The Twenty-Sixth KEKB Accelerator
Review Committee Report
19 January 2023

Introduction
The Twenty-Sixth KEKB Accelerator Review Committee meeting was held on 13-14 December
2022. Appendix A shows the present membership of the Committee. Seven committee members
attended the 26th meeting in person, and several others on zoom. The meeting was held in hybrid
mode and featured two days of oral presentations by KEKB staff members, plus discussions
between the Committee members.
The Committee welcomes the new Heads of KEK Accelerator Divisions III (Makoto Tobiyama,
formerly Head of Division IV), IV (Mika Masuzawa) and V (Hiroyasu Ego).
The agenda for the meeting is shown in Appendix B. The slides of the presentations are available
at https://www-kekb.kek.jp/MAC/2022/ .
By now, SuperKEKB has produced an integrated luminosity close to 0.5 ab -1. Since the 25th
review, SuperKEKB has set a new world record luminosity of 4.7x10 34 cm-2s-1, which is asignificant accomplishment, namely more than twice the previous KEKB record, but it still is morethan a factor 10 below the design. Appendix C compares the present machine parameters withthe design values and with those of the previous KEKB. Improvements towards the design value,such as the installation of a nonlinear collimation system, are being carried out during the present
long showdown. Beam operation is expected to restart in October 2023. Allocating sufficientmachine time for beam studies and tuning will be important to understand and mitigate the present
limitations and to further increase the luminosity. Adequate beam diagnostics will help in thisendeavor. During its 26th meeting, the Committee has examined the progress of the project andthe present challenges.
The ARC commends the establishment of an International Task Force for SuperKEKB, which has
engaged a significant number of experts from around the world and already put forward several
new ideas for improving SuperKEKB performance. While the volatile electricity prices are aconcern, a recent MEXT review lends great support for the future pursuit of a vigorousSuperKEKB/Belle-II programme.

As always, the high standard of the presentations impressed the Committee. Already highlighted
in a previous report, the next generation is important for the success of SuperKEKB operation
over the coming decades.
The most important recommendations of the Committee were presented to the SuperKEKB staff
members before the close of the meeting. The Committee wrote a draft report during the meetingthat was then improved and finalized by e-mail among the Committee members. The report isavailable at http://www-kekb.kek.jp/MAC/2022/ .

Contents
A) Executive SummaryB) Key RecommendationsC) Findings, Comments, Recommendations
1. SuperKEKB Status
2. 2021c-2022b
3. Belle II Status
4. MDI (BG)
5. Control
6. Monitor
7. Injector
8. BT
9. Injection
10. Vacuum
11. MR Magnets & QCS
12. RF
13. Helium Refrigerator for SRF
14. Optics Issues
15. Collimator Issues
16. Impedance Issues
17. Sudden Beam Loss
18. NLC
19. ITF Summary
20. Beam-Beam
21. LS2, IR Modifications
22. Others, QCS Tour, etc.
Appendix A: KEKB Accelerator Review Committee Members
Appendix B: Agenda of the 26th KEKB Accelerator Review Committee meeting
Appendix C: Required and achieved SuperKEKB parameters and comparison with KEKBA) Executive Summary
In summer 2022, SuperKEKB achieved a new world record luminosity of 4.7x10 34 cm-2s-1, which
is a significant accomplishment, more than twice the previous KEKB record. Among the various
issues encountered were (1) the interplay between a single bunch instability and the feedback
system, which was successfully solved by feedback tuning, (2) still unexplained sudden beamloss, which is tentatively attributed to “fireballs”, (3) large vertical equilibrium emittances in bothrings, (4) emittance growth in the BT, and (5) poor or unstable injection conditions. Many upgrade
activities are underway in the current Long Shutdown (LS) 1. Beam operation is expected toresume in October 2023. The age profile of the SuperKEKB personnel is a concern. About a thirdof the team is rehired retired staff, which is largely outnumbering the staff below 40 years of age.
The ARC has formulated recommendations on how to address the above issues and supports anambitious luminosity goal for the year 2024.

B) Key Recommendations
Recommendations: The Committee has made recommendations throughout the different
sections below. The most significant of these recommendations and a few more generalrecommendations are summarized here:
1. The ARC committee recommends actively recruiting new young staff members to help
with the wide-ranging accelerator work of SuperKEKB and to prepare for the next decadesof operation (R1.1).
2. Find a mechanism to engage additional PhD students (perhaps also already masterstudents and even undergraduates), from Japanese universities or from abroad in the
exciting and real-time accelerator environment of SuperKEKB, in both experimental andtheoretical accelerator-physics studies (R1.2).
3. Develop a new algorithm to correct the orbit in a way that is insensitive to possible motionof BPMs without capacitive sensors and their calibrations (R2.4).
4. Simulate the effect of the measured vertical floor deformation by up to 40 mm on thevertical emittance in the LER and HER (R11.1).
5. Consider realigning the whole ring, especially the Southern part (R11.2).
6. Simulate the injection efficiency to the LER, by changing the injection offset, which may
affect the efficiency or the lifetime for large amplitude particles (R9.6).
7. Quantify the expected integrated luminosity improvements for all specifically proposed
LS2 projects. Update annually (R21.2).
8. Investigate through a combination of electro-magnetic, mechanical and shower
simulations (as performed for LHC collimators) whether a sudden collimator jaw
deformation by ~100 micron could occur, triggered by HOM heating or small beam impact,which could then lead to a self-amplifying increasing deformation and catastrophic sudden
beam loss (R17.5).

9. Given the ongoing extremely high concern for the cost of electrical power, study
configurations for the RF system in SuperKEKB that will allow for full or nearly full beam
operation while significantly lowering the cost of RF power generation (R12.2).
10. Complete a comprehensive study of the possible advantages coming from inserting an
ECS in the e- BT before proceeding with the real device construction (R8.2).

C) Findings, Comments, Recommendations
1.SuperKEKB Status
Findings & comments:
In the summer 2022 SuperKEKB established a new luminosity world record of 4.7x10 34 cm-2s-1.
Over the past three years, it has delivered an integrated luminosity close to 0.5 ab -1 to Belle-II,which is a significant accomplishment. The peak results were obtained while colliding 2346
bunches in each ring with 1.15 A in the HER and 1.46 A in the LER and a 𝛃y* of 1.0 mm. A 𝛃y* of
0.8 mm was tried in preparation for a possible future mode of operation.
A recent MEXT review covered two topics: The results of the past ten years and the proposedplan for the next operational period for SuperKEKB and Belle-II. The MEXT review strongly
endorsed the future pursuit of a vigorous SuperKEKB/Belle-II programme. The ARC congratulates
the SuperKEKB team on this important successful review.
The SuperKEKB Accelerator Divisions III to V gave reports on staffing issues. In overall numbers,
these divisions are missing at least 10-20 staff. At present the personnel is about half of what wasavailable at the time of KEKB and, for example in the RF group, only a quarter of the number
present during the TRISTAN era. About 35 of the presently active persons, or ~30% of the total,
are retired, rehired staff. Only three staff members are younger than 35 years and another fourare between 35 and 40 years old.
The cost and availability of electrical power had a strong influence on the beam run 2022b, arisingfrom higher oil prices, surcharges, power contract changes, and the overall Japanese power
infrastructure. This run was stopped about a week early due to sudden changes with these power
cost issues. Such influences will extend into the future. The SuperKEKB divisions found many
clever ways to save power costs over the SuperKEKB complex during the ongoing downtime. The
overall implementations for future beam operations will necessitate new relations with Japanese
power companies and potentially new modes of beam operations.
Recommendation:
R1.1 The ARC committee recommends actively recruiting new young staff members to help withthe wide-ranging accelerator work of SuperKEKB and to prepare for the next decades ofoperation.

R1.2 Find a mechanism to engage additional PhD students (perhaps also already masterstudents and even undergraduates), from Japanese universities or from abroad in the exciting
and real-time accelerator environment of SuperKEKB, in both experimental and theoreticalaccelerator-physics studies.
R1.3 Optimize RF configurations for SuperKEKB to allow for full beam operations while saving
RF related power costs to allow extended running hours, given a likely reduced but fixed powerbudget.

2. 2021c-2022b
Findings & comments:
Tremendous efforts were undertaken to improve the luminosity of SuperKEKB during this period,
resulting in new peak and daily luminosity records. Some of the improvements were increased
beam currents with more bunches, improved field reproducibility of interaction-region
quadrupoles, two bunch injection, and improved field quality in the injection kickers. Some of the
obstacles encountered were sudden beam losses, low injection efficiencies, current dependenceof the orbits, and damaged collimators. These obstacles implied medium values for the verticalbeam-beam parameters 𝜉𝑦 in standard operation, whereas higher 𝜉𝑦 values were reached inoperation with a few hundred bunches.
So far the best specific luminosity has been achieved with a nominal value of y*=1 mm. However,
the estimated actual value of y* for that case was about 0.8 mm, due to the optics error causedby an orbit shift at the final-focus sextupoles. An open question is why such an opticallymismatched 0.8 mm resulted in better performance than a matched 0.8 mm. The latter led to ashorter beam lifetime than the mismatched 0.8 mm.
The daily integrated luminosity exhibits a large day-by-day fluctuation during this period. Asidefrom injection efficiency and its stability, also the bunch current limit set by sudden beam losses,
the abort frequency and optics degradation affect the daily integrated luminosity.
One of the causes of poor injection after an abort may be the sudden orbit change in the rings
due to the varying beam-induced thermal effects. The loss of heating of the beam vacuum
chambers may push the quadrupole positions after an abort. Although some BPMs havedisplacement gauges to correct the offsets of these BPMs relative to the nearby sextupoles, other
BPMs without sextupoles just move together with quadrupoles. So, advanced algorithms may beneeded to correct the orbit by guessing/estimating the displacement of each BPM.

Recommendations
R2.1: Investigate the reason why the beam lifetime was shorter for the matched y*=0.8 mm
configuration than for the mismatched 0.8 mm case.

R2.2: Summarize the reason for the degradation of the integrated luminosity, looking at theoperation log and create a top-10 list of causes for the losses of integrated luminosity.
R2.3: Understand why adding more bunches does not produce the expected improvements interms of beam current.
R2.4: Develop a new algorithm to correct the orbit in a way that is insensitive to possible motionof BPMs without capacitive sensors and their calibrations.
R2.5: Examine the history, step size, and frequency spectrum of IP feedback signals.
R2.6: Check, using the stored recorded position data from the last run, the magnitude of themeasured vertical motion (jitter) of the individual two beams at the IP as compared with theamplitude changes of the vertical deflection collision to see if the jitter positions are adequately
corrected for optimum luminosity. Are the position jitter levels for the case of y*=0.8 mm different
from those at 1.0 mm?

3. Belle II status
Findings & comments:
The goals of Belle II are to support the highest possible SuperKEKB luminosity, to ensure optimum
and efficient particle physics data collection, to help minimize the beam backgrounds whileallowing the peak luminosity to increase, and to significantly reduce unnecessary beam aborts.
The detector team has worked very hard to improve the data collection efficiency of Belle II. They
have found that there has been a drift in the center-of-mass energy Ecm by as much as 4 MeV,
that has cost the equivalent of 5-7% of integrated luminosity over a period of 1.5 yrs. They are
now more often checking the Ecm, in order to stay closer to the peak of the 4S resonance. One of
the major concerns is the backgrounds generated by bad injection. They have had to increase
the DAQ deadtime for injected pulses, that require an extra-long time to finally damp down to thestored beam. This has increased the overall deadtime of the detector to approximately 10%.
There are several improvements planned for the detector during the LS1 downtime. The TOP
detector will have several PMTs replaced with new tubes that have much longer lifetime. The
current PXD (PiXel Detector nearest to the beam pipe) is currently a partial installation with only
2 ladders out of a total of 12 installed in the 2nd layer. The plan is to install a complete new PXD
detector including the first layer of 8 modules as well as to improve the observed heat-related
expansion issues. At the same time, a new beam pipe is being fabricated, that features improved
shielding from secondary synchrotron radiation (SR) scattering. These two items are the driving
terms for the long shutdown. In addition, more shielding will be installed near the front of thecryostats in order to reduce detector backgrounds from shower debris of beam particles hitting
the beam pipe in this area. More neutron shielding will be installed with the intention of reducing
the number of FPGA failures. In addition, the software in the FPGA modules is being upgraded
to be less sensitive to data degradation issues which seem to arise from neutron background.

The detector and background teams are assisting the accelerator team in improving the beamabort system and in attempting to detect the starting locations of the fast beam loss events.
Monochromatization, with the help of either nonzero vertical IP dispersion (Frascati proposal fromthe early 1970s) or through a chromatic waist shift (Raimondi proposal from 2022), can lower theeffective collision energy spread and increase the event rate in cases where the beam energy
spread is comparable to (or larger than) the width of the Upsilon resonance. Upsilon (4S) has a
Breit-Wigner width of 20 MeV. The natural rms collision energy spread is around 5.5 MeV, asmesaured by Belle II. For zero collision energy spread, obtained by a perfect monochromatization,
the event rate would increase by about 10%.
We applaud and encourage the collaboration of the detector groups with the accelerator groups.
The detector team should be able to help the linac and injection teams to understand which
injection bunches are extra lossy and which are better. The detector team has developed several
beam loss detectors that can help the accelerator team in discovering if there are specificlocations in the rings where the fast beam abort events originate as well as shorten the time
between beam instability detection and the ensuing beam abort.
Recommendations:
R3.1: Determine the origin of the shift in the collision energy. Consider collision energy feedback
based on the SuperKEKB dipole-based energy spectrometer or the Belle II dimuon events, forexample.
R3.2: Explore the possible merits of “monochromatization” in terms of increased event rate andbeam-beam performance.
R3.3: The detector team should more closely work with the linac and injection teams tounderstand the varying quality of injection bunches.

4. Machine Detector Interface (MDI) and Backgrounds (BG)
Findings & comments:
Belle-II beam backgrounds did not limit the beam currents in 2021 and 2022, thanks to successful
mitigation by collimators, by the vacuum scrubbing progress, etc. The TOP counter is the subdetector most vulnerable to beam backgrounds. The recent TOP background breakdown reveals
that beam-gas scattering in LER, beam-gas scattering in HER, Touschek scattering in LER and
HER, respectively, luminosity and injection all contribute at roughly comparable levels to the TOP
background, with slightly higher contributions from the LER than from the HER. Data and MonteCarlo simulations are in fair to good agreement, with higher than expected single-beam
contributions from the LER and lower than expected ones from the HER. A Snowmass White
Paper predicts that backgrounds will remain acceptable for luminosities up to 3x10 35 cm-2s-1. For
the target value of 6x1035 cm-2s-1 there is quite some uncertainty, however.

For the HER, the measured Touschek background rate is 5 times lower than expected from theMonte-Carlo simulation. The reason for this discrepancy is unclear.
A new machine learning application was developed, which can be used to monitor the real-timecomposition of the background sources. A “feature attribution” method of this application indicates
which parameter has contributed to a change of background rate. In the future, this application
can be used for the accelerator tuning. The ARC applauds this excellent development.
At higher beam currents, the duration of the injection background has increased. Other major
issues are: the sudden beam loss, which is addressed by abort timing analysis using fast loss
monitors; the bad injection, e.g., due to linac energy drift, poor two-bunch injection etc.; and thelimited stability of good injection conditions. LER large beam loss aborts caused QCS quenches
8 times in 2022. A much higher rate of bad injections occurs at smaller y*. It takes about 20-30
minutes to recover after each abort, which results in a significant loss in integrated luminosity.
Earthquakes have also resulted in 26 beam aborts. Several collimators were damaged by beamlosses. The beam orbits before and after a beam abort are completely different, but the earlier
orbits are reestablished by the orbit feedback.
To better understand the sudden beam loss events, 7 new beam loss sensors were installedaround the ring, and 6 more are due to be installed. The loss monitor at D6V1 was the first sensor
to see beam loss in most cases, but not always. This might indicate that the beam disturbance inthese cases occurred upstream of the D6 section. The planned installation of additional monitors
will help further pin down the origin of the sudden beam loss.
Recommendations:
R4.1: Further improve background models to reduce the last factor of 3-5 discrepancy between
measured and simulated Touschek rates.
R4.2: Implement as many diagnostics as possible to identify the origin of the sudden beam loss,
including more or different loss monitors, vibration sensors on/near collimators, thermocouples,etc. If possible, trigger and record data from available diagnostics, such as beam size monitors,
bunch profile detectors, and turn-by-turn BPMs, for the last few turns before beam abort.

5. Control
Findings & comments:
The successful commissioning and development of the SuperKEKB facility, with many complex
and inter-related sub accelerators and diagnostics, is only possible because of the functionality,
performance and reliability of the distributed control system. It is a mix of legacy functions withmany new requirements and control needs still being added or enhanced. The SuperKEKB
Control team can be proud of their contributions to the project.

The review presented some performance enhancements to the system timing functions, as well
as some updates on the maintenance and technology upgrades to the core networked computer
systems.
The core synchronization and timing functions throughout the SuperKEKB complex and KEK lightsources are implemented in Event Timing Generators and Receivers. To better control thedamping ring extraction the delay setting in the event was upgraded from 8.7 ns resolution to a
400 ps resolution, which reduces some jitter, and “cogging” in the extraction synchronization. This
improves the top-up operation in SuperKEKB for 2 bunch injection and allows the implementationof the BCE (Bunch Current Equalization). This timing improvement is also used for injection into
the Photon Factory and PF-AR.
A White Rabbit based timing distribution and event recording system has been added. Thisfunctionality is used to improve the time-stamp resolution for injection, beam loss and abort
diagnostics. Many examples of this improved functionality were presented, including IP loss
monitors near Belle II, injection background studies, bunch ID and history for beam loss events,
beam abort sequence history and the like. The flexibility and utility of the new hardware andsoftware are very useful. This is a successful and valuable upgrade.
The core computing hardware and software platforms require periodic upgrades. The SuperKEKB
servers, network and storage systems have all been maintained and selectively upgraded.
The use of the White Rabbit timing systems is an excellent pragmatic choice, as sharing theimplementation of this general-purpose timing backbone with CERN and other user labs is
economical as well as an opportunity for KEK to contribute their software applications to the larger
accelerator community. KEK’s recent addition of the EPICS code to the White Rabbit server is an
excellent example of this two-way benefit. Sharing these kinds of projects and developments isgood for everyone in the worldwide accelerator community.

Recommendations:
R5.1: Continue to test and validate the timing and time stamp functions used in the abort and loss
diagnostics - when you need them, you want to be sure the system works flawlessly to get thevalue of the recorded event.
R 5.2: There are some diagnostics implemented in commercial general-purpose instruments(such as the picoscopes used in the loss monitors). Because the product lifetime of commercial
instruments is probably much shorter than the operational life of SuperKEKB, be sure that
adequate spares are maintained. If a product is discontinued, you do not want to write newsoftware and invest manpower in a different commercial instrument to duplicate this function.
R 5.3: The Committee supports the idea to extend the bunch current equalizer capability in thebunch selection to the two-bunch injection operation mode. It will be beneficial for improving
injection efficiency and machine operation stability as well.

6. Monitor (Beam Instrumentation)
Findings & comments:
The operation of SuperKEKB requires state of the art beam instrumentation and beam feedback
systems. The review focused on recent new developments in the four subsystems.
BPM systems - An intensity-dependent orbit shift has been observed in the VXI-based HER BPM
receivers. These are legacy BPM instruments originally used in KEKB and it is likely this effecthas been there for a long time. Studies suggest that this intensity dependent effect is coming fromthe very wide dynamic range receiver front end, which uses a series of programmable attenuators
to scale input signals into the receiver channel. Investigations are underway, if the effect is due
to finite RF matching at some attenuator settings, the proposed installation of circulators in thesignal path may improve this measurement. The turn-by-turn BPM systems have needed repair.
At present the hardware is being repaired by KEK. There is a future upgrade of remaining BPM
cards by a commercial vendor.
Beam Imaging Diagnostics - Both the X-ray and synchrotron light monitors were reviewed. The
X-ray based beam imaging has had to replace CCD cameras due to radiation damage; also some
scintillator damage was seen. A Cu absorbing filter has been installed to reduce the radiation fluxin the detectors. There has been a leak in the HER X-ray path; a copper-beryllium window appears
to have been damaged by moisture. This is still under investigation.
The visible light imaging uses a diamond mirror to reflect the visible image. This mirror has
suffered from heat deformation due to the X-ray flux. The development has moved from a single
crystal to polycrystalline mirror materials with gold and platinum surfaces. The imaging system
includes a coronagraph, with optics that can block the central beam spot to allow measurementof the less-intense beam halo. Many images were shown. The technique offers a promising pathto understand possible contributions of the beam halo to backgrounds, etc. The capabilities
include single bunch and single turn images, which are valuable for injection studies.
The bunch-by-bunch feedback systems are critical for SuperKEKB, and these have been indevelopment since the early days of KEKB. The technology is a shared design used at many
facilities, with the benefit of shared operational and instrument codes for beam instability
diagnostics. In the last year, the noise floor of the transverse receiver has been investigated. To
reduce the noise floor a different processing configuration with re-partitioned RF and baseband
gain, as well as with a comb generator of a different design, has been prototyped. This
modification may help increase the feedback gain and may change the potential emittanceincrease from extremely-high-gain configurations. This is an ongoing study. The flexibility tochange the processing filter bandwidth and phase slope via the filter tap length and coefficients
may also give extra capability to add feedback gain and flexibility in tuning. Initial tests show good
damping with the alternate filters.
Tunnel environmental monitoring systems have been implemented. These allow remote logging
of temperature and humidity.

Recommendations:
R6.1: The orbit intensity shift may be due to mismatches in the attenuators, but in any case, theselegacy HER BPM electronic modules may be difficult to maintain over the lifetime of SuperKEKB.
An alternate plan might be to use the same BPM modules as were developed for the LER; thiswas suggested in the presentation. This path should be considered a fallback option withadequate spares of the LER type module.
R6.2: The information coming out of both the X-ray and synchrotron light monitors is potentiallyvaluable in diagnosing beam properties. It is clear the halo studies require labor and time to better
understand what they show and how they could be helpful. Similarly, interpreting the X-ray beam
size data may take workforce and time investments. With the recent and sad loss of John
Flanagan, it is important that adequate scientific personnel is made available to continue
developing these techniques.
R6.3: The capabilities of the instability feedback will really be tested as the currents increase to
the design values. There may also be issues with heating of the beam line kicker components.
Adequate workforce and machine time investments to study the damping systems as the beam
currents are increased will be important. Understanding the interactions of the LER longitudinalbunch-by-bunch feedback with the low-mode feedback in the LLRF systems will also be important
as currents increase and the instabilities become more challenging.

7. Injector
Findings & comments:
A description of the injector layout is given with emphasis on simultaneous top-up injection in 4+1
rings. The parameter table shows an exceptional improvement in performance from 2018 until
the last run, which ended in the summer of 2022. This reflects a deep knowledge of the injector
subsystems and the great jobs done until now. However, for the injector, the design value of thepositron charge (currently at 85%) and electron charge (currently at 50%) as well as a stable 50
Hz operation with two electron bunches still remains to be achieved. The emittances measured
at the linac end (BT1 before ARC1) of both species are at, or better than, the design value forelectrons while they are close to the design value for positrons.
Electrons: The thermionic DC gun works as specified. The beam quality in terms of electron
emittance from the RF gun with photocathode could be improved by installing a DOE in the laserline to improve the spatial distribution. Charge stability was improved too, by installing charge
feedback. High charge operation (5 nC) was also tested, reaching the design value of 4 nC at theend of the linac. The charge losses in the J-ARC and at the target location must be improved.
The stability of the second bunch must also be improved. An emittance growth at the linac end
with time (a sort of emittance growth drift) is observed if the bunch is not reoptimized while theemittance in the RF gun region remains constant.

Positrons: The positron charge is almost at design value ! However, the transmission efficiency
between the target and the end of the linac is 60%. Horizontal emittance after the Damping Ring
(DR) is larger than the design value and a new low-emittance optics for the DR will be tested.
Implementation of optimizers for linac tuning can improve performance, but sometimes at the costof not understanding the machine physics behind the optimized result.
In general, the stability and the beam quality are critical questions for a linac and the following
transfer line. Implementation of beam-based feedback with dedicated diagnostics can improvethe beam stability and stabilize the beam quality.
Recommendations:
R7.1: Perform systematic measurements of the orbit jitter of the two electron bunches andcorrelations with possible sources. Advance the synchronous data acquisition between Linac andBT.
R7.2: More generally, perform a feasibility study for the implementation of synchronous beamdata acquisition, which will be extremely useful for studying drift and instability.
R7.3: Implementation of an orbit feedback if the use of pulsed magnets allows for this.
R7.4: Continue with the upgrade plan as presented in the summary slide.
R7.5: Concerning the emittance growth of the second electron bunch, study the effect of longrange-wakes in the linac.
R7.6: Identify the causes limiting the charge of the electron bunch along the injector.
R8.7: Discover the loss locations and causes for the positron transport in the linac.

8. Beam Transport (BT)Findings & comments:
Survey results from 2018 for magnets of the LER and HER BT quadrupoles in the injection region
showed large misalignments that were mitigated via optics design changes (bend angle in ARC4for LER and septum angles for LER) at the time. During LS1, a few large quadrupole offsets in
HER and residual misalignments with respect to the new reference orbit will be corrected.
It has been found that a fringe field near the septum plate has a sizable multipole component.
The field quality would be improved by reducing the pulse width and improving the septum shim
shape. Installation of a new shim will be scheduled in summer 2024.
A new vacuum chamber for QI4E was designed with sufficient clearance for the injected beam.

A leakage of LER injection orbit bump was observed, which might arise from differences of theceramic duct shape. They may need to manufacture new K2-type ceramic ducts and replace K1
ducts by new ones in 2024.
In line with the budget rules, components of a new Energy Compressor System ECS andfabrication of components for a cooling water system were approved and could be fabricatedwithin three years.
A new BT line for the HER has been proposed, aiming at keeping the CSR/ISR emittance growthunder control. ISR induced emittance growth of the new BT line decreases to 1/3 of that in thecurrent BT line.
It was highlighted that benefits from most of the upgrades proposed will be reaped in mid-2024
or later, so about one year after the re-start up.
Recommendations:
R8.1: Evaluate the impact, if any, of the new beam pipe for the QI4E quadrupole in terms ofimpedance.
R8.2: Complete a comprehensive study of the possible advantages coming from inserting an ECS
in the e- BT before proceeding with the real device construction.
R8.3: Quantify, by numerical simulations, the reduction in terms of CSR emittance growth fromthe new BT line as well as you already did it for the contributions coming from ISR.

9. Injection
Findings & comments:
An overview of the injection issues clarified the injector status, achievements, and planned next
developments, intended to improve it towards the target performance and to support the design
luminosity. Injection has a strong correlation with the injector and Beam-Transport BT optimizationand the speaker gave a positive impression on the collaborations and common efforts between
the groups that work on the various parts/issues of the SuperKEKB accelerator complex. Afterthe overview, the speaker focused on two specific issues not discussed in other presentations:
The emittance growth in the BT line and the injection efficiency into the main rings. Emittancemeasurements showed a horizontal emittance growth in the Arc 1, along with a vertical emittance
growth in Arc 1 and Slope 1. An effort was devoted to understanding if the cause of the horizontal
emittance growth was due to ISR and CSR effects. They found an agreement between
simulations and measurements by changing a few of the beam parameters and machine settings,
such as the rf phase and the charge. They observed that the emittance growth is consistent with
the CSR effect, but the measured blowup is still larger than that of the simulation.

As a suggestion, they should use a 3D model for the CSR because the 1-D model in Elegant
provides an underestimation of the CSR effects.
After this comprehensive campaign of measurements and simulations for the e - BT, a couple ofcountermeasures were suggested and one of them was that the best solution for the BT line is toinstall both the new straighter line and the Energy Compensation System ECS, in order tosuppress ISR and CSR with minimum longitudinal emittance increase. Other proposals are to
generate a CSR shielding by operating with a vertical bump or by reducing the beam-pipe
aperture, or to change the BT layout into a new straight one either with or without a smaller
aperture.
A new beam transport (BT) line is a big effort, and a detailed study of the new layout should be
conducted. In the simulations, a model of IBS should also be included and the effect of CSR on
the beam distribution along the BT should be considered, too. However, before a final decision istaken on the implementation of a new beam transport line, a similar detailed study should be done
for the vertical emittance growth in BT to understand the cause of the emittance blowup. It is notclear what the source is at this moment. Furthermore, narrowing the vertical aperture in the BT
line permanently may be risky, as the stronger wake fields may possibly enhance the fluctuationof the vertical orbit.
One of the outstanding issues is to increase the injection efficiency for the positron beam to reachthe target luminosity. Reduction of the positron emittance in the BT is one of the most important
improvements to be done together with the investigation of the beam loss during the first 1000
turns after injection. It may be worth trying to simulate the injection efficiency to the LER, bychanging the injection offset, which may affect the efficiency or the lifetime for large amplitudeparticles.
Re-alignment of the transport lines of both LER & HER, especially the parts just upstream of theinjection points, can help improve the injection efficiencies and mitigate the emittance dilution.
Recommendations:
R9.1: Emittance growth: a 3D model for the CSR should be employed in the BT simulations. IBS
should also be considered.
R9.2: A detailed optics investigation should be executed to understand the source of the verticalemittance growth.
R9.3: Look at the history on the vertical orbit deviation of first and second bunches before
narrowing the vertical aperture of the BT line.
R9.4: To suppress the CSR & ISR, as mentioned in the presentation, a new straight transport line
could be considered. The following straight transport line might be approximately optimized:

A more ideal transport line (red curve) with minimum ISR & CSR effects.
R9.5: Simulate the injection efficiency to the LER, by changing the injection offset, which may
affect the efficiency or the lifetime for large amplitude particles.
R9.6: We suggest that some efforts should be made to ensure that if a single injection thyratron
spontaneously fires, all the other injection thyratrons will be forced to fire too. This would result in
the event becoming a closed orbit kick at the injection point and it would eliminate the possibilityof an open beam oscillation and resulting damage to a collimator head. This issue will become
even more important as the beam currents increase.

10. Vacuum
Findings & comments:
The dynamic pressure rise is decreasing as the beam dose progressively increases in both LER
and HER. In the latter, the pressure rise is four times lower, because 82% of the beam pipes andbellows had already been installed in KEKB (memory effect) and the ring has been less vented
during the last years. At the end of the 2022a/b run (6000-7000 Ah), the equivalent desorption
yields are 2x10-8 molecules/photon in the HER and 4x10-7 in the LER. The slope in the high dose
region of the log(P)-log(dose) plots is around -0.8 for both rings, which is a typical value for highenergy electron rings. An integrated beam dose of 5x105 Ah (i.e., more than 50x today’s
accumulated dose) is necessary to reach the same desorption yields in the LER as today in HER.
In the DR, the vacuum requirements are reached: Today, with an accumulated dose of 67.6 Ah,
the desorption yield is 2x10-6 molecules per photon.
The LER dynamic pressure does not increase linearly with beam current as it should be if
synchrotron light impingement were the only cause of gas desorption. Fitting the experimental
data, it seems that thermal effects also play a significant role at high currents; this could indicate
a local heating of vacuum components by Higher Order Modes (HOMs).

The beam lifetime is not affected by the residual gas in the HER and only marginally (20-40%)
affected by the residual gas in the LER. The Touschek effect is the main limitation of beam
lifetime. In addition, electron-cloud effects are not anymore measurable.
Among the troubles that were presented, the one related to malfunctioning of a water-coolingpump caused the most serious issue. The problem should not happen again as the coolinginterlock was changed, now inducing a beam abort when multiple water-flow alarms are received.
Problems found in two gate valves were solved, but not fully understood. In the future runs,
particular attention should be paid to gate valves to understand if the issues were isolated events
or signs of persistent weaknesses.
An intense work is ongoing during the LS1, including the replacement of several collimators,
modifications of the QCS cryostat and replacement of 50 m of beam line for the NLC.
In conclusion, today, vacuum is not a showstopper and most of the issues were solved andunderstood.
Recommendations:
R10.1: Continue the excellent operation, monitoring and data analysis of the vacuum systems.

11. MR Magnets & QCS
Findings & comments:
No time has been lost due to the main ring magnet system for the last 31 months.
Owing to difficulties in procuring hollow copper conductor, the required new skew sextupoles willfeature coils recuperated from spare normal sextupoles. A good decision.
Around the IP, the tunnel continues to sink, which is being carefully monitored. The main eventfor the final quadrupole QCS has been the appearance of a vacuum leak, requiring the additionof pumps. After cold examinations with the help of the vacuum group, this leak was found to beon the service cryostat: it will be repaired towards the end of this financial year.
An investigation into flux creep in the superconducting magnets has revealed that this is moremarked if the power supply is left in the up-ramp position: in the down-ramp position the flux-creep
is much less; so this will henceforth be taken to be the standard for operation.
Recommendations:
R11.1: Simulate the effect of the measured vertical floor deformation by up to 40 mm on thevertical emittance in the LER and HER.
R11.2: Consider realigning the whole ring, especially the Southern part, during LS1.

12. RF
Findings & comments:
The SuperKEKB RF system is operating well. In June 2022, it was supporting beam currents of
1.15 A of electrons in the HER and 1.46 A of positrons in the LER, distributed over 2346 bunches.
The upgrade of the RF system from KEKB to SuperKEKB is complete but several conversions of
ARES stations from single klystrons feeding two ARES cavities to a set up with 1 klystron per
ARES cavity still need to be completed. The RF team has studied how to maximize the stored
beam current without adding more klystrons, including beam loading, cavity power limits, andcavity phasing.
The new Coupled Bunch Instability (CBI) damper system is working well. The Auto-level Control
Loop (ACL), the Phase Lock loop (PLL), the Direct RF Feedback (DRFB), and the Zero-ModeDamper (ZMD) are being optimized to maximize the allowed stored beam currents. The studies
indicate to increase the voltage of the ARES cavities, to extend ZMD to all stations, and tominimize the gains of the PLL and the ACL. As currents increase, these studies will be critical tounderstand how to configure the numerous regulator loops for the best station stability as well as
for optimum beam stability. There will be many decisions to make on the unique configurations to
run the 2 cavities per klystron stations and the 1 cavity per klystron stations.
The design choice of superconducting and ARES-type energy storage cavities means that thehigh stored energy helps mitigate gap transient effects. As higher currents are reached,understanding the gap transients in HER and LER, and the match between them, may be
important. The LLRF design with the Direct Loop means that during the beam gap the direct loop
will attempt to drive the klystron with extra power to cancel the transient from the missing beam
current in the cavity and estimating this transient power overhead may be important. Similarly, an
operational strategy to run the machine with one or more parked RF stations will be necessary.
With such parked unpowered cavities, the gap transients will be more pronounced. Estimatingthese situations and their management seems helpful.
At the achieved currents the RF stations are not yet highly stressed. As currents increase therequired performance of the direct feedback and the dedicated mode -1,-2 and -3 loops will
increase. If the PEP-II and LHC experience is a guide, the model-based configuration methods
may help with optimally configuring the stations. This helps with both the station performance,
and the time required for the RF experts to oversee the RF system configurations and operation.
There were seventy-two beam aborts (about 0.6 per day) attributed to the RF systems in the
beam run 2022a/b with about ⅔ due to aging controls and ⅓ due to cavity breakdowns in either
ARES or SCC cavities. The issues of aging components in the RF control system are dealt withas they arise. The overall RF system is regularly inspected to reduce the rate of these trips.
The HOM power generated by the beam in the SCC cavities is about twice what was expected.
In order to run at the needed higher beam currents in the HER, additional HOM SiC dampers arebeing added to the downstream beam lines near the SCC cavities. The presently installed units
have been shown to be effective at absorbing the extra HOM power.
The ongoing high concern for the cost of electrical power will cast a shadow over the beam
operations of SuperKEKB for the foreseeable future. The RF system is the largest power
consumer in SuperKEKB. In view of these concerns, the RF team should investigate alternative
configurations for the RF system in SuperKEKB that will allow for full or nearly-full beam operation
while significantly lowering the cost of RF power generation. The alternative configurations may
involve parking a number of RF stations, running with reduced voltage to allow longer bunch
lengths, running with different bunch patterns to reduce the RF overhead, reconfiguring the RF
overhead while operating the longitudinal feedback systems, and running fewer klystrons but at
higher power levels where the klystrons and HVPS are more efficient.
Recommendations:
R12.1: Study in simulations whether variations in filling pattern could reduce the overall generated
cavity HOM power (see the procedure and results in PRAB 21, 071001, Phys. Rev. Accel. Beams
21, 071001 (2018) - High order mode power loss evaluation in future circular electron-positron
collider cavities (aps.org) )R12.2: Given the ongoing extremely high concern for the cost of electrical power, study
configurations for the RF system in SuperKEKB that will allow for full or nearly full beam operation
while significantly lowering the cost of RF power generation.
R12.3 Develop an estimate of the gap transients at the design current with nominal RF
configuration, and for operating scenarios with one or more parked stations.

13. Helium refrigerator for SRF
Findings & comments:
The helium refrigerator system for the SRF is vintage, having initially been constructed for
TRISTAN in 1988. The 2 K function required for the SRF was developed by KEK. The system issufficiently powerful but requires regular maintenance to ensure conformity with the regulations.
The original manufacturer has left the business, requiring the procurement of generic spares.
There are three spare turbines. The team is evidently very competent.
Recommendations:
R13.1: Undertake a survey to establish which parts may require replacing in the near future.

14. Optics issues
Findings & comments:
In recent high-beam-current operations SuperKEKB struggles to keep the machine performance.
Optics aspects contributing to this degradation were presented:
In 2021 a tune drift from QCS field drift along with large vertical beta-beating had been observed.
The source was identified as the ‘flux creep’ effect and the implemented mitigation was to exceed
operational current and ramp down.
Resistive wall introduces a tune shift versus beam current. However, tuneshift changes on a dayby-day basis. Horizontal orbit changes at the sextupoles shift the tune, and it is seen that the orbit
depends on intensity. The horizontal tune shift versus intensity agrees with resistive wall
calculations. Vertical tune shift shows a certain consistency with expectations from orbit shifts at
sextupoles, when ignoring the expected effect of the vertical resistive wall. This could be just a
coincidence, but the origins of the vertical tune shift with current should be further studied. Thistune shift with current is accompanied by up to 20% beta-beating, inadvertently reducing y* from
1.0 mm to 0.8 mm. Orbit adjustment at sextupoles improves efficiency and background.
The HER beam becomes unstable after larger earthquakes, needing optics corrections. Also,
even without earthquakes, a degradation of emittance and injection efficiency is observed duringhigh intensity operation, calling for optics corrections every 2 or 3 days. A plausible reason is asmall orbit fluctuation as the residual orbit correction is of order 20-30 m only. The BPM readings
can be affected by drifts of gain, temperature changes, etc. Reliability of orbit correction iscompromised by these drifts. It should be explored how to correct only the real orbit shifts.
Using comparisons between 3 BPM orbit readings reveals significant discrepancies fromexpectation that could be explained if HER QC1 had 3% transfer function errors. Accurate
magnetic modeling from magnetic measurements is not implemented. Oide-san commented thatthe observed discrepancies could be due to assuming only a 1D model, while both x-y coordinates
should be considered for the coupled transport matrix.
Dynamic aperture measurements have not been carried out.
The ARC committee suggests a look at a possible theorem of tune shifts against the closed orbit
deviation for a chromatically corrected optics.
Recommendations:
R14.1: Perhaps the step size for lowering y* should be a smaller one, i.e., one could consider afirst step from 1 mm to 0.90 mm and then to 0.8 mm, etc.
R14.2: Refine the 3 BPM method with x-y readings and possibly more BPMs, taking 4x4 matrices
into account.

R14.3: Explore the possibility of additional orbit correctors around the SLY's to reduce the orbit
deviation during operations.
R14.4: Consider developing optics measurements from turn-by-turn BPM data with pilot bunches.
These measurements would allow one to monitor and then correct the optics during high intensity
operation, reducing the amount of dedicated operation time required for optics corrections with
low intensity in the current approach.
R14.5: Implement a more refined lattice modeling using magnetic measurements.
R14.6: Perform more non-linear measurements, including global and local chromatic coupling,
detuning with amplitude etc., as this nonlinear domain remains largely unprobed.

15. Collimator issues
Findings & comments:
A great deal of effort and study has gone into the collimators located around each ring. The HER
accommodates old KEKB collimators while the LER has a series of newer collimators. There are
currently two major reasons that collimator heads become damaged. One reason is the
spontaneous firing of an injection thyratron, creating an unclosed orbit kick to the beam, which
subsequently strikes a horizontal collimator jaw. The other is a sudden beam loss event, which
ends up depositing a large fraction of the beam onto a collimator and consequently damages a(mostly vertical) collimator jaw.
A large study has been launched in order to find a material or combination of materials to use asa collimator head. This material should be robust enough to either survive a direct strike by thestored beam or, if damaged, to not significantly affect the beam. Various materials from tungsten
to titanium to carbon-fiber-reinforced carbon have been considered. Several of the original
collimators from KEKB, that are in the HER, show evidence of damage, and it is likely that severalof these collimators were damaged while SuperKEKB was running. These damaged collimators
do not seem to adversely affect the stored beam.
As mentioned in the presentation, it is suggested that all of the collimator heads, no matter whichmaterial is used for the head, should be coated with Cu, and the ARC agrees with this suggestion.
The coating will minimize the impedance to the beam from the collimators.
We recognize that the effort to further understand the collimators and to maximize protection of
the detector and machine components (i.e., QCS cryostats), while minimizing the effect on thestored beam is a difficult problem with several conflicting constraints. We encourage continuing
collaborative efforts with the background team, the accelerator team and the vacuum team toresolve this complicated issue.

Recommendations:
R15.1: To track the minimum transverse impedance contributed by a collimator, the product of
local y and the kick factor is used as a figure of merit. We recommend that for each collimator
installed, a special effort in minimizing this product be pursued, considering all impedancecontributions (geometric and resistive).
R15.2: Concerning the damaged collimators in the HER, although they do not seem to affect thestored beam, it would be good to repair as many of these damaged heads as possible, in orderto keep the impedance from these collimators to a minimum.

16. Impedance Issues
Findings & comments:
A “-1” mode single-bunch instability, which blows up the vertical emittance, occurred at highbunch current, above 0.9 mA, with narrow collimator aperture (resulting in Qy>0.01) and bunchby-bunch feedback active. This instability could be suppressed by tuning of the feedback, up to
the design bunch current of 1.4 mA or beyond. The instability strength depends on the conditionof the feedback and is affected by impedance increase due to collimator damage. The observed-1 mode instability could be reproduced in simulation considering the transverse wake and a highgain (G~0.1) multi-tap feedback. The tune range Qy>0.58 is preferred for suppresing the
instability, but the injection is worse for these higher vertical tunes.
The ARC commends the understanding and successful suppression of this instability. The
threshold current for the -1 instability (0.9 mA) was similar to the threshold for the sudden beamloss (0.75 mA). A coupled bunch head-tail instability, with somewhat lower threshold than the
single bunch instability, as had been studied by Scott Berg and others, might be one possible
explanation for the sudden beam loss.
Recommendations:
R16.1: Study the possibility of a coupled-bunch head-tail instability.

17. Sudden Beam Loss (SBL)
Findings & comments:
Sudden Beam Loss (SBL) produces large current losses in a batch of bunches around either ring,occurring over just a few turns, which then causes full beam loss (abort) and may lead to severe
hardware damage, mainly in the LER, to IR quadrupole quenches, and to large IP backgrounds.

These losses and aborts occur independently in both HER and LER. An SBL event only occurs
every few days during SuperKEKB luminosity delivery making it difficult to study this effect.
The sources or causes of the SBL are under active investigation. SBL is a major limitation toincreasing the beam currents and luminosity in SuperKEKB. An international task force has beencreated to look at SBL.
New and existing instrumentation have been developed or used to study this important effect turnby-turn: Bunch Orbit Recorder (BOR), Bunch Current Monitor (BCM) (4096 turns), timed loss
monitors, bunch size monitor (XRM), and acoustic monitors.
The observations show that the orbit changes are less than 1 mm (which needs to be further
checked), the bunch sizes seem not to change, and there seems to be no bunch oscillations.
There is a vacuum pressure change at the place where the beam is lost, but this may be aconsequence of the loss, not its cause. Again, studying the cause of SBL is difficult as an SBL
occurs only every few days.
The cause of SLB is still under study, but it does NOT appear to be related to vacuum pressure
issues, dust events, vertical abort kicker misfire, bunch-by-bunch feedback, coupled bunch
instability, or RF feedback issues.
Present potential causes are Electron Cloud (EC) effects in LER collimators, electrical discharges,
or “fireball” explosions related to beam heating of metal sputtered particles. However, how afireball or local electron cloud could cause an extremely fast beam loss without instability has notbeen carefully explained. To cause a significant beam loss in one turn by bremsstrahlung off
nuclei, the beam particles would need to pass through about 1 cm of a solid metal, like copper ortantalum, or through at least several meters length of a typical plasma.
Future plans to study SBL are adding more loss monitors, improving the abort trigger, improving
BOR resolution, adding a second BOR at 90 degrees (in betatron phase space), and making
collimator jaws from high sublimation materials.
The turn-by-turn bunch-by-bunch vertical size measurements are hard to perform, and thesemeasurements should be rechecked to verify that the bunch sizes do not change with SBL.
Even though the SBL shows no obvious time correlation with the top-up injection events, theinjection bunch sequence in SuperKEKB could be checked for all the bunches in a row to see ifthey are not filled in such an order that some adjacent bunches might be overfilled so that thecorresponding bunches could drive an SBL. The SBL may not appear to be caused by injection
directly but could arise from over-injection into many bunches.
An investigation should be performed whether a beam-beam flip-flop effect could happen. Sucha flip-flop might initiate a rapid vertical beam size increase in one beam and, thereby, cause rapid
beam losses on the tight vertical collimators. This could happen to either beam.

One of the slides (p. 8) may show a relatively large (50 sigma) vertical orbit deviation during the
SBL. Simple beam scraping cannot generate such a large effective orbit deviation. The betatronphase relation between the BOR and the beam lost point may be interesting, too.
Recommendations:
R17.1: Recheck that bunch-by-bunch turn-by-turn vertical beam size measurements are accurate
and do not change during SBL.
R17.2: Check whether the injection bunch filling sequence may overfill bunches and, thus, could
be related, in some indirect way, to the SBL bunch current loss pattern.
R17.3: The ARC fully endorses the installation of new BORs in different locations to observe theorbit deviations. With a few more of these, it may be possible to guess the location where thegrowth or the beam loss started.
R17.4: Investigate whether a beam-beam flip-flop could trigger an SBL given the tight verticalcollimators.
R17.5: Investigate through a combination of electro-magnetic, mechanical and shower
simulations (as performed for LHC collimators) whether a sudden collimator jaw deformation by~100 micron could occur, triggered by HOM heating, electron cloud, or small beam impact, which
could lead to a self-amplifying increasing deformation and catastrophic sudden beam loss.
R17.6: Install weak solenoids around the collimators, as planned, to suppress any local
multipacting, electron-cloud formation or fireball/plasma creation.

18. Nonlinear Collimator (NLC)
Findings & comments:
The nonlinear collimator consists of a pair of skew sextupoles, separated by a quasi -I transform,
which surround a collimator, D05Va, with variable aperture. Its main merit is a dramatic decreaseof the collimator impedance, possibly reducing the overall vertical collimator impedance by afactor of two. The installation requires the removal of about 20% of the damping wiggler units(10/56), implying ~10% reduced damping time, and also some additional shielding. The Touschek
lifetime is reduced by about 10% due to the additional skew sextupoles.
Displacement monitors will be installed at the BPMs next to the NLC skew sextupoles.
As an option, if these skew sextupoles are placed on vertical movers, the distance between the
beam core and the beam tail at the collimator can be enlarged or the strength of these sextupoles
be reduced. By moving the sextupoles, the distance between the beam core and the deflection(extraction) field would be smaller. The compensating sextupole would also have to moveaccordingly.

Recommendations:
R18.1: Study the possibility to vertically offset the nonlinear element (sextupoles) (movers?) to
make the magnetic collimation more efficient.

19. International Task Force(s) (ITF) Summary
Findings & comments:
ITF was established with the goals to achieve 10 35 cm-2s-1 after LS1 without major modifications
and to consider ideas to increase this luminosity by a factor of 6. The summary report has been
finalized and distributed. Now it has been decided to establish an overall new ITF chaired by
Onishi-san.
There were two Optics proposals: Sextupole configuration proposed by Yunhai Cai and adding
new sextupoles at the IP-image point as proposed by Pantaleo Raimondi. Explorations of these
proposals did not yield immediate or obvious improvements.
The Beam-Beam task force made the proposals to increase the momentum compaction factor
and explore crabbing ratios, however the machine tunings were instead focused on achieving a
new record luminosity beyond 5x1034 cm-2s-1 before LS1.
TMCI: Progress was presented in the dedicated presentation by Ohmi-san.
LINAC: The Injector linac sub-group was formed at the 2nd ITF general meeting on Sep. 2, after
receiving a recommendation to extend the focused task-force groups to the other key areas. No
international members joined. Maybe a smaller subgroup should be established to attract
international collaborators.
Sudden Beam Loss: This group was only formed in July 2022. Its progress was presented in a
dedicated talk.
The new proposal foresees 5 ITF focus groups: Beam Tuning, Collective Effects, Beam Injection,
Sudden Beam Loss and IR Upgrade. Discussions ensued on how to involve and attract
international collaborators.
Recommendations:
R19.1: Consider appointing deputy chairpersons from outside KEK for each subgroup of the ITF.

20. Beam-Beam Experiment and Simulations
Findings & comments:
SuperKEKB operates with the nanobeam plus crab-waist scheme, with a large Piwinski angle.
Vertical emittance is critical. At the time of the luminosity record, the vertical emittances were oforder 50-60 pm, still much larger than design. After tuning of the bunch-by-bunch feedback theagreement of simulations and measurements has much improved. Actual specific luminosity issome 5%-50% worse than predicted by simulations, perhaps due to insufficient tuning during thehigh bunch current machine study, but the almost constant value of the specific luminosity towards
high bunch-current products is consistent with expectation. The measured specific luminosity
does not seem to depend on the number of bunches. There is no evidence that SuperKEKB has
already reached the beam-beam limit. In machine studies with fewer bunches, vertical beambeam tune shifts of 0.056 and 0.043 could be achieved. A correlation between top-up injection
and specific luminosity was observed. The LER injection kicker contributed to ~3% of luminosity
loss.
Multi-physics beam-beam simulation code development including the nonlinear lattice, spacecharge, and impedance effects, etc., is proceeding at KEK. Similar complementary efforts areunderway in Europe, China and the US.
A path to the design luminosity was outlined, with 3.3 times smaller y*, 2.5 times higher beambeam tune shift, and 2.5 times higher LER beam current.
A significant IR upgrade is required to achieve 0.3 mm y*. The IR upgrade should avoid
overlapping solenoid and quadrupole fields. It could, e.g., be based on CCT magnet technology.
Recommendations:
R20.1: Develop a concrete IR upgrade proposal and demonstrate the expected performance gain
through comprehensive simulations including beam-beam, nonlinear lattice, and impedance.

21. Long Shutdown 2 (LS2) and IR modification
Findings & comments:
The SuperKEKB should be congratulated on achieving a luminosity of 4.7x1034 cm-2s-1 in 2022.
With the LS1 work completed, the SuperKEKB will have the capability to reach a luminosity
approaching 3x1035 cm-2s-1. With LS2 work completed (the exact work and scope are still to be
determined) the SuperKEKB may have the capability to reach a luminosity approaching the targetof 6x1035 cm-2s-1.

The largest presently envisioned task for LS2 is the replacement of QCSR and QCSL, enlarging
the beam pipe aperture from 13.5 mm towards 18-20 mm, and of the nearby compensation
solenoids. These changes must encompass all the conditions of the IR, including several
constraints and requirements from the accelerator and the detector. Topics include beam
aperture, backgrounds, SC quadrupole construction, cryogenics, cooling, collimation, antisolenoids, and many more.
Other work being investigated for LS2 includes a new e- transport line for better HER injection, eECS, and large-scale vacuum chamber modifications. The expected integrated luminosity gains
need to be quantified.
Recommendations:
R21.1: Make a detailed SuperKEKB parameter table showing the accelerator parameters that areavailable presently, then after LS1, and, finally, the possibilities after LS2.
R21.2: Quantify the expected integrated luminosity improvements for all specifically proposed
LS2 projects. Update as needed; the committee suggests annually.
R22.2: Determine the minimum vertical emittance generated by synchrotron radiation in the fringe
fields of the proposed new solenoid/anti-solenoid fields.

22. Others, such as the QCS tour etc.
Findings & comments :The ARC review committee took a tour of the QCS cantilever rafts near the Belle-II detector in
the Tsukuba IR at the end of the review. The rafts had been pulled back to allow work on BelleII. The amount of work on the rafts and Belle-II is impressive. The complicated interfaces between
SuperKEKB and Belle-II show the careful design of the two apparatuses. It is clear that muchwork will be needed if in LS2 the rafts are changed to include newly designed improved QCS
magnets.
Recommendations:
R22.1: None.

Appendix A: KEKB Accelerator Review Committee
Members

Frank Zimmermann, Chair

CERN

Ralph Assmann

DESY

Paolo Chiggiato

CERN

Paolo Craievich

PSI

John Fox

Stanford University

Andrew Hutton

JLab (excused)

In Soo Ko

POSTECH

Catia Milardi

INFN-LNF

Evgeny Perevedentsev

BINP

Katsunobu Oide

UNIGE/CERN and KEK (ret.)

Qing Qin

ESRF

Bob Rimmer

JLabJohn Seeman

SLAC

Michael Sullivan

SLAC

Tom Taylor

CERN (ret.)

Rogelio Tomas

CERN

Tadashi Koseki

KEK, Director of Acc. Laboratory, Ex Officio Member

Makoto Tobiyama

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

Mika Masuzawa

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

Hiroyasu Ego

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

Appendix B: Agenda of the 26th KEKB Accelerator
Review Committee meeting
December 13 (Tuesday)
08:30 - 09:00

Executive Session

09:00 - 09:10

WelcomeM. Yamauchi

09:10 - 09:40

SuperKEKB Status

M. Tobiyama

09:40 - 10:10

2021c-2022b

Y. Ohnishi

10:10 - 10:30

Belle II Status

K. Matsuoka

10:30 - 11:00

MDI (BG)

H. Nakayama

11:10 - 11:30

Control

H. Kaji

11:30 - 12:10

MonitorM. Tobiyama

13:30 - 14:00

Injector

M. Satoh

14:00 - 14:30

BT

M. Tawada

14:30 - 15:00

Injection

N. Iida15:10 - 15:40

Vacuum

K. Shibata

15:40 - 16:10

MR Magnets & QCS

Y. Arimoto

16:10 - 16:40

RF

M. Nishiwaki

16:40 - 17:00

Helium Refrigerator for SRF

K. Nakanishi

17:00 - 19:00

Executive Session
December 14 (Wednesday)

08:30 - 09:00

Executive Session

09:00 - 09:30

Optics Issues

H. Sugimoto

09:30 - 10:00

Collimator Issues

T. Ishibashi

10:00- 10:30

Impedance Issues

K. Ohmi10:30 - 11:00

Sudden Beam Loss

H. Ikeda

11:00 - 11:30

NLC

A. Morita

11:30 - 12:00

LS1 Status

K. Shibata

13:20 - 13:40

ITF Activity Summary

M. Masuzawa

13:40 - 14:10

Beam-beam

D. Zhou

14:10 - 14:40

LS2, IR modification option 3’

M. Masuzawa

14:40 - 15:30

Others, QCS Tour

15:00 - 20:00

Report writing / Executive Session

11:00 - 12:00

Close-outAppendix C: Required and achieved SuperKEKB
parameters and comparison with KEKB.

parameter

KEKB w BelleSKB 2022b

SKB design

LER

HER

LER

HER

LER

HER

E [GeV]

3.5

8

4

7

4

7

x* (mm)

1200

1200

80

80

32

25

y* (mm)

5.9

5.9

1.0

1.0

0.27

0.30

x (nm)

18

24

4.0

4.6

3.2

4.6

y (pm)

150

150

~50

~50

8.6

12.9

I (mA)

1640

1190

1321

1099

3600

2600

nb

1584

Ib (mA)

1.04

y

0.098

2249
0.75

0.059

2500

0.587

0.489

1.44

1.04

0.04070.0279

0.069

0.060

30 -2 -1 -2
Lsp (10 cm s mA )

17.1

71.2

214

34 -2 -1
L (10 cm s )

2.11

4.65

80

The beam-beam parameter is computed without the hourglass factor or geometric factor for the
luminosity.