The Fourteenth KEKB Accelerator Review Committee ReportFebruary 27, 2009
Introduction
The Fourteenth KEKB Accelerator Review Committee meeting was held on February 9-11,
2009. Appendix A shows the present membership of the Committee. Warren Funk, Eugene
Perevedentsev, and Wang Shuhong were regrettably unable to attend this meeting.
The meeting followed the standard format, with two days of oral presentations by the KEKB
staff members, followed by discussion between the Committee members. The Agenda forthe meeting is shown in Appendix B.
The Committee was as always impressed by the high standard of the talks, both the technicalcontent and the presentations themselves. The 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 theCommittee members.
Contents
Executive Summary
Recommendations
Findings and Recommendations
1) KEK Roadmap and SuperKEKB Upgrade
2) Progress in Crab Crossing3) Belle Status
4) Cooling of Crab Cavities below 4K5) Simultaneous Injection6) Issues on the Upgrade
7) Luminosity with Travel Focus Scheme8) IR Design9) Design of New Superconducting Quads
10) Special Studies with Crab Cavities
11) Evaluation of Detector Background12) IR Beam Pipes and Assembly13) SuperKEKB Ring Optics
14) Evaluation of X-Y coupling / requirements on correction15) Issues and Plans on Beam Diagnostics
16) Plans for the next run17) Studies on Beam Pipes & Electron Cloud18) Upgrade of RF Power Sources & Cooling System
19) Crab Cavities for SuperKEKB

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A) Executive Summary
The performance of KEKB improved over the last year, but not as much as had been hoped.
A calibration error has been discovered in the luminosity monitor, so that the previous
luminosity records have been increased by a few percent. The peak luminosity that has beenachieved with crab crossing is now 1.68 x 1034 cm-2s-1, extremely close to the previous record
without the crab cavity (1.76 x 1034 cm-2s-1).
The Belle detector has accumulated a total integrated luminosity of 891.2 fb-1 of which 98.2
fb-1 was accumulated in 2008, compared with 145.4 fb-1 in 2006 and 86 fb-1 in 2007. The
accumulated luminosity was limited due to reduced running because of budgetary constraints(162 hours in 2008 compared with >220 hours in 2006 and 2007). The BELLE detector has
published, or submitted for publication, a total of 294 papers in refereed journals (242 at thelast meeting). This year the main focus of the physics program has been on Υ(1s), Υ(2s),Υ(3s), and Υ(5s) running, requiring that the accelerator be set up at non-standard energies.
The Users seemed to be happy with the beam quality that was provided.
The Committee congratulates the recent Nobel Prize winners Nambu, Kobayashi and
Maskawa. Their theoretical work was tested and proven at KEKB and PEP-II, which has
demonstrated the importance of the KEKB physics program.
A major focus of the Review was the SuperKEKB Upgrade. The funding outlook forSuperKEKB is extremely positive given its scientific merit. The possible timelines forfunding SuperKEKB show the start of construction, at most two years away, and at best, only
one year away. This makes this meeting of the KEKB Machine Advisory Committee
extremely timely in helping chart a course for KEK towards a major new accelerator project,SuperKEKB.
It will be important to choose a suitable goal for SuperKEKB. It is suggested that this goal
be related to the annual integrated luminosity rather than the peak luminosity. KEKB was
funded to produce a total delivered luminosity of 1ab-1 and is expected to reach this goal
within the next two years, depending on the running schedule. A reasonable initial goal for
SuperKEKB would be 1-2ab-1 per year (for comparison, in the best year of KEKB, theintegrated luminosity reached ~0.2ab-1). The exact value of the goal is extremely important:
if the initial goal is too low, the physics reach will be compromised, if it is too high, it may
force the machine design towards solutions that are too expensive.
In order to significantly increase the luminosity of KEKB, there are two possible approaches:increase the current (more intense beams), or decrease the emittance (brighter beams). To
date, the KEKB team has concentrated on the increased current option. Recent developments(particularly in Frascati) have renewed interest in the low emittance option. It is
recommended that the KEKB team evaluate the low emittance option to see whether it would
provide a suitable basis for SuperKEKB. Particular attention should be paid to limiting boththe construction costs and the operating costs of the machine, an important consideration for
the funding agency. This initial study is vital to enable the timely development of a complete
design for SuperKEKB, preferably one with many options for later luminosity improvements.
A great deal of component R&D has been carried out over the last few years and great strides
have been made. Most of this R&D would be applicable to both design solutions, so thiswork should carry on. However, it is suggested that the machine design work concentrate on
the low emittance option for the next few months, with a focus on identifying any possible
showstoppers. The design should be brought to a point where an informed decision between
the two options can be made.

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B) Recommendations
The Committee has made recommendations throughout the different sections below.
Highlights of these recommendations are summarized here.
KEKB Operations
1) The Committee suggests that the studies with higher voltages in the crab cavities, reduction of thevertical IP beam sizes by chromatic corrections, reduction of the x-y IR coupling, and studies
related to the low emittance option be given high priority.

2) The Committee recommends that studies of the beam-beam effect continue with the goalof matching the simulations and the experimental results, and increasing the luminosity.
3) Pursuing these studies is not only important for the BELLE physics program but isparticularly important for the future performance of SuperKEKB.
SuperKEKB
4) The Committee recommends that a detailed timeline for SuperKEKB preparations be
developed, including milestones and decision points. This should be consistent with thetimescale for the start of construction presented by the lab director.
5) The Committee recommends that the machine design work concentrate on the low
emittance option for the next few months, with a focus on identifying any possible
showstoppers.
6) The Committee recommends that component R&D continue, particularly for thosecomponents that would be used in either option. This list includes superconducting andpermanent magnets, low SEY beam pipe and other electron cloud mitigation studies,
positron source upgrades, masks and collimators, etc.
7) The Committee encourages the study of high-efficiency klystrons, for example using thedepressed collector potential concept.

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1

KEK Roadmap and SuperKEKB Upgrade

Professor Suzuki presented the road map for the KEK Laboratory, which consists of a multipronged approach to accelerator-based science. J-PARC will become a world leader in long
baseline neutrino studies, and will soon initiate material science studies using neutrons andmuons. KEKB will be upgraded to study CP asymmetry and new sources of flavor mixing in
the quark sector. The physics from SuperKEKB will be complementary to that which can beobtained from LHC. KEK will remain involved in preparing for energy frontier experiments
at ILC, and is involved in LHC. SuperKEKB will play a large role in these preparations, bothin the physics aspects and the technologies. KEK will also remain in the forefront of photon
physics with the Photon Factory and later an ERL.
The outlook for SuperKEKB is extremely positive, with the worst case scenario being
construction starting in JFY2011 and in the best case; construction would begin a year sooner.
The total construction cost envisioned by the funding agency is $300-400M and the operating
cost should not significantly exceed the present annual operating budget of $60M.
2 Progress in Crab Crossing
Findings:
Much progress has been made in understanding the discrepancy between predictions andmeasurements of luminosity with crab compensation. During FY2008 the daily integrated
luminosity with crab has approached the high values of past years. Peak luminosity was
1.5x1034cm-2s-1 in 2007 and 1.68 x1034cm-2s-1 in 2008, compared with 1.76x1034cm-2s-1 in
2006. More importantly, the primary factors preventing much higher luminosities are better
understood (see below).
We give a rather detailed description of findings here since a clear understanding is essential
to interpretation of the Committee's recommendations.
A negative 4.5% error in luminosity measurement, presumably slowly growing over time,caused a re-evaluation of luminosity records over the past year, particularly with crab cavity
operation, increasing peak luminosity with crab (Υ4S) from 1.61 to 1.68 x1034cm-2s-1.
Two primary limitations are observed in crab cavity performance - a lifetime imposed limit tobunch current and a drop in specific luminosity at higher currents well below the values
indicated by simulation.
The decrease in lifetime at higher bunch currents is believed to be caused by a horizontal
physical aperture at the LER crab cavity (aperture limited at the location of a mask). The
modification to the beam optics functions from the coherent beam-beam focusing causes thehorizontal beta size at the crab cavity to increase by nearly 5 times. A trial of raising thehorizontal beta function at the Interaction point allowed colliding bunches at the SuperKEKB
design current, although the luminosity was lower as a result. The observed dependence of
lifetime on horizontal offset of the beams at the Interaction Point is interpreted to be aconfirmation of this model. Options for orbit bumps around the crab cavity are constrainedby the need to minimize beam loading of the crabbing mode, exacerbated by a misalignmentof the HER crab cavity found during a recent shutdown. It was also concluded from thiseffort that the aperture center and field center of the LER crab cavity are offset by 4mm.
Plans to mitigate the effects of reduced lifetimes include: reducing βx around the crab cavity
while raising the crab voltage by lowering cavity temperature, increasing βx at the Interaction
Point (from 0.9m to 1.5m), and improving injection rate and stability of beam currents forPage 4 of 20

tuning by achieving pulse-to-pulse switching between electron and positron injection.
With βx =1.5m, no beam lifetime problem is found at 1.1 mA2, but the problem recurs at 1.5
mA2. Using the crab cavities at βx =1.5m leads to an increase of specific luminosity of 20%,
while the expected geometric factor gain is 10%. This is a possible indication that the crab
cavity has contributed to a higher beam-beam parameter. The achieved specific luminosity is
consistent with a ξy (HER) =0.09.
The specific luminosity should be related to the vertical size of the beams and accordingly
some effort to relate the two has been made. A direct measurement of the vertical beam sizes
yields values ~67% larger than expected from luminosity calculations (crab off). A hint ofsome of this discrepancy came from single beam measurements in April, 2008 showing acurrent dependent increase in vertical beam size. This effect could not be reproduced in
November, and is felt to have been caused by some coherent bunches motion. The primarysource of this discrepancy is unresolved at this time.
The increase of luminosity of a factor of ~2 due to crab cavities predicted by simulations isreconfirmed independently by Yunhai Cai using a completely different code. Both codes are
therefore basically in agreement, although a large difference remains when compared withmeasurements. In addition to beam-beam effects, this independent code has also included theeffects of wake fields. The simulations indicate a microwave instability threshold of 1/3 ofdesign current in the LER causing a 20% increase in energy spread at 1.0 mA. This was
supported by a cross-section measurement by Belle on the narrow Υ(2S) resonance.
Luminosity optimization using a computer–based Downhill Simplex Method, and the manualscans that were presented last year have been continued with larger (4 or 5 units) starting
errors on 12 coupling and dispersion parameters at the Interaction Point. Application of the
Downhill Simplex Method plus the manual scans could not recover luminosity above 60% of
the no-error prediction. These simulations have apparently demonstrated that this level of
optics errors prevents effective compensation by the existing methods. In addition the growthin energy spread in the LEB will further increase the impact of chromatic errors.
Other tests were made in the search for luminosity limiting effects, including horizontal
emittance of both beams (the present values are the best), negative α optics (unstable beam),and increased number of bunches (vertical beam blowup with 2 bucket LER spacing). The
last trial suggests that better suppression of the electron cloud effect would benefit
performance appreciably.
Operation statistics show a much higher availability (but with reduced scheduled hours) thanin FY2007 where crab cavity commissioning was a major effect.
Committee Response:
The Committee congratulates the KEKB staff for impressive advances in understanding
luminosity limitations during operation with crab cavities, as well as overall performance of
KEKB. While delivering a larger fraction of operating time to the Physics run some key
factors in performance limitations have been better understood.
We approve the direction of effort to further improve performance of KEKB and plan for
SuperKEKB upgrade. Better understanding of KEKB performance will directly contribute tohigher SuperKEKB performance. In addition we have the following suggestions.
Explore further bumps around the crab cavities to simultaneously achieve the target beamloading and minimize beam losses. Adding angle bumps to displacement bumps may give
better results.

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Are the larger errors used in simulation of tuning plausible given the known alignment andfield tolerances? If not an open-minded search for other stray fields or errors may be
productive.
We endorse continuing efforts to resolve the discrepancy between measured beam sizes andthose derived from luminosity.
We endorse the pursuit of chromatic coupling effects using skew sextupoles as proposed by
the KEKB team.
3 Belle Status
The Belle detector has been producing excellent physics results, dividing up the running time
as follows:
√s

Υ(1s)

Υ(2s)

Υ(3s)

Υ(4s)

Υ(5s)

Luminosity (fb-1)

9.8

10.1

13.5

74.6

31.0

There have been some significant improvements to the BELLE detector, notably the trigger
data is now read out via COPPER and there is a new Global Decision Logic. The rate at
which papers are being published in peer-reviewed journals continues to be impressive.
There is also a strong effort to develop the hardware that will be required for SuperKEKB,
which is proceeding in parallel with data-taking. This involves most of the detectors and is
aimed at accepting an event rate ten times higher than now, but with twenty times morebackground. The idea for Super Belle vertexing is a silicon strip vertex detector with six
layers and an inner double layer of pixels. The pixels will be based on DEPFETs originallydeveloped for the ILC by groups in Germany, Czech Republic and Spain. The double sided
silicon strip detectors will be readout with the APV25 chip, originally developed for CMS. A
prototype of the barrel time-of-flight detector (TOP) is currently undergoing tests at the FUJI
beamline, as is a prototype of the Aerogel Ring-Imaging Cerenkov detector. The TOP isintended to replace the current aerogel and time-of-flight detectors in Belle. The TOP
measures times and images rings from internally reflected Cherenkov light produced inquartz bars.
All of these projects seemed to be proceeding well, with prototype hardware already available
for testing. The Committee was impressed with the quality and quantity of the work being
carried out.
4 Cooling of Crab Cavities below 4K
Findings:
The restricted physical aperture in the vicinity of the crab cavities is one of the lifetime
limitations in KEKB when operating with crab cavities. To improve the situation, optics
solutions have been found that reduce the beta function in this region and hence the physicalsize of the beam. Since the reduction in beta also reduces the crabbing kick, it is necessary tocompensate with an increase in crab voltage. The cavities already operate near theirmaximum values, believed to be limited by the Q0 of the cavities. Previous measurements
have shown that the Q0 can be increased by lowering the temperature below 4K, in fact afactor ~4 increase has been measured when going to 2.8K. Modifications have therefore
been made to the cryogenics system to enable the cavity helium bath to run with a vapor
pressure of 72 kPa corresponding to a temperature of 3.7K and yielding up to a 20% increasein crab kick voltage.

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These significant modifications have been gradually implemented and include, amongst
others, considerably increased water cooling chiller power, improved helium bath pressure
control, and cooling of the pump oil and discharged He gas.
Unpowered cavity tests have shown that the pressure can be accurately controlled and thetemperature maintained at 3.7K for many hours even with 100 W added to represent dynamic
loading, the expected maximum operational value for the LER ring.
Tests will be made with RF power in March/April and these should confirm the hypothesisthat the Q0 change will increase the available field, though only beam use will provide theultimate test.
Comments:
Operation at reduced pressure renders the system vulnerable to small leaks and theintroduction of purging equipment would be advisable.
Although the situation with oil loss has improved, it is possible that the loss could be reduced
further to maximize the times between oil refills.
Experience gained in other laboratories that have many years experience working with warm
Helium pumping (such as Jefferson Laboratory) could be profitably used for further
improvements to the 3.7K system.
5 Simultaneous Injection
Over the last few years, the Injector Linac Group has successfully developed subsystems andcontrol programs required for simultaneous injections to all four rings (HER, LER, PF and
PF-AR). These include a fast bend magnet to PF, a bypass hole for electrons at the positrontarget, phase-controlled acceleration and deceleration in the common lattice, orbit feedbackby optical measurements with wire scanners, and an event system. Up to now, the group
achieved fast alternating beam injection to HER and LER. Prior to the introduction of the
new system the typical switching time from HER to LER filling was more than 30 seconds.
This time has now been reduced to about 2 seconds, providing a ten-fold decrease of thebeam current variations in KEKB. Simultaneous injection has significantly improved
performance, has shortened the time for luminosity tuning, and the impact of the short beam
lifetime on the integrated luminosity has been compensated. The group plans to provide
pulse-to-pulse switching between HER and LER in April, and pulse-to-pulse switching
between PF, HER and LER in October this year.
The Committee is convinced that the group will achieve the final goal of pulse-to-pulse
injections to all three rings this year, a requirement for the SuperKEKB project. The
Committee extends congratulations to the Injector Linac Group for the successfulachievement of simultaneous injection.
6 Issues on the Upgrade
Findings:
The present phase of operation of the KEKB B-Factory is coming to an end. It has been avery successful run of about a decade. Many significant particle physics results were
produced.
The plan forward for the KEKB upgrade has several significant choices to be made. One
possible design option has been studied by the KEKB team, but additional study is needed.
The two general directions appear to be a) high current and high beam-beam parameters or b)

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low emittance, small vertical IP betas and modest currents. Both need to be studied to arriveat the best option.
Coherent Synchrotron Radiation (CSR) is a critical effect for short bunch, high peak current
machines with curved orbits. An analysis of CSR effects on a SuperKEKB beam wasrecently performed from first principles, making only those approximations appropriate to theSuperKEKB conditions. A realistic beam pipe shape was used and calculation of the wakefield was extended in distance. These calculations show that the CSR wake field dominated
all others, and tracking simulations using this wake field indicate large (1.6-1.8) increases inboth energy spread and bunch length. Reasonable changes in ring design were explored anddid not change the outcome significantly. If uncompensated, the effect of CSR is to lower the
luminosity from 5 to 2 x1035cm-2s-1.
Several options are under consideration to recover the performance target. Limitations to theratio of β*y to bunch length are exacerbated by bunch lengthening, but may be partiallyovercome by the “traveling waist” scheme. A combination of crab cavities and sextupoles
moves the vertical waist of the beam at the Interaction Point correlated with the longitudinal
position along the beam. While difficult to implement in the HER, the need is more acute in
the LER and the implementation is easier. Traveling waist optics have been computed andfound to have acceptable dynamic aperture. The successful implementation of crab waist
compensation would recover most of the luminosity lost by the increase in the LER bunch
length. However, traveling waists or crab waists are affected by the dynamic aperture of theaccelerator. The Committee believes that additional studies are required, which should
include limitations due to the dynamic aperture.
The beam-beam parameter sets an important limit on the operation of KEKB. Operationally,
this limit appears to be about 0.09 for the present machine. Computer simulations of thisparameter limit have been extensive and attacked from many different directions. Many
features of the beam-beam limits appear to be correctly simulated by the codes but not all.
The current dependence of the luminosity on current is one area where the computer codes do
not seem to match the present machine. This is for both "with" and "without" the crab
cavities. One suspicion is that there is some beam physics that is not represented in the rightway in the computer codes. These additions or corrections should be pursued. Can the ~0.4
beam-beam parameter limit predicted by the computer code be tested in the present KEKB?
With a beam-beam parameter limit of the order of 0.1, the expected luminosity ofSuperKEKB is 1 to 2 x1035cm-2s-1. This luminosity level should be discussed with the SuperBELLE collaboration to get their comments. A minimum expectation should be established.
These issues, along with the high beam currents requiring major RF system upgrades andhigh operational costs, have prompted a recent look at ultra-low emittance options such as thelarge crossing angle scheme proposed for the Italian SuperB project. Preliminary estimatessuggest this option would have lower capital and operating costs compared to the originalplan. Choosing this option would require redirection of some, but not all, areas of design
work for the upgrade.
The design of the Italian SuperB project has several features which could be applied to theSuperKEKB design. Some of these are low emittances, large crossing angles, crab waist andlongitudinal polarization at the interaction point. However, some of these will requiredifferent optical lattice layouts, vibration suppression, different IR layouts, and electron spin
production equipment and transport. Not all may be practical for SuperKEKB.
Comments:

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Given the recent information on limitations to beam parameters, particularly in the LER, andthe cost analyses presented, the Committee agrees with the decision to investigate thefeasibility of a low emittance, large crossing angle option for SuperKEKB. Given theschedules presented by Prof. Suzuki, investigation of options requiring major shifts inemphasis for preparatory work should be carefully considered. The Interaction Region
design has significant differences with very low β* functions and the arcs must bereconfigured to reduce beam emittance. Stability of elements, particularly in the Interaction
Region, becomes a major concern and the injector should provide very low emittance beams.
However, the potential benefits of higher luminosity and lower capital and operating costs arevery significant. Accordingly the Committee recommends investigating the possibilities ofvery low emittance with large crossing angle as soon as possible, while continuing design
work for systems in common to all options.
In order to plan the studies related to the SuperKEKB proposal, a timeline should be
developed to start upgrading the collider on the timescale suggested by the lab director.
Working backward will help set priorities.
7 Luminosity with Traveling Focus Scheme
The questions addressed in this talk were: to use sophisticated beam-beam simulations to
explain the present KEKB beam-beam results, to address the needed lengthening of the LER
bunch in SuperKEKB design because of the Coherent Synchrotron Radiation CSR effectwhich increases the energy spread of the LER beam, and to look at the Super-bunch/microbeta approach to beam-beam effects.
Beam-beam computer simulations of the present performance of KEKB have been performed
for several years. Many different situations have been studied. With the recent addition ofmachine errors to the simulations and recent identification of hardware errors in the crab
cavity apertures, the simulations now agree better with luminosity observations. The
discovery of the small physical aperture near the crab cavity helps explain the beam-beamand lifetime effects in the present machine. With the discovery and cure of this aperture
restriction, it may be possible to store higher currents and the KEKB luminosity might beincreased beyond the present 1.6 x 1034 cm-2s-1with crab cavities. However, it will not be
possible to re-align the apertures which are within the crab cavity cryostat.
From observations, a vertical beam-beam parameter of ~0.09 has been achieved in the HER
since last spring which is a very nice result. This beam-beam parameter seems to remain
constant as the beam current is increased. A related observation is that without crab cavities,
the vertical emittance appears to be lower than was initially anticipated. This low verticalemittance saturates the beam-beam parameter at a lower current. However, more work isneeded to make the beam-beam-simulations agree with this saturated beam-beam parameter,
matching the variation observed in the KEKB specific luminosity versus the product of thebunch currents. This is most likely coming from additional machine optics errors which,when identified, should be added to the computer code.
A traveling focus scheme was simulated and appears to be the solution to regain theluminosity lost from CSR effects on the longitudinal bunch dimensions in the high current
option for SuperKEKB. This code simulates the most likely parameters of the SuperKEKB
design with an increased LER bunch length. A traveling focus scheme in SuperKEKB needs
four crab cavities and eight sextupoles to cover both rings. The results show that aluminosity of about 5 x 1035 cm-2s-1 can be achieved, but that the beam-beam parameters are

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very large at 0.3 to 0.4. Further studies are ongoing. The dynamic aperture effects of thesetraveling focus sextupoles have not yet been studied and this should be done.
Simulations were also made of the beam-beam effects with the crab waist scheme from the
Italian Super-B design with micro-betas. It was confirmed that this scheme can reduce theeffects of the beam-beam forces and allow reduced beam currents to be collided to make thesame luminosity. However, there are questions concerning the tighter tolerances oncomponent errors, the more difficult beam tuning with the small emittances, and theheightened sensitivity to component vibration in this scheme.
The Committee highly recommends continued beam-beam simulations with a strongemphasis on matching existing data.
8 IR Design
The new IR design for SuperKEKB needs to provide lower beta functions at the collision
point, and be compatible with higher beam currents. These constraints are challenging given
the congested space, a background-sensitive detector, precise tolerance requirements, andhigh power levels. This presentation gave a status report of ongoing work for this new IR
design. The impact of the beam-beam effect on large dynamic betas and emittances are being
included in the design. The required physical apertures, power deposition locations, beamline separation, realistic magnet strengths and diagnostics are included. However, a realistic
solution to provide the desired low horizontal beta function (20 cm down from 40 cm)
presently does not exist, though work continues. Soon the IR lattice design will be mergedwith the whole ring lattice and the beam-beam effect included, which will broaden the design.
However, some important requirements for this design are still a struggle to achieve.
The presented optics requires a set of new quadrupole magnets with the QCSLD working at
1.9K and the QCSLFE using permanent magnet technology similar to the PEP-II design.
Synchrotron radiation effects and beam induced heat loads on machine components were
presented in a later presentation, by Iwasaki, and have a very strong impact on the final
solution. As the beams will pass with a large offset through the strong magnets, a hardsynchrotron radiation spectrum is produced. This power must be accounted for in thevacuum design.
From a vacuum point of view a 'cold' bore at 80 K does not appear to be very useful, as
presented by Kanazawa, and the chamber will likely represent a problem due to gas
adsorption/desorption under changing beam loading conditions. A possible alternative for aroom temperature vacuum system needs to be investigated. An iterative approach will be
required to find a compromise between the optics and the space required for essential
hardware like pumps, bellows and position monitors.
A beam position monitor BPM is likely needed inside the cryostat in the near IR design.
There are likely cold BPM designs developed for other machines. This design should be used
to develop the hardware basis for the SuperKEKB design.
For the low emittance option the tolerances on stability of position-sensitive components suchas quadrupoles, sextupoles, and BPMs will be much more demanding than for the present
KEKB, particularly in the Interaction Region. Sufficient resources must be committed tounderstanding these issues and providing both passive and active solutions to meet the very
stringent requirements. These should be initiated early in the evaluation/design process.
Thus, the Committee recommends a continued strong focus on this interaction region design.

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9 Design of New Superconducting Quads
The presented optics requires a set of new quadrupole magnets with a pair of (H) defocusing
SC quads pushed very close to the interaction point. Both have room temperature bores. Oneof them (QCSLD) is cooled to 1.9K with reduced stabilizing copper to achieve the required
121.1 T/m gradient at the magnet center and the effective magnet length of 192.6 mm. Four2-in-1 SC quadrupoles with modest design parameters and 80K bores make the transition to
separate beam pipes and normal conducting magnets. A single permanent magnet
quadrupole (QCSLFE) in the HER line completes the complement of near-IR quadrupoles. It
is 0.52 m long and is made from R32HS (Samarium-Cobalt) with 43.3 T/m gradient.
The QCSLD quadrupole operates at a moderately high current density (~1975A/mm2) andwith low (1.3) Cu/SC ratio in a high magnetic field of 7.6T before the Belle andcompensating solenoids are added. For this magnet, and the QCSRD magnet, a reliable andeffective quench protection system will be essential. Other SC quadrupoles are part of 2-in-1
designs, but with lower fields (gradient 34.79 T/m) and a reasonable operating point.
While the QCS quads have a room temperature bore, the QC1 and QC2 quads have a bore
temperature of 80K in order to save space. We have some concern about the effect on thequads cryogenic system from possible beam mis-steering events on top of the load of imageand HOM currents.
Quite a bit of development is required on all of these magnets. Depending on the choice ofoptions for SuperKEKB, there will likely be some changes in magnet requirements so
finalizing the choice of machine philosophy soon will be helpful.
10 Special Studies with Crab Cavities
The crab cavities built for KEKB are operating extremely well. One of the future upgrade
paths for the CERN LHC includes the use of crab cavities. Operation in a proton machinepresents specific challenges such as the possible influence of noise in the crab RF system on
the un-damped beam. A series of studies has been proposed, and is being implemented, tomeasure these effects in KEKB. It is believed that these tests, done in collaboration with
CERN, could also benefit understanding of the beam dynamics issues of crab cavity
operation in KEKB.
In a video conference linking the Review Committee with CERN, initial studies were
presented describing the measured effect of phase noise on the beam. Single low intensity
beams showed emittance increase with excitation but behaved differently in collision. Highintensity beams, more bunches, had yet a different behavior. None of this is yet explained. It
was shown that the effect of the crab cavity kick when the beam was excited longitudinallywith phase noise was to couple the longitudinal motion into the transverse plane.
This collaboration and the associated studies are just getting under way. The benefits to bothmachines could be significant and so the Committee supports their continuation.
11 Evaluation of Detector BackgroundFindings:
While several sources may contribute to background in the experimental detector, the highcurrents and Interaction Region design constraints in SuperKEKB make synchrotron
radiation (SR) background the strongest and, most likely, the most difficult source to control.
Compared to KEKB, the SuperKEKB design moves the first quadrupoles closer to theinteraction point, making masking and handling of the deposited power more difficult. The

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critical energy of the SR increases from 2 to 14 keV and the width of the SR fan increases
also.
Other background sources to be investigated include beam-gas, radiative Bhabha, andTouschek scattered particles. A study of HOM and image current induced heating has juststarted.
Studies of SR related background using GEANT4 code include avoiding direct hits from the
HER, energy deposition in the IP beam pipe, and backscattered photons. Round beam pipes
are used for analysis of direct SR, and detailed beam pipe features, especially the central
chamber and adjacent transitions, are used for backscatter analysis.
After careful consideration of options, the central beam pipe will be oriented parallel to theHigh Energy Beam (HEB) to aid in achieving suitable masking. Extensive ray tracing was
employed to determine the appropriate geometry to prevent primary photons from striking
chamber walls near the central beam pipe. A mask protruding 4 mm from the tapered
chamber wall was found to be necessary to stop the photons from the HEB, whose spectrum
extends well beyond 50 keV. This mask, combined with the beam pipe alignment describedabove, prevents most secondary photons from striking the central chamber. Extensive
simulation showed just 4 cases of photons entering the detector region with energies around50 keV, though statistics have been too low to study SVD occupancy.
Power in the photons from the HEB striking the taper and mask may approach 1 kW withpower densities from 10 to 100 W/mm2. A calculation suggests that the temperature at thesurface of the mask would exceed 400ºC under SuperKEKB design conditions. The heatingfrom power deposition requires further study.
Comments:
The analysis of photon backgrounds and heating in the interaction region has been well
executed and has guided the decision on beam pipe orientation. While the detector appears
reasonably well protected from photon background, the heating of the 4 mm mask isexcessive.
More heat will be added from beam HOM and wall currents. Clearly analternative design will be needed.
If a low emittance option for SuperKEKB is chosen the SR protection in the Interaction
Region will be simplified. A decision earlier, rather than later, would reduce the amount ofeffort here that may turn out to be unnecessary.
The other background sources must be studied soon, as well as the possibility of tertiary
photons and additional checks to be sure no paths are missed. The effect of a misaligned
beam on the SR fan should be a priority in these studies. Many of these also will depend on
choice of design.
12 IR Beam Pipes and Assembly
An overview of the IR layout was presented showing the proposed layout of the insertion
magnets and schematics of the beam pipes, with possible locations for bellows and BP
monitors. Pumps have not been included at this stage. The layout of the vacuum system
consists of copper pipes with a 50 µm gold coating to reduce photon scattering. In QC1RE
and QC2R the beam pipe is in contact with the 80 K liquid nitrogen bath due to design
constraints of these magnets. Aperture requirements for the beam pipes take into account therequirement that synchrotron radiation from the QCSR should clear the section up to 10 mfrom the IP and are compatible with continuous injection.

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In principle, the 80 K surface proposed for the QC1R-QC2R cryostat could be a usefulpumping surface. Unfortunately, as shown to the Committee, nearly all the gas species
desorbed by synchrotron radiation are not cryosorbed at this high temperature. In addition,
during operation the cold bore chambers will be exposed to scattered photons, electrons andions from the beam-gas interactions and from the electron cloud. Therefore, it is not
sufficient to use conventional vapour pressure data to evaluate the vacuum performance ofsuch a cold bore. Condensed gas molecules may be desorbed by thermal radiation, and soft
photons with a few eV are sufficient to crack chemical bonds. To avoid stimulated
desorption it is a common requirement to screen a cryosorbing surface.
It is proposed to provide some means of heating the cold chambers in case of an accidental
exposure to atmospheric air, and specifically to water vapour. The Committee suggests that,
if such an accident occurs, warming up of the cryostat will in any case be necessary.
Flushing with warm nitrogen gas could be a simple method to raise the temperature.
Under the expected high power load of the beam pipe, the proposed gold coating will be
subjected to large thermal stress cycles. Any detachment of the gold film from the copper
substrate must be avoided, since it would destroy the coating. It is therefore proposed tocarry out tests with electron bombardment to qualify the coating method.
In the present proposal, two beam position monitors near the IP are located inside the QSC1R
and QC2R cryostat. KEK has no direct experience with the design of cold beam monitors.
However, there are many machines with cold beam position monitor and corresponding
designs could be adopted.
A very rough and preliminary assembly procedure has been presented to the Committee.
Conclusions: The design presented to the Committee is very preliminary. In view of thehigh synchrotron radiation load and the estimated parasitic mode power near the IP chamber,the proposed 80 K beam pipe proposal should be reconsidered. A detailed pumping scheme
should be included in the next design phase.
13 SuperKEKB Ring Optics
Findings
The main design considerations for the SuperKEKB ring lattice include a lower beamemittance than KEKB, a need for a new coupling correction scheme, and the traveling focus.
Low emittance is obtained in HER by tuning the quadrupole strengths, while in the LER, thebending magnets are replaced by longer ones, in addition to quadrupole tuning. The design
emittances achieved are 6nm for the HER and 2nm in the LER, both of which are sufficiently
small for the high-current option for SuperKEKB.
Coupling correction in KEKB is made by controlling the vertical closed orbit in the
sextupoles. The range of vertical orbit however is limited in SuperKEKB due to synchrotron
radiation considerations. To provide coupling correctors in the SuperKEKB design, therefore,
auxiliary normal and skew quadrupole windings with individual power supplies are installedon the sextupole magnets.
Coupling chromaticities can be corrected by skew sextupoles. If this is shown to beimportant at KEKB in its next run, these skew sextupoles will also be installed inSuperKEKB. The Committee considers the test on coupling chromaticities a significant step
towards the understanding of beam-beam limitations to both KEKB and SuperKEKB, andlooks forward to seeing the results of these KEKB tests.

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An initial survey of what is needed for the traveling focus scheme in SuperKEKB is made. It
is found that 24 fitting conditions are required. The number of quads in the region involved
is enough for the job.
Suggestions
In the emittance requirement, it is suggested that an evaluation be made with and without theeffect of dynamic beam-beam distortion so that maximum flexibility can be maintained.
The idea of quadrupole and skew quadrupole windings on sextupole magnets seems adequate
and effective.
With multiple fitting conditions to fulfill, the optical matching to provide the traveling focus
is expected to be an operational complexity if all conditions must be fulfilled with equal
accuracy. It is suggested that the criticality of these conditions be evaluated, including therequired tolerances.
A set of real-time tuning techniques, procedures and diagnostics should be developed toestablish and fine tune the traveling focus scheme.
14 Evaluation of X-Y coupling / requirements on correction
Findings
Vertical beam sizes at the Interaction Point derived from luminosity assuming equal beam
sizes in LER and HER yield 1.1 microns at 0.6 mA2 and 1.6 microns at 1.5 mA2. These
values disagree with measurements derived from synchrotron light monitors (1.8 microns at
0.6 mA2and 2.8/2.4 microns at 1.5 mA2), and correspond to very small x-y coupling
parameters.
Chromaticities as well as chromatic aberrations of the Twiss functions are measured withcare by scanning the RF frequency. The results disagree with those obtained by SAD using
an ideal lattice.
A clean and effective algorithm is developed to measure the coupling coefficients R1, R2, R3,
and R4, particularly at the Interaction Point. In this effort R3 and R4 are found to be large forthe LER Interaction Point. By varying the RF frequency, the coupling chromaticities are thenobtained. These results again disagree with SAD calculation. The Committee is impressedby this study and concurs that these results can potentially play a role in IP optics
optimization. It was noted that the machine conditions during these measurements were not“virgin” conditions but were the result of extensive tuning for maximum luminosity.
In the process of these studies, it is found that even for a single non-colliding beam, the HER
has a surprisingly large non-vanishing αx* for HER and βx* is also distorted. On the other
hand, αx* is close to zero for the LER.
Also as by-product of these studies, one can obtain damping rate as a function of beam
current. This result then yields the transverse impedance. It is found that tune spreadsmearing is weaker than radiation damping by a factor >10. This is an indication that the
sextupole arrangement at the KEKB is well balanced so that their tune spread contribution issmall.
Suggestions
The vertical beam size derived from the luminosity should be reconfirmed allowing the HER
and LER to assume different beam sizes, and also assuming beam distributions other thanGaussian.

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The disagreement between all the measured chromatic effects and the SAD calculations is
disturbing. It is suggested that an attempt be made to compare the chromatic aberrations withSAD calculation using a lattice as closely resembling the operational lattice as possible andnot the ideal lattice.
The non-vanishing αx* in the HER is presumably a result of optimizing the luminosity.
Whether that is true and if so, why it is required should be pursued. In particular, such a large
distortion to the on-momentum linear optics should definitely be corrected if it is not required
for the purpose of optimizing the luminosity.
15 Issues and Plans on Beam Diagnosticsa) SuperKEKB parameters
SuperKEKB is different from KEKB with respect to the vacuum chamber, bunch separation,
beam currents, IR and radiation levels. This requires new developments in several areas ofbeam diagnosticsb) BPM (Beam Position Monitor) System Upgrade.
While a number of components can be reused from KEKB, new BPM modules adapted toSuperKEKB vacuum chambers are being developed with smaller buttons to deal with thehigher beam currents. Their linearity has been analyzed, and found satisfactory. As in
KEKB, they will be equipped with displacement sensors to compensate for thermal motion of
the vacuum system.
The BPM receiver frequency was changed from 1018 MHz to 509 MHz because of the lower
waveguide cut-off frequency of the beam pipes with an ante-chamber. A new superheterodyne receiver module has been developed and undergone laboratory tests. These tests
indicate that their signal-to-noise ratio should be improved to achieve the required resolutionof 1 µm at a current of 30 mA. The use of the same receiver frequency as the RF system
makes the BPM system more sensitive to parasitic RF leakage from the RF system, and thisissue is being addressed, as it slightly degrades the desired accuracy, particularly at low beam
currents.
A commercial Libera Brilliance BPM receiver module has also been acquired for tests in
KEKB and might be used for special use like the IR orbit feedback.
Cold bore magnets are proposed near the IR, which may require cold BPMs to be developed.
The power loss in the cold coaxial cables has been calculated to evaluate whether the power
lost is acceptable for the cryogenics system or not.
The number of BPM’s equipped with turn-by-turn capability for measuring response matrices
for lattice optics diagnostics will be increased from 38 to 120.
Radiation resistant PEEK (polyetheretherketone) cables have been developed to deal with thehigher radiation levels in the tunnel, and have been undergoing irradiation tests.
c) Synchrotron Light Monitori) Visible light monitor
From the standpoint of the expected beam sizes, the interferometers, streak camera and gatedcamera developed for KEKB can be reused. The issue of deformation of the extraction
mirror is being addressed through reduction of the bending radius of the source magnets aswell as through new mirror structures, such as a beryllium coating on a thin water-cooled
copper plate.

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ii) X-ray monitor
An X-ray monitor using a coded aperture imaging technique developed by X-ray astronomers
is being developed in collaboration with the University of Hawaii and CesrTA at Cornell.
This should enable high-resolution turn-by-turn measurements of individual bunch profiles.
A high speed readout device (STURM - Sampler of Transients for Uniformly Redundant
Masks) is being developed to sample the InGaAs sensor array.
iii) A beamsstrahlung monitor is being developed to diagnose the quality of the beam-beamcollisions and the beam sizes at the IR.
d) Development of fast gated module.
A fast gated module is being developed, which can select individual bunches separated by
2ns to measure the tune, orbit and longitudinal phase. A module will be tested with beam in
the next fiscal year.
e) Suggestions and comments.
The KEKB team has developed advanced beam diagnostics for many years and the proposed
upgrade scheme looks very reasonable.
The RF parasite pick-up in their BPM system seems to have its origin in RF leakage in theconnectors and cables, and may need further shielding. But the Committee believes that thedominant source of the parasitic RF emission in the high power RF systems should also be
addressed. In the CERN AD (Antiproton Decelerator), parasitic RF leakage was a major
problem during early commissioning, and was solved by improving the shielding of BPM
connectors and cables by 25 dB, and by reducing the RF leakage from the high power RF
system by 25 dB, for a total gain of 50 dB.
Regarding the problems with thermal deformation of synchrotron light extraction mirrors,
Jefferson lab has developed extraction mirrors on thermally stable sapphire substrates andthis alternative may be suitable for SuperKEKB.
The luminosities to be measured in SuperKEKB will be up to 50 times larger than for KEKB.
This measurement is crucial to the real time tuning and the success of the accelerator. The
Committee suggests that the accelerator instrumentation group take an active role in thespecification and development of the luminosity monitor for SuperKEKB.
16 Plans for the next run
The operation of KEKB has been very busy for the past few years. This accelerator has
produced a lot of data for the BELLE detector while allowing the accelerator physicists
sufficient time to carry out experiments to improve the machine operation. The accelerator
has made great advances over the years. The next fiscal year will be the same, but with theadditional problem of reduced running due to limited funding levels and the need to do a thefull load of machine studies as well as particle physics.
The number of accelerator studies to do is quite large. They should be prioritized to makesure that the most important experiments and improvements are done first if time runs short.
The Committee suggests that the studies with higher voltages in the crab cavities, reductionof the vertical IP beam sizes by chromatic corrections, reduction of the x-y IR coupling, andstudies related to the Italian option be given higher priority. The Committee feels that what ismeant by the "Italian Option" should be better defined. Furthermore, any new test whichaims at demonstrating that the crab cavities work as designed for the beam-beam effectshould be done.

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The Committee posed a new question to the KEKB team as to whether the traveling focus IR
scheme to improve the beam-beam effect can be done with a single crab cavity in the ring in
conjunction with the ring sextupoles. This study may lead to a new way of operating the ringor a realization that this has been implemented by accident with the present tuning scheme
used during operation.
If the Y2S running planning for JFY2009 is only for a brief time, it may not be worth
spending a lot of accelerator tuning time increasing the luminosity. This subject opens thelarger suggestion that the entire run be planned carefully with BELLE included to maximize
the combined outcome.
17 Studies on Beam Pipes & Electron Cloud
The Committee was presented with a detailed progress report of recent studies for theSuperKEKB vacuum. Since the last MAC, good progress has been made with copper beam
pipes. Several pipes with an antechamber have been extensively tested under LER beam
conditions. In addition to straight pipes and quadrupole chambers, several bending magnet
chambers have been successfully produced. To simplify the fabrication, it is proposed toleave out the e-beam welding of the screen inserted between the beam channel and thepumping duct. A further simplification and cost reduction is obtained with the use of copperalloy flanges, which make copper-to-stainless steel transitions redundant. A bending magnet
and Q-type chamber will be installed in the LER for comprehensive tests this year.
Results obtained with various coatings for low secondary electron yield have been presented.
Several vacuum chambers have been coated with 200 nm thick TiN coating and installed in
the LER. The achieved SEYmax of about 0.84 is similar to that achieved with the coating from
BNL. Compared to a bare copper chamber, the near-beam electron cloud density is reduced
by about a factor of ~3. A combination of antechamber design and TiN coating should meetthe requirements for the SuperKEKB.
Following the recommendations of the previous MAC, the KEK vacuum group has continued
to study the surface graphitisation in a new laboratory set-up. Values of ~ 1.1 for the SEYmax
have been measured in the lab. The reduction factor observed in the machine is lesspronounced, and this is tentatively attributed to an insufficient thickness of the layer. This
uncertainty should be clarified with a second test.
The study of clearing electrodes, which can be installed inside magnet vacuum chambers(wigglers), has provided encouraging results. In order to be ‘invisible’ to the beam, theelectrode consists of an insulator (0.2 mm alumina) and a 0.1 mm tungsten layer, both
sprayed onto the beam pipe. An external bias voltage (300V) can be applied with an electricfeed through. A test system installed in a wiggler magnet chamber has demonstrated thelarge potential of this technique since reduction factors between 10 and 100 have beenreported.
A more ‘passive’ approach to reduce secondary electrons has been tested by replacing theclearing electrode with a grooved surface (a sample produced at SLAC). A feed through andthe external bias can be avoided but the gain is also much reduced (factor 10 reported).
The preliminary result of this work has been that the clearing electrode is by far the best,
followed by a grooved surface with TiN coating and then a grooved surface without TiN
coating. The bare Cu surface is worst.
The near-beam electron density, and specifically the effectiveness of a solenoid field, hasbeen studied with a newly designed detector installed in the LER. With the new detector

Page 17 of 20

design, only those electrons which originate near the beam and spiral out in the solenoid field
are measured. The measurement confirms that with a 50 G field, the electron density can be
reduced by a factor of 100 and that the solenoid remains the preferred solution for the drift
space.
Summary: The Committee is impressed by the large amount of work which has been done on
many important vacuum issues for SuperKEKB. The chamber design has made significant
progress and many components have been successfully installed in the machine. The mostrecent progress on the electron cloud is impressive and gives confidence that this crucial issue
for the SuperKEKB - or for that matter, for all machines, which are plagued by electron cloud
effects- has a solution. The basic understanding has progressed enormously and, at thismoment, a considerable range of options have become available to suppress the electron
cloud. The clearing electrode solution may still require some development but one important
aspect, its compatibility with a high intensity bunched beam, has been demonstrated. The
agreement between simulations and measurements of electron densities is particularly
encouraging.
18 Upgrade of RF Power Sources & Cooling System
All the RF-subsystems, such as klystrons, RF cavities, klystron power supplies, waveguide
networks, cooling systems, have already been operated for KEKB. It will be simple toincrease the number for SuperKEKB step by step. The only issue of concern is the total
electric power of about 100 MW for the machine operation. More than 75 MW is required
for the RF system.
If we are able to improve the klystron efficiency by 20%, by incorporating a depressedcollector potential, it would be a great opportunity to save both construction and operating
costs. Since the fusion research group developed the concept for high-power gyrotrons, thisCommittee strongly recommends the RF group to explore the feasibility with industries as
early as possible.
19 Crab Cavities for SuperKEKB
Several designs under consideration, but no choice has yet been made. However, thetraveling waist crab crossing scheme appears to be the current reference design, and to beready to run in 2013 or 2014, a choice would have to be made soon.
On the other hand, since the impact of the current crab cavities on the luminosity is not wellunderstood, there is probably still time to choose a design. The Committee recommends
continued study of the various crab cavity options with the aim of making a selection within
the next year. This timescale corresponds well with the proposed timeline on a decision on
the low emittance option which would not require the crab cavities.

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Appendix A
KEKB Accelerator Review Members
Andrew Hutton

JLabChairmanAlexander Chao

SLAC

Warren FunkJLab (Unable to attend)

Oswald Gröbner

CERN (retired)

Trevor Linnecar

CERN

Won Namkung

PostechFlemming Pedersen

CERN

Eugene A. Perevedentsev

BINP (Unable to attend)

David Rice

CornellJohn Seeman

SLAC

Wang Shuhong

IHEP (Unable to attend)

Katsunobu OideKEK Secretary, Accelerator

Masanori Yamauchi

KEK Secretary, BELLE (unable to attend)

Page 19 of 20

Agenda of the 14th KEKB Accelerator Review Committee
Date / Time
2/9 (Mon.) 8:30-9:00
9:00-9:20
9:20-10:10
10:10-10:40
10:40-11:00
11:00-11:30
11:30-12:00
12:00-13:30
13:30-14:10
14:10-14:50
14:50-15:10
15:10-15:50
15:50-16:30
16:30-17:00
17:00-18:10
18:102/10 (Tue.) 8:30-9:00
9:00-9:30
9:30-10:00
10:00-10:30
10:30-11:00
11:00-11:30
11:30-12:00
12:00-12:30
12:30-13:30
13:30-14:00
14:300-14:30
14:30-15:00
15:00-15:30
15:30-16:30
16:302/11 (Wed.) 8:30-11:00
11:00-12:00
12:00

Subject
Executive Session
KEK Roadmap and SuperKEKB Upgrade
Progress in Crab Crossing
Belle Status
CoffeeCooling of Crab Cavities below 4K
Simultaneous Injection
Lunch
Issues on the Upgrade
Luminosity with Travel Focus Scheme
CoffeeIR Design
Design of New Superconducting Quads
Discussion
Special Studies with Crab Cavities(TV
conf with CERN people)
Reception
Executive Session
Evaluation of Detector Background
IR Beam Pipes and AssemblySuperKEKB Ring Optics
CoffeeEvaluation of X-Y coupling / requirements oncorrection
Issues and Plans on Beam Diagnostics
Plans for the next run
LunchStudies on Beam Pipes & Electron Cloud
Upgrade of RF Power Sources & Cooling
SystemCrab Cavities for SuperKEKB
CoffeeQ&A on Specific Issues
Executive Session
Executive Session
Close Out
Adjourn

Page 20 of 20

Presenter
A. Suzuki
Y. Funakosh
iI. Nakamura
H. NakaiN. IidaK. OideK. Ohmi
H. Koiso
N. Ohuchi
Y. Morita

M. Iwasaki
K. Kanazaw
aA. Morita
Y. Ohnishi
H. Fukuma
H. KoisoK. Shibata
M. OnoK. Nakanishi