The Twenty-Seventh KEKB
Accelerator Review Committee
ReportApril 28, 2024

Introduction
The Twenty-Seventh KEKB Accelerator Review Committee (ARC) meeting was held on 2527 March 2024. Appendix A shows the present membership of the Committee. 12 committee
members attended the 27th meeting in person, and 2 others on zoom. The meeting was heldin hybrid mode and featured two days of oral presentations by KEKB staff members, plus
discussions between the Committee members, and a final half a day for another executive
session, report drafting, and close-out.
The Committee welcomes the incoming Heads of Accelerator Divisions III and IV, Dr. Kyo
Shibata and Dr. Makoto Tobiyama.
The agenda for the meeting is shown in Appendix B. The slides of the presentations areavailable at https://www-kekb.kek.jp/MAC/2024/ .
During its 27th meeting, the Committee has examined the progress of the project with works
during Long Shutdown 1 (LS1) and restart of the collider since end of January 2024, thepresent challenges, and long-term upgrade plans. As always, the high standard of thepresentations impressed the Committee.
Already highlighted in a previous report, the next generation is important for the success ofSuperKEKB operation over the coming decades. Although during the past year a total of 6
SuperKEKB staff members retired (5 academic and 1 technical staff), a total of 7 (namely 4+3)
new people could be recruited, reversing a negative downward trend of overall staff. The ARC
is also pleased that by now 3 PhD students are involved with the SuperKEKB accelerator.
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 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/2024/ .

Contents
A) Executive SummaryB) Key RecommendationsC) Findings, Comments, Recommendations
Welcome1. 2024 Schedule and Summary of Upgrade Work (LS1)
2. 2024a Run
3. Belle II Status
4. Injector Overview
5. RF Gun
6. Accelerating Structure
7. Pulsed Magnets / Power Supplies
8. Linac Control Monitors
9. Injection
10. Optics
11. Nonlinear Collimator
12. Sudden Beam Loss (SBL)
13. Monitors
14. Damping Ring
15. MDI
16. BT
17. Control
18. Magnet
19. QCS
20. Safety
21. Vacuum
22. Cryogenics
23. RF (ARES + SC)
24. High Power RF
25. Overview of Collective Effects
26. LS2
27. ITF
28. Luminosity Expectations
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
Appendix D: Glossary of acronyms

A) Executive Summary
SuperKEKB has successfully restarted operation after an 18-month long shutdown. The ARC
congratulates the SuperKEKB team on the smooth recommissioning, including the newnonlinear collimation section, and achieving vertical single-beam emittances in the storage
ring which are lower than in 2022. The vacuum system conditions smoothly and both beam
currents are increased stepwise as planned. A vertical beta function of 1 mm has alreadybeen established, and the specific luminosity at present bunch currents of ~0.3 mA is
comparable to the one of 2022, but lower than the one obtained in some beam-beamsimulations. Sudden beam loss events still occur, but additional diagnostics already provide
new information that should help track down their origin.
Electricity price and power consumption determine the cost of SuperKEKB operation and limit
its running periods. The biggest energy consumer of SuperKEKB is the collider RF system.
One or two modifications to the klystron controls might lead to noticeable energy savings.
The ARC would like to emphasize that the SuperKEKB accelerator is a frontier machine andis a world leader in Accelerator Technology with ambitious goals for high peak and integrated
luminosity. This accelerator is led by a highly dedicated group of experts who haveencountered and overcome technical obstacles, and who will find new issues as they
approach the ultimate accelerator design goals. The achievements accomplished by this teamand the KEK laboratory are already being incorporated into future collider designs and theworldwide accelerator community is carefully watching the impressive progress of this very
exciting enterprise.
The ARC has formulated recommendations on how to address the above issues, as well as
various others, and supports the ambitious luminosity goals for the year 2024 and beyond.

B) Key Recommendations
The Committee has made recommendations throughout the different sections below. The
most significant of these recommendations, and a few general recommendations are
summarized here:
1. Try to establish a long, uninterrupted beam run for autumn 2024, in order to fully qualify
the benefits, and gains from the LS1 works through a step-wise approach. (R1.1)
2. Schedule a dedicated injector study period, when Photon Factory (PF), PFAccumulator Ring (AR) and SuperKEKB are not operating, and without access to the
Fuji area of the SuperKEKB tunnel; this could be two weeks before the summer
shutdown. (R1.2)
3. Study, and reduce, the vertical emittance growth after the electron Beam Transportline (BT1e). (R9.4).
4. Include the error of the QCS “cancel coils” in the base lattice of the High Energy Ring
(HER) used for the regular operation and investigate how to mitigate its impact insimulations and experimentally. (R9.5 and R10.3)
5. Further investigate the optimum setting of the Non Linear Collimator (NLC)
parameters, such as betas, apertures, sextupole strength, betatron phases from theinjection point to the NLC, considering the collimation performance, injection efficiency,
Transverse Mode Coupling Instability (TMCI) threshold, etc. (R11.2)
6. Further improve the Sudden Beam Loss (SBL) diagnostics such as by additional beam
oscillation monitors, bunch-by-bunch beam-size monitors, and even more beam-loss
monitors around the ring. (R12.1)
7. Re-visit the “usual” effects that could explain the SBL - for instance, coupled-bunch
TMCI, electron cloud, arcing or trapped electrons in ion pumps, dust in the beam pipe,
Non Evaporable Getter (NEG) pumps heated by Higher Order Modes (HOMs), power
supply regulation errors, feedback-related perturbations, beam-induced collimator
motion or jaw deformation, … , and motivate their rejection by experimental
observations. (R12.2)
8. Based on the PEP-II experience at SLAC with NEG pump heating from HOM power,
monitor a few NEG strips feedthroughs with thermocouples for temperature increases
with stored currents. Select NEG pumps that are near collimators or other HOM
generating structures. (R21.1)
9. Develop a contingency plan as to how one could act should the 35-year-old cryogenics
equipment fail. (R22.2)
10. Determine which SuperKEKB accelerator studies need to be done during the next few
years, and when, to allow technical decisions related to the future upgrades to be madeat an appropriate time. (R26.1)

C) Findings, Comments,
Recommendations
WelcomeFindings & comments:
During the past year a total of 7 (namely 4+3) new people were recruited, reversing a negative
downward trend of overall staff, as only six persons retired during the same period. The SAC
is also pleased that 3 PhD students are now involved with the SuperKEKB accelerator.
We commend the SuperKEKB and KEK laboratory for their outreach to young scientists andthe effort to bring Ph.D students, undergraduate and master students to KEK. The
commissioning of SuperKEKB is a unique opportunity to train the next generation ofaccelerator scientists and engineers. The field critically needs this new cadre to advance thefuture machines, and SuperKEKB can only benefit from the energy and contributions of early
career scientists. We suggest expanding the program with EAJADE, including US universities
and labs, possibly through the US Particle Accelerator School, and also strengthening ties
with the CERN Accelerator School.

1. 2024 Schedule and Summary of Upgrade Work (LS1)Findings & comments:
The present 2024 SuperKEKB operations schedule is quite limited after summer, due to an
extremely tight budget. With the initial budget allocated by MEXT, SuperKEKB may only

operate for three months until July (due to huge electricity costs). An additional operating
budget by KEK would allow operation for another 1.5 months after the summer. A further,
supplementary budget from MEXT, could finally also allow operation in March 2025 (andprobably in December 2024). Operation in January and February 2025 is in any case notpossible due to scheduled work on a 150 MW power substation.
In JFY2024 the SuperKEKB management will change. Prof, Makoto Tobiyama, presently
Head of Accelerator Division III, will become the head of the Accelerator Division IV. Prof. Kyo
Shibata will take the reign of the Accelerator Division III. Prof. Hiroyasu Ego will remain theleader of the Accelerator Division V.
The age profile of the SuperKEKB staff still looks precarious, especially for the technical staff.
External partners are involved in the frameworks of MNPP-01, the US-Japan collaboration,and the EU EAJADE project.
Vacuum and other work during LS1 included the Interaction Region (IR) chamber work, IR
radiation shield modifications (e.g. new shield on IR bellows), replacement of damaged
collimator heads in both rings, collimator relocations, repair of an IR QCSR cryostat vacuum
leak, aperture enlargements of the injection channel, installation of a non-linear collimator in
the LER OHO straight section, installation of beam loss monitors / acoustic sensors and bunch
oscillation recorders to monitor the sudden beam losses. For the QCSR re-installation, theremote vacuum connection (RVC) worked very smoothly – no vacuum leak was found. Many
collimators were either replaced or upgraded (with copper coating). The majority of thecollimators are of the old KEK type, based on a movable vacuum chamber.
In the injector linac, 16 old linac accelerating structures have been replaced with new ones
(with larger energy gain, and without water leaks). The fabrication has started for 12 additional
structures. A new fast kicker has been installed, which can deflect the first and second bunch
independently. Several larger-aperture pulsed quadrupoles have been installed. A successful
beam tuning using Bayesian optimization has been implemented, greatly increasing thepositron transmission. The source of BT beam emittance blowup is still not understood, but
Coherent Synchrotron Radiation (CSR) could be proven innocent, for the positron beam. A
new BT energy compressor (Bte-ECS) is planned for 2024-25.
Recommendations:
R1.1: Try to establish a long, uninterrupted beam run for autumn 2024, in order to fully qualify
the benefits and gains from the LS1 works through a step-wise approach.
R1.2: Schedule a dedicated injector study period, when PF, PF-AR and SuperKEKB are notoperating, and without access to the Fuji area of the SuperKEKB tunnel; this could be two
weeks before the summer shutdown.
R1.3: Prepare a performance-ranked list of the anti-ageing measures that had to be delayedand establish a plan for implementing them over the next couple of years.

2. 2024a Run
Findings & comments:
The LS1 shutdown started in June 2022 and was completed in January 2024. Many upgrades
and maintenance activities were completed during LS1. These upgrades affected almostevery accelerator system such as the IR cryostat, RF systems, vacuum, magnets, controls,magnets, and cryogenics. The 2024a beam run for SuperKEKB started in February 2024. By
March 25, 2024, at the time of this review, the peak luminosity had reached 1.9 x 1034 cm-2s1, and an integrated luminosity of about 8.4 fb-1 was delivered to Belle II in Run 2024a.
The ARC congratulates the SuperKEKB team for successfully completing LS1, and resuming
beam operation. The ARC supports the planned priority for raising peak luminosity towards 1
x 1035 cm-2s-1 and beyond, e.g. by gradually increasing beam currents, and by further
squeezing the vertical beta* to 0.8 mm, and correcting the IR optics errors, as fast as possible.
This might imply accepting large injection background or turning off the detector.
Beam scrubbing has proceeded without complications.
Early during the turn-on SuperKEKB the vertical beta function at the IR, betay*, was set to 8
mm to help ease IR issues. Over a period of a month the betay* was reduced to 1 mm in steps.
One of the goals of this run is to try again a betay* of 0.8 mm to see if work during LS1 helpedwith former issues here and to see what items need to be worked on in the upcoming summerfall shutdown. A further goal beyond 2024 is reaching a betay* of 0.6 mm.
The number of bunches in use now is 2346 which is very close to the design number. The
beam currents have reached 800 mA in the LER, and 640 mA in the HER. These are about ahalf of the goal for beam currents for this run.
The LER vertical emittance has been tuned to 10 to 20 pm down from 25-35 pm from the lastrun, which is an excellent result.
The crab waist has already been switched on, helping increase the luminosity. The
theoretically optimum crab waist value in use is 80% in LER and 40% in HER, which are thesame values as during the last run.
The vertical fractional tune was changed from about 0.59 to about 0.56, which increased thevertical tune shift from about 0.26 to 0.31. This is a very promising improvement.
After work done on the injection system in LS1, the LER injection efficiency presently reaches
values of up to 90%, at intermediate beam currents. However, the HER injection efficiency is
still poor at about 20%. The HER injection efficiency is under careful study. The stored beam
lifetime is about 7 to 10 minutes.
The vertical emittance improved in this run thanks to the new IR Be beam pipe that allows forflexibility in the orbit control and possibly the improved Beam Based Alignment (BBA) at strong
sextupoles. However, it seems that the beam-beam effect blows up the emittances to thesame level as in the previous run. The specific luminosity has improved with the new lower
vertical fractional tune. The use of crab waist clearly improves the specific luminosity.
The Dynamic Aperture (DA) measurements have not yet been fully analyzed as, e.g., thelocation of losses could not yet be identified. Similar measurements with open collimators andlow beam current could help distinguish between the physical and dynamic apertures.
The measured DA is smaller than in simulations. The DA simulations need further work.
The crab waist sextupoles clearly affect the measured DA. This implies an existence ofaberrations from the crab sextupoles.

The Non-Linear Collimator (NLC) system is still under commissioning. So far, the NLC helps
the backgrounds from the stored beam, but hurts the background of the injection beam.
There are three proposed “Plans” for reaching the luminosity goal in the 2024a-b run.
Plan A: Positron (LER) current = 2.08 A, electron (HER) current = 1.48 A, betay* = 1.0 mm,
luminosity = 8 x 1034 cm-2s-1.
Plan B: Positron current = 2.26 A, electron current = 1.61 A, betay* = 1.0 mm, luminosity = 9.4
x 1034 cm-2s-1.
Plan C: Positron current = 2.08 A, electron current = 1.48 A, betay* = 0.8 mm, luminosity = 1.0
x 1035 cm-2s-1.
Recommendations
R2.1: Push peak luminosity (using a combination of Plans A, B, and C) towards 1035 cm-2s-1
and beyond, e.g. by gradually increasing beam currents and/or by further squeezing thevertical beta* to 0.8 mm and correcting the IR optics errors, as fast as possible. This might
imply accepting a large injection background or turning off the detector for the initial tuning
period.
R2.2: Consider operating (perhaps briefly) early in the 2024a run at a betay* of 0.8 mm toobserve what are the next technical steps needed to be done during the 2024 summer
shutdown to run stably at this value in the fall.
R2.3: As progress allows (just before or after summer), perform accelerator studies related topotential LS2 projects that need early lead-time decisions, e.g. by pushing betay* down to 0.6mm or to the nominal value, at low current.

3. Belle II status
Findings & comments:
The Belle II detector has improved nearly all detector subsystems in anticipation of
accumulating more data and reaching a higher peak luminosity. The pixel detector (PXD) has
been fully installed onto a new central beam pipe that has been improved to minimize
backgrounds from scattered synchrotron radiation. The Central Drift Chamber (CDC) gas
system has been significantly upgraded and monitoring of the gas quality has been greatly
improved. The performance of the CDC has improved and now appears to be in very good
shape throughout the CDC volume. Most of the conventional Photo-Multiplier Tubes (PMTs)
used in the TOP detector have been replaced with extended-life PMTs. The background levelin these PMTs has, in the past, been a limiting factor for beam operation. The new PMTs are
more robust, but they need to survive as the beam currents and luminosity increase. The gas
monitoring system in the KLM has improved to avoid the same mishap in the gas flow system
as happened during Run 1. After a careful study of the detector backgrounds, the nose conesof the cryostats have been modified; the tungsten W shielding pieces have been replaced withstainless-steel pieces as these should reduce the shower debris generated by the W pieces.
The data acquisition DAQ system has been improved and made more robust and this should
improve uptime for the detector. Machine learning is being used to assist the detector team incorrelating backgrounds with machine configurations.

Recommendations:
R3.1: Encourage continued collaboration with the linac and accelerator groups to understandbackgrounds as they arise and as beam currents and luminosity increase.

4. Injector Overview
Findings & comments:
From 2020 to 2021, the failure rates on the gun increased, while the radiofrequency (RF)systems remained pretty unchanged. In 2022, they reduced the repetition rate from 50 to 25
Hz, the failure rate on RF systems remained unchanged, while on the RF gun it increased asin recent years. In 2022, they also had a problem with the controller of a pulsed magnet thatwas the cause of about 40% of the downtime. Countermeasures on RF systems are to replace
the most critical accelerating structures with new ones and to reinstall a new RF gun. However,in 2022, the beam loss time ratio was less than 1% indicating a reliable injector. The ARC is
happy to know that the world’s first Diffractive Optical Element (DOE) was installed and put in
operation at the 1st/2nd line laser, and the DOEs were also updated to the high bunch charge
operation. The ARC is also glad to know both the thermionic DC gun and the photocathodeRF gun work well. The thermionic DC gun needs to inject a beam to PF, PF-AR, and the linacof SuperKEKB for different beam requests. The goal of e+ bunch charge, 4nC, is nearlyobtained with 3.5nC at linac end and BT1. With the help of machine learning, the e+ bunchcharge could reach ~5.5 nC at the end of Sector 2. 65% of e+ production efficiency, evenhigher than the simulation value, was achieved.
In BT2, an important emittance growth was observed for both the positron and electron bunch.
Excluding CSR for the positron bunch, it is thought that this may be caused by field errors in
the magnets, due to changes made for 4 GeV operation. This will be investigated andmitigated. The electron bunch #2 has a problem with the trajectory stability: an injection testof bunch #2 into the MR will be carried out soon.
To better control the optics in sectors 1 and 2 and J-ARC, additional pulsed quads andassociated power supplies were installed.
To mitigate the instability of the trajectory of the second electron bunch, fast kickers were
installed at the end of the linac and in BT to correct the orbit. There remains an issue to deal
with fast kickers, because they remove the offset but increase the jitter of the trajectory.
A point to clarify is what is the source of the amplification of trajectory jitter? Is it due to longrange wakefield? The amplification of jitter should be investigated considering the transverse
wake of the RF structures. Long-range wakefields in current accelerating structures should be
calculated.
As a general comment, it seems that the injector is reliable but several open issues need tobe addressed in order to provide the nominal charge for both species to be injected into the
MR. In the discussions on the injector, the question of stability, in particular of electron
bunches, and the emittance growth in BT for both species continually arises.
Emittance values should be consistent between the different presentations. Sometimes it isdifficult to identify the absolute values of the normalized emittances and their evolution along
the injector and beam transport line. In general the emittances of both beams are (too) large
as is illustrated in the following table. For the electron beam, both emittances are much higher
than the target emittances at 4 nC (40/20 H/V mm mrad). For the positron beam, theemittances have small values although still high compared to the target emittances at 4 nC
(100/15 H/V mm mrad). Optimization of the injector for the electron beam should have a higher
priority than that of positrons.

Norm. emittances (micron)2022 BT22024 BT2e- x (R56=0) at 2.2 nC~250(~50 in BT1)

~200(~100 in BT1)e- y (R56=0) at 2.2 nC~120(~60 in BT1)

~350(~100 in BT1)

e+ x

N/A
(~125 in BT1)

~175(~150 in BT1)

e+ y

N/A
(~10 in BT1)

~24(~10 in BT1)

Recommendations:
See R1.2 above.
R4.1: Priority should be given to optimize the injector and injection for electron bunches.

5. RF Gun and electron-beamFindings & comments:
The RF gun has been operated successfully during the last run. Up to 6 nC have been
produced, and charge feedback has been implemented to stabilize the long term current. The
in-house produced Ir7Ce2 cathode material has proven to be very robust with a very long
lifetime. The DOE optics is working effectively. Temporal shaping is being considered as well.
Laser positioning feedback has been implemented to improve pointing stability. Somedegradation of the laser windows was observed, apparently due to carbon contamination. The
source of the carbon is unclear. Improved pumping close to the window did not completely
solve the problem. A new, improved design has been developed and will be installed toeliminate this problem as well as a larger DOE to cover the whole cathode. The RF gun has
increased the number of breakdowns in the region behind the cathode, where the choke islocated. To solve this problem for the operations, it is planned to install a new rf gun in thenext summer with some modifications in the cathode region to address an issue with excessive
breakdown rate in the choke. Essentially, the cell geometry of the RF gun will be the sameand the emittance should be the same as for the current RF gun. Beam dynamics simulations
predict an emittance of 7 mm mrad, which is much lower than the emittance measured fromthe present rf gun in sector B. The electron source should provide a certain value of normalized
emittance, which can then only increase along the linac. This point should be investigatedbecause the RF gun with its small (8 mm) aperture could be the cause of the large emittancevalue measured in sector B and also generate trajectory jitter due to the transverse wakes. In
order to estimate the wake effects in the RF gun, it is necessary to consider the beam size
and the laser pointing jitter.
Electron beam emittance along the injector for different bunch charges. BT is at the end of thebeam transport line (BT2). In the legend, the charge values refer to charge in BT2. Normalized
emittance in sector B at 5.2 nC from the RF gun (4 nC in BT): x = 57.5 mm mrad, y = 86 mm

mrad (blue diamonds). Goal values for the normalized emittances at 4 nC are: 40/20 (H/V)
mm mrad.

The ARC commends the production of a functional and state-of-the-art injector laser complex
for the production of high-charge bunches. However, the cause of the large emittance should
be further investigated by comparing the results of the simulations with the measurements
performed in linac sector B.
The impact of the laser power variation and RF phase drift on the injected charge has been
seen in the operational history. The group has a PID based feedback for this and is developing
new instrumentation. The control development for this system may be a good opportunity fora new staff or Ph.D researcher who is interested in state space or modern control.
Recommendations:
R5.1: Install the new RF gun to restore reliable operation.
R5.2: Perform beam dynamics simulations and compare the results with measurements in
sector B.
R5.3: Install the energy compression at the linac end. There are several advantages.

6. Accelerating Structure
Findings & comments:
The linac has performed in the main reliably during the last run, however many of thestructures are original 40-year old PF type sections. Three S-band structures developed waterleaks in the last year. Inspection of the extracted structures showed significant breakdown
damage in the input coupler and the beam irises. 21 out of 142 structures are thought to havethis problem. A new design has been developed with rounded corners at the input and lower
peak field on the irises. 4 prototypes and 12 new production sections have been made andinstalled. Another batch of 12 will be ordered (subject to available budget). No spare SLEDsystems are available and they are very expensive to manufacture. A new less expensive
spherical SLED cavity has been developed, which costs about half as much, and this wastested successfully up to 80 MW peak power. Several dummy loads have experienced

breakdowns and damage, as well as some waveguide flanges. New loads will be ordered in
the coming year.
The new structures were designed with a linear detuning of the dipolar mode in order tostrongly suppress instabilities due to long range wakes.
Recommendations:
R6.1: Proceed with 12 new linac structures and new dummy loads to replace aging structures(or structures in sector B) to increase the beam stability.
R6.2: Dedicate machine shifts to verify the electron beam stability in the new accelerating
structures.

7. Pulsed Magnets / Power Supplies
Findings & comments:
During the shutdown several Linac areas were upgraded and changed from DC quadrupole
magnets to pulsed quadrupole or bending magnets with new power supplies. There were 4
major areas covered in the presentation:●

Pulsed quads at the J-ARC matching section

The motivation for this change is to allow interleaving of beams for multiple facilities on a cycleby-cycle basis. The arc quad system has been demonstrated to work, though there are issues
with overheating of the air-cooled design. Better temperature control is in progress.
●

Pulsed quads in the Linac e+/e- compatible optics section

The motivation for this new set of pulsed quads is to allow better optimization of the lattice forboth e- and e+ (at rather different beam energies), to suppress emittance growth and theimprove e+ transmission through the path.
●

A ceramic Linac fast kicker for bunch orbit tuning

The motivation for this new fast kicker is to allow independent control on the 2 e+ bunch
injection with 96 ns separation between the bunches. There are small differences in bunch
orbit that if corrected improve and equalize the charge transfer of both bunches. A ceramicvacuum pipe is required for this fast kicker, as is a new design of fast pulsed power supply.
Tests show the kicker has adequate bandwidth to kick a single bunch, but there is an impact
on orbit jitter that is still being investigated.
●

A pulsed beam line for beam diagnostics (planned for summer 2024 installation)

This new pulsed beam line for dedicated diagnostics will allow parasitic extraction of beam forstudies in the diagnostic beamline. Emittance tuning for SuperKEKB is time consuming andthe interleaving of diagnostic beams with production beams can be helpful for operating
efficiency. This beamline with 2 pulsed bending magnets will be installed in the summer of2024.
The use of the new machine learning tuning method is an excellent application of new ideas
and the committee is excited to see the contributions of the students and new staff. Thesemethods are being applied in both the J-arc and the e-/e+ linac sections.

Discussion:
The arc quads are implemented to allow cycle-by-cycle changes in the optics to deliver various
beams to different end facilities. This dynamic configuration change means the magnets must
not retain hysteretic memory effects from previous cycles – the magnets seem to be nearsaturation from the data? How significant will these effects be for realistic beam patterns?
The DC quads had current regulated power supplies. The new pulsed magnets are driven“feed-forward” from a dedicated IGBT switch arrangement. The drive current is controlled by
the time duration of a gate signal. It was not clear if there was a current transductor or flux
measurement that is used to control the pulse width based on past cycles. The data show
there are effects from rep rate (1 Hz vs 50 Hz) as well as temperature. What is the plan forcontrol and monitoring?
The J-ARC pulsed quadrupoles allow different settings for the six different beam
configurations. The present pulser circuit must provide the full current for each setting, whichmakes the needed pulser quite strong. Perhaps, depending on the shared base current in thequadrupole settings, the power supply could be hybrid where most of the current is DC andonly a small part (say, 10 to 20%) is an added pulsed current. This hybrid mode would makethe pulser power supplies simpler and less costly, and the quadrupole eddy current heatingproblem far less.
Recommendations:
R7.1: Ensure that the pulse width modulation power supplies for the quads have adequate
current monitoring and consider implementing feedback to better control the field seen by thebeam.

8. Linac Control Monitors
Findings & comments:
After the increase of the positron yield it became important to install diagnostics for optimizing
positron versus electron settings and intensities. Beam diagnostics installation is challenging
due to the high radiation environment, the very limited space and the difficulty tosimultaneously and separately detect both secondary e- and e+ bunches within a very shorttime distance. As a solution, Wideband Beam Monitors (WBMs) were installed in the e+source. The WBMs were successfully commissioned, and they help observing and optimizing
the positron Intensities. The motivations for the development of this device were also
comprehensively described. The bandwidth of the entire system is the key parameter and it
depends on the cutoff frequency of the vacuum chamber where the pickups are allocated, as
the propagation of the wake field generated upstream perturbs the measurement itself. In thisspecific case, the vacuum chamber diameter is 38 mm and corresponds to a cutoff frequency
of approximately 5 GHz. However, for qualitative measurements, such as RF phase tuning, it
can become an essential device. On the other hand, for absolute charge and bunch length
measurements, the device should be equipped with wake field mitigators to overcome thewake effects in the measurements. Furthermore, the RF-phase scan pointed out the possibilityof increasing the charge of the positron bunch. This point should be investigated, while also
keeping in mind the final positron yield in the damping ring. This is because a larger positroncharge per bunch could be produced, but with a larger energy spread, leading to greater
losses on the way to and into the damping ring. In general, the ARC is pleased to see theprogress with the linac control monitors and looks forward to learning more about its use andbenefits in operation.
Recommendations:
R8.1: Use this device (WBM) during machine tuning and eventually design the wake dampers.

9. Injection
This talk presented a comprehensive and detailed analysis of the Injector complex status after
the commissioning that followed the LS1 shutdown. During LS1, several modifications havebeen implemented and many measurements have been prepared in order to meet the injector
design parameters, as requested, in order to achieve higher luminosity and to guarantee
operational stability and optimized background during the Belle II detector data taking.
The ARC is impressed by the quantity of measurements that were made, and by the accurate,and exhaustive analysis presented. This led to a very clear description of the injector status,
and allowed understanding of several key points concerning emittance degradation, and beamtransport line setup.
In this initial stage, after the implementation of the ring optics with betay* = 1 mm the effective
injection efficiency still needs to be optimized.
Injection into the HER is performed at the rate of 25 Hz, only one bunch is injected, effective
injection efficiency is of the order of 20%, and a large injection background is observed.
Injection in the LER is performed at the rate of 23 Hz, only one bunch is injected, effective
injection efficiency is in the range 10% to 90%, shorter lifetime and injection saturation are
observed at high current, and a large injection background is measured.
Background is evaluated using diamond detectors installed at the QCS and central beam pipein each ring.
Injection stability is low in both rings.
Concerning Beam Transport sections BT the situation was summarized as follows:
Primary sources of horizontal emittance blowup in BTe1 are Incoherent Synchrotron
Radiation ISR (~ + 30 um), and Coherent Synchrotron Radiation (CSR) (~ + 60 um at 2 nC),
as already outlined in the previous ARC meeting.
Vertical emittance blow-up still remains unidentified.
Emittance blow-up sources in LER-BT are not fully understood, but now at least partly
attributed to local nonlinear magnet fields and coupling sources.
Recent measurements show large bunch-charge dependent emittance growth, which might
be related to longitudinal spiky bunch profile causing unexpected CSR effects.
In order to confirm unexpected CSR contributions most of the dipole sections in ARC1 weredisplaced in the vertical plane, this in turn introduced vertical dispersion, and large resistive
wall impedance. To cope with these side effects a proper value for the vertical displacement
was identified, 13 mm vertical offset, to maintain emittance growth under control, and optics
featuring vertical dispersion suppression has been computed and applied in the ARC1.
Then emittance dependence on vertical bump amplitude applied at selected position wasstudied:No symptom of CSR or Resistive Wall (RW) wake have been detected in BTp–- ARC1, andin BTp – ARC2,3, on the contrary multipole field errors, and X-Y coupling were observed in
BTp – ARC2,3, and at ARC3 entrance respectively.

Relying on experimental results a realistic model of beam transport lines has been developedin the optical lattice simulation code SAD including multipoles in bending, lattice errors, andthe permanent skew quadrupoles embedded in the dipole magnets for dispersion correction.
Simulations with the upgraded optics model showed that multipoles of bending magnets, andsextupole components inferred from the measured XY coupling can reproduce measured X/Yemittances in BT2p.
About e+ beam emittance at 1.1 GeV in RTL, no relevant emittance blowup, and bunchcharge dependence of beam emittances are detected after the Damping Ring (DR).
Optical Transition Radiation (OTR) based quadrupole scans returned horizontal normalized
emittance x comparable with the nominal one, and less than half the nominal value for y.
7-GeV Electron Beam Transport channel beyond the well-known x blow-up induced by ISR
and CSR, shows an increased harmful x blow-up which has been partially cured by varying
R56 factor (J-ARC) from 0 to 0.3.
Simulation of the injected beam in HER showed that QCS cancel-coil errors reduced the onmomentum dynamic aperture by ~25%.
-- The dependence of the energy spread, and emittances on the RF phase, measured at the
BT line, seems interesting and puzzling. There may be some contribution from a possible
vertical dispersion at BT location MSP15, but not all the observations could be explained. A
measurement after the correction of the vertical dispersion at this point will be valuable.
The ARC supports the decision of the SuperKEKB team to eliminate the vertical emittance
blowup before starting high intensity electron current operations.
Recommendations:
R9.1: Study an implementation of dispersion-free steering and of BNS damping, which
simultaneously minimizes energy spread at the end of the linac, and wake-field induced
transverse emittance growth along the linac.
R9.2: Attempt to measure the longitudinal bunch profile at some selected points along LER-BT channel.
R9.3: Clarify if the resolution of the OTR diagnostics used for the quadrupole scan iscompatible with the measured beam dimensions.
R9.4: Study, and reduce, the vertical emittance growth after BT1e .
R9.5: Investigate if it is possible to mitigate the impact of QCS cancel-coil errors on HER beam
dynamics, and include this effect in the ring optics model.
R9.6: Evaluate again the dynamical aperture for HER after including the QCS cancel-coilerrors in the optics if this had not been already done.
R9.7: Consider refining terminology of “raw” and “effective” injection efficiencies to be moreintuitively understandable.

10. Optics Tuning in 2024a
Findings & comments:
Extra effort was put into orbit tuning, obtaining a smoother HER orbit in 2024. The orbit control
at sextupoles has also been improved with automatic tools. Vertical emittance is improved byabout a factor 2 in both rings, probably due to the improved orbit, but this is not confirmed.
Vertical emittance does not depend on betay*, yet maintaining the low vertical emittance isharder at the lower betay*.
The non-linear collimation system based on strong skew sextupoles is being commissioned.
Beta-beating and vertical emittance strongly depend on the orbit deviations at thesesextupoles. Commissioning has started with lower beta functions in this region. For increasing
sextupole strength larger apparent vertical beta-beating is observed in this region, up to
180%.. This is an artifact coming from the strong deformation of the phase-space due to theskew sextupoles (the orbit response becomes parabolic). This is reproduced in simulations.
The linear components can be extracted from a fit; however, this is time consuming. An
analytical formula could be used with partial success. Measured beta function would still havea 10% uncertainty.
Amplitude detuning has been measured in the HER via turn-by-turn data of vertically kicked
data. Without crab waist, the vertical detuning versus vertical action is measured to be positive,
while it is negative in the simulation. One source missing in the simulations is the
manufacturing errors in the cancel coils that cause skew sextupole and skew octupole
multipoles. After including this source in the model the agreement with measurement
significantly improves. The exact mechanism is not yet identified; it could e.g. be via a beam
offset at the skew octupole. Horizontal data exists but it is not yet analyzed. Data were taken
also with crab waist on. A vertical kicker is only available in the HER and not in the LER yet.
Vertical dispersion, beta-beating and coupling (rms) in 2024 were measured to be similar to2022. Second order dispersion was not yet measured.
The value of information sharing and coordination within the collaboration cannot be
overemphasized.
Recommendations:
R10.1: Investigate and correct the IR optics aberrations (QCS correction coils,
sextupoles, etc.) with the help of optics measurements in different configurations.

crab

R10.2: Attempt to carry out optics measurements at low current with all collimators open (ifpossible and safe).
R10.3: Include the error of the QCS correction coils in the base lattice of the HER used for the
regular operation.
R10.4: Perform a center-of-mass energy scan.R10.5: Install a vertical kicker for beam
dynamics studies in the LER.

11. Nonlinear Collimator
Findings & comments:
The study on the Non-Linear Collimator (NLC) is ongoing and not yet finished. It was installedin the OHO straight section. The skew sextupole magnets,“"SNAP.”" and“"SNAP.2”" arealready there. Without the NLC the detector background rises quickly with the LER beam
current, imposing a limit at about 1.6 A. Also, the LER vertical beam size blow-up withintensity increase, with a bunch-current limit at 1.25 mA. The expectation is that the use of the
NLC reduces the impedance and will help overcome some of the current limitations. For
example, the TMCI threshold should be raised to 2.1 mA. During the first beam tests theD05V1 device (this is the NLC). a larger gap was used to replace the D06V1 device (standard
collimator). The following observations were made:1) The beam lifetime is the same in two cases.
2) There is a better background reduction from ring losses with the NLC.
3) The injection background is worse with the NLC, as expected.
4) No beam size blowup was seen in the LER up to 1.4 mA and with the NLC.
5) The current dependent tune shift is 40% smaller with the NLC.
Overall, the initial beam observations are very encouraging, demonstrating several benefits of
the NLC. The reduced cleaning of the injection background is a limitation, however.
The committee congratulates the team for the excellent work done and the successful
installation and first commissioning of the NLC. The NLC seems to be a highly promising
device and sufficient beam time should be invested to fully optimize its settings and to exploit
its potential for improved performance.
Recommendations:
R11.1: Consider some combination of D05V1 (NLC) closed and D06V1 at a somewhat opened(but not fully open) gap to see if an optimal compromise can be found, reducing impedance
while also providing acceptable injection background conditions.
R11.2: Further investigate the optimum setting of the NLC parameters, such as betas,
apertures, sextupole strength, betatron phases from the injection point to the NLC, considering
the collimation performance, injection efficiency, TMCI threshold, etc. R11.3: Calculate theexpected reduction in tune shift with the NLC and compare with the observed value.

12. Sudden Beam Loss
Findings & comments:
Since RUN1, the team is performing specific studies and has upgraded the machine during
LS1 to better understand the origin of Sudden Beam Loss (SBL) events. Despite all these
efforts, the origin of the SBL is not understood yet. An SBL is defined by a significant particle
loss occurring within a turn (10 microsecond) in a duration less than the abort trigger time (20microseconds). SBL were observed during RUN1 in both LER and HER. SBL triggered
quenches of QCS, beam dumps etc. Multiple SBL events may strongly deteriorate the beam
operation via background to Belle II detector or beam dumps, and severe SBL may ultimately
result in the damage of components such as QCS, collimators etc. The observed loss at

collimators seems to be insensitive to the location of the loss inducing mechanism. Before an
SBL, beam oscillations within 0.1 mm are sometimes observed but not systematically.
Pressure bursts are also observed but most probably consequently to the SBL.
Following previous studies, damage of vacuum equipment such as broken RF fingers, dust
and electron cloud were ruled out as possible reasons for SBL. Instead, another mechanism
is proposed, which involves an interaction with the electromagnetic field of the beam, leading
to excessive heating of some metallic debris such as tungsten or tantalum originating frompreviously damaged collimator heads. In the so-called “fireball” hypothesis, a Cu+ based
plasma is created within 0.1 microsecond (following the sublimation of the Cu vacuum
chamber by the hot metallic debris) leading to large transverse kicks (a few 10 microrad) of
the beam bunches.
During LS1, the damaged collimators during RUN1 were replaced and some Ti, Ta, C
collimators heads were Cu-coated (hoping) to mitigate the SBL effect. Permanent magnets tosuppress any electron cloud build up were also installed at all horizontal collimators. In
addition, several instruments were installed: new Beam Oscillation Recorders (BOR), loss
monitors (PMT, EMT) for timing resolution, and acoustic sensors at some collimators and at
the QCS to assess the “fireball” hypothesis. Existing BORs were also upgraded.
The ARC appreciates the strong effort made during LS1 on installing additional diagnostics toidentify the origin of the sudden loss events.
Since the restart of the machine, checks, calibration, and adjustments took place. A first SBLlike event was observed on the 8th of March and others later, but the analysis performed so
far could not confirm the “fireball” hypothesis. Focus is presently on defining and establishinga systematic analysis method using beam oscillation, beam loss and acoustic data.
The ARC is very pleased to see the efforts and careful analyses made on this important topic
that is limiting the performance of the collider.
Recommendations:
R12.1: Given the promising results from the new monitors, further improve the SBL
diagnostics such as by additional beam oscillation monitors, bunch-by-bunch beam-size
monitors, and even more beam-loss monitors around the ring.
R12.2: Revisit “usual” effects that could explain the SBL - for instance, coupled-bunch TMCI,
electron cloud, arcing or trapped electrons in ion pumps, NEG pumps heated by HOMs, power
supply regulation errors, feedback-related perturbations, beam-induced collimator motion or
jaw deformation, …- and motivate their rejection by experimental observations.
R12.3: Continue the further development of the “fireball” model using realistic inputs andbased on experimental data acquired with SuperKEKB and other experimental systems, anda parallel simulation effort.
R12.4: Continue the development for the acquisition of a set of experimental data withsystematic analysis and categorize the different beam loss observations.
R12.5: Identify potential weak areas around the machine and define a set of measures toprotect them, while maintaining the integrated and peak luminosity objectives.

R12.6: Try to establish a statistics of the number of SBLs versus time and beam current, inorder to see if there is any kind of cleaning effect visible or a dependence on intensity.
R12.7: Study the correlation, if any, between SBL and uneven bunch current filling along thebatch.

13. Monitors
Findings & comments:
The ARC is very much impressed that various kinds of monitors, including BPMs (T-B-T andnarrowband), bunch oscillation recorders, beam profile monitors, displacement meters,
feedback system, beam loss monitors, and ML-assisted beam tuning methods, are maintainedor newly installed in LS1, and some are updated. Among these different monitors, the beamprofile monitors and feedback systems work well.
The BPM at the HER injection region was improved for better performance. The visible-lightbeam profile monitors perform well on different beam profile measurements in LER.
Gated T-B-T monitors and beam loss monitors need further maintenance, and a machinelearning assisted beam tuning tool is being developed to optimize the beam injection efficiency
and lifetime, etc. The system shall come into operation by 2024.
Recommendations:
R13.1: Continue the effort of adjusting the threshold of the beam loss monitors to avoid
unnecessary beam abortion due to small beam losses.
R13.2: Start maximization of injection efficiency with machine-learning (ML) from a random ora lower value than the human-optimized one, to develop the tool of ML.

14. Damping Ring
Findings & comments:
The Damping Ring (DR) of the SuperKEKB positron injection system has been working in asmooth and reliable way since it started operating in February 2018.
The DR is used to store a positron beam at 1.1 GeV for 40 msec. The DR must feature a large
acceptance in order to accommodate the high-emittance and high-energy-spread beam
coming from the Linac. The nominal beam consists of 2 pulses each including 2 bunches. The
ring, 135.5 m long, is based on a reverse-bend FODO cell. This lattice allows short damping
time, of the order of 12 msec, while using low magnetic field strength dipoles, which are themost suitable for a beam pipe having 34 mm diameter.
In recent years, unexpected emittance growth after DR and the small dynamic aperture in the
LER, suggested to reduce the beam emittance in the DR by about 20%.
Moreover, beam orbit drifts and fluctuation made optics modification difficult.
Lastly, an extracted beam orbit drift had been observed, stemming from the temperaturedependence of output current of extraction kickers.

Several attempts to reduce DR emittance resulted in large optics distortion, beta beating andhorizontal and vertical dispersion that could not be recovered by tuning quadrupoles which, inthe present DR configuration, are grouped in six families independently powered.
In this context, during LS1 several studies have been done to better understand the DR optics
and to fix orbit drifts.
It was found that the excitation curve of arc quadrupoles had not been properly evaluated. The
corrected values are consistent with the optimization factors identified during optics
corrections. However, this improvement did not yet result in a reliable lower emittance optics.
Magnetic field measurements have been done on spare dipoles and quadrupoles by using arotating stretched-wire coil instead of the harmonic coil used in the past. New measurements
revealed significant correction factors of the order of few % and 5 -10 % for dipole andquadrupole magnets respectively. The DR team has planned further magnetic fieldmeasurements on the remaining different kinds of spare magnets.
Beam orbit fluctuations and drifts were studied, while storing the beam in the DR. It wasobserved that fluctuations depend on the temperature of the vacuum chamber, which is cooled
by a water chiller unit. Reducing the temperature range of the cooling water slightly improved
slow orbit drifts, but did not eliminate the effect.
Dispersion measurements taken by varying the RF cavity frequency also unveiled a relevant
dependence of orbit on stored beam current.
Extracted beam orbits measured at the LINAC BPM uncovered a slow drift varying with theextraction repetition rate, drift that is more evident for the 2nd extracted bunch. This drift was
induced by temperature dependence of the extraction kicker.
Successful tests demonstrated that the kicker deflection dependence on temperature could
be reduced to about 1/3 by adopting different technical precautions:● using mica capacitors instead of ceramic capacitors,● switching from natural oil cooling to forced oil cooling, which was successfully
operated for over 100 hours, at a rated voltage with the mica capacitors.
Unfortunately, these very promising tests could not be completed in time to be implemented
on the DR kicker system before the beam startup.
The vacuum level of the RF cavity has been significantly improved by replacing an O-ring.
For the nominal optics, the emittances from the damping ring were determined by quadrupole
scans. According to these, the horizontal emittance is consistent with expectation, while thevertical emittance is significantly (about 3 times) smaller than the design value. The
synchrotron light monitor in the DR seems to give a far too large value for the verticalemittance.
The ARC endorses the decision to complete magnetic field measurements on the remaining
kinds of spare magnets, with a special attention to sextupole magnets.
Recommendations:
R14.1: Use turn-by-turn BPM data to study DR optics errors, esp. for the lower emittance
optics.
R14.2: In the case optics studies will not return improvements, try to obtain an optimized lower
emittance optics by introducing more independent quadrupole families.
R14.3: To establish priorities for DR studies, developing an optimized optics with lower
emittance may be more important than understanding orbit & optics dependence on the beam
current.

R14.4: Implement all the technical solutions successfully tested for reducing the extraction
kicker deflection dependence on temperature, which is harmful for the second bunch.

15. MDI
Findings & comments:
The MDI team continues to impress this committee with its multi-pronged effort to further
understand and improve the machine related backgrounds. There have been impressive
improvements in background simulation in tracking down local neutron sources, and in adding
shielding to mitigate the neutron backgrounds. In addition, working together with theaccelerator team, a very strong effort has been presented to attempt to track down the sourceof the Sudden Beam Loss (SBL) events. Many more detectors of various types have been
installed around both rings which should help in tracing the start of the SBL event. The
damaged collimator heads in the LER have all been replaced with new heads, and most of
the HER collimators with damaged heads have also been replaced. Efforts are ongoing tofurther shorten the time between detecting an unstable beam and aborting the beam. Acoustic
sensors have been installed around some of the collimators, and have been tested with beam.
More Beam Oscillation Recorders (BOR) have been installed including a new recorder using
RFSoC (Radio Frequency System on Chip) technology. More beam loss sensors have been
installed, and work is ongoing to link these detectors directly to the beam abort system, thereby
further shortening the time required to abort the beam. Injection backgrounds are also being
analyzed, and related information and signals can be used by the linac team to improve
injection backgrounds and injection efficiency.
Recommendations:
R15.1: Continue to attempt to shorten the time between detecting an unstable beam andaborting the beam.
R15.2: Make a controlled test of the BOR modules by applying a known kick to the beam.
R15.3: Synchronize and build a general display package for all of the loss monitors to makeit easier to identify possible locations of SBL sources.

16. BT
Findings & comments:
Several crucial aspects concerning the Beam Transport system have been addressed, andmany solutions were tested, and in large part implemented during LS1. Upgrades andpreparation work were intended to improve injection efficiency in the main rings, to keep undercontrol emittance growth along the BT, to eliminate spontaneous firing of the ring injection
kickers, and to stabilize machine operations.
A sizable multipole component on the HER septum plate SE1 generated by a fringe field waseliminated by shimming. This multipole component was already identified during the 2022 run.
Its mitigation enlarged the septum magnet stay clear aperture, and improved the septum field
uniformity by 54%. Regardless of such positive results, septum field uniformity is stillconsiderably lower than expected from simulations, indicating that the origin of the nonlinearity
was not completely identified.
QI4E next to SE1 was equipped with a new beam pipe providing a 0.8 mm wider clearance
for the injected beam.
BPMs in the final part of the BTp have been equipped with I-tech Libera-SPE, which arepresently under test. The new hardware will provide the possibility to independently measure
the trajectory of two consecutive trains of two bunches separated by 96 nsec.
A new vertical kicker featuring polarity inversion was installed in the HER Fuji-straight section.
A similar device is in production for the LER. These devices are intended for dedicateddynamic aperture studies.
17 dipole sections located in the ARC1 have been vertically misaligned, in order to mitigate
CSR induced emittance growth in the BTp. It turned out that emittance growth of the e+ beam
was not due to CSR, which has given more weight to the hypothesis that the emittance growthis due to high order components in the magnetic fields in ARC3.
A diagnostic for CSR direct measurement has been installed in ARC1 and preliminarily tested.
Several OTR monitors with 60 m resolution have been added in the BT for emittancemeasurements. However, as the rms beam sizes are comparable with the OTR resolution,these diagnostics need to be upgraded.
A leakage in the LER injection orbit bump, stemming from differences in the injection kicker
ceramic duct shape, was cured by a new K2-type ceramic duct and replacing the old device.
Several accidental spontaneous firing events, ASF, affected the LER injection kicker operation
during the initial phase of the 2024 run. This type of failure can be mitigated by directly
triggering all the other paired kickers when an accidental spontaneous firing is detected.
Recommendations:
R16.1: Render the OTR and CSR diagnostics fully operational, and complete theimplementation of the new BPM acquisition; all the tools should be ready before the summer
shutdown, in time to be used during machine study periods dedicated to injector optimization.
R16.2: Plan a systematic study to unequivocally identify and mitigate the source of the large
emittance growth in the BT, for both beams.
R16.3: Implement mitigation techniques tested in the laboratory to avoid injection kickers ASF.

17. Control
Findings & comments:
The control system for the SuperKEKB facility is literally the “backbone” on which every othersystem depends. This is a legacy system with many layers of hardware and software, andupgrades that are integrated in the existing system over time. The Controls group can be very
proud of their contributions to the success of the SuperKEKB program.
The presentation highlighted the multiple IOC subsystems which are implemented using 6
different IOC platforms. These become obsolete over time. The group has identified a path forthe obsolescent PLC modules and is evaluating what to do about the MVME5500 VME
controller. The MicroTCA platform is being considered for future implementation.
The EPICS software platform is used widely. This is an excellent choice and allows the groupto share software for various functions with other labs.

The new bunch current equalization system was presented, the bucket selection software andhardware for 2 bunch injection is ready. The event trigger and monitor functions have been
upgraded and can support turn by turn BPM triggers and synchronization functions such as
loss monitors with BPM signals. The general purpose nature of these functions can be usefulfor Belle II, too.
There has been a development effort on a White Rabbit based event recorder/timestamp
system for abort monitoring. The hardware effort is largely complete but the software effort isongoing. This system will improve the time resolution of the abort monitors to 8 - 100 ns.
The computing systems are being maintained and expanded, as are the backbone networking
switches and infrastructure. This is a pragmatic task to preserve the functionality as old
computing architectures become obsolete and newer platforms are brought into use. The
Controls group deserves our praise for this balancing act.
Several very modern web-based software tools are being implemented for staff
communications, software development, code version management, etc. An exciting idea isto develop an automated shift log system and to modernize the existing powerpoint based
shift report system.
Recommendations:
R17.1: Be careful with developing new functions to ensure compatibility with legacy functions
that may still need to operate. An example is the integration and operation of the new andoriginal abort monitor systems.
R17.2: Many of the new interesting web based tools may have very short commercial lifetimes,
probably much shorter than the operating life of SuperKEKB. When developing tools on theseweb platforms, try to ensure the codes, databases and records they create can be accessedwith general purpose software, as in the future someone may want to look at historic shift
reports and be able to seamlessly use the legacy and new functions.

18. MR Magnets
Findings & comments:
New skew sextupole magnets, built from new laminated yokes and recuperated coils, for nonlinear collimation, were installed within schedule. They are powered in series with anacceptable difference in transfer function. These magnets have skew quadrupole trim coils. A
new configuration of power supplies is needed for the new optics, and to power the skew
sextupoles and the trim coils. Currently, operation is effected with spare power supplies while
awaiting delivery of the new power supplies.
The IR was refurbished. The initial collision beam offset was about 100 μm which could becorrected swiftly.
It was reported that a problem with the power supply QFROE_4 (quadrupole in the OHO
straight section) is not yet fixed and needs further investigation.
Recommendations:
R18.1: Fix the power converter for QFROE_4.

19. QCS
Findings & comments:
The main work during LS1 on the IR superconducting magnets QCS concerned 1)
modifications of the head of QCS cryostat to reduce background and 2) the repair of thevacuum leak at the QCSR service cryostat. Both activities were completed successfully. The
leakage rate of the QCSR cryostat was high before the repair work. A cooling test in December
2023 confirmed that after the repair the same vacuum pressure could be reached as at the
QCSL, both at room temperature and at low temperature. The very good operational status of
the cooling and of the magnet power supply was presented. Two possible plans for anti-ageing
measures on the old system were discussed and compared, namely 1) overhaul of the electricmotor for the helium compressor and 2) system renewal of the refrigeration control system. It
was decided to overhaul the electric motors for helium compressors annually. The renewal of
the refrigeration control system is under consideration, as well as additional heat exchangers
to reduce usage of liquid nitrogen and to improve the cooling margin of the refrigerator.
The team is congratulated for the stable operation of the QCS magnet power supply andcooling system from the start of the 2024 run. The ARC supports the plans presented.
Recommendations:
R19.1: Add a heat exchanger to reduce liquid nitrogen consumption as proposed.

20. Safety
Findings & comments:
Radiation levels in the Beam Transport (BT) line, in the Fuji and Tsukuba experimental halland in the Oho experimental hall can exceed the limit of <0.2 μSv/h for general areas(averaged over 1 hour). The radiation level in front of the most upstream BT escape exit was
0.8 μSv/h in 25 Hz 2- bunch operation before tuning, mainly due to electron beam losses. The
injection tuning reduced this to an acceptable level of 0.13 μSv/h. The beam losses along the
BT line are monitored using an optical fiber. When a screen monitor is inserted the radiationlevel strongly increases, and the repetition rate must be reduced to 1 Hz. Additional radiation
shields are considered for some of the escape exits. The Fuji and Tsukuba experimental halls
become radiation monitoring areas during beam operation. In the Oho experimental hallradiation level increased due to the installation of the NLC. As a mitigation, new concrete
shielding towards the hall was added during LS1. The beam pipe of the collimator was
surrounded by a 5 cm thick lead shield over a length of 5 m. Simulations of the radiation levelin the experimental hall as a function of length of the lead shield motivate a plan to increase
the length of the shielding to 11-13 m, and to not only use lead, but a combination of lead andstainless steel or polyethylene (for neutrons). The additional neutron shielding is motivated bymeasured neutron radiation levels caused by NLC beam losses from injection. Indeed, theradiation to be shielded is caused in roughly equal parts by gammas and neutrons.
The planned installation of a bunch energy-spread compression system (ECS) in the BT line
should not compromise the BT tunnel safety.

In 2023, at J-PARC, two fires were caused by power supplies. One fire (25 April 2023)
occurred in a newly developed power supply for the MR, due to incompatible parts(transformer) in the startup circuit (initial charge). The second fire (22 June 2023) suddenly
occurred in an old power supply, manufactured in 1985, where parts in the polarity changer
deteriorated due to long-term use. Countermeasures for both types of events were taken KEK
wide. In the near future, power supply equipment related to accelerator operation shall be
regularly maintained and inspected by the manufacturer. Power supply units that are older
than ~40 years, especially those with polarity changers, should be updated or replaced.
The Personnel Protection System was modified so that even if there are people in the Tsukuba
Area (Belle II), a beam can be transferred all the way to BT End; that Injection beam tuning ispossible even during long-term shutdown of Belle II (LS1); and that even if there is tunnel
access for Belle II work or work on Tsukuba accelerator hardware, the injection beam can betuned during the day. This change allows for more flexible operation.
Additional radiation shielding will be needed as the beam currents are increased. The scopeof added shielding is smaller and less expensive if it is added as close to the radiation source
as is possible, for example, around the beam pipe. Accessibility for maintenance must be
taken into account.
Recommendations:
R20.1: Continue monitoring and mitigating radiation levels in the BT line, in the experimental
halls, and in the collider tunnel. Always put safety first.
R20.2: Periodically check that the safety inspections of the KEK and J-PARC power supplies
are done on the agreed-on schedule.

21. Vacuum
Findings & comments:
During LS1, an impressive amount of work was successfully completed by the Vacuum team.
Besides the IR work, the collimator repair, upgrade and relocation, each known issue wassystematically tackled and solved. Since the machine restart, the vacuum scrubbing has been
carefully monitored for both MR rings and the DR. After a beam dose of 310-360 Ahr
accumulated at the date of 2024/03/31, the vacuum system of the MR is ready to accept 1.1
A and 1.5 A for the HER and LER, respectively. R&D is ongoing to improve the BPM
supporting system, the collimator robustness, and the pressure monitoring.
Recommendations:
R21.1: Based on the PEP-II experience with NEG pump heating from HOM power, monitor a
few NEG strips feedthroughs with thermocouples for temperature increases with stored
currents. Select NEG pumps that are near collimators or other HOM generating structures.
R21.2: In view of maintaining the robustness and availability of the vacuum system, promote
the early implementation for the detection of pressure anomaly in the daily pressure follow-up
and set-up a consolidation program of the vacuum system when required.

R21.3: In connection with the sudden beam loss investigation, look for the eventual presenceof debris inside the vacuum components dismounted during LS1 (or later), and routinely
inspect the vacuum chambers for particulates.

22. Cryogenics
Findings & comments:
We were presented with the long history of the cryogenic system used to cool thesuperconducting cavities. It was explained that while it still works satisfactorily, spare parts,
notably the turbines, are not readily available, as the manufacturer no longer produces thisequipment. In addition, the last update of the control system was in 2012. Ideally this should
now be updated, but the group hesitates due to the cost. The group plans to continue to use
the present turbines with care, and notes that there is some redundancy in that with two of thethree turbines working there would still be sufficient cooling capacity. The cryo-plant has 8 kW
capacity, upgraded from the original 4 kW. This is more than what is needed for the present
SRF configuration. The plant power consumption is ~2.2 MW. A more modern control system
configuration and bypassing some of the elements could reduce the total power consumption.
The ARC underlines the importance of a working cryogenics system.
Recommendations:
R22.1: Apply for funding to upgrade the control system.
R22.2: Develop a contingency plan as to how one could act should the 35-year-old cryogenics
equipment fail.
R22.3: Consider possible energy savings by improving controls or reconfiguring the plant.

23. RF (ARES + SC)
Findings & comments :The RF system of SuperKEKB uses superconducting cavity stations as well as the normalconducting energy storage (ARES) stations. Since the last review there have been incremental
improvements in the silicon carbide HOM loads used in the superconducting systems, as wellas improvements in the water cooling systems in anticipation of higher HER currents. As
configured now, the systems are adequate for 2A HER current.
During LS1 the damping ring RF cavities received new metal vacuum gaskets which show
improved vacuum in operation. However, a periodic vacuum bursting was observed during
operation that is well below the trip threshold, but which should be monitored. The gas seems
to be mainly hydrogen, but the source is not understood. The ARES C-cavity systems have
improved water cooling systems which should support 3.6 A LER current. 4 cavities received
new power couplers and have been commissioned at full power.
One ARES cavity (DO5-A) has historically exhibited breakdown. Explorations in LS1 showedarcing damage at an internal braze. This cavity has been replaced with a spare unit. In thereassembly, a sealing area between the SC and CC cavities required a metal gasket but a 0.7
mm gap between the copper cavity flanges was left, as the metal weld bellows was notavailable after two previous reconfigurations. The assembled cavity system has been power
tested at full power.
Power testing of the silicon carbide HOM loads was cut short due to problems with a klystronpower supply. As a result, the LER power limits “are guaranteed at 2 A and maybe OK for 2.5-2.6 A”. This is consistent with estimates from observed heating prior to the shutdown.
Recommendations:
R23.1: Instrument and carefully watch the temperature of the flange in DO5-A which may haveanomalous heating from both accelerating RF currents as well as beam induced HOM power.
R23.2: Repair the klystron test stand to allow resumption of off-line testing.

24. High Power RF
Findings & comments :The two-ring RF systems have been mostly reliable and supported beam operations during
the last run. One of the oldest original Tristan klystrons suffered end of life failure after 130,000
hours due to increasingly frequent internal breakdowns and was replaced with a spare one.
There are several other old tubes, raising concerns about their life span. However, only one
other tube has failed over the last 3 years. There are six spare klystrons, three of original type
and three of upgraded type. Rebuilding of existing tubes is no longer economical. New
klystrons can still be purchased but delivery is ~2 years. Anti-ageing maintenance has been
continued to keep the RF stations operating reliably.
One Klystron Power Supply (KPS) failed at the rectifier cable connection on restarting after
LS1. The cause is under investigation. There are 21 KPSs, 19 of which were built in the 1980’s,
but they have had continuous updates and maintenance. There is an ongoing plan to replaceobsolete components.
High Power RF (HPRF) cooling systems have suffered from severe corrosion, but have beenstabilized with external bracing. This situation should be monitored closely because coolingsystem failure could be serious.
As beam currents progressively rise, the stress on the RF systems will increase. Systems
should be carefully monitored and anti-ageing updates should continue.
RF systems are a major consumer of electricity. The klystrons have modulation anodes toallow active control of output power; however, the old analog controls are not automated. A
new control system is proposed that will allow automatic adjustment to maintain best
efficiency. A possible savings of 1.5 MW over all systems is estimated. The HVPS also havea remote control of the klystron voltage. If maximum power demand is below specification due
to reduced current operation perhaps this can be adjusted to keep the klystrons just below
saturation for maximum efficiency while maintaining headroom for feedback.
Recommendations:
R24.1: Test at least one of the spare klystrons to make sure there is always a readyreplacement in case of failure during the run.

R24.2: Closely monitor the klystron inventory for any further signs of aging.
R24.3: Continue anti-aging maintenance and updates of RF stations to preserve reliability.
R24.4: Continue to seek energy savings such as modulation anode automated control, andremote adjustment of klystron voltages. Study which amounts of energy could be saved, andestimate the effort and time scale to implement the more sophisticated control.

25. Overview of Collective Effects
Findings & comments :The SuperKEKB beam operation has made several breakthroughs: (1) the injection
background effect on the luminosity measurement was removed; (2) the single-beam verticalemittances are <20 pm; (3) the FB noises were significantly suppressed; (4) the LER
impedance was reduced by the NLC; (5) the orbit around the CW sextupoles is better
controlled; and (6) both LER and HER can operate around their design working point in thetune diagram (.53, .57).
At present, the vertical beam-beam tune shift is about 0.017 in the HER and 0.025 in the LER.
These values are lower than previously reached, and a factor 3.5 or 2.5, respectively, below
design.
The luminosity of SuperKEKB is determined by an interplay of different effects.
The combined effect of all known impedance sources yields a CSR-driven MWI threshold at
an LER bunch current around 1.2 mA, which is less than the design value of 1.44 mA.
A “-1 mode” instability is caused by the interplay of vertical impedance and resistive bunchby-bunch feedback with a threshold around 0.5 mA. This instability can be cured by fine-tuning
the feedback (introducing a reactive component), as had been done in the 2022 run.
Measured coupled-bunch instability rise times are 2-3 times longer than expected from theory.
No significant electron-cloud effects are seen in the LER, since installation of solenoids in drift
spaces in 2017.
A larger number of tune scans were carried out both with single beams, and in collisions withand without crab waist.
Recommendations:
R25.1: Explore the limit of the beam-beam tune shift at higher intensity in operation with a few
bunches.
R25.2: Optimize the crab waist strengths for actual operating conditions.
R25.3: Apply ML/AI techniques to IP optics correction and luminosity optimization.

26. LS2
Findings & comments :The planning has started for long range upgrades for SuperKEKB to be done during a future
shutdown, labeled LS2. This future shutdown will likely be needed to increase the luminosity
from about 2 x 1035 cm-2s-1 (possible after LS1) to the ultimate level of 6 x 1035 cm-2s-1. Exactly
what future work needs to be done has not yet been decided, as operational results from thepresently upgraded SuperKEKB and injector linac have to come to fruition as well as detailed
calculations, simulations, and advanced engineering are needed before these decisions canbe made. There are three possible upgrade work scopes: 1) Moderate upgrades (relatively
short down), 2) Large scale upgrades (medium length down, e.g. new IP magnets, andcryostats to shorten distance between IP and the first low- quadrupole L* from 935 mm to
835 mm) or 3) Very large scale (very long down with significant preparation time, underinvestigation). Which option is chosen will likely change when the upgrades in LS2 aredecided, scheduled, and pre-shutdown preparation can be specified. A study group involving
magnet, vacuum, diagnostics, Belle II, and industrial teams has been formed to study the
proposed changes and upgrades to the IR. The LS2 project team will give an interim report to
MEXT in roughly JFY2026.
We were shown a plan to use a newly developed Nb3Sn superconductor with 5 micron
filaments, in new stronger quadrupoles with larger apertures to replace the existing first units
in the QCS, to allow a reduction in beta* following a proposed new long shutdown, LS2. A trial
winding was circulated. We encourage this development.
Recommendations:
R26.1: Determine which SuperKEKB accelerator studies need to be done during the next few
years, and when, to allow technical decisions related to the future upgrades to be made at an
appropriate time.
R26.2: Encourage laboratory tests of crucial technologies (e.g. thin Nb3Sn coils) to be done
over the next few years.
R26.3: Periodically monitor the composition of the upgrade study group to make sure all the
relevant skills sets are included.
R26.4: Continue the development work on the new Nb3Sn quadrupoles.

27. ITF
Findings & comments :The two primary goals of the international task force are to define a realistic path to 1035 cm2 -1
s and to develop ideas for reaching 6⨉1035 cm-2s-1.
75 researchers have joined with about 52% from abroad. The key points to address are
sudden beam loss, impedance, beam tuning (time fluctuation and current dependance), beam
blow-up and injection issues. For the long-term strategy a large modification of the IR might
be needed but this is a difficult topic.

Many activities are on-going with successful results: Mitigation of CSR (FERMI, Italy),machine learning to improve injection performance, sudden beam loss understanding, planof simulating beam-collimator interaction (under consideration at CERN), new methods tocorrect local coupling at IP (CERN, applied at LHC), TBT BPMs for optics measurements(CERN), stability of cooling water system, strong-strong beam-beam simulations with fulllattice (KEK, IHEP, and CERN), and "PyHEADTAIL" estimates for transverse-mode-coupling
instability (TMCI threshold is ~1.8 mA/bunch and introducing a non-linear collimator system
will raise the threshold to ~2.1 mA/bunch.)
Support from EAJADE for trips has allowed numerous visits from collaborators.
There is a tentative proposal to reorganize ITF into a Working Group with chairpersons being
the three KEK Division Heads, aiming at organizing one workshop per year with summary
reports addressed to the ARC. The next step is still to be defined.
A framework for joint hardware development is more complicated but should be explored. The
next onsite workshop could profit from the occasion of the collider conference eeFACT 2025
in Tsukuba.
Recommendations:
R27.1: Define and implement the next steps for the ITF-IWG transition soon, perhaps inconsultation with the ITF WGs.
R27.2: Further increase engagement from the international community in all aspects of theSuperKEKB effort.

28. Luminosity expectations
With current operational conditions (Plan A) it is not possible to reach 1035 cm-2s-1. Reaching1035 cm-2s-1 at betay*=1mm would require 2.26 A in LER and 1.61 A in HER (Plan B). The
Plan C is to squeeze betay* to 0.8 mm. The challenges at injection would be the 2-bunch
scheme, and the high repetition rate of 23-25 Hz.
The requirement from the Belle II detector of 150 fb-1 per month was established assuming
100% efficiency and they would be happy with 200 fb-1 by summer. Highest priority is runningstably at 1035 cm-2s-1 to justify future runs.

Appendix A: KEKB Accelerator Review
Committee Members
Frank Zimmermann, Chair

CERN

Ralph Assmann

DESY

Vincent Baglin

CERN

Paolo Craievich

PSI

John Fox

Stanford University

Andrew Hutton

JLab (excused)

In Soo Ko
committee)

POSTECH (excused & now retired from

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. Lab., Ex Officio Member

Makoto Tobiyama
Member

KEK, Head of Acc. Division III, Ex Officio

Mika Masuzawa
Member

KEK, Head of Acc. Division IV, Ex Officio

Hiroyasu Ego
Member

KEK, Head of Acc. Division V, Ex Officio

Appendix B: Agenda of the 27th KEKB
Accelerator Review Committee meeting
March 25 (Monday)
08:30 - 09:00
Executive Session
09:00 - 09:10
Welcome09:10 - 09:40
Summary of Upgrade Work (LS1)
09:40 - 10:10
2024a Run
10:10 - 10:40
Belle II Status
10:50 - 11:20
Injector Overview
11:20 - 11:50
RF Gun13:10 - 13:40
Accelerating Structure
13:40 - 14:10
Pulsed Magnets / Power Supplies
14:10 - 14:40
Linac Control Monitors
14:40 - 15:10
Injection
15:20 - 15:50
Optics
15:50 - 16:05
Nonlinear Collimator
16:05 - 16:35
Sudden Beam Loss (SBL)
16.35 - 17:05
Monitors
17:05 - 17:35
Damping Ring
18:00 - 20:00
Executive Session
March 26 (Tuesday)
08:30 - 09:00
Executive Session
09:00 - 09:30
MDI
09:30 - 10:00
BT
10:00- 10:20
Control
10:30 - 11:00
Magnet11:00 - 11:30
QCS
11:30 - 12:00
Safety13:00 - 13:30
Vacuum
13:30 - 14:00
Cryogenics
14:00 - 14:30
RF (ARES + SC)
14:30 - 15:00
High Power RF
15:10 - 15:40
Overview of Collective Effects
15:40 - 16:10
LS2
16:10 - 16:40
ITF
16:40 - 18:00
Report writing
19:00 - 21:00
Executive Session
March 27 (Wednesday)
08:30 - 11:00
Executive Session
11:00 - 12:00
Close-out

M. Tobiyama
Y. Ohnishi
K. Trabelsi
M. Satoh
M. Yoshida
H. EgoT. Kamitani
T. Suwada
T. Yoshimoto
H. Sugimoto
S. Terui
H. Ikeda
G. Mitsuka
M. Tawada

H. Nakayama
T. Mori
H. Sugimura
S. Nakamura
T. OkiT. Mimashi
K. Shibata
K. Nakanishi
T. AbeK. Watanabe
D. Zhou
M. Masuzawa
Y. OhnishiAppendix C: Targeted & achieved Super-KEKB(SKB) parameters, comparison with KEKBParameter

KEKB w/ BelleSKB 2022b

SKB 1 April 2024

SKB design

LER

HER

LER

HER

LER

HER

LER

HER

E [GeV]

3.5

8

4

7

4

7

4

7

x* [mm]

1200

1200

80

80

80

60

32

25

y* [mm]

5.9

5.9

1.0

1.0

1.0

1.0

0.27

0.30

x* [nm]

18

24

4.0

4.6

7.0

4.9

3.2

4.6

y* [pm]

150

150

~50

~50

~38

~31

8.6

12.9

I [mA]

1640

1190

1321

1099

845

680

3600

2600

nb

1584

2249

2345 + 1

2500

Ib [mA]

1.04

0.75

0.587

0.489

0.36

0.29

1.44

1.04

y

0.098

0.059

0.04070.02790.02770.0197

0.069

0.060

Lsp [1030cm-2s-1mA-2]

17.1

71.2

82.9

214

L [1034 cm-2s-1]

2.11

4.65

2.03

80

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

Appendix D: Glossary of acronyms
ARES
ASF
BPM
BOR
BT line
BTe
BTp
CSR
DOE
DR
ECS
HER
HOM
IP
IR
ISR
ITF
KEKB
LER
LS1
LS2
MDI
MR
MWI
NEG
NLC
OTR
PEP-II
PF
PF-AR
QCS
RF
RFSoC
SBL
SC
SRF
TBT
TMCI

Accelerator Resonantly coupled with an Energy Storage (warm RF cavity
system at KEKB and SuperKEKB)
Accidental Spontaneous Firing
Beam Position Monitor
Beam Oscillation Recorder
Beam Transport line (between linac and collider rings)
Beam Transport for electrons
Beam Transport for positrons
Coherent Synchrotron Radiation
Diffractive Optical Element
Damping Ring
Energy Compressor System
High Energy (electron) Ring of SuperKEKB
Higher Order Mode
Interaction Point
Interaction Region
Incoherent Synchrotron Radiation
International Task Force (set up to boost SuperKEKB performance)
Former B factory at KEK, predecessor of SuperKEKB
Low Energy (positron) Ring of SuperKEKB
Long Shutdown 1 (from summer 2022 to end January 2024)
Long Shutdown 2 (expected towards the end of this decade)Machine Detector interface
Main Ring (at J-PARC)
Micro-Wave Instability
Non-Evaporable Getter
Non-Linear Collimator
Optical Transition Radiation
Former B factory at SLAC/Stanford in the United States
Photon Factory
Photon Factory Accumulator Ring
Final focusing superconducting quadrupole magnet system
Radio Frequency
Radio Frequency System on a Chip
Sudden Beam Loss (not fully understood beam loss events at
SuperKEKB)Super Conducting
Superconducting RF
Turn-By-Turn
Transverse Mode Coupling Instability