:25
14:25-14:50
14:50-15:15
15:15-15:40

Crab cavity overview
HOM damper
Input/Coaxial coupler
CryostatK. Hosoyama
Y. Morita
K. Nakanishi
H. Nakai

15:40-16:00
16:00-16:15
16:15-16:40
16:40-17:05
17:05-17:30

Break
Refrigeration system
RF control
Crab optics & dynamics
Magnets, installation, orbit control

H. NakaiK. Akai
A. Morita
M. Masuzawa

17:30-18:30

Executive session

18:30-20:00

Reception

Crab cavity

Feb. 22 (Tue.)SuperKEKB
8:30- 9:00
9:00- 9:20
9:20- 9:45
9:45-10:10

SuperKEKB overview update
Beam-beam update
Vacuum upgrade
ARES upgradeT. Mimashi
M. Tawada
Y. Suetsugu
T. Abe10:10-10:30
10:30-10:55
10:55-11:20
11:20-11:40
11:40-12:05

break
SCC upgrade
Lattice and dynamic aperture at SuperKEKB
IR overview
IR vacuum chamber

T. Furuya
Y. Ohnishi
Y. Funakoshi
K. Kanazawa

12:05-13:25

lunch

13:25-14:00
14:00-14:25
14:25-14:45
14:45-15:10
15:10-15:30

IR vacuum system upgrades plus a short status of PEP-II
Secondary electron yields from vacuum materials and in-situ surface
characterization
QCS
Background
Impedance

15:30-15:45

breakPage 25 of 26

N. Kurita
S. KatoN. Ohuchi
O. Tajima
K. Shibata

15:45-16:05
16:05-16:30
16:30-16:50
16:50-17:10
17:25-18:30

Coherent radiation
C-band R&D status for SuperKEKB
Tungsten mono-crystalline target for a high-intensity positron source
KEKB controls: Status and upgrade plan
Executive session

18:30-20:00

Working dinnerFeb. 23 (Wed.)
8:30-10:30
Executive session
10:30-11:00
Interaction with KEKB staff members
11:00-11:30
Closing

Page 26 of 26

T. Agoh
T. Kamitani
T. Suwada
N. Yamamoto