The Seventh KEKB Accelerator Review Committee report
Introduction
The seventh KEKB Accelerator Review Committee meeting was held on February 25-27,
2002. The Committee welcomed one new member: Peter Kneisel from JLab. The
Committee expressed their appreciation to outgoing member, Ron Sundelin for his hard work
and wishes him well in his retirement. Appendix A shows the present membership of theCommittee.
The seventh Committee meeting followed the usual format of oral presentations by the KEKB
staff members and discussion by the Committee members. However, the because of theincredible success of KEKB operations, the Committee focused on the options for improving
luminosity in the long term. The Agenda for the meeting is shown in Appendix B. The
recommendations of the Committee were presented to the KEKB staff members before theclose of the meeting. The Committee wrote a draft report during the meeting that was thenimproved and finalized by e-mail among the Committee members.
Contents
1) Executive SummaryA) ForwardB) SummaryC) Comments
2) Findings and RecommendationsA) Overview of KEKB
B) Belle StatusC) Luminosity Boost
D) LER Blowup and SolenoidE) Antechamber PrototypeF) MasksG) High CurrentH) Physics Motivation
I) ParametersJ) OpticsK) Special Magnets
L) QCSM) RF SystemN) ARESO) Superconducting Cavities
P) Crab Cavities
Q) Vacuum, Antechamber, Masks, Bellows, etc.
R) Facilities
S) Monitors and FeedbackT) Linac UpgradeU) Linac RF Sources
V) Damping Ring and Beam TransportW) Control System
X) Beam Dynamics

11) Executive SummaryA) Foreword
KEKB has made excellent progress in the year since the last Committee meeting. The peakluminosity has increased to a new world record of 6.65 x 1033 cm-2s-1 (at this time last year it
was 2.5 x 1033 cm-2s-1). The BELLE detector has accumulated 56 fb-1 compared to 13 fb-1 ayear ago and published 23 papers in refereed journals. This is an extraordinary achievement,
and required overcoming many issues during the year. In the two and a half day meeting,about one third of the talks dealt with present status of KEKB, and the remainder covered theplans for upgrading KEKB to a luminosity of 1035 cm-2s-1. The KEKB machine group had
done an enormous amount of work, and is to be congratulated on the impressive results thathave been achieved.
Summary
KEKB has made spectacular progress over the last year, breaking almost all of the previous
worldwide luminosity records: highest peak luminosity: 6.6 x 1033 cm-2s-1, highest daily
integrated luminosity: 329.5 pb-1, highest 7 day integrated luminosity: 2.06 fb-1, and highest
30 day luminosity: 7.56 fb-1. These values are now above those obtained at PEP-II (maximum
peak luminosity of 4.5 x 1033 cm -2s-1). The KEKB team has had to overcome a series ofproblems, both hardware limitations and tuning improvements, in order to reach thisperformance and they are to be congratulated on this excellent result.
The BELLE detector has now accumulated 56.3 fb-1, compared with 72 fb -1 logged by BaBar.
The BELLE detector is working extremely well and has had no major difficulties.
Backgrounds are low, and losses during beam injection are generally small; this makes
continuous injection acceptable to the detector. However, this will still require close
collaboration between the BELLE detector group and the KEKB machine team. Overall, the
BELLE collaboration is extremely pleased with the quality and quantity of the beam being
provided by the accelerator.
The beam current limitations in the LER were obtained by installing additional solenoid
windings (an additional 50% compared to last year). This completes the installation
everywhere where it is reasonable. The last solenoids had a large effect and demonstrated theimportance of finishing the task. Optimization of the machine parameters included moving
the horizontal tunes closer to the half integer and improving the optics correction. A faster
abort kicker also permitted an increase in the number of bunches from 1153 to 1223. At
present the HER beam current is 822 mA (design value is 1.1 A); the LER beam current is1333 mA (design value is 2.6 A) at peak luminosity.
Super KEKB was introduced. This is an upgrade of the entire KEKB facility with a goal of1035 cm -2s-1. It would collide 10 A in the LER against 4 A in the HER, requiring major
improvements to almost every system. Many details of this upgrade were presented during
the meeting and are discussed below.

2B) Comments
The committee would like to see a road map for the present KEKB that showed the planned
hardware and procedure improvements, and the resulting luminosity improvement that may
be expected. The graph of integrated luminosity that was shown was a lower limit and serves
well as an estimate of promised performance, but does not serve well as a guide for decisionmaking (the KEKB Project Leader agreed, and later updated this graph). The Committee
recommends that the KEKB Project Leader prepare a list of machine parameters for each year,that will indicate where improvements are likely to be needed and when.
The R&D required for Super KEKB should also be planned in parallel. This will help the
KEKB Team to organize their work so that the limited KEKB manpower can be effectively
used. It will also serve as input to the KEK directorate in making decisions about the SuperKEKB parameters and schedule.
The decision to exchange the ring polarities, putting electrons in LER and positrons in HER,
would minimize beam blow-up due to the electron cloud instability. However, the solenoids
have now demonstrated that it is possible to control this instability, so the choice is not so
obvious because the fast ion instability that affects the electrons has the potential of being
even more dangerous, with extremely fast growth rates. The KEKB Project Leader is
encouraged to evaluate both options for the ring polarities and to determine which seems thebest. The impact of this choice on the injector should also be evaluated and, ideally, asolution found which can support both options.
The KEKB Project Leader predicted that the conservative lower limit for the monthly,
integrated luminosity would increase to 9 fb-1; the Committee agreed that this is the lower
limit and that more may be obtained. The impression of the Committee is that the ultimate
luminosity of the present KEKB is likely to be in the range (1.5 – 3) x 1034 cm -2s-1. The goalof the Super KEKB should probably be about 10 times higher than the ultimate luminosity of
KEKB, so it is important to accurately estimate what might be achieved by continuous
improvement of the present machine. The committee recommends that the Project Leader
make a careful evaluation of the ultimate luminosity limit of KEKB, including those
improvements that can be added to the machine without requiring a total rebuild (e.g. adding a
few more RF cavities, providing continuous, two-beam top-up capability, etc.).
In parallel, the theoreticians should develop estimates of the integrated luminosity needed to
study the different aspects of the Super KEKB physics so that the machine parameters can bematched to the physics requirements. The range of integrated luminosities of interest is from
1,000 – 30,000 fb-1, although the highest range will be a challenge for the machine to reach.
This information will serve as input to the KEK directorate in making decisions about theSuper KEKB parameters and schedule.

32) Findings and Recommendations
KEKB 2001-2002
A) KEKB overview
KEKB has made spectacular progress over the last year, breaking almost all of the previous
worldwide luminosity records: highest luminosity: 6.65 x 1033 cm-2s-1, highest daily integrated
luminosity: 329.5 pb-1, highest 7 day integrated luminosity: 2.06 fb-1, and highest 30 day
luminosity: 7.56 fb-1. These values are now above those obtained at PEP-II (maximum
luminosity of 4.5 x 1033 cm-2s-1). The KEKB team has had to overcome a series of problems,
both hardware limitations and tuning improvements, in order to reach this performance andthey are to be congratulated on this excellent result.
The beam current limitations in the LER were obtained by installing additional solenoid
windings (an additional 50% compared to last year). This completes the installation
everywhere where it is reasonable. The last solenoids had a large effect and demonstrated theimportance of finishing the task. In addition, some HER masks were replaced with a moreresistant design and faster abort kickers were installed. A whole host of other minor, but
important, hardware improvements were made in almost every system.
Optimization of the machine parameters included moving the horizontal tunes closer to thehalf integer, managing the tunes with a gated betatron meter, and improved optics correction.
The faster abort kicker also permitted an increase in the number of bunches from 1153 to
1223. At present the HER beam current is 822 mA and the LER beam current is 1333 mA atpeak luminosity.
In addition, several procedures that will improve the luminosity have been tested, but have notyet become standard operating procedure: continuous positron injection, which should raise
the integrated luminosity by ~15%, and injection of 2 bunch per linac RF pulse, which has
already demonstrated a 50% increase in injection rate.
Some improvements will be installed in the near future. There are some new operating modes
which will become standard following machine development in March: continuous injection,
2 bunch injection of positrons, and 3.5 bucket spacing will increase the LER current to 1.5 A
and the HER current to 0.9 A. After the summer shutdown in 2002, the bunch spacing will be
reduced to 3 buckets, the HER beam current will increase to 1.1 A (design value) and the LER
beam current will be increased up to the design value or as long as the luminosity is improved.
The KEKB Project Leader predicted that the conservative lower limit for the monthly,
integrated luminosity would increase to 9 fb-1; the Committee agreed that this is the lower
limit and that more may be obtained.
Super KEKB was introduced. This is an upgrade of the entire KEKB facility with a goal of1035 cm -2s-1. It would collide 10 A in the LER against 4 A in the HER, requiring major
improvements to almost every system. Many details of this upgrade were presented during
the meeting and are discussed below.
The committee would have liked to see a road map for the present KEKB that showed theplanned hardware and procedure improvements, and the resulting luminosity improvement

4that may be expected. The graph of integrated luminosity that was shown was a lower limit.
This serves well as an estimate of promised performance, but does not serve well as a guidefor decision-making (the KEKB Project Leader agreed, and later updated this graph). The
Committee recommends that the KEKB Project Leader prepare a list of machine parameters
for each year, that will indicate where improvements are likely to be needed and when. The
R&D required for Super KEKB should also be planned in parallel. This will help the KEKBTeam to organize their work so that the limited KEKB manpower can be effectively used. It
will also serve as input to the KEK directorate in making decisions about the Super KEKB
parameters and schedule.
The impression of the Committee is that the ultimate luminosity of the present KEKB is likely
to be in the range (1.5 – 3) x 1034 cm -2s-1. The goal of the Super KEKB should probably be
about 10 times higher than the ultimate luminosity of KEKB, so it is important to accurately
estimate what might be achieved by continuous improvement of the present machine.

B) BELLE Status
The BELLE detector has now accumulated 56.3 fb-1, compared with 72 fb -1 logged by BaBar.
The BELLE detector is working extremely well and has had no major difficulties.
Losses during beam injection are generally small and this makes continuous injection
acceptable to the detector. However, this will still require close collaboration between the
BELLE detector group and the KEKB machine team.
The beam abort kickers have occasionally misfired, with each event causing up to 5 krad doseto the silicon vertex detectors. The silicon detectors have now received an integrated
radiation dose corresponding to about one third of the expected lifetime. This detector willsoon be replaced by a detector package that is about ten times more resistant to radiation
damage. In addition, improvements to the electronics that triggers the abort kicker have madethese accidents less frequent. This does not appear to be a problem at this time.
Overall, the BELLE collaboration is extremely pleased with the quality and quantity of thebeam being provided by the accelerator.
The BELLE collaboration has started to evaluate the upgrades required to profit from theincreased luminosity in KEKB. The designs involve a smaller interaction region beam pipe
surrounded by four layers of silicon detectors (three in the present detector) and a small-celldrift chamber. There appears to be no major interference between the new machinecomponents and the detector but this will need to be confirmed as the designs evolve.

C) Luminosity Boost
Since the last KEKB accelerator review committee meeting in February 2001, the peakluminosity has been increased from about 2.5 to 6.6 x 1033cm-2s-1. This remarkableachievement is also reflected in an increase of the integrated luminosity per day, which
increased from 140 to 329 pb-1/day, in the total integrated luminosity, which increased from12 to 56 fb-1, and in the beam currents, which increased from 0.7 A to 1.4 A in the LER and

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from 0.6 A to 0.9 A in the HER. All these improvements are remarkable and show how well
KEKB has operated.
The increase in luminosity was due, in part, to an advantageous choice of the betatron tunes.
The decision to change the vertical fractional tune from 0.08 to above the half integer has
been very much worthwhile. Also, the very careful correction of the optical functions, which
made it possible to bring the horizontal tune to only 0.014 above the half integer, has paid off
in luminosity.
The increase in luminosity was also due to the increased beam currents that were possible
after the electron cloud effect was suppressed by extending the total length of vacuum
chambers with solenoid windings from 1300 to 2200 m.
Among the innovations that support increasing the luminosity, two should be mentioned inparticular: (a) circumference control in the LER and (b) continuous injection. The committee
suggests implementing a chicane for circumference control in the HER. The committee
strongly recommends making continuous injection the standard mode of operation, since thiscan lead to more stable operating conditions and to a further optimization of parameters.
The operation of KEKB has been very efficient and regular with little downtime. However,injection into the HER is often difficult and time consuming after the HER beam has beenaborted. The committee recommends that the reasons for these difficulties be investigated.
The suspected culprits include tune and orbit drifts in the times when the feedback systems
have no beam to act on, and tune shifts with intensity.

D) LER Blowup & Solenoid
Throughout the commissioning, the Electron Cloud Instability in LER has been one of themost important problems at KEKB, causing the vertical blowup in the positron beam size. To
reduce the electron cloud density, a large number of solenoids with the field of 40-50 Gauss
were installed in the year 2000, resulting in a clear improvement of the blowup thresholdcurrent in the single-beam mode. Installation of the solenoids continued in 2001 in several
stages as was recommended by the last KEKB Accelerator Review Committee. It wascompleted in January 2002 with 2836 coils with bobbins and 5736 bobbin-less coils covering
practically all the available space in LER drift sections (~95%).
The blowup was weakened after each stage of the installation, and careful tune-up of thecollision resulted in steady growth of the KEKB luminosity over the year since the lastCommittee. The effect of the solenoids on the specific luminosity was recently confirmed bydirect measurements with the solenoids on and off.
In spring and summer 2001, the effect of the solenoid was demonstrated in the gated tune
meter data. This showed a variation of the horizontal and vertical tune shift along the bunchtrain caused by the electron-cloud. Machine studies dedicated to the interferometermeasurement of the current-dependent vertical beam size in single-beam mode in the LER,
confirmed the scaling of the blowup threshold and beam-size growth rate with the line densityof the beam charge for the fill patterns with 4 and 3 bucket spacing.

6The specific luminosity and the luminosity per bunch were improved after the 4th and 5thsolenoid installation in September 2001 and January 2002. In the latest measurement of thebeam size versus current in February 2002, no blowup was seen within the presently available
current range of 1.3 A in the 4-bucket spacing fill pattern used for physics. An important
indication of the improved situation with the electron cloud is that in this measurement, as
well as in physics runs, the chromaticity setting was moderate (~4) compared with the highchromaticity settings (~8-10) used a year ago to suppress the beam size blowup, and whichwere believed to help in stabilizing a head-tail-type instability caused by the electron-cloudwake.
An urgent question remains of how much safety margin is provided by this apparent cure of
the beam size blowup by the solenoid field. This should be answered by a dedicated study ofthe blowup thresholds for fill patterns with 4-, 3- and 2-bucket spacing; the Committee
strongly recommends doing this study.
The effect of solenoids was also seen in the recent measurement of the coupled-bunch
instability transient, triggered by opening the transverse feedback loop and recorded by the
Bunch Oscillation Recorder. The observed growth rate was reduced when the solenoid is
excited. Less clear is the mode spectrum change caused by the solenoid. This study provides
plenty of data to be compared with the simulation and seems to be very interesting.
The 5% space remaining in the LER not covered by solenoids contains many BPM regions. It
is suggested that one possible way to cover these regions might be to apply high voltages tothe BPM buttons in such a way that they serve as clearing electrodes. In this way thecoverage against the electron cloud problem may approach 100% of the circumference notcovered by magnets.
Among other instabilities we note the low-frequency coupled-bunch instability (supposedly aresistive-wall type) causing operation problems due to the temporarily disabled low-frequency
module of the transverse feedback and which should be cured when it is re-installed.
In view of the Super KEKB challenges, the Committee recommends enhancing the machine
study program devoted to electron cloud studies and other collective effects, taking the fulladvantage of powerful diagnostics available at KEKB. More beam studies of the prototype
antechamber-type vacuum chamber are also encouraged.

E) Antechamber prototype
The antechamber is expected to provide high conductance, a reduction of the power density
on the vacuum chamber, and a separation of the photoelectron source from the beam space.
A prototype of an antechamber was constructed and installed in the KEKB positron ring toevaluate photoelectron reduction. It was made of copper and composed of three kinds ofchamber: bend, quadrupole, and straight section chambers. In the straight section, thechamber had a tapered wall to absorb the synchrotron radiation. The vacuum group built andinstalled photoelectron monitors in the bend and straight sections to measure the secondary
electron current. They compared electron currents in this antechamber with the current in thenormal, single-cell chamber used now. In the bend section, it seems that the production rateof electrons in the antechamber is 1/6 to 1/10 of those in the normal, single-cell chamber. The
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reduction due to the solenoids is a factor of 9. In the straight section the reduction due to thesolenoids is estimated to be a factor of 10. The measured electron current includes bothphotoelectrons and secondary electrons produced by the multipacting process. As theseparation between the two kinds of electrons is not sufficient, it is difficult to estimate thephotoelectron production in the antechamber. It is therefore unclear how effective theantechamber structure is in reducing the electron effect. The committee hopes that further
experiments are carried out using the prototype antechamber to clarify these questions, as theantechamber (together with the solenoid windings) is one of the most promising ways toreduce the photoelectron effect.

F) Masks
The mask designs for KEKB have evolved with time over the life of the project. The designs
are now on Versions 4 and 5. The present designs are well constructed and are working well.
However, there are a few problems left to resolve; in particular, the grooves generated byerrant aborted beams, and the HOM heating of nearby bellows and NEG pumps. There are
solutions to these issues including rapid abort signals, beam-phase aborts, screens added tonearby NEG pumps, and changes of the length and material of the mask heads. These cureshave been, or soon will be, implemented.
These masks are required to reduce the backgrounds in the BELLE detector. The BELLE
group reports that the present backgrounds are acceptable to them, which is very
commendable. Furthermore, these backgrounds are slowly being reduced with time due to theimproved masking and sustained beam scrubbing of the accelerator.
The masking for the Super KEKB will be even harder than at KEKB. The committee
recommends that an initial study of the collimation scheme be carried out.

G) High Current
At present, KEKB is operated with 4-bucket bunch spacing and up to 1.4 A beam current in
the LER and 0.9 A in the HER. As much as 95% of the field-free circumference has been
covered with solenoids. In this situation the beam blow-up due to the electron-cloudinstability has completely disappeared. The presently installed RF system (16 ARES cavities
in LER, 10 ARES cum 8 SC cavities in HER) still offers a comfortable reserve, because thesystem is run with less overvoltage than originally foreseen. The most severe problems withmasks and absorbers have been solved by new designs (Versions 4 and 5), and also by atighter beam-abort system (e.g. beam abort triggered by as little as 1° beam phase error).
It can, therefore, be expected that the installation of the full RF system (4 more ARES
systems in LER, and 2 more ARES systems in HER, all constructed already) permit the full
design currents of 2.6 and 1.1 A. This should enable KEKB to reach at least the design
luminosity of 1 x 1034 cm -2s-1 in the coming year. If may well happen that the electron-cloudinstability will cause problems again at only slightly higher current and/or reduced bunch
spacing. Since a further extension of the solenoids is impossible, electrostatic clearing of atleast some of the remaining 5% of space might be considered. In particular, the beamposition electrodes, which take some of this space, might be given a high-voltage bias for that
purpose.

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A number of miscellaneous problems and observations (such as damage of the masks by the
dumped beam, 5-bunch spacing which had to be avoided because of the fortuitous excitationof a resonance in the detector beam pipe, extreme heating of bellows or NEG pumps just near
the masks) give a warning that new problems may occur at every step of increased beam
current. It seems advisable, therefore, to consider upgrading the performance beyond thedesign currents to be a continuous process, which should be systematically pursued during thecoming years.

Super KEKBH) Physics Motivation
The physics motivation for KEKB is divided into three phases. Phase 1, “Discovery of CP
Violation in B System” requires an integrated luminosity of 30 fb-1. This has already been
completed, with BELLE obtaining a value of sin 2φ1 = 0.99 ± 0.14 ± 0.009 (BaBar has
obtained a value of sin 2φ1 = 0.59 with the same error bars). This discrepancy should be
resolved soon.
Phase 2, “Precise Tests of KM Scheme” requires an integrated luminosity of 300 fb-1. This
phase should be completed in the next few years with the present KEKB and PEP-II.
Phase 3, “Supersymetric Flavor Physics” requires an integrated luminosity of at least 3000 fb1. This is the regime that the Super KEKB should be designed to cover. The integrated
luminosity per month is about 100 fb-1 at a peak luminosity of 1035 cm -2s-1, so an integrated
luminosity of 3000 fb-1 would require about 3 years operation.
It will be important for the theoreticians to come up with estimates of the integrated
luminosity needed to study the different aspects of the supersymetric flavor physics so that themachine parameters can be matched to the physics requirements. The range of integrated
luminosities of interest is from 1,000 – 30,000 fb-1, although the highest range will be achallenge for the machine to reach.
The physics capability of Super KEKB was compared to hadron colliders, particularly LHC-B.
The committee recommends that the detector group (Super BELLE) define the physics goals
so that the luminosity scope of Super KEKB can be specified. Should the physics scope ofSuper KEKB compete for the first years with LHC-B, or compete on only a few specificphysics topics, or compete fully on all physics fronts including data statistics? The committee
had a consensus that Super BELLE should be designed to compete fully with LHC-B.

I) Parameters
A set of basic parameters was given for the Super KEKB. The driving parameter is theluminosity of 1035 cm-2s-1. To reach this luminosity, the beams are designed to have very highcurrents (9.4 A in LER and 4.1 A in HER). In addition, the bunch length has to be very short(3 mm rms). The high intensity and the short bunch length combine to significantly enhancethe effect of coherent synchrotron radiation (CSR), which may present a serious challenge.
The reported conclusion that the total radiated power is negligible is based on the assumption
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that the CSR is well shielded by the conducting walls of the beam pipe. This is true provided
that the orbit bend radius is constant, and that the beam surroundings are uniform. An
improved estimate of CSR shielding may be in order, taking account of non-uniform orbit
bends, and variable beam pipe cross-sections in a real machine.
More serious problems come from the CSR near fields, whose reactive component of the
impedance is not well shielded, thus contributing to the broadband longitudinal impedance.
Therefore, a careful feasibility study of the bunch length is recommended for Super KEKB.
Some of the issues associated with short bunch length, including how serious the HOM
heating will become and whether there will be a CSR instability, can be studied in the present
KEKB by lowering the momentum compaction factor. The committee suggests that such
experiments be carried out.
In this design, the beams are assumed to be flat at the collision point. This means that adecision was made not to benefit from the potential gain of the round beam configuration.
Although such a decision is reasonable because a round beam configuration requires a much
more complicated lattice design, and no recent e+e- colliders have succeeded in obtaining thispotential benefit, the committee urges further examination of this option so as not toprematurely forego its potential.
One of the key design features is to switch energies of the two beams, i.e. to store e- in LER
and e+ in HER. This has the advantage of reducing the strength of the electron cloudinstability, and it also shortens the filling time. However, the committee wishes to note that adown side is the potential for a serious fast ion instability in the LER. A preliminary estimateof the fast ion instability gives a growth time of 0.3 ms for 1 nTorr. Unlike the electron cloudinstability, which can be cured by solenoid windings, the fast ion instability, once it occurs, isdifficult to control. It is therefore recommended that the fast ion instability be examined moreclosely for Super KEKB. In particular, the committee recommends that some experiments
should be carried out as part of this study.

J) Optics
The Super KEKB lattice design work has just started. At this early stage, no actual lattice waspresented, but a design approach was described. The main emphasis in this design approachis the flexibility of this lattice. In particular, there is a wide range of variability in the
momentum compaction factor as well as in the natural emittance. The committee agrees
completely with this emphasis on flexibility, and urges the lattice effort in this direction.
Another important issue will be that smaller values of both x* and x* are required, which inturn will impose more demanding chromatic correction, and/or tighter dynamic aperture. It isalso suggested that the crab-crossing scheme be considered in the early lattice design.

K) Special Magnets
A first round of parameters for the interaction region magnets for Super KEKB have been
determined from the lattice studies. The overall interaction region design looks promising.
The crossing angle has been increased from ±11 mrad to ±15 mrad to allow better clearance
for the magnets and vacuum chambers on either side of the interaction region. There has also
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been an initial look at the mechanical and magnetic designs needed for these magnets. The
concepts are similar to the present designs used in KEKB, including septum quadrupoles asneeded.
In order to reduce the y*, the first quadrupoles must be moved closer to the collision point.
Thus, the strength of these magnets must increase, and the compensation solenoid, which used
to be in front of the quadrupole, must now be placed over the quadrupole. As with otherrecent insertion designs, overlapping coils should not be a problem.
One issue of concern is the heating of nearby vacuum chambers by synchrotron radiation athigh beam currents. The beams exiting from the interaction region are strongly bent by being
off-axis in the quadrupoles, and generate many kilowatts of power. These chambers havebeen an ongoing concern in the present electron-positron colliders and special attention mustbe paid for Super KEKB.
Furthermore, an initial semi-detailed vacuum design of the beam chambers at and around thecollision point is important, particularly for the reduction of backgrounds in the new detector.
The committee suggests a first round of investigation for these chambers.

L) QCS
The beta functions at the collision point must be reduced in order to produce the higher
luminosity desired for Super KEKB. This requirement forces the final quadrupoles to be
moved closer to the interaction point than those in KEKB. This also requires that the magnet
strength be significantly increased, forcing the use of superconducting quadrupoles.
The design of these superconducting quadrupoles for the Super KEKB is well advanced. The
design is an extension of that used for KEKB, but with higher gradients. The work done so
far looks reasonable and is very detailed for this phase of the upgrade project. The committee
recommends that further design studies include vacuum chamber and detector backgroundissues as well as prototyping.

M) RF System
The upgrading of the RF system for 9.4 A (LER) and 4.1 A (HER) beam current is a major
project. The total number of cavities and associated RF systems must be twice that of KEKB.
There will be a total of 56 klystrons and the AC input power for the RF system and theassociated infrastructure will be 73 MW.
In HER, the mixture of ARES and SC cavities will be maintained. One very desirable featureof this hybrid solution is the possibility to arrange the relative phases so that the ARES
cavities deliver more than their share in beam power, while the higher voltage persuperconducting cavity is used optimally.
In spite of the fourfold increase in power, it turns out that the basic design of the present RF
system can be maintained, and most of the present existing equipment designs can be usedwith suitable modification. The main development issues concern the higher-mode dampers
and couplers, but since many of these components have spare capacity already the11improvements may often be limited to strongly reinforced cooling. The vacuum-pipe tapers
between SC cavities will have to be eliminated. In addition, a program is aimed at improving
reliability and reducing the crowbar trips. It should be noted, however, that the reliability ofthe present systems is already excellent now that the inevitable teething troubles have been
overcome. The RF system fault rate is reported to be about once per day.

N) ARESThe main features of the ARES system of coupled cavities have been pointed out in previous
reports. Their exceptionally high stored energy is already required in HER at the SuperKEKB design parameters. Two types of input couplers have been tested up to 900 kW. Thisshould already be marginally sufficient for super KEKB, but improvements will be made incooling and in the use of low-loss alumina for the vacuum window. Of the two types ofcoupler-window assembly, one (“choke-type”) has suffered two cases of window failure. The
other type (“over-and-under cut”) has performed satisfactorily and will be maintained.
The parasitic 0 and π modes of the two-cavity system (acceleration and storage) are extracted
into an external water-cooled load that will be given increased cooling capability. Different
SiC higher-mode absorbers (bullet-shaped and tiles) will be employed with enhanced watercooling. In order to make the necessary detuning within the tunable range of the present tuner,
the resonance frequency of the fundamental mode is planned to be lowered by machining theinner surface of the present ARES cavity.
All in all it appears that the ARES system offers an excellent basis for the required upgrade
needed for the increased beam currents.

O) SCCThe 8 superconducting cavities in the HER have been operating reliably, with only minor
problems, since their installation 4 in 1998 and other 4 in October 2000. Two cavities (D10A and D11-C) have suffered from helium leaks at the indium joints; the cavities were taken
off-line in January 2001 and October 2001, respectively, and re-installed after repair inAugust 2001 and January 2002. Only minor degradations in cavity performance wereobserved. The leaks did not represent a systematic weakness in the design, but could betraced back to problems during the assembly process. During these repairs, the voltage andthe beam current were supported by the other cavities, which had been conditioned up to 2
MV, a 25 % margin over the operational goals.
Between April 2001 and February 25, 2002, the superconducting cavities have supported
beam currents up to 870 mA in 1225 bunches with up to 380 kW of RF power transferred tothe beam. The HOM power generated, and successfully damped in the ferrite dampers, hasbeen up to 8 kW per cavity.
For the Super KEKB, the circulating current in the HER needs to be increased to 4.1 A. This
increase requires large RF power levels to be transferred to the beam, and very high HOM
losses at the cavities. The input couplers have to sustain 460 kW of incoming power, and the
HOM power generated by the beam increases to 80 kW. The present ferrite dampers cannot
handle such power levels. The route towards a technical solution has been identified:

12a) Removing the tapered beam line sections between the cavities will reduce the loss
factor of the dominant wide range HOMs, reducing the generated power toapproximately 50 kW per module.
b) The damping material of choice would be SiC because of its higher thermal
conductivity. This would provide better cooling capabilities and possibly reduce thesurface temperature at the dampers to tolerable levels.
It has been recognized that the development of appropriate HOM dampers is essential for thesuperconducting cavities for Super KEKB; the committee fully endorses such R&D efforts to
be carried out as soon as possible.
The input coupler power for Super KEKB increases from 380 kW (demonstrated) to 460
kW/cavity. The KEKB input couplers seem to have sufficient design margin, and incremental
changes such as reinforced cooling and improved interlock protection might solve theincreased demands on the couplers. Such investigations should also be started as soon as
possible.

P) Crab Cavity
The development of the crab cavity system is continuing, and a full-scale superconducting
prototype cavity has been tested. It has exceeded the design specifications by a margin of
approximately 40 %, and had a coaxial coupler for frequency tuning installed in the beam line.
As has been mentioned in the sixth KEKB Accelerator Review Committee Report, thepriority of this program is not considered extremely urgent since no dramatic degradation of
luminosity due to the IP crossing angle has been observed. The committee was told that the
crab scheme cannot be applied to the Super KEKB because of HOM power limits. However,the crab cavity is a very complex design, and no operational experience with asuperconducting crab cavity in a collider is available yet. The committee recommends thatthe existing crab cavity be installed in the HER close to the superconducting cavities to study
the high-current behavior. If the results are positive, the decision could be made to fabricate
the full crab cavity system for installation at an opportune date to take advantage of thepossible luminosity gain.
The crab cavity program continues with interesting technical developments, such as sputteredniobium on copper substrates, and manufacturing of thin copper bellows to be sputter coatedwith niobium. These might be of interest to a wider community. This demonstrates KEK
management’s commitment to support SRF technology as one of the core competencies of
KEK, and is strongly encouraged by the committee.

Q) Vacuum, Antechamber, Masks, Bellows, etc.
1. Antechamber
The total gas load in Super KEKB is estimated to be three to four times higher than the design
value of the present KEKB. Enhancement of the pumping speed is important for the very13

high current and high gas loads produced, and an antechamber can provide a higher effective
pumping speed. This enhanced pumping will be provided in the antechamber proposed, with
NEG strips, localized NEG pumps, sputter ion pumps, and sublimation pumps all integrated
into the design. The pumping slots proposed have RF shields that are not the usual slit type,but are the rectangular or hole type which are effective in minimizing HOM penetration. The
KEKB vacuum group has considerable experience in NEG operation in the present ring, so
the committee is confident that the slit structure in the new Super KEKB will be designedwith low conductance for HOM penetration and high conductance for pumping.
An antechamber was proposed, which has a long, shallow side chamber along the beam orbit.
The width of the side chamber is limited by the geometrical structure of the quadrupole andsextupole magnets. The magnets will not be changed; thus it is impossible to adopt thestandard type of antechamber used in synchrotron radiation storage rings. In the standard
antechamber, the beam chamber is separated from the antechamber by a thin neck, preventing
the photons from escaping from the beam chamber. Photon irradiation in the beam space canbe minimized and photoelectron production should also be minimized.
In the proposed antechamber with a shallow side chamber, all the photons irradiate thevacuum chamber, especially in the shallow side chamber. However, this chamber roughly
doubles the distance between the source point of the synchrotron radiation and the first
irradiation point (1300 mm to 2900 mm). Because of the vertical divergence of thesynchrotron radiation, this increased length reduces the vertical synchrotron radiation power
density by a half. As a result, the proposed antechamber gives a factor 4 reduction of SR
power on the chamber that is 134 W/mm2 to 30 W/mm2, which is acceptable for copperchamber.
Experiments to reduce photoelectron yields were done in the synchrotron radiation beam linein the Photon Factory, and the results showed that a saw tooth structure is effective in
reducing photoelectron production.
2. Photon stop
A slant type photon stop was discussed as an option. In Super KEKB, the bending radius is16.3 m for the LER (e-) and 104.5 m for the HER (e+). The angle of incidence on the bendchamber is 3.2 degree in the LER. If the photon stop is installed in the LER ring 2.1 m
downstream of the entrance to the bending magnet, the slant angle should be less than thisvalue because the angle of incidence on the shallow-pocket antechamber is also less than thisvalue. This type of the photon stop is successfully used in high-energy synchrotron radiation
rings, so the design and operational experience in such machines is very relevant.
The photon stop option is closely correlated with an aluminum alloy chamber. The power
density is too high to permit the high power radiation from abnormal orbits caused by magnet
trips and operational errors to strike the aluminum alloy chambers. The aluminum alloychamber and the photon stop are useful, and work well, only when the machine operation iscorrect and sound. The Super KEKB vacuum system should be designed to have sufficientmargin to be stable against operational errors.

14

3. Wiggler
The synchrotron radiation power from the damping wiggler is estimated to be 27 kW/m in the
wiggler vacuum chamber. The chamber in this section has cooling channels on both sides,
and this power level is acceptable. There is a bending magnet section about 30 m downstreamof the wiggler. The power density at this section increases to 50 kW/m because the angle ofincidence increases going from the straight section to the arc section. The power density inthis arc section is estimated to be 5.7 W/mm2, so a special photon stop is not proposed
because this power density is acceptable in normal operational conditions. The committee
agrees with this assessment in normal, correct operation of the machines without any trippedmagnets or other conditions with abnormal beam orbits. More attention should be paid todesigning the vacuum chambers for an accidental beam orbit shift, when the synchrotron
radiation may irradiate the un-cooled surface of the vacuum chamber.
4. Bellows
It is difficult to make a reliable RF shield that is capable of supporting the extremely highcurrents. Since the number of bellows is high, new bellows should be developed with an
R&D program. The design presented, with longitudinally short slots and a double finger
structure with thin cuffs of 0.1 mm thickness, seemed promising. In the design of the wholevacuum system, every effort should be made to reduce the number of bellows in the ring,
though it is understood that bellows are necessary in some areas (e.g. the interaction region)
to help in the installation process and to absorb mechanical deformation and thermal
expansion. Designing the vacuum chamber to reduce the number of bellows would favor the
adoption of an antechamber design based on photon stops, because the high current beams
have less effect on the chamber temperature.

R) Facilities
For the Super KEKB, the increased currents in both rings require a dramatic increase ofelectric power consumption and cooling capacity. However, the estimated wall plug power ofabout 100 MW is less than that was required for TRISTAN, so it would be covered by the
installed total power of 150 MW at the KEK site. Even though the power demands may not
be evenly distributed among the five substations, this could be resolved by carefularrangement of the subsystems in groups, and/or connecting power cables between thesubstations.
The efficiency of the klystrons and energy recycling should be looked at carefully in order toreduce the operating costs. Even though this issue has been looked at before, an expert groupshould investigate this issue to see whether there are any new technologies that have become
available.
Since the cost of new buildings can be a significant fraction of upgrading the RF and coolingsystems, the committee recommends that the total cost of the project be minimized. This maylead to a change in the technical solution that provides the minimum cost (for example, a 2
MW klystron would take up less floor space than two 1 MW klystrons).
In general, there are no critical issues in the facilities and infrastructure for Super KEKB
operations.

15S) Monitors & Feedback
The beam position monitor system of KEKB has a resolution of about 1 µm. An analysis ofboth rings has shown that this very good resolution can also be obtained for the Super KEKB
parameters. Also, the use of antechambers should not degrade the resolution since anappropriate detection frequency for the BPMs has already been found.
Operational experience has shown that the synchrotron radiation monitor is currently quitesensitive to HOM heating. Therefore, it cannot be used for the high currents of Super KEKB
without some improvements. An innovative design using a mirror with a grazing incidence
would reduce the direct heating due to the synchrotron radiation power to the KEKB values.
An antechamber was proposed to reduce the strength of the beam-induced electro-magnetic
field at the extraction window that would be a source of the HOM.
Currently, a transverse coupled bunch mode with a rise time of 1 ms is the strongest
instability. It is compensated by a feedback with a damping time of 0.5 ms. If the particle
types were switched for Super KEKB to avoid the electron cloud effect for the positron beam,then the fast ion instability for the electron beam in the LER will become the strongest
instability. The vacuum pressure for today’s operating currents is below 1 nTorr. A worstcase estimate of 5 nTorr for Super KEKB currents would lead to a fast ion instability rise timeof 0.05 ms, requiring a drastic improvement of the feedback system which be not betechnically feasible. A careful analysis should be carried out to determine whether the fastion instability or the electron cloud effect is easier to deal with, and whether changing the ringpolarities is the best option for Super KEKB.
The committee recommends that the recently removed low frequency feedback system bereinstalled in the HER to control the slow instability which is observed and, possibly, tostabilize injection after a beam abort.

T) Linac UpgradeContinuously topping up the positrons reduces the importance of the positron filling time,
which is then only relevant following beam aborts. The injector group has successfully tested
the positron top-up mode in the last few months and this is expected to become standard
operation this summer. However, in order to take full advantage of this technique, the top-up
mode should be made available for both beams, which would require a bypass to be built
around the positron converter. The committee recommends that the KEKB Project Teampursue this capability with a high priority.
There are two separate issues in the injector linac for the Super KEKB operations: theintensity upgrade and, if the ring polarities are inverted, an energy upgrade of the positronlinac. Positron beams of 8 GeV with 1.2 nC would be required. There are two options
presented for the energy upgrade scheme, both using a damping ring of 1 GeV to reduce beamlosses near the detector following injection into the KEKB rings.
Option 1 is to replace 24 units of the existing S-band structures with C-band structures having
twice the accelerating gradient. These would fit within the existing building andinfrastructure. In this case, the new damping ring would also be necessary to reduce the16positron emittance in the C-band structures. Option 2 is to re-circulate the 1 GeV positron
beam from the damping ring back to Sector A for acceleration to 8 GeV. It requires an extra
beam transfer line, a separate bending arc, and a target-bypass line, in addition to the dampingring.
For the intensity upgrade, the positron injection rate is to be increased from 1.5 mA/sec to 3.0
mA/sec for the HER, while the electron injection rate is to be increased from 3.0 mA/sec to15.0 mA/sec for the LER. The capability for producing 5 nC electron bunches has alreadybeen demonstrated, since 10 nC bunches are routinely accelerated for positron production.
For the positron beams, the capture efficiency must be doubled from 0.6 nC to 1.2 nC. Even
though it is feasible to increase the focusing strength at the positron generator, there are otheroptions such as increasing the electron energy at the positron converter or positron bunch
stacking in the damping ring which should be investigated.
While both schemes are feasible, Option 1 has an advantage in reliability and the Option 2
seems to have an advantage in cost.
The Committee would like to recommend the use of C-band structures for a reliable injector
to the Super KEKB if the construction cost is not excessive. Therefore, the committee
recommends carrying out a detailed cost analysis for both schemes in the coming year.

U) Linac RF Sources
The C-band RF system for the injector would be consistent with the line of development ofSPring-8 SASE FEL Project (SCSS), and the linear collider(JLC). The exact frequency
would be slightly different (5712 MHz, exactly twice that of the present S-band linac, hasbeen chosen for KEKB to allow staged installation of the new systems), and the structures
themselves would be different from SCSS and JLC. However, the RF sources and modulators
could be very similar and it would be valuable to obtain operating experience at thisfrequency. The committee therefore supports the use of C-band, 5712 MHz, for the SuperKEKB Injector upgrade.
The preliminary design of the C-band RF source for the e+ beam energy upgrade to 8.0 GeV
has already been developed. The strategy of combining 2 klystrons and 2 modulators to forma single RF source unit seems reasonable. It is possible to decrease the total capacitance by
30% to get a 2 µs flat top on the pulse.
The use of available, existing components for this new RF source is also quite reasonable.
Examples are using the existing C-band 50 MW klystron to be the final amplifier, using theexisting 200 kW C-band klystron to be the sub-booster amplifier, and using temperaturecontrolled rectangular waveguide (WRJ-5 or 6) as the driveline phase reference.
It is important to fully evaluate the phase stability tolerances of the C-band RF sources.

17V) Damping Ring & Beam Transport
The Damping Ring is needed for both linac upgrade Options (C-band and recirculated Sband) to reduce the beam emittance and energy spread to the required small values. An
Energy Compression System (ECS) and a Bunch Compression System (BCS) are necessary
for positron injection into the Damping Ring from the linac and into the C-band linac
structure from the Damping Ring, respectively. The zero-order fundamental parameters forthe Damping Ring and the Bunch Compression System have been developed. The
preliminarily designs of the Damping Ring and Beam Transport systems seem reasonable.
Obviously, further studies are needed: the committee identified the following areas:•••

The effects of machine errors on the dynamic aperture of the Damping Ring;
The possibility of stacking more linac bunches into the Damping Ring bucket toincrease the pulse charge and provide a higher injection rate into the Super KEKBpositron ring;
The effect of nonlinearities on the design of the Energy Compression System and theBeam Transport system.

W) Control System
The decision to join the EPICS collaboration has lead to an efficient front end of the KEKB
control system. Also, the strategy of basing the control system as much as possible oncommercially available components, and outsourcing much of the programming, has worked
very well. For simplification, the same control system is used for KEKB and for the PF-AR.
All in all, the KEKB operating system is working very effectively, and the successful
integration of simulation software like SAD seems to have lead to very helpful capabilities.
Some of the old TRISTAN CAMAC field busses have been used for KEKB. In spite of thefact that they work well, developing new systems with CAMAC is problematic, because
designers are becoming rare and components are often no longer available. The committee
therefore recommends moving slowly away from CAMAC technology for obsolescencereasons, and not to use it for the new systems of Super KEKB. If a digital RF feedback
should be needed, CAMAC components would not be used for technical reasons.

X) Beam Dynamics
At present, the main thrust of the beam dynamics studies at KEKB is to understand theelectron cloud instability, and its impact on the machine luminosity. One of the main
limitations of the KEKB luminosity was the blowup of the LER vertical beam size duringcollisions, which limits the presently achieved vertical beam-beam parameter in HER to about
0.04. This effect has been regularly observed during the past two years, both byinterferometer and from the current dependence of the specific luminosity.
Before the 4th installation of solenoids in September 2001, the LER current limit for thephysics runs was set slightly above the threshold of the LER blowup in the single-beam mode
and, at the beginning of the luminosity runs, the increased vertical beam size could be
attributed either to the electron cloud instability, or to the beam-beam effect, or to theinterplay of both of these effects. However, even now that LER current of 1.3 A is below the18

single-beam blowup threshold, bringing the beams into collision results in an increase of thevertical beam size in LER.
One possible way of explaining this effect is to consider the combined action of the coherent
beam-beam interaction and the machine impedance. For KEKB LER, the electron cloud isbelieved to dominate the transverse machine impedance budget, and the vertical instability
threshold is lower than the horizontal.
Linear theory predicts a head-tail type instability in a system of colliding bunches. Thisinstability, in combination with the strong non-linearity of the beam-beam force which
provides a decoherence mechanism for the growing bunch oscillations, is a candidate to
explain the experimentally observed “heating” of the vertical beam size during collisions,which occurs below the threshold of the single-beam instability.
The simulation made for KEKB seems to confirm this hypothesis. Although the firstsimulation of the combined effect of the electron cloud and beam-beam involves some
simplifications (e.g., the colliding bunches are now sliced into transversely rigid Gaussian
disks), the instability growth and saturation can already be seen. The indication that thesynchrotron tune spread may cure the growth in the beam size seems to be very important.
Further work in this direction, with more realistic “soft” strong-strong models, is strongly
encouraged.
Concerning the LER blowup in single-beam mode, the past year brought more confidence inthe simulation results of the electron cloud buildup in LER. The thresholds obtained from thehead-tail instability model with the simulated cloud densities are in good agreement withexperimentally observed LER blowup thresholds; their variation with the momentum
compaction factor and the effect of the machine chromaticity are also explained by this model.
Recent simulations of the electron cloud buildup in the antechambers with and withoutsolenoids predict stability of the Super KEKB beams with the solenoids, however the safetymargins are not wide. Refinement of these calculations may be in order, given the importanceof the result.
Another important issue where the simulation can help in understanding the real situation withthe electron cloud is to explain the experimental results on the spectra and growth rates of thecoupled-bunch instability in LER. The committee recommends that these studies be pursued.

19Appendix A
KEKB Accelerator Review Members
Andrew Hutton

JLabChairmanAlexander W. Chao

SLAC

Eugene A. Perevedentsev

BINP

Masanori Kobayashi

KEK

Won Namkung

PostechFleming Pederson

CERN (excused)

David Rice

Cornell (excused)

Wolfgang Schnell

CERN

John Seeman

SLACPeter Kneisel

JLab

Georg Hoffstaeter

DESY

Wang Shuhong

IHEP

20

Appendix B

Agenda of Seventh KEKB Accelerator Review Committee
February 25(Monday)
9:00-10:00
Executive session
10:00-10:15
KEKB overview
10:15-10:35
Belle Status
KEKB 2001-2002
10:35-10:55
Luminosity Boost
10:55-11:15
LER Blowup & Solenoid
11:15-11:35
Antechamber prototype
11:35-11:55
Masks
11:55-12:15
High Current
12:15-13:30
Lunch
Super KEKB
13:30-13:50
Physics Motivation
13:50-14:10
Parameters
14:10-14:30
Optics14:30-14:50
IR Design
14:50-15:10
Special Magnets
15:10-15:30
QCS
15:30-15:50
Break
15:50-16:10
RF System
16:10-16:30
ARES
16:30-16:50
SCC
16:50-17:10
Crab Cavity
17:30-18:30
Executive session
18:45-20:45
Reception

K. OideF. Takasaki
Y. Funakoshi
H. Fukuma
K. Kanazawa
Y. Suetsugu
K. Akai

M. Yamauchi
Y. Ohnishi
H. KoisoY. Funakoshi
M. Tawada
N. Ohuchi
K. AkaiT. Kageyama
T. Furuya
K. Hosoyama

February 26
8:30- 9:00
9:00- 9:40
9:40-10:00
10:00-10:20
10:20-10:40
10:40-11:00
11:00-11:20
11:20-11:40
11:40-12:00
12:00-13:00
13:00-13:20
13:20-13:40
13:40-18:00
18:00-20:00(Tuesday)Executive session
PEPII status
Vacuum, Antechamber, Masks, Bellows, etc.
Break
Facilities
Monitors and Feedback
Linac Upgrade
Linac RF Sources
Damping Ring and BT
Lunch
Control Systems
Beam Dynamics
Executive session
Working dinner

February 27
8:30-11:00
11:00-11:30
11:30-12:00(Wednesday)
Executive session
Interaction with KEKB staff members
Closing
21

J. Seeman
Y. Suetsugo
Y. Ohnishi
H. Fukumu
T. Kamitani
S. Fukuda
M. Kikuchi
T. Katoh
K. Ohmi