The Twenty-First KEKB Accelerator Review Committee Report
June 15, 2016
Introduction
The Twenty-First KEKB Accelerator Review Committee meeting was held on June 13-15,
2016. Appendix A shows the present membership of the Committee. Two members of theCommittee, In Soo Ko and Matt Poelker, were unable to attend. The meeting followed thestandard format, with two days of oral presentations by the KEKB staff members, followed bydiscussion between the Committee members. The Agenda for the meeting is shown inAppendix B.
The amount of progress that has occurred since the last review is impressive, but the expectedbudgets may delay the following steps. As always, the high standard of the presentations
impressed the Committee, particularly the remarkable evolution of the new hardware thatisnow installed in the tunnel, and the excellent results from the phase 1 commissioning. The
Committee examined the progress of the project.
The most important recommendations of the Committee were presented to the KEKB staff
members before the close of the meeting. The Committee wrote a draft report during themeeting that was then improved and finalized by e-mail among the Committee members. The
report is available at http://www-kekb.kek.jp/MAC/.
Contents
A) Executive SummaryB) RecommendationsC) Findings and Comments
1. Overview of SuperKEKB, status and plans
2. Belle II physics and construction status
3. Overview of Phase 1 commissioning
4. Overview of lattice
5. Optics correction and low emittance tuning
6. Vacuum system, beam scrubbing
7. Instabilities: simulation and observation
8. Beam instrumentation and bunch feedback systems
9. Photon monitors
10. Operation Status of RF System
11. Normal-Conducting RF Cavities for SuperKEKB / MR & DR
12. Integrated vacuum control system for RF and Operation status of SCC
13. Beast II, beam background
14. Ring magnet system
15. Beam transport and injection
16. Beam abort system
17. Control
18. Beam Injection Control
19. RF Gun Report
20. Injector Linac Overview
21. RF Gun and Electron Beam
22. Laser and RF Gun

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23. Positron Generation and Beam
24. Injector Alignment
25. Injector Commissioning (Phase1&2)
26. Positron damping ring
27. IR construction
28. Overview of IR magnets and construction status
29. Measurements of field quality for quadrupole magnets using harmonic coils
30. Measurement of the magnet center with single stretched wire system
31. Cryostat performance
32. Quench protection system for superconducting solenoids
33. Tour (KEKB control room, MR tunnel, Linac, Damping ring, QCS)
A) Executive Summary
Since last year’s ARC meeting the SuperKEKB project has made spectacular progress. The
ARC warmly congratulates the SuperKEKB team for a successful fast beam commissioning.
In only a couple of months this commissioning has resulted in storing close to 1 A of beam
current in both HER and LER, encouraging vacuum conditioning with more than 500 Ah of
stored beam in both rings, accurate optics correction, HER injection using the electron beamfrom the RF gun, top-up injection, and a characterization and optimization of the Belle II
background conditions using the BEAST-II Phase 1 set up and various ring collimators.
It is important that Phase 2 commissioning starts in the fall of 2017 and phase 3 data taking
with the complete Belle II in 2018, so as to enable Belle II data to make competitive
contributions to frontier flavor physics.
The Belle II construction and integration with the machine are on track, but the schedule istight. Phase 3 of the accelerator commissioning can begin when the Belle detector is complete.
Some data taking with the Belle II outer detector could take place during the accelerator
commissioning Phase 2.
The construction of the damping ring is progressing well and should be completed in time forPhase 2. Beam commissioning of the damping ring is now planned to begin in October 2017,
just ahead of, and continuing during, the Phase 2 commissioning run.
The vacuum scrubbing is proceeding well. The electron cloud build up has been well controlled
for Phase 1 operation, and can be further mitigated, e.g. by extended surface conditioning orby the installation of additional solenoids. The refurbished RF systems are performing well andhave supported all the Phase 1 goals.
Finances have had a significant impact on the progress of the SuperKEKB project. The project
has been very efficient in using the past available funding to manufacture the absolute basic
needed hardware to get to Phase 1 commissioning. Substantial beams have been stored, keeping
the project on a solid but delayed path forward. The 2016 budget is again a challenge to get
ready for Phase 2 over the next year for the October 2017 commissioning start. The ARC
supports the KEKB team’s efforts for securing supplemental funding in 2016 and to promotea more satisfactory operation budget in the next fiscal year. This improved funding would help
Belle II to be in more effective competition with the LHCb and other similar experiments at
the CERN LHC collider and worldwide.

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B) Recommendations: The Committee has made recommendations throughout thedifferent sections below. The most significant recommendations are summarized here.
1) The Committee believes that SuperKEKB needs to remain the top priority of the KEK
Tsukuba campus in the next few years if the commissioning and operation are to be
successful in a timely fashion.
2) Despite the strong support of KEK management to increase staffing, including a few new
junior hires and rehiring senior, experienced retired staff, the shortage of qualified staff
continues to be a risk to successful completion of the project, the commissioning andoperations. The number and skills of the additional staff members going forward shouldbe optimized for the commissioning and operations phases of the project.
3) We encourage transfer of knowledge from more senior to younger staff to ensure continuityof system knowledge over the full lifetime of SuperKEKB.
4) Continue and expand involvement of international and domestic collaborators in thecommissioning of all SuperKEKB accelerator systems (linac, damping ring, LER, HER).
5) Develop a complete, detailed commissioning schedule for the guns, linac and rings for thecommissioning Phases 2 and 3. Follow the recommendations of the SuperKEKB RF Gun
Committee.
6) Study the unexpected HER vertical emittance value. If possible, the question whether theobserved large emittance is real should be clarified before the long stop of operation
following Phase 1.
7) The SuperKEKB/Belle II interaction region (IR) is extremely complex and, althoughenormous progress has been made over the past year, the Committee recommends
continued attention to the issues of the beam-beam interaction, beam lifetime,superconducting magnets, vacuum pressure, backgrounds, assembly, and machine detector
interfaces.
8) Strengthen quality assurance of procedures for installation of magnets in the cryostat.
9) While an optics solution exists for Phase 2, reaching the final luminosity in Phase 3 depends
on solving a challenging set of interconnected beam dynamics problems. Despite
considerable progress, a complete overall solution has only partially been obtained. The
Committee recommends that studies continue to optimize the luminosity.
10) The electric power consumption is the main cost driver for operation. Electricity cost per
MWh for SuperKEKB is significantly higher than at the time of KEKB. Strategies should
be devised by the KEK management to lower the electricity costs of SuperKEKB operationin the longer term.
11) The Committee recommends that the online version of the SuperKEKB Technical Design
Report be regularly updated.

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C) Findings and Comments
1. Overview of SuperKEKB, status and plans
The construction of SuperKEKB is well advanced, aiming at 40 times the luminosity of KEKB
with a goal of 8×1035/cm2/s. This new accelerator uses the techniques of a nano-beam schemewith βy* of 0.3 mm and doubled beam currents in each of the rings at 4x7 GeV. There will bean 83 mrad crossing angle at the IP with 2500 bunches collided, having beam currents of 2.6
A for electrons and 3.6 A for positrons. Many of the accelerator systems are the same forSuperKEKB as for KEKB but there are also many new systems including LER magnets, LER
and HER vacuum chambers, IR layout and magnets, LER and HER wiggler sections, RF
configuration, diagnostics, photo-injector-gun, and positron damping ring.
The Phase 1 installation of the two rings was completed in early 2016. Over 1000 new vacuum
chambers have been produced. Many pre-running system checks were completed before thebeam was turned on. Commissioning for Phase 1 for both beams started in February 2016. As
planned, Belle II and the final IR magnets and vacuum chambers were not installed. Beamcommissioning has proceeded very well with progress more rapid than the corresponding
period with KEKB. Vacuum scrubbing is well under way. Currents of 830 mA of electrons in
HER and 910 mA of positrons in LER have been achieved. Initial data on IP backgrounds weretaken by the BEAST-II Phase 1 configuration. Many beam parameters have been measured
with most being what was predicted. The RF system has been running at about 70% of full
specifications. The use of electrical power has to be watched carefully as this cost determines
the running time. More commissioning will be done over the next few days before theupcoming 18 month shutdown. The IR will be taken apart in July 2016 to start the final IR andBelle II installation.
Phase 2 commissioning with the full IR and Belle-II (but without the vertex detector) will startin November 2017. Belle II may take a little production data during Phase 2 running if thetarget luminosity of 1×1034/cm2/s can be reached.
Phase 3 will have the full accelerator and full Belle II detector installed. It will start around
November 2018 after all upgrades are finished.
The proposed schedules for Phases 1 and 2 depend on anticipated yearly funding and any
funding supplements allocated to SuperKEKB and Belle II operations.
Recommendations:
R1: Develop a few alternative high-level spending profiles in case the anticipated funding
profiles do not arrive as planned.

2. Belle II Physics and Construction Status
A very impressive display of the physics questions Belle II could address was presented. There
are several hints that there is something beyond the Standard Model in flavor physics studies.
The decay of heavier particles, and especially of the bottom quarks, is sensitive to the existenceof new particles, which allows for the probing of New Physics (NP) in high statistics
experiments such Belle II or LHCb. We were presented with several decay channels, firststudied by the previous B-factories, which hint at the possibility of NP where more data is
crucial in settling the issue. It is vital that the detector start collecting data as soon as possible
in order to remain competitive.

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The status of the detector was also shown and the schedule for finishing the final subsystems(primarily the SVD and PXD vertex systems and the Aerogel RICH (ARICH)) indicated thatthe timeline was tight but possible. There have been a few difficulties with the ARICH HV and
HAPD photo-sensors and an issue of signal cable space between the SVD and PXD wasrecently uncovered but there are no apparent showstoppers at present.
Recommendation:
R2: Maintain the present schedule for completion of the detector and IR magnets. Try toforesee possible issues and strive to produce mitigating strategies for keeping to theschedule. Further delays will weaken any impact the Belle II data could have on the heavy
flavor physics searches for NP.
R3: If Phase 2 commissioning goes well and is successful, we suggest that the Belle II and
SuperKEKB teams discuss the possibility of including an early physics run during thePhase 2 time.

3. Overview of Phase 1 Commissioning
An overview of the Phase 1 commissioning of SuperKEKB was presented. The goals included
basic machine tuning, vacuum scrubbing with 360 – 720 Ah and optics tuning for low
emittance. Total beam currents of above 0.8 A were reached both in the HER and the LER.
The beam lifetimes reached 600 min (HER) and 75 min (LER) with the difference explainedby the vacuum pressure. Significant progress in zero current LER emittance was achieved,
approaching near design values. The X-Ray monitor in the HER has some issues and is being
investigated. The commissioning progress was faster than in KEKB, illustrating the excellent
preparation and learning experience. Several important issues were encountered and addressed:(1) Strongly nonlinear vacuum pressure rise and a vertical beam size blowup as a function ofbeam current were found in the LER. As a countermeasure solenoids were installed at the Al
bellows in the LER. The solenoidal fields strongly improved the vacuum behavior and werealso effective in raising the blowup threshold by a factor 1.5.
(2) Frequent beam aborts associated with vacuum bursts were observed in the LER. The beam
current where the bursts occurred has gradually increased. Possible causes are the discharge atpoor electrical contacts by the wall current and the collision of dust (small particles) with thecirculating beam. Dust particles were provoked by hammering the vacuum chambers and could
reproduce the signature of the observed vacuum burst events.
(3) Longitudinal coupled bunch instabilities were observed in LER. Studies are ongoing.
(4) A few hardware problems were induced with the high beam currents, in particular a vacuum
leak and damage to the feed-through of the transverse FB kicker.
(5) Coupling and dispersion correction are important for achieving a low vertical emittance andwere indeed studied in the context of emittance tuning. While the HER corrections went well,a difficulty was encountered for the LER corrections. A leakage magnetic field from theLambertson septum magnet with a skew quadrupole main component was identified as cause.
Skew quadrupole magnets (ferrites) were installed for canceling the leakage field from theLambertson, allowing much improved optics and almost nominal zero current emittance.
The committee congratulates the team for the excellent achievements. Rapid progress wasdemonstrated, major milestones were achieved, a few issues identified and problems addressedin a highly professional attitude.

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Recommendations:
R4: Options for local shielding and correction of the Lambertson leakage field should befurther investigated and, if possible, implemented.
R5: Try to reach beam currents above 1 Ampere in the last days of operation.
R6: Study whether the unexpected HER vertical emittance increase with current is due tobeam-gas fast-ion effects on the beam size.
R7: The consistency of the measured vacuum pressure and the observed beam lifetimes
should be shown in an analytical or numerical analysis.
R8: A survey of any other suspicious places for electron cloud in addition to the aluminum
bellows should be performed.
R9: The longitudinal and transverse impedance measurements should be compared with the
impedance model of the machine. This can allow early identification of limiting hardware
for higher current operation.
R10: The tune spread along the bunch train should be measured, analyzed and compared to
expectations.
R11: The current-dependent vertical emittance blow-up should be further analyzed andcountermeasures developed.
R12: The residual coupling problem in the LER should be studied further.

4. Overview of Lattice
Upgrade of the KEKB Main Rings optics aimed at implementation of the novel NanoBeam
collision concept with the expected 40-fold luminosity gain will proceed in 3 phases. Phase 1
is devoted to single-beam problems, without final focus QSC’s and detector solenoid.
Two main tasks of Phase 1 commissioning are i) beam scrubbing of the vacuum components
to achieve an acceptable background level for the Belle II detector before its installation; ii)
tuning the lattice to the minimum vertical emittance (of the order of 10 pm), that is essential
for the NanoBeam collision regime. The latter is only possible with a good understanding of
the optics model, including betas, dispersions, x-y coupling, sextupole families etc.
The staged strategy of the optics tuning includes the wiggler-off optics tuning by local bumps,
obtaining the gain map, adjusting and equalizing the ring circumference with the wigglers-on
the, and finally, a fine tuning of the beta-beat and of the betatron phase advance plus
minimizing the x-y coupling.
The arc lattice includes non-interleaved pairs of sextupoles equipped with skew-quadrupole
correction coils. These pairs are connected by a “–I” transformation, in order to preserve thedynamic aperture of on-momentum particles, while the skew-quad coils are used to locallycorrect the vertical dispersion and the coupling.
An important step in LER optics studies was identifying the leakage field of the Lambertson
septum magnet in the beam abort line as a source of strong coupling. After installing the insitu correctors to cancel the skew-quadrupole component of the leakage field, this problem was
greatly mitigated.
The local coupling correction was supported by the vertical beam size monitoring using the
new X-ray imaging monitor with coded aperture (XRM). From these measurements, the

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vertical emittance of LER is successfully minimized down to 10 pm (5 pm target), that isroughly consistent with the optics measurement data. From the latter an emittance of 7.2 pm is
expected.
However, in HER the XRM data indicate a vertical emittance of 120 pm (corresponding to thex-y coupling ratio of 2.8%!), in contradiction to the HER optics data predicting some 9.1 pm.
Further study of XRM data adequacy is recommended to find the reason for this discrepancy.
Of equally high priority is the planned measurement of the Touschek lifetime in HER, in orderto obtain an independent estimate of the vertical emittance.
The Committee is impressed by the rapid progress of the Phase 1 Main Ring optics during therelatively short commissioning period. As for the vacuum scrubbing task the target dose of~700 A hrs can be reached by the end of Phase-1, with currents of about 1 A each beam, butthere are some concerns about many installations scheduled in sequence and requiring vacuum
venting: new BPMs, collimators, low-beta quadrupoles and eventually the final detector setup.
All these operations will have a negative impact on the achievement of the optimal vacuum
conditions, slowing down the overall machine optimization.

Recommendations:
R13: During the remaining period of the Phase 1 commissioning concentrate on the
improvement and understanding of the X-ray beam size monitor accuracy.
R14: For the Phase 2 optics, the Committee recommends preparing robust optics tuning
procedures for curing the effects of the final focus system on the dynamic aperture. A
series of IR optics should be prepared for the step-by-step beta* squeeze, with
understanding of how the dynamic apertures can be recovered at each step.

5. Optics correction and low emittance tuning
The optics commissioning during Phase 1 has been carried out very efficiently in a rather shorttime. SuperKEKB features a new lattice, new magnets and new instrumentation. All therequired tools had been well prepared before the commissioning.
These allowed the identification of a large skew quadrupolar error source at the LER
Lambertson magnet, which was quickly corrected by powering selected coils in nearby
sextupoles and by inserting a new skew quadrupole permanent magnet next to the source.
The fast reaction time and the suitability of the mitigation techniques demonstrate theoutstanding capabilities of the SuperKEKB commissioning team.
Both LER and HER optics measurements imply a vertical emittance at the level of 10 pm. Thisis seen as satisfactory for the phase 1.
The LER vertical emittance has been confirmed with beam size measurements.
However, and only very recently, the HER X-ray monitor has shown a vertical beam size
measurement an order of magnitude larger than expected. This could be due to limitations in
the measurement device or to actual optics issues. Another traditional figure of merit for thevertical emittance is the Touschek lifetime.
Current measurements might support a larger vertical emittance than expected but with a large
uncertainty.

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Investigations are ongoing to clarify the situation. This is of crucial importance for the
performance of the collider.
HER measured horizontal and vertical chromaticities are in agreement with model predictions,while offsets of 2 and -4 are found for LER. This could be explained by sextupolar components
in the new dipoles, however first estimates from magnetic measurements suggest too weak aneffect.
Recommendations:
R15: Compare the HER optics measurements using different techniques. Coupling from turnby-turn data might be less sensitive to BPM errors such as incorrect calibrations or tilts.
R16: Develop a correction strategy for the residual Lambertson leakage field.
R17: Measuring LER natural chromaticity, resonance driving terms and amplitude detuningmight help understanding the source of the chromaticity offset. Also, some optics
measurements could be repeated after switching off all sextupoles and associatedcorrection coils.
R18: Measure dispersion in the LER after switching off the closest magnets of the HER, inorder to clarify a possible interference in the Tsukuba section, which might be responsible
for the anomalous dispersion in the LER.
R19: Repeat Touschek lifetime measurements for both rings after a reasonable dynamical
vacuum level has been achieved. HER Touschek lifetime might provide an independentmeasure of the vertical emittance.
R20: Critically revise the calibration procedure of the HER X-ray monitor.
There are two recommendations from the 20th review committee that still remain valid and areeven more important now, having the IR magnetic measurements, and in view of the opposite
sign of the leak compensation magnet tilt angle:
R21: Identify all relevant IR non-linear optics errors affecting the dynamic aperture (DA) anddevelop a suitable correction scheme.
R22: On the basis of realistic LER/HER optics model simulations with and without beam-beaminteractions, develop correction procedures aimed at the DA control, suitable for thecontrol-room implementation.

6. Vacuum system, beam scrubbing
As expected, the vacuum system was ready to take beams from February 2016 onward. The
NEG activation performed at the end of the last year was the conclusive step of five years of
production and installation.
Beams approaching currents of 1 A are now (10th of June) circulating in both rings. The
lifetime are 600 and 75 minutes for HER and LER, respectively; the smaller value for the latteris due to an average pressure one order of magnitude higher: 2×10-6 against 2×10-7 Pa at 500
Ah. The difference is essentially due to the fact that in the LER most of the beam pipes are
newly manufactured and were never exposed to beams before; on the contrary, most of theHER vacuum chambers were fully conditioned during the KEKB operation.
The conditioning of the vacuum walls is progressing faster in the electron ring: photodesorption yields as low as 10-7 have been recorded (500 Ah). In the positron ring, the

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attenuation of the pressure rise (Pa/A) is delayed because electron clouds develop in theuncoated Al bellows (5% of the ring length) and electron stimulated desorption is consequently
generated. The gas release was localized and eradicated by the installation of solenoids andpermanent magnets in all uncoated Al bellows.
The combination of antechamber and TiN coating has shown to be very effective in the
mitigation of electron cloud at low current. However, a non-linear pressure increase ismeasured at higher beam current (i.e. at I>0.5 A in present operational conditions, or alreadyat I>0.2 A in tests with the design bunch spacing of 2 buckets). This could be the result of an
incomplete conditioning of the coating; higher beam doses are necessary to better evaluate sucha behavior.
As it was recommended last year by this committee, the effect of venting and long storage on
the secondary electron yield of TiN, after months of operation, has to be measured to better
understand the need of possible re-conditioning of the coating.
Results obtained by clearing electrodes seem to indicate that photoelectrons are the dominantsource of electron clouds and that the different bunch filling pattern used until now do not
generate significantly different electron density, at least in the wigglers. This result and its
interpretation – which may be in conflict with the clearly pattern-dependent threshold for thevertical blowup – need to be probed with higher beam currents.
In LER, unexpected and frequent positron beam losses (“bursts”), and consequent beam aborts,
with concomitant pressure spikes have been experienced. Such events are mostly localized in
D1 and D2 around Tsukuba straight section. They seemed to initially increase in frequency andto spread to the nearby dipoles with increasing beam current; a conditioning effect has been
observed. A possible explanation is dust particles intercepted by the beam. Grooves could be
one of the sources of such particles. The effect is seen mostly, or entirely, for vacuum chambers
from one manufacturer.
A vacuum leak appeared in a flange of a tapered beam pipe. The explanation given is electron
beam collision on the vacuum wall of the flange due to beam instabilities. A possible mitigation
is an optimization of the collimator positioning or a local beam mask. Photons reflected fromthe tapered area should also be considered.
Other issues appeared during the commissioning, but most of them are already solved or do
not seem to be showstoppers for the following phases of operation.
In conclusion, the recommended conditioning phase is almost completed with encouraging
results. The only potential obstacles to proceed to the next operational phases are the non-linear
electron-cloud effect at currents above about 200-700 mA, depending on the filling scheme,
and the sudden vacuum-pressure bursts – both in the LER. No problem with the collimation
system has been found. All issues encountered have been isolated, studied and solved or
highlighted for further analysis.
Recommendations:
R23: The experimental study of SEY degradation after venting or long storage is an important
subject that should be tackled as soon as possible to understand the need of reconditioning
between operational phases.
R24: The origin of the dust particles that generate beam losses and beam aborts should be
identified. The venting-pumping cycles from the coating to the final installation should
be analyzed to find any anomalous event that can be correlated with the incriminated dust
particles.

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R25: We recommend that, for Phase 2, permanent or solenoid magnets are deployed on themost critical beam pipes, as planned, so that no showstoppers correlated to electron cloud
hinder the achievement of nominal operational parameters. A factor of five higher
positron current is still needed.

7. Instabilities: simulation and observation
The Committee is positively impressed by the effective and successful effort aimed atunderstanding the behaviour of the instabilities as well as their impact on the transverse beam
sizes in both rings, with special attention to the LER.
In the first stage of the commissioning beam dynamics in the LER has been clearly dominatedby the electron-cloud related phenomena, regardless of the many precautions adopted in thedesign to cope with these effects: grooves in the dipole vacuum chambers, TiN coating,solenoidal windings and electron clearing electrodes in the wiggler magnets. Experimental
evidence for electron clouds has been clearly and quickly recognized. The most harmful
consequences were mitigated already during the commissioning, e.g. by installing permanent
magnet solenoids around the Al bellows of the vacuum chamber, which were identified as amajor source of electron cloud. These solenoids have increased the threshold for vertical blowup by about 50%.
Extensive simulations concerning the vertical beam size threshold evolution along withmeasurements of the electron cloud intensity have helped guide the progress and helped keep
under control the vertical beam size growth with beam current. The work done allowed, alreadyafter a short scrubbing period, operations with 1 A positron beam current in a 4 bucket
separation pattern.
A transverse coupled bunch instability induced by the electron cloud was identified
experimentally and its growth rate measured; both aspects are consistent with expectation.
Tune shift measurements indicate that the two rings have transverse impedances comparable
to those measured in the old KEKB rings.
A longitudinal coupled bunch instability of still unknown origin has been observed in the LER.
This instability was suppressed by the longitudinal feedback system.
Longitudinal impedance has been investigated by measuring bunch length as a function of
single bunch current and for several collimator setups. Data are not yet much revealing. At firstglance the microwave threshold behavior which had been observed at a single bunch currentof the order of 0.4 mA in the KEKB LER has disappeared. On the other hand the measured
bunch lengthening with bunch current (about 25-50% for a bunch current of 1.4 mA) appears
to be significantly larger than what had been expected from the impedance model.
Simulations can predict the threshold in the electron density where vertical beam size growthoccurs. Achieving a sufficiently low electron density is a challenge for the vacuum system.
The Committee believes that the work done is well founded and represents a solid prerequisite
for guaranteeing stable vertical emittance values of the order of 10 pm, as required to reachhigh luminosity in the NanoBeam collision scheme.
Recommendations:
R26: Validate simulation results against measurements in every way possible, with the aim ofestimating high current challenges. The simulations can be very valuable in estimating

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limits, thresholds and high-current dynamics if they are carefully validated against real
physical data from the machine.
R27: Coordinate with feedback studies, measure bunch tune vs. position along the train, e.g.
by analyzing the grow-damp data shown in the presentation. This diagnostic can help
distinguishing HOM driven motion from ion or electron cloud driven motion.
R28: What is the source of the unexpected longitudinal instability? A study with various filling
patterns, and observation of the unstable modal patterns can help sort out possible HOM
frequencies related to impedance-driven instabilities.

8. Beam instrumentation and bunch feedback systems
In Phase 1 several BPM systems are available. The old KEKB narrowband 1 GHz detectors
are being used in the HER. For the LER with a new vacuum chamber (lower cutoff), new 509
MHz narrowband detectors are employed. All VXI main frames were replaced by new ones.
Wrong cable connections were found and corrected for 25 BPMs. BPM offsets have been
determined by beam-based alignment. The measured position resolution has been measured tobe better than 3 micron in LER and better than 5 micron in HER. The stability of the relative
gain was inferred using the “consistency” method (comparing the 3 against 1 BPM button).
While these consistencies are stable for the new BPMs in the LER, some BPMs in the HER
show drifts or sudden jumps (possibly due to old feedthroughs and/or old cables). A larger
noise level was found in the 1 GHz detectors located in site building D7, where RF equipment
is located. The BPM resolution in the Oho and Nikko straight sections is probably affected by
the same cause. A mitigation measure would be to replace the 1 GHz detectors with the new
509 MHz detectors, which appears to be better shielded against such noise. Some minor
problems were found and corrected for five of the new 509 MHz BPMs.
A gated turn-by-turn (GTBT) monitor is based on a fast switch (with 6 ns gate width). In total
117 units of these gated BPMs are installed, i.e. more than 50 such detectors per ring. The
commissioning of the GTBT included timing adjustment with the injection beam, and a finetiming adjustment using a pilot bunch during scrubbing run. The system is ready to use.
The IP orbit feedback is not needed during Phase 1. However, the BPMs of this feedback canbe tested with beam. The specifications foresee a resolution of better than 1 micron (and a
feedback bandwidth below 100 Hz). With increasing beam current, for the LER the relative
quality of the y signal is seen to degrade compared with the x signal. One possible explanation
is a vertical vibration of the BPM head. The size of the effect is a few micron rms.
Beam loss monitors are used to trigger a fast beam abort in case of unexpected sudden losses.
The SuperKEKB beam loss monitor systems include 32 ion chambers and 114 PIN detectors.
A few examples of beam aborts triggered by beam losses illustrate that the actual aborts occur
typically a few 100 microseconds after the start of the loss.
Obtaining the betatron tune with a global tune measurement for multiple bunches works wellat low beam current. However, at higher current the beam response is strongly damped by the
feedback. Here a gated tune measurement is available for a pilot (or selected) bunch, excitedby a PLL using the digital filter iGp12. The iGp12 is the successor of the iGp digital filter
developed under US-Japan collaboration with SLAC.
The SuperKEKB transverse feedback systems have been developed in SLAC-Japan and INFNKEK collaborations. The longitudinal feedback uses a kicker made from Al-alloy. The
feedback systems support a variety of applications/functions. The injection kicker can be

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adjusted using the transverse feedback system as detector. The bunch current information fromthe current monitor is sent through reflective memory (real time) to the bucket selection systemat the linac during the injection period (<20 ms).
At a very early stage of the commissioning, strong transverse coupled-bunch instabilities wereobserved in both rings and both planes even for fairly low stored current. In the LER the verticalinstability was much stronger than the horizontal one; in the HER the instability strength was
about the same in both planes. All these instabilities were fully suppressed by the feedback
systems, which thereby enabled vacuum scrubbing with high beam current, as well as studies
of coupled-bunch instabilities (e.g. ones driven by electron cloud or ions). Instabilities havebeen characterized by growth-damp experiments. The LER vertical instability shows the fastest
growth rate. In operation with 3-bucket spacing above the threshold current of 660 mA alongitudinal coupled-bunch instability is seen, with an instability mode number around 500.
Such a longitudinal instability was never observed during KEKB operation.
The water-cooled dummy loads for LER of the longitudinal feedback systems burned out due
to a stopped water chiller. Now an automatic abort is triggered when a chiller stops.
The new transverse feedback kicker for LER seems to be broken. A complete short-circuit
appeared after a short maintenance period. There had been no indicator of any trouble prior tothis event, i.e. no abnormal temperature rise, no reflection, and no vacuum burst.
The BPMs and the feedback kickers exhibit only a very slight increase of temperature withbeam current, of order 1-2 degree at 1 A, which suggests that any HOM heating will remain
acceptably small even as beam currents are raised further.
Most of the beam instrumentation prepared for the SuperKEKB rings (Phase 1) is working
well, including the BPM system (narrowband position monitors, gated turn-by-turn monitors,
and IP position monitors), the beam current and bunch current monitors, the loss monitors, andthe bunch-by-bunch feedback systems (tune monitors, transverse FB (HER and LER),
longitudinal FB (LER)).
The LER transverse kicker needs to be repaired. The root cause of its breakage needs to beinvestigated. In preparation for Phase 2 operation, also a few damaged BPM heads will be
replaced. An advanced kicker is being developed with collaboration of SLAC. Simulations of
the IP feedback performance (with dithering) and an implementation of IP feedback algorithms
are foreseen.
The impressive commissioning experience profited from the investments made in the past years
and validates the design choices. The BPM consistency checks (“3 bpm method”) are extremely
helpful for assessing the performance of thee hundreds of monitors and processors. Possibly,
these could be made to run in the background as a diagnostics for the system operational state.
New systems, such as gated turn by turn monitors, IP position detectors, and loss monitors are
just starting their commissioning. Manpower is tight, but initial results suggest these systems
are ready to be tested with beams. When they are fully understood, they can be further
commissioned and used as part of operations.
The transverse and longitudinal feedback systems are building on KEKB expertise. The
benefits of a flexible architecture and multiple kickers are evident from the smooth operationeven with one damaged kicker unit.
Recommendations:
R29: Check interlocks and instrumentation in light of future high operational currents.

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R30: Use diagnostic information from feedback studies. Try to understand modal growth rates,
and compare them with estimates (tie to simulations and models).
R31: Develop a plan for obsolescence of the HER BPM processors – if they are to be run formany decades, spares and a test jig plan are needed.
R32: IP feedback commissioning and system development will also be manpower limited. We
encourage collaboration with sister labs and the use of passive measurements as a means
to begin to understand this critical system.
R33: Measure the mechanical vibration of the IP feedback BPMs and/or adjacent beam pipe;
where available compare the modal spectrum with ANSYS simulations and withpresently observed (and future) IP BPM noise spectra.

9. Photon monitors
Since the last review meeting great progress has been made for the photon monitors, including
X-ray monitor (XRM), synchrotron radiation monitor (SRM) and large angle beamstrahlung
monitor (LABM). The committee enjoyed the very detailed talk, with an abundance of results
ranging from installation and calibration to the measurements on the beam blow-up due toelectron cloud during the Phase 1 commissioning. All the basic functionalities were welldemonstrated.
Transverse bunch sizes were measured by different monitors, as well as the emittances andcoupling coefficients. The XRM calibrations yielded excellent results on the geometricmagnification factors, which agree well with the beam-based measurements at both rings. An
emittance knob calibration was applied. The overall performance is reasonable for LER, andresults consistent with expectations based on the measured optics. The XRM in LER was used
to study the electron-cloud blow-up, which provided an opportunity for further checks of the
instrument.
For the HER, a light-level dependence was seen, with some possible sources analyzed. Also a
possible x-y tilt at the source point was observed, which is different from the LER. The analysesof this problem are reasonable, and more efforts will be made in the near future.
The following will be done next: examining masks after exposure to the beam, improving
vacuum pumping ability around the optics box and getting faster pixel readout.
Interferometers and streak camera are set up for measuring transverse beam sizes usingsynchrotron radiation light in both LER and HER. Diamond mirrors for visible light monitors
were installed in the extraction chambers, and well aligned. The measurement results are
displayed in the control room, which is of great help in the commissioning.
In the calibration of SRM, the ratio of slit separations between interferometer slit plane andextraction mirror plane is about a factor of two larger than expected. Analysis is ongoing andmore measurements are needed to understand this discrepancy. Bunch lengthening wasmeasured in both LER and HER, but the measurement errors are large for all bunch currents.
Raising the light intensity will be a way to lower these errors. A shift of the optical axis isobserved in the HER, but not in the LER. All the problems found during the system setup andcalibration are now being investigated.
The LABM system was installed at the IR, with the DAQ and EPICS records developed for thePhase 1 commissioning. Some measurements were performed using the synchrotron light fromfar side of the IP. Work for the LABM transition from Phase 1 to Phase 2 is being carried out.

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Recommendations:
R34: In view of a possible coupled bunch instability, using a large bunch spacing for themeasurement of bunch sizes in the HER with the XRM might minimize the beam current
dependence.
R35: Continue to study the problems encountered in the calibration of the SRM and LABM,
and be ready for the Phase 2 commissioning.

10. Operation Status of RF System
The refurbished and reconfigured RF systems are working well and supported all the Phase 1
goals. Some nine stations are using the new uTCA based digital LLRF system. This is also
working well and includes built in diagnostics such as fault logs etc. that will maketroubleshooting and analysis of trips much easier. The old analog LLRF systems are stillworking well but it is planned to phase them out as the new LLRF system is implemented.
New analysis and simulations of the gap transients suggest that partial filling of the gap andpossibly altering the overlap of the gaps between the two rings might reduce the residual error
in the collision point to less than 0.5 degrees. The ringing of the transient caused by the threecavity ARES system is reproduced in the simulation. The mix of NCRF and SCRF cavities inthe HER results in a different shape of the transient compared to the LER. This is one reason
for the residual error. It is not clear if this small residual error will cause significant loss inluminosity, but the new collision scheme will be more sensitive than the old KEK-B. The bunch
top off scheme should be able to keep the fill pattern stable even if some bunches on either sideof the gap experience shorter lifetimes. In addition it may be possible to adaptively modify thefill profile in order to learn and minimize the residual transient by controlling individual bunch
intensities. It may also be possible to correct the residual error with RF feed-forward if there issufficient headroom available in the systems.
Recommendations:
R36: Continue the transient analysis, quantify the effect on luminosity from the residual
mismatch between the rings. Continue to study the influence of fill pattern shaping or
adaptive feed-forward on the residual error.
R37: During beam tests in Phase 1, inject a bunch train with the largest number of bunches atthe correct spacing and full design bunch charge.

11. Normal-Conducting RF Cavities for SuperKEKB / MR & DR
The reconfigured NCRF ARES cavities have performed well during the Phase 1 run. Some
issues have been found including one ARES cavity that had a crack near the coupler flange.
This was sealed with an O-ring and continued operation. It will be repaired or replaced before
Phase 2 operation. There have been several coupler arcs and the couplers are being monitoredby cameras in addition to the normal arc detectors.
At the last review it was reported that 13 couplers were conditioned off line to 850 kW. This is
still the same number today, while there are 5 more waiting to be conditioned. The installed
couplers have not seen maximum power in Phase 1 because of the low beam current during
scrubbing. It may be possible to push individual cavities by phasing, to increase the demand
while reducing others. This could be a good test to demonstrate robust coupler operation before

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full current is available and test the couplers for any weaknesses. With care this could be doneparasitically during scrubbing.
The chillers for the NCRF systems have been replaced but there is an issue with the anticorrosion additive in the new systems precipitating out. This has caused down time due toclogged filters. One possibility is low pH caused by exposure to the tunnel environment
affecting the chemistry of the water system. Similar problems in PEP-II were solved byintroducing a nitrogen blanket over the water system to exclude oxygen.
Two cavities are parked in LER to save electricity, another two are off due to faults. Detuningof these parked cavities is important because they can drive low-order coupled-bunch modes.
The -1 and -2 modes have already been seen during commissioning. Balancing pairs of cavities
detuned in opposite directions may be one way of minimizing this problem. Other coupledbunch instabilities have been observed, possibly due to the additional impedance in the LER
due to cavity relocation, but have been well controlled by the bunch to bunch feedback system.
Most of the scrubbing has been performed in the every third bucked filled pattern. As vacuum
conditions improve it would be good to attempt high current running in the design fill pattern.
Recommendations:
R38: Consider gradually pushing individual RF systems up to high beam loading power to testfor robustness. This could be done parasitically during scrubbing or as a dedicatedexperiment at the end of Phase 1. If there is enough voltage headroom some additional
stations could be turned off. This could increase power in the remaining stations and may
improve klystron efficiency, saving electricity costs.
R39: Optimize the detuning of parked cavities to minimize excitation of low mode coupled
bunch instabilities. Consider parking cavities in pairs, detuned in opposite directions.
R40: As opportunities allow, continue conditioning the remainder of the couplers to full power.

12. Integrated vacuum control system for RF and Operation status of SCC
The new integrated vacuum control for the LLRF system is operational in the D4 and D5 areasand performs as expected. It is based on PLC embedded with ladder-CPU and EPICS-IOC andits purpose is the protection of the RF cavities against excessive pressure increase. The PLC
based system is flexible and can be extended to other cavities and vacuum components. The
pressure-rise alarm is sent to the beam abort and vacuum control systems. No issue is identified.
In general the RF characteristics of the SCC modules have been preserved after the long shutdown. The operation of the SCC modules is stable and fulfils the requirement for Phase 1. Eightsuperconducting cavities, inherited from KEKB, are installed in the HER. Despite the higher
beam power required for SuperKEKB, by the help of the ARES cavities, the beam loading is
kept similar to the one of the previous accelerator.
Two issues have been identified and addressed:• New high power SiC beamline absorbers are needed in between the cavities for the
expected higher HOM power at full current. These have not yet been installed but can be
added outside of the gate valves, so that there is no need for venting and the cavity
conditioning should not be affected. The implementation will be carried out either for
Phase 2 or Phase 3. The cleanliness of this installation will be very important to avoid
introducing new sources of particulates close to the cavities.

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• Two cavities have shown a degraded performance (i.e. a reduced Q at the operational
field or a reduced gradient). This is attributed to particulate contamination. One cavity is
known to have suffered a vacuum leak. This cavity was recovered by horizontal highpressure rinsing (HPR) without any need for module disassembly except for coupler and
HOM dampers. The second cavity may be exchanged against the spare prior to Phase 2.
More conditioning time may help. In case the spare cavity will eventually be installed
the cavity that is taken out should be refurbished as a usable spare.
Beam aborts due to RF trips have been reported 7 times during the last 4 months (quenches incavities and discharges in couplers). Intriguingly, it was observed that the cavity trip rateincreased after experiments with vacuum bumps were performed elsewhere in the ring (to study
backgrounds in the BEAST II detector). Pressure at the cavities was not seen to rise, so that theexact cause is unclear.
Problems have been experienced with some piezoelectric tuners (failure due to electricalbreakdown). The Piezo manufacturer suspects that rapid voltage fluctuations as seen in thisapplication may be harmful to the piezo actuators. Filtering with a decreased voltage-change
rate will be tried to mitigate this effect. In the medium term, an alternative supplier could beselected.
New vacuum interlocks have been developed to monitor and protect the SCRF cavities. These
have been installed and can quickly turn off the RF in case of vacuum transients close to thecavities. Such vacuum transients always are a concern for the RF couplers. The protection of
the latter will be even more important at the new high power level.
Recommendations:
R41: We recommend that the planning for the Q recovery of the affected SC RF cavities isdetailed and integrated into the global planning of Phase 2 or Phase 3. Consider trying to
improve the performance of degraded cavities by horizontal HPR (as alreadydemonstrated) or by further in-situ conditioning.
R42: It would be useful to study if the availability of additional spare modules is necessary
and the actions to tackle in the short term for achieving a good availability in Phase 3.
R43: The increase of the trip rate after the pressure bumps needed for the BEAST study is
puzzling. We recommend that this correlation is studied in details so that possible issues
are identified (dust generation?).
R44: If the spare cavity is installed then consider timely refurbishment of the extracted cavity
to be returned as a usable operational spare.
R45: Develop an installation procedure for the new beamline HOM loads so as to minimize
the risk of introducing new sources of particulates.

13. Beast II (Phase 1), beam background
The aim of the BEAST II detectors is to measure the background signals from the beam during
scrubbing and to compare these readings with expectations from simulations. This is in orderto validate the simulation model of the IR and the installed detectors. In addition, the team is
attempting to separate out various background sources and see if installed collimators are
effective in controlling some of these backgrounds. The BEAST team has negotiated special
running times for specific running conditions with the accelerator team. The organization has

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been impressive and the coordination with the accelerator team is very well planned. Some
excellent measurements have been made and the team has some very interesting data to
analyze. This effort should be adequate to validate the simulation as was hoped. The BEAST
team has acquired significant experience in collecting data from various detectors and thisexpertise will be very important when the Phase 2 part of BEAST II is installed in the detector
and the real machine will start up. It will become important for the team to quickly collect andanalyze new data in order to make sure there are no unexpected background surprises. The timeinterval between Phase 2 and Phase 3 is not very long and there will be very little time torespond to surprises.
Recommendations:R46: Suggest the BEAST team to use the experience it gains from Phase 1 running to improvetheir efficiency in analyzing new data so that they will get information about backgroundsources from the Phase 2 data promptly.
R47: Additional data could be obtained by inducing aperture limits with closed orbit bumps orcollimators around the IP, anticipating reduced apertures with the low beta optics for datataking.
R48: The loss calculations should include deteriorating effects from linear errors (orbit, betabeat) and reduced available aperture.

14. Ring magnet system
As the most important system in Super KEKB, main magnet installation during last year was
successful, and there have been great achievements on magnet survey and alignment in
anticipation of the Phase 1 commissioning. All this was very clearly expressed in thepresentation, with many detailed results from magnet installation, survey and alignment, as
well as magnetic field measurements. A lot of work is still waiting for the team in preparationof Phase 2, in particular the installation and alignment of QCS in the IR.
Quite big changes have been made to the main magnet system in SuperKEKB compared withthe one of KEKB. In order to get lower beam emittances, new long bends replaced the original
KEKB bends in the LER, and 280 new wigglers have been installed in the LER. A total of 36
new wigglers were added to the HER. About 220 new vertical steering magnets, together with~300 modified horizontal steering magnets were installed in both rings. All this installation
work was successfully finished by May 2015, with vacuum chambers installed in the Northtunnel. Checks of magnets and related power converters plus cables were carefully done.
A survey of the main magnets along both LER and HER has been performed, with the results
cross-checked by 3 teams. A deformation of the rings has caused its Northern and Westernparts to expand towards the outside by up to about 10 mm, yielding a smooth, low order
perturbation. Data analysis of the tunnel motion reveals a correlation with earthquake activity.
A continual tunnel sinking is observed, enhanced after the big earthquake in 2011. The rings
are larger than the design. The expected 16.4 mm difference between the LERs of SuperKEKB
and KEKB was confirmed after the beam injection in February 2016. The motion of the tunnel
is also monitored by a hydrostatic leveling sensor (HLS) system.
Crosstalk between adjacent steering magnet and quadrupole was measured. The results could
be used to improve the magnet models.

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The magnet power supplies for SuperKEKB are a mix of refurbished converters from KEKB,
and a smaller number of newly fabricated supplies. One important item of this last group is a
new type of QCS supply.
The successful machine start up is a result of the care and careful work of the power-convertergroup. The smooth machine start up is built on the careful testing and characterization of themany power supplies, cables, sensors, software codes, etc. A few technical issues wereencountered during the testing and checkout. The presentation included some test data. The
histograms of the measured output currents demonstrate that the ripple and resolution of themain ring supplies is at the several PPM level. This impressive result testifies to the care in
design and fabrication.
The new QCS supplies were designed and a prototype system fabricated plus tested. The design
has implemented a hybrid analog-digital regulator, where an analog PWM system runs closed
loop with a setpoint that is adjusted via a digital feedback from an 8 1/2 digit DVM. Resultsfrom this prototype reveal the action of the digital feedback to reduce current drifts and ripple
to the 2 PPM level over 8 hours. This is a significant result.
One question that was not clear from the presentation is what physics requirements set the
specifications for the power supply noise and ripple? Is there a specification for each class of
supply for long-term stability, harmonic content or broadband noise in some bandwidth? Eachclass of supply has been tested. Our question is how well the power supplies meet the necessary
specifications.
Recommendations:
R49: Simulation of beam orbit distortions due to the misalignments determined by the survey
could be compared with the orbit measurements from Phase 1 commissioning.
R50: The differences between the magnetic field measurement results made in theexperimental hall and in the IR (Tsukuba) should be considered.
R51: Close cooperation with the accelerator physics group will help to understand the resultsof the survey and the magnetic field measurement as well as their implications.
R52: Analyse the failure causes for the various power supplies to determine any common
factors that can be addressed before run 2 to reduce the impact on running time.
R53: Compare the achieved power-converter performance with the specifications.

15. Beam Transport and Injection
The beam transport (BT) system connects the end of the linac to the LER, the HER as well as
to the PF-AR. The beam line system for injection into the LER/HER consists of a total of 6
arcs. Half of the BT lines have a double-story structure (for the electron and positron beams).
Prior to the last arc, sections with a significant download slope transport the two beams,
separately, to the KEKB collider tunnel, which is located 11 m underground. The BR comprises
about 60 dipoles and 50 quadrupole magnets each, for the electron and positron beams. In
addition 3+3 kickers and 4 (or 2) septa are used for injecting the electrons (or positrons). Two
sextupole magnets for each beam will be used in phase 2 to correct the higher orderdispersion.in addition to 60 BPMs each, the beam lines are equipped with 20 screen monitors,
as well as several wire scanners and collimators.
Upgrades for SuperKEKB included (1) an increase of the positron magnet strengths requiredby the higher beam energy, and (2) new septa. The change of beam energy also necessitated a
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change of the layout of the beam switchyard 3. Some dipoles for the positron required new
power supplies for the higher beam energy. Another solution was applied in arcs 2 and 3: here,
the gaps of the dipoles were narrowed, which avoided upgrading their power supplies or watercooling. The beam switchyard is directly connected to an energy compression system for the
positrons. The latter is also being modified for the higher beam energy, with construction stillongoing. To reduce the injection error, the effective septum thickness was lowered from 5 to
3.5 mm (only for HER in Phase-1, and for LER in Phase-2).
A large part of the new BT has been commissioned. The positron beam has a huge emittance,since the damping ring is not yet in operation, and the positron beam comes directly from theflux concentrator. A large part of the positrons is lost at collimators (used for cavity protection)and in the linac. Some 10% of electrons and (remaining) positrons may be lost in the BT, but
the decrease in charge may be within the large error bars of the measurement. The injection
efficiency is close to 100%, for the positrons, and fluctuates, with an average perhaps around
50% for the electrons. The electron injection efficiency is affected by a large vertical orbit jitterat the end of the BT, of a few mm peak-to-peak. Groups of 4 wire scanners are installed atseveral locations in the linac and the BT. These wire scanners are used for emittancemeasurements and optics matching. Wire scan measurements in the BT indicate a spurious
horizontal dispersion and also a vertical jitter coming from the linac. Measurements were
performed for the beam from the thermionic gun and from the RF gun. The latter produces asmaller emittance. The vertical jitter was found to be about the same, i.e. independent of theelectron of the gun. The jitter corresponds to about 1 sigma of the beam size from the RF gun.
The measured electron-beam emittance is about 100-200 μm, and therefore 5-10 times above
the design value (20 μm). The measured emittance increases in the BT compared with the
upstream linac (by about a factor of 2), which is attributed to spurious dispersion.
Some technical issues were encountered during the commissioning. A water interlock trips the
dipole magnets 2-3 times per week, due to filter clogging by CuO dissolved from the coolingwater pipes. Also the water interlock is triggered for some of the dipoles whose gap was
narrowed, due to the increased leakage magnetic field (20~30 Gauss) and/or due to aging of
the switches.
An abnormal x-y coupling and occurrence of unexpected vertical dispersion is observed in thepositron line, between arcs 2 and 3. Possible explanations like quadrupole rolls and sextupole
components in the dipoles have been suggested, but none of them appears realistic.
On the whole, the first results of the BT commissioning satisfy the demands of Phase 1.
Recommendations:
R54: The origin of the abnormal x-y coupling and vertical dispersion in the e+ BT line should
be understood.
R55: The water interlock problems need to be addressed (perhaps involving shielding of theleakage field and replacement of the switches).
R56: The leakage dispersion seen at the BT wire scanners should be corrected.
R57: Accurate monitoring of charge transmission in the BT lines should be made available.

16. Beam Abort System
The SuperKEKB beam abort system is challenged by the increased beam power and theshortening of the abort gap. A thorough upgrade of the system has been put in place to cope

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with the new requirements. Previous damage tests have been used to define the allowed current
density in the extraction window, which determines the kicker sweep parameters. The ceramicchamber could not be coated with vapor and, instead, the sputtering technique has been used.
The abort system has been equipped with appropriate diagnostics to ensure it is correctly
functioning. In particular, the extracted beam orbit and beam size have been experimentally
verified as expected. The temperature increase due to image currents has been verified to bewithin tolerances. The most critical system failure modes have been identified to be the verticalkicker discharge and circuit fault. The probability of these failure modes are suppressed by
duplicating the electrical circuit.
Recommendations:
R58: Revisit the horizontal capacitor breakdown (discharge) failure mode to, hopefully,
discard the possibility of damage.
R59: Investigate the source of the losses towards the end of the train.

17. Controls
The SuperKEKB Control systems are mature systems based on well-developed and wellfunctioning KEKB systems. The rapid machine commissioning of the last year was an exampleof the excellent technology base in the machine controls and experienced highly-skilled staff.
The use of commercial networks, commercial computer systems and extensive standard
software platforms means that the transition of the KEKB system to the SuperKEKB era canbe incremental and allows the existing expertise to be brought to new accelerator systems.
The upgrades center on higher-performance networks, host computers and new IOCs. Many of
the original KEKB sub-system interfaces, such as for power supplies and the VME sub-systems
are carried over. In future years there can be upgrades to some hardware backplanes. The
reviewers do not think there is urgency in replacing the operating I/O subsystems. As the newIOCs and backplanes are implemented the EPICS structure facilitates integration and testingof new hardware as incremental upgrades.
The Fault files and data logging features have been expanded, and upgraded. The Alarm andmonitoring system has also been upgraded using EPICS functions.
The new abort trigger system with new modules is partially commissioned. The newfunctionality is very helpful and will be fully implemented in later machine phases.
Recommendations:
R60: Develop in situ test methods for the beam abort modules, and have a robust automated
way to be sure the system is properly configured and ready to function when it is reallyneeded. In addition, there should be redundant triggers to the abort system.
R61: Can physicist staff write python scripts for new diagnostics or machine measurements?
Does this help with commissioning?
18. Beam Injection Controls
The presentation showed the timing system and LINAC injection had excellent operational
readiness and these critical systems were stable in the first commissioning – a great job.

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The group can be proud that they were ready to accommodate the first commissioning needs,
including the beam scrubbing top-up tools.
The Event timing system is complex and uses new modules. We stress the importance of goodtools to write timing programs, to efficiently operate and understand the timing system, and toallow for rapid integration with many accelerator sub-systems.
The presentation highlighted some of the tricky issues in synchronizing the various oscillators.
Some functions were not completely integrated between LINAC and Ring timing needs. A new
system configuration used during the Phase 1 commissioning was shown, but the future
integration of the damping rings, including techniques to efficiently inject and extract, and tosynchronize to the main ring, etc. still need to be developed.
Regarding diagnostics, timing programs using TDC functions have been tested. Can these be
run in the background? Maintenance issues with cables, etc., need to be considered. Some
members of the committee were concerned that the module fibers, cables etc. might get
misconnected in some late-night diagnostic or maintenance situation. Any real time changes
should be documented.
Recommendation:
R62: It is critical to check the timing programs, and to use the TDC as operationaldiagnostics.
R63: Begin coordination process with damping ring needs; plan now for later integrated
operational phases.

19. RF Gun Report
The RF guns, cathodes and lasers have been followed by a separate Review Committee, theSuperKEKB RF Gun Committee, which has been meeting every six months. The reports fromthe most recent three meetings were presented to the Accelerator Review Committee. The Gun
Committee established a “Philosophy:”•

Any component integrated into the accelerator must be reliable, maintainable, andreproducible.

•

Start with a simple, robust system. Improve it and add features step by step afterwards.

•

Cost is not irrelevant.

The Gun Committee has consistently recommended to reduce the number of options being
pursued in parallel, and this advice has not always been followed. At this time, there are two
RF guns (one of which, the Quasi-Traveling-Wave RF Gun, has now delivered beam
successfully through the linac to the SuperKEKB rings), at least three lasers and several
cathodes. The Accelerator Review Committee is also concerned with the proliferation of
options. We would like to see better prioritization of the upcoming tasks that includes all items
required to meet the parameters of Phase 2 and Phase 3. This should also include diagnostics
for determining that the gun is delivering the required performance as well as software andscreens for the operators to ensure that the gun is working correctly with the required stability.
Recommendations:
R64: Follow the recommendations of the SuperKEKB RF Gun Committee.
R65: Provide a detailed prioritized list of tasks required for Phase 2 and a second prioritizedlist of tasks required for Phase 3.
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20. Injector Linac Overview
The overall progress on the injector system for SuperKEKB has been good. Upgrades on photoguns, positron source, linac controls, linac stability, emittance wire scanners, and utilities havealready been made. More improvements are coming including a new damping ring, RF gun
and positron target upgrades. During Phase 1 beams were successfully injected into
SuperKEKB from both the thermionic and the RF gun.
Ultimately, the injector must keep the SuperKEKB beam currents constant with ring lifetimesof about 6 minutes. Twice the beam currents are needed compared to KEKB. For the newcollider, the injected beams have to be more stable, of higher charge, and of smaller emittances.
In a few years, beam injection into five rings on site will be needed, varying pulse by pulse.
An event generator for the linac is needed to control the generation of bunches to be injected
into specific SuperKEKB buckets. The control of the bunch timing for injection into the rightbuckets of the SuperKEKB rings will need more work on the low level RF LLRF system forthe linac, especially to help with synchronization delays longer than 2 milliseconds. A reporton the status and improvements of the LLRF should be given at the next meeting.
After the Damping Ring is on-line, bunch trigger timing changes from 490 to 2000
microseconds are needed. There should be simulations to determine which timing requirements
are needed and what errors can be tolerated.
The tolerances for beam pulse to pulse jitter in position, angle, emittance, and charge that canbe accepted into SuperKEKB and the photon factories should be tabulated. Indications fromrecent measurements are that the beam position jitter and pulse-by-pulse charge jitter certainly
need work in the linac.
The KEK light sources will need top up injection over the next few years. Though this task
should be significantly easier than the injection needed for SuperKEKB, plans are needed forthe light sources that are compatible with SuperKEKB injection.
Recommendations:
R66: Identify the solution for the Low-Level RF (LLRF) system upgrades required andformulate the specifications.
R67: Determine if the longitudinal location of the positron bunches in the Damping Ring canbe shifted during the storage damping time to facilitate the bunch timing for injection into
SuperKEKB. (This was done successfully and easily in the SLAC damping rings.)
R68: Continue the work on reducing linac beam jitter sources to bring them to the needed
specifications.
R69: The project should continue to monitor the linac upgrade schedules as there is much to
do and the time to perform the work is short while complicated by required injection during
the photon factory operating periods.

21. RF Gun and Electron Beam
Great progress has been made since the last review. The QTW RF gun has been installed andcommissioned and delivered beam to the HER for extended periods during Phase 1 running.
There is some evidence that the emittance from the RF gun is significantly smaller than thethermionic gun as expected but there is a large residual jitter in the horizontal plane. This ispossibly coming from laser pointing jitter since the laser comes in at a wide angle in the

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horizontal plane, but this is not well understood and is currently still under investigation. The
simplified drive laser has worked well in operation, running for long periods without
intervention. R&D on pulse shaping has been deferred until later to concentrate on making thebasic system robust for operations.
The new Ir5Ce cathode has performed well and a new press-fit cathode holder has eliminatedthe breakdown problems experienced with the previous design. A new Ir7Ce2 cathode,possibly a single crystal, may improve the quantum efficiency and provide better uniformity.
In parallel another new RF gun has been developed. The cut disk RF gun has a simpler
geometry and a large enough aperture for normal illumination of the cathode. This may reduce
the jitter if it is indeed coming from the laser transverse fluctuations. This gun will be installed
off to the side on the 45° line during the summer shutdown to allow further testing.
The RF gun has not yet produced high enough bunch charge to be used for the positron
production. The thermionic gun has performed this task well during Phase 1 and will remainin place until one or other of the RF guns has proven capability.
Recommendations:
R70: Maintain focus on stable operation of the RF gun and laser system to support the Phase
2 run. Work with whole injector and linac team to understand and mitigate the sourcesof jitter.

22. Laser and RF Gun
The present laser system of the RF gun was discussed, along with planned upgrades for
obtaining the higher charge and repetition rates required for Phases 2 and 3. All laser systems
are original developments from KEK. The final SuperKEKB laser should deliver a pulse energyof 50 or 500 microJoule depending on the cathode material (Ce2Te or Ir5Ce, respectively). It
must produce two bunches, spaced by 96 ns, at 50 Hz, allow for temporal pulse shaping andfor continuous operation.
The laser employed for the Phase 1 operation.is a Yb doped solid state laser, realized as a
hybrid system which combines fiber-based amplifiers and Yb:YAG thin-disk multi-pass
amplifiers at a wavelength of 1030 nm. The development of this underground laser is complete.
The stability of the laser is acceptable. A spectral range adjustment allows temporal pulse
shaping. The pulse shape was optimized with the help of a streak camera.
The temperature increase of the thin-disk amplifier is limiting the repetition rate of the present
laser to 25 Hz, where the pulse energy at the fundamental frequency and at the wanted 4th
harmonic (generated by a nonlinear crystal) reaches a maximum value of 10 mJ and 230
microJoule, respectively.
Monitoring the stability of the laser pulse energy reveals a power variation at the level of 10%
on short time scales, along with a smaller long-time drift. The effect of regular elevator
maintenance can also be noticed in the history.
It is difficult to meet the 50 Hz double bunch repetition rate with the existing system. A new,upgraded laser is being developed for the future higher power operation. Here, after the fiber
amplifier, the light is split into two paths, towards a fiber + Yb:YAG thin-disk and a fiber +Nd:YAG rod regenerative amplifier, at wavelengths of 1030 and 1064 nm, respectively. The
Menlo system commercial oscillator is available only for the 1030 nm of the Yb:YAG laser.
Chirp pulse amplification is used to compress the Yb:YAG pulse to a length of 30 ps. Lowering

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the temperature of the Yb:YAG thin disk laser to a few tens of centigrade below zero increases
the amplifier gain, reduces the noise level, and yields a more stable signal. A 50 Hz test
operation at low temperature has already been done. A plan A and a simpler plan B have been
prepared for the future evolution of the laser system.
The reported laser technology is complex, with several systems being developed in parallel,
and multiple reviews in the past year. With one short presentation, we cannot try to advise orpretend to have the expert knowledge from the system reviews. Instead, we wish to highlightthe need to begin to integrate this system as part of the operational SuperKEKB facility.
The diagnostics need to be integrated with the operational needs. A photo-diode and a scope is
not a path to robust operations. What technical specifications for pulse temporal shape, energy,
jitter and controlled parameters drive the operational needs? What diagnostics are needed while
the system is in development? And in commissioning? And in operation? How much can be
automated? Can one write log files?
As an example, if the system incorporates feedback regulators, a logging system that samples
the feedback actuator/correction signals over time, and allows correlations of disturbances
against other machine states, might be very useful. Similarly, knowing the typical range of the
feedback actuators may be insightful when in operation they are about to exceed their limits
because of some unexpected disturbance. It is good to know something is amiss before it
becomes a loss of the injector.
Recommendations:
R71: One person should develop the user control strategy and interface that will be needed byoperations now – and start to understand necessary functions for operations as well as
system experts. The system and technology will need to transition away from a 10 person
R&D project towards a day to day operational system, critical for daily use.
R72: Visit a lab with an operational laser injector, see how it is operated and what experts do,what operators do. Start to develop procedures for commissioning and operation with thislaser gun system.
R73: Specify the operational diagnostics and a commissioning plan. What instruments or test
equipment is needed to become part of an operational injector? Clarify the specifications
for the SuperKEKB injector (e.g. charge stability).

23. Positron Generation and Beam
For the Phase 1 commissioning, the Positron Source is used without the Positron Damping
Ring (DR). Hence the full potential of its upgrade is not yet available. On the other hand, theperiod of 2015 till 2017 can be used for step-by-step testing of key components of the positron
production chain.
Last year the operation started with limited performance: the Flux Concentrator (FC) used forcollecting the positrons worked at 50% of its design current (6 kA instead of 12 kA) with atemporary pulse modulator, the DC solenoid current was 370 A (instead of 650 A), theaccelerating voltage in the capture section was 10 MV/m (design target 14 MV/m). As a result,a positron yield of only 20% was obtained at beam switchyard 2, at the entrance of the DR
injection line. The first positron charge available for SuperKEKB was only 0.12 nC per bunch.
A number of technical problems were encountered and resolved, resulting in implementing the
FC tri-plate feeder line, new coaxial cables, the snubber circuit, and a full-scale 12 kA pulse

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modulator. These enabled the successful test run of the spare FC head #4 at the full
specification of 12 kA (but without the bridge coils) for continuous 200 hours.
By the end of last summer these new components were installed for beam operation in the linac
tunnel, together with the FC assembly #1 (head #3), bridge coils, DC solenoids and capture
accelerating structure already in place. The FC processing, with a bridge coil current of 750 A,
was interrupted, last September, by an FC breakdown resulting in an unrecoverable damage.
The FC current was only 9.8 kA. However, in previous tests without the bridge coils, the FC
was stable even at 11.5 kA.
The solution suggested by SLAC positron experts was to apply “work hardening” to the OFC
copper block of the FC. The multi-cycle repeated plastic deformation of copper would resultin stability of the spiral gap, and its spacing is expected to recover after breakdown. The crucial
tests are planned for this summer (1) with the FC head #4, not “work hardened”, trying to
reproduce the breakdown conditions, and (2) with the newly made FC head #5 to be tested for
operability after the “work hardening” at full current and with the bridge coils at their nominal
current.
After the beam tuning at the end of last year, with an FC current of only 6 kA, the positronyield was raised up to 30%, and the positron charge was 1.9 nC at the beam switch yard 2. The
yield enhancement from the FC was only 1.8.
A complete simulation of the positron yield including all operational conditions is being
prepared. At present the expected loss in the positron yield caused by the limited performanceof almost all components of the Positron Source is not yet known.
The positron injection conditions now are as follows: 7 nC electrons at the target; 2 nC
positrons at the DR injection line entrance (actually, 1.3 nC due to collimators), 0.75 nC at the
linac end, and 0.3 nC at the Beam Transport line end, the latter two intensity drops are causedby beam acceptance limitations when the direct injection is performed without the DR. Since
February 2016 stable injection into the LER was achieved with a positron charge 0.3 nC ineach of 2 bunches.
According to the plan, after testing, the new FC assembly with “work-hardened” copper head
will be installed in the beam line by the end of 2016. DR commissioning and LER injection forPhase 2 will start in November 2017.
The Committee is satisfied with the efforts of the Positron Source team to overcome someserious problems and yet to supply the LER with enough positrons for the Phase 1
commissioning. A few comments in conclusion: i) restoring the old linac injector called the“double deck’ is a good addition and helps with operational flexibility; ii) the work on
increasing the Flux Concentrator current is very good and has shown promising progress forenhancement of the positron yield; iii) the new effort to implement the “work-hardening” of
the Flux Concentrator should help a lot.
Recommendations:
R74: A simulation of all the effects influencing the positron yield should be finished to estimate
the expected beam losses and their distribution after the target.
R75: The linac group should estimate the losses of positrons after the target while beingtransported to and injected into the positron Damping Ring.

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24. Injector Alignment
Achievements and status of the injector alignment were presented. The alignment of theinjector linac is important for delivering a high quality beam into SuperKEKB. The target
emittance of 20 mm-mrad requires the local rms alignment to be better than 0.1 mm, and theglobal rms alignment to be better than 0.3 mm.
The alignment methods rely on a 500 m long straight laser reference line, laser trackers andtargets on walls, girders and machine elements. In January 2015 a complete injector alignment
was done to an accuracy of better than 0.1 mm. The hardware alignment on the girders hasremained within 0.1 mm and a standard deviation of about 0.05 mm. Some minor problems
were found on the girders but the overall stability is as required.
Radiation damage to the alignment system has been observed in the collimation sections. The
team is looking for alternative equipment with better radiation hardness.
Alignment surveys for the linac have been performed regularly since 2015. It was found thatthe linac alignment has deteriorated since 2015 with deviations reaching up to 1 mm. The worst
drifts occur at the expansion joints, where changes of up to 0.5 mm per year were observed,
influenced by weather, temperature in the tunnel, flow of underground water, etc. There also
seems to be a yearly cycle. Simulations are being performed to better understand the actualalignment requirements.
The Phase 1 studies will be continued during Phase 2 commissioning and will be complemented
by studies on beam effects of misalignments, beam-based cures and studies on the feasibilityof magnet movers. For Phase 3 commissioning it is envisaged to have either active or passivealignment systems in place, to steer to a golden orbit that minimizes emittance growth, and todevelop compensation methods like offset injection.
The committee notes the very good work done on monitoring the injector alignment and on
identifying critical issues.
Recommendations:
R76: Perform a detailed analysis of measured and simulated emittance growth, taking into
account alignment errors. Review alignment tolerances in view of the observed emittance
growth.
R77: Study methods of beam-based alignment and the required hardware for achieving andmaintaining the alignment within tolerances. Closely couple these studies to the work on
the injector commissioning and the team pursuing this.
R78: Continue the studies on radiation hardness for alignment equipment.

25. Injector Commissioning (Phase 1 &2)
The results from the injector commissioning were presented. The 600 m long injector must
provide beams of electrons and positrons for four (or five) different storage rings. The
transverse emittance for SuperKEKB must be a factor 15–70 reduced with respect to KEKB.
Precise trajectory measurements and control as well as simultaneous top up operation are
required. After the positron target and up to the end of sector 2 the electron and positron beams
must be transported through quadrupoles with the same DC fields. Pulsed correctors areavailable for independent e+/e- trajectory correction.

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The main ring injection both into HER and LER started in this February with the thermionic egun. The RF gun was used on May 31, 2016 with success at the first attempt. The overall
injection efficiency is 10–20% without bunch compression and increases to 70–100% withbunch compression. The normalized emittances with the thermionic gun at the end of the linac
are 160/300 mm-mrad (x/y e-) and 1000/1200 mm-mrad (x/y e+). With the RF gun earlymeasurements indicate linac emittances at sector 5 of 107/77 mm-mrad (x/y e-). This would
mean an emittance growth by 500-1000% for beam from the RF gun. Further studies areneeded.
The bunch charge fluctuation from the RF gun is about 4–5%, down from 20% in 2014. This
must be compared to 1%~2% stability with the thermionic e- gun. Fluctuations in beam
position with the thermionic gun vary from 0.1 mm to 1 mm, the horizontal fluctuations being
larger than the vertical.
The Committee congratulates the team for successful injection into SuperKEKB, enabling the
successful Phase 1 commissioning. The beam quality is being assessed in a professional andcareful way. Several limitations have been identified and are being properly addressed.
Recommendations:
R79: Commission the common electron/positron optics between the positron target and end of
sector 2.
R80: Perform a detailed analysis of normalized jitter, taking also into account the dispersion
and effects of energy jitter.
R81: Perform a detailed analysis of emittance growth and its sources. Develop correction andoptimization strategies for optics, energy, RF phase and trajectory.

26. Positron Damping Ring
All the design concepts of the Damping Ring have been defined and assessed since long time
and the construction is progressing at the right pace. We congratulate the team on theimpressive installation work that we had the opportunity to visit during the meeting.
All magnets in the transfer lines from (LTR) and to the linac (RTL) have been installed as wellas those in the DR. Power supplies and the related cabling are already in place and connected.
Vacuum chambers are already installed in the DR arcs and the ones of the straight session will
follow soon. RF systems have already been assembled in the tunnel for subsequent in-situ
checks; the installation will be complete in December. The presented planning foresees the
beginning of the beam vacuum pumping in one year’s time, and the beginning of the beamcommissioning four months later. The planning is tight and ambitious considering the lack of
manpower, which has affected the DR construction since the beginning of the project.
Still we notice how a detailed study aimed at integrating the RF systems and the injectionextraction kickers of the DR in the general timing has never been presented.
Two cavities have been conditioned off line and have recently been moved into the tunnel tobegin assembly of the RF station. Electropolishing was very beneficial in permitting rapid
conditioning. A few light emitting events were seen early in conditioning but the cavities
quickly attained full gradient. The off-line tests met all performance goals. The klystron andpower supply for the damping ring RF are in place. The RF system is expected to be fully readyto support commissioning later this year. The third damping ring cavity has been postponed

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since two cavities should be capable of providing all the voltage and power needed. It would
be good to have the third cavity as a spare when the machine is in full operation.
Recommendations:
R82: It is important that the DR is ready for Phase 2. We recommend to prepare a planaddressing a subset of activities essential to start the commissioning and at the same timecompatible and consistent with the assigned resources, in terms of funds and manpower.
Maintain schedule to support damping ring commissioning ahead of Phase 2.
R83: In order to let the DR accomplish its duty efficiently, a timing system properly integrated
with the corresponding devices of the linac and of the rings is required. We recommend
to prepare in time such DR timing developments.

27. IR Construction
A very good presentation was made concerning the IR construction. The work accomplished
has been remarkable. A plan has been made and the steps are well thought out on how toassemble the two final focus cryostats, the final vertex detector assembly and the main detector.
According to the schedule, the detector will roll on line this coming December. Then thegeneral assembly of the cryostats inside the main detector can begin. The team is waiting forthe second cryostat to arrive and, once here, will proceed to characterize and then prepare it forassembly. Phase 2, which will start in the fall of 2017, will not yet have the vertex assembly
but will have a suite of temporary detectors in the same volume to study detector backgrounds
prior to installing the vertex subsystems. The innermost vertex tracker (the PXD) is assembled
directly onto the central beam pipe and, hence, needs to have the final beam pipe early in orderto start a long, complicated and delicate assembly. This precludes any possible modifications
to the central beam pipe if some issues arise after Phase 2 background data is collected. The
connecting bellows sections between the cryostats and the central chamber still need to be madeand these are crucial elements of the assembly. It was also pointed out that because of the tightspace constraints there will be no vacuum pumping near the collision point which will result ina pressure bump at the central interaction point. This is not uncommon in this region near thecollision point where space is at a premium.
Recommendations:
R84: Try to ensure the second cryostat arrives on time. This will assist in getting the complete
assembly sequence thoroughly understood and tested.
R85: Try to get the connecting bellows sections fabricated promptly so they can be tested andcertified.

28. Overview of IR Magnets and Construction Status
The 8 main quadrupole magnets, the 8 leak field coils, and the 36 corrector coils are made, as
are 3 of the 4 compensation solenoids. The 4th solenoid is due to be delivered in July. The
electrical insulation problems encountered last year with the assembly of the magnets into the
QCSL cryostat were resolved and the complete unit was delivered to KEK in December 2015.
Since then the system cooled down using helium from a 1000 liter dewar. Now testing of thecomplete system is well advanced. All the magnets and correction windings have been powered
up to full current without problems (one magnet required 3 training quenches). The fields have
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been measured with harmonic coils and using stretched wires in order to verify the load lines
and to locate the alignment targets on the cryostat body. It was discovered that while the leakfield cancelling magnets took out the normal components, the skew components appear to bein the wrong direction. The reason for this, due to an apparent inversion of the coil is being
investigated. Uncorrected, this could lead to a halving of the Touschek lifetime, but according
to the beam optics group it should be possible to correct the global effect using devices outside
the IR region. The measurement campaign continues, and is now addressing the solenoids.
The QCSR cryostat is presently being equipped with its magnets, and should be delivered to
KEK in November this year.
Following the problems last year, it is reassuring to see the progress made with the QCSL
assembly. It is however regrettable that the cancel coil was installed reversed and every effort
should be made to find the cause so as to avoid a repetition of this, or a similar error on the
QCSR assembly.
There remains an immense amount of work to do on this system, and this will have to be donewith the utmost care at every step, given the difficulty in making repairs. The functioning of
the complete system cannot be fully verified until installation in the field of the BELLE
solenoid in phase 2 of the programme. The team is therefore encouraged to continue simulation
work, exploring all working (and failure) conditions.
Recommendations:
R86: Determine as to why the cancel coil was installed reversed, so as to avoid future
occurrence.
R87: Strengthen quality assurance of procedures for installation of magnets in the cryostat.
R88: Ensure redundancy of signal wiring.
R89: Compare measured quench behavior with that predicted by simulations.

29. Measurements of field quality for quadrupole magnets using harmonic coils
The array of superconducting quadrupoles which form the final focus system in the
SuperKEKB has high gradients (70 T/m) and tight tolerances on the higher-harmonic content(0.05%). A careful magnetic measurement has been performed in order to assure the target
field quality.
Rotating coil technique is used, with three types of different radius (12, 25 and 33 mm) coils
fitting different quadrupole sections. Each coil has a long winding to measure the field integral
and a short winding to scan the field profile longitudinally.
The QCSL magnets, including the leak-field compensator, are assembled and installed in their
final cryostats in the lab. The assembly is mounted on the moving stage, with appropriate
position monitor and inclinometer to control the magnet roll. Rotation is done by a motor witha rotary encoder.
The field integral measurement revealed deviations from the design by +0.08% for QC2LP and–2.69% for QC2LE. Each excitation curve was obtained, including the hysteresis (which,actually, is very small in the operation current range). The harmonic amplitudes are all below
0.05% with a reported measurement accuracy of 0.01%.

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The measured field profiles are in reasonable agreement with those calculated. Measurements
also revealed some skew and “forbidden” normal harmonics, whose effects on the beam
dynamics should be checked in simulations.
Recommendations:
R90: Clarify the sources of systematic errors affecting the harmonic content measurement, e.g.
the effect of the rotation axis offset or inclination with respect to the magnet axis.
R19: Consider calibration of the coils in a reference uniform-field magnet in order to avoid thesystematics.
R92: In the next stage of measurements, on the beam line together with the solenoid fields,
check the repeatability of the measured data, especially after a quench-recovery andcomplete warm-up–cool-down cycles.

30. Measurement of the magnet center with single stretched wire system
The stretched wire system enables the precise (± 0.1 mm) determination of the magnetic centerand angular orientations of a quadrupole. The method was pioneered and has been extensively
developed by Fermilab. A system was delivered to KEK in May and a Fermilab expert (Joe di
Marco) came to help with is use. Initial results show the centers of QC2LP and QC2LE to havehorizontal offsets of about 1 mm and vertical offsets of 0.1 to 0.6 mm with respect of the design
values, and roll-angle errors of about 2 mrad. Measurement results will be compared with thosecoming from the harmonic coils. This work is ongoing. It is important to realize that theapparatus itself is subject to harboring systematic errors and should be checked.
Recommendations:
R93: Verify the apparatus with a warm (resistive) quadrupole that can be measured physically.

31. Cryostat Performance
The final focus cryostats are extremely complicated and critical assemblies combining tightalignment tolerances, heavy shielding elements, large magnetic forces and over 50 separate
current connections. Enormous progress has been made since the last review. The QCS-L has
been tested off-line at KEK and has performed very well. Extensive magnetic field mapping isunder way. However, since the magnet is not connected to the final helium refrigerator, it has
to be tested in batch mode, with daily filling from a 1000 liter dewar. The LN2 shield was
initially configured with the flow in the opposite direction to the planned final installation. The
shield temperature was significantly higher than expected, which probably contributes to an
overall higher static load than design, although this is still within operating margin. The shieldflow will be put back to the design configuration in the near future to see if this improves thetemperature. The current leads are cooled by return gas. The temperature of the intercepting
shield and those of the cold mass mechanical supports were higher than expected. This could
affect the static load to the cold mass significantly. The current lead gas flows were set toachieve stable operation but there has not been sufficient time to optimize them. Once the shieldtemperature is reduced they may be fine-tuned to reduce overall helium consumption and toextend the batch running time.

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Very recent measurements have shown discrepancies in the electrical center of the magnets
between the stretched wire measurements, the rotating coil and the mechanical data. Further
investigation is ongoing. The QCS-R will be delivered in November and will only have warm
checks off-line before being installed and fully measured in the tunnel.
Recommendations:
R94: Continue measurements and optimization of the static loads to try to get closer to design
values.
R95: Continue magnetic field mapping measurements to understand the discrepancies in theelectrical center.
R96: Start to measure and evaluate the mechanical response with resonant modes in order tocompare with simulations.

32. Quench protection system for superconducting solenoids
The integrated field of the Belle II solenoid on the beams is cancelled by solenoids installed in
the cryostats housing the magnets of the low-beta insertion, and surrounding those magnets.
While for the solenoids ESR2 and 3 which have low inductance andwork at about 10% of
current carrying capacity, protection is trivial, that required for solenoids ESL and ESR1, which
provide the main compensating field and have inductances of 2.5 H and 8.8 H respectively,
requires careful attention. It is proposed to use the classical approach of detecting the quench
by monitoring the voltage across a coil, upon which a (solid state) circuit breaker opens so as
to allow a significant fraction of the stored energy to be extracted into an external resistor. The
value of the resistance is calculated to ensure that the voltage to ground does not exceed an
acceptable limit for the coil, in this case 200 V. Each of the solenoids is divided into 2 circuits,
powered by 2 power supplies, via 3 leads. The temperature of the quenched coil should notexceed 200 K. Calculations show that this scheme works for ESL, but that for ESR1 cold diodestacks must be installed across the coils to ensure that the temperature does not rise above avalue of 200 K.
Recommendations:
R97: Check that expected radiation level at the location of the cold diodes can be tolerated.
R98: Test the diodes in cryogenic conditions before installation.
R99: Foresee the possibility of replacing diodes in case of failure.

33. Tour (KEKB control room, MR tunnel, Linac, Damping ring, QCS)
The Committee was treated to a tour of the Main Control Room, the SuperKEKB tunnel, theInjector, the Damping Ring and the superconducting quadrupole. The careful engineering andthe quality of the installation were really impressive. The tour made it easier for the Committee
to visualize the topics that had been discussed during the presentations.
The Committee would like to thank the many members of the SuperKEKB team for taking thetime to show us the facilities.
Recommendations:
None.

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Appendix A
KEKB Accelerator Review Committee Members
Frank Zimmermann, Chair

CERN

Ralph Assmann

DESY

Paolo Chiggiato

CERN

John Fox

SLAC

Andrew Hutton

JLab

In Soo Ko

POSTECH (unable to attend)

Catia Milardi

INFN-LNF

Evgeny Perevedentsev

BINP

Matt Poelker

JLab (unable to attend)

Qing QinIHEP

Bob Rimmer

JLabJohn Seeman

SLAC

Michael Sullivan

SLAC

Tom Taylor

CERN (ret.)

Rogelio Tomas

CERN

Seiya Yamaguchi

KEK, Director of Acc. Laboratory, Ex Officio Member

Katsunobu Oide

KEK, Former Director of Acc. Lab, Ex Officio Member

Kazunori Akai

KEK, Head of Acc. Division III, Ex Officio Member

Kazuro Furukawa

KEK, Head of Acc. Division V, Ex Officio Member

Haruyo Koiso

KEK, Head of Acc. Division IV, Ex Officio Member

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Appendix B Agenda of the 21st KEKB Accelerator Review Committee
June 13 (Monday)
08:30 - 09:00 Executive Session
09:00 - 09:05 Welcome
09:05 - 09:45 Overview of SuperKEKB, Status and Plans
09:45 - 10:15 Belle II Physics and Construction Status
10:35 - 11:05 Overview of Phase I Commissioning
11:05 - 11:20 Overview of Lattice
11:20 - 11:45 Optics Correction and Low Emittance Tuning
11:45 - 12:25 Vacuum system, beam scrubbing
13:25 - 13:45 Instabilities: simulation and observation
13:45 - 14:10 Beam instrumentation and bunch feedback systems
14:10 - 14:25 Photon monitors
14:25 - 14:45 Operation Status of RF System
14:45 - 15:05 Normal-Conducting RF Cavities for SuperKEKB / MR
& DR
15:05 - 15:20 Integrated vacuum control system for RF and Operation
status of SCC
15:40 - 16:10 Beast II, beam background
16:10 - 16:50 Ring magnet system
16:50 - 17:05
17:05 - 17:20
17:20 - 17:35
17:35 - 17:50

Beam transport and injection
Beam abort system
ControlBeam Injection Control

June 14 (Tuesday)
08:30 - 09:00 Executive session
09:00 - 09:15 RF Gun Report
09:15 - 09:25 Injector Linac Overview
09:25 - 09:45 RF gun and electron beam
09:45 - 10:05 Laser and RF Gun
10:25 - 10:45 Positron generation and beam
10:45 - 11:05 Injector alignment
11:05 - 11:25 Injector commissioning (phase 1&2)
11:25 - 11:55 Positron Damping Ring
13:00 - 13:20 IR construction
13:20 - 13:40 Overview of IR magnets and construction status
13:40 - 13:55 Measurements of field quality for quadrupole magnets
using harmonic coils

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S. Yamaguchi
K. Akai
T. Browder
Y. Funakoshi
Y. Ohnishi
H. Sugimoto
Y. Suetsugu
K. OhmiM. Tobiyama
J. Flanagan
T. Kobayashi
T. AbeM. Nishiwaki
H. Nakayama
M. Masuzawa,T. OkiN. IidaT. Mimashi
T. Nakamura
H. KajiY. Honda
K. Furukawa
M. Yoshida
X. Zhou
T. Kamitani
T. HigoM. Satoh
M. Kikuchi
K. Kanazawa
N. Ohuchi
Y. Arimoto

13:55 - 14:10
14:10 - 14:25
14:25 - 14:40
15:00 - 17:30
18:30 - 19:00

Measurement of the magnet center with single stretchedwire system
Cryostat performance
Quench protection system for superconducting
solenoids
Tour (KEKB control room, MR tunnel, Linac, Damping
ring, QCS)
Report Writing / Executive Session

June 15 (Wednesday)
08:30 - 11:00 Executive Session / Report Writing
11:00 - 12:00 Close-out

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H. Iinuma
Z. Zong
X. Wang