ARC Intermediate ITF Review
March 7, 2022; revised March 19, 2022

On 25 February 2022, the 5th meeting of the International Task Force (ITF) forSuperKEKB was combined with an intermediate ITF review by the ARC.
The ARC participants at this review were R. Assmann, J. Fox, A. Hutton, C. Milardi,
K. Oide, E. Perevedentsev, Q. Qin, R. Rimmer, J. Seeman, M. Sullivan, T. Taylor,R. Tomas, and F. Zimmermann (Chair). P. Chiggiato and M. Poelker had sent excuses.
The review featured reports from the ITF working groups, by A. Morita, T. Ishibashi,
D. Zhou, and M. Satoh. In a closed ARC session on 28 February, an additionalpresentation was delivered by M. Masuzawa, which summarized highlights and lessons
from the fall 2021 run.

General Remarks
1. Future ITF subgroup meetings could be announced to all KEK accelerator divisions tostrengthen the cooperation and to better include the other divisions in the ITF activities.
2. It is important to assign sufficient time to machine studies together with extra manpower(from inside and outside KEK) in order to unravel the limitations in optics, impedanceand beam-beam.

OpticsThe optics sub-group has organized 3 meetings where the following topics were presented:● 1st sub-group meeting @ 2021/09/22: https://kds.kek.jp/event/39396/– Jacqueline Keintzel, “Turn-by-turn optics measurements in SuperKEKB”– Leon Van Riesen-Haupt, “Experience in porting SKEKB lattice to MADX”– Hiroshi SUGIMOTO, “Optics Correction at SuperKEKB - Overview and Issues -”● 2nd sub-group meeting @ 2021/10/13: https://kds.kek.jp/event/39767/– Pantaleo Raimondi, “SuperB Final Focus chromatic correction section with the IPphase sextupoles”● 3rd sub-group meeting @ 2021/11/17: https://kds.kek.jp/event/40129/– Yunhai Cai, “SuperKEKB LER Dynamic Aperture analyzed by using LEGO”
The turn-by-turn optics measurements presented by Jacqueline showed compatible results withthe usual COD technique used in SuperKEKB for the linear optics parameters, however withapparently lower resolution. Further developments of the technique could improve theperformance but due to lack of time no further measurements were taken.

In the non-linear optics regime Jacqueline uncovered some discrepancies between model
predictions and beam measurements, e.g. in the amplitude detuning and the non-linear
chromaticity. The source of these discrepancies should be examined further.
Leon showed important progress with the modeling of FCC-ee and SuperKEKB in MADX;
however, a final lattice reproducing the SuperKEKB optics in MADX was not reported yet.
Sugimoto-san presented an extensive view of the optics related limitations that may affect theSuperKEKB performance. Long and non-reproducible optics tuning was reported as one of thelargest concerns. For example, it was observed that the quadrupole strength of the final doublet
changed with time after a quench.
Pantaleo proposed to insert a new weak sextupole (SLW) around the βy waist between QC2Pand SLY. First attempts of using such a sextupole did not improve lifetime in a simplified modelof the machine. Results should be discussed with Pantaleo.
Yunhai Cai proposed a new arc sextupole arrangement with a larger momentum acceptance(increasing from 0.9% to 1.4%) for LER, and first measurements with these sextupole settings
were performed in the real machine. Disappointingly, the measured Touchek lifetime of thislattice is lower than for the operational SuperKEKB lattice. Extensive tuning of the machine with
the new configuration might be needed before a real conclusion can be drawn. This fact that theTouschek lifetime decreased, instead of greatly increased, implies that existing machine models
are incomplete and that unknown machine errors play a critical role in the machine
performance.
For comparing different sets of DA, plots showing the DA in a 2-D plane such as Z-X, produced
from a common code, are necessary. Just a single number such as "momentum acceptance" isnot enough to fully characterize the situation.
The beam-beam sub-group reported that reliable measurements of chromatic coupling at the IP
are not available. In addition, the SuperKEKB IR modeling is difficult, which prevents assessing
their codes and slows down developments. They would benefit from more complete models ofthe machine based on both ideal descriptions and measurements. They also reported that
reducing the vertical IP beta functions will decrease the vertical beam-beam tune shift andhopefully improve performance. A larger optics change is requested to increase the LER
momentum compaction factor by 36%. This lattice has not yet been developed.

Recommendations:
1. Increase effort on non-linear modeling of SuperKEKB rings to improve model
predictability. This comprises off-momentum optics and physical aperture
including collimators. Beam measurements, e.g. off-energy optics
measurements, might be key to improving the model. K-modulation could help
understand final doublet misalignments and the applied orbit corrector
strengths.

2. Improve interaction and communication with collaborators for the preparationand follow-up of specific studies (e.g. the studies for Yunhai’s and Pantaleo’soptics).

TMCI
The vertical blowup of the single LER beam above about 0.8 mA / bunch, related to the -1
head-tail mode, is a major concern, and this phenomenon also seems to affect the beam-beamperformance. The exact mechanism causing the vertical blow up has not been identified.
Much effort has gone into improving the impedance model of the machine and the codes havebeen modified to allow the transverse impedance to be split into separate regions and scaledwith the local beta functions. This effort has revealed two main regions of concern, the IR andthe collimators. It is still not clear which of these is dominant. The QCS is split into two sections;however, the beta function is still varying strongly inside these two regions. It might beappropriate to subdivide those regions further to increase accuracy. Bunch lengthening andtune shift measurements closely agree with the simple formulae. However, these data do notexplain the mode -1 instability. Both beam-beam collisions and feedback can suppress the blowup, to some extent. Repeating the tune shift measurements with collimators retracted (carefullyafter injection) may help determine whether collimators or IR are the dominant source ofimpedance. One could also open or close individual collimator gaps.
Comments:● Congratulations on the excellent progress and systematic studies.

Recommendations:
1. Complete the revision of the impedance budget mainly for the terms comingfrom the IR. A detailed understanding of the different impedance contribution willbe crucial in view of the IR upgrade. Perform impedance calculations by
combining both sides of the IP.
2. Establish the frequency-dependent transverse impedance and simulate thesingle-bunch dynamics with the corresponding realistic short-range wakefield.
3. Develop a theoretical framework beyond a usual wake(?) for a region with alarge variation of beta function, like the IR.
4. Future beam studies aimed at investigating impedance coming from IR and atevaluating -1 mode instability threshold could be done, if possible, by opening
collimators in order to sort out different contributions.
5. Perform tune spread measurements along the LER bunch train, in single
beam operation mode, in order to further investigate the source of the beam blowup.

6. Further investigate the instability threshold dependence on the transverse
feedback configuration.
7. Study whether operation with negative chromaticity (plus feedback) cansuppress the vertical blow up.
8. Examine the RF voltage needs for planned large-momentum-compaction
optics.
9. Measure and quantify the magnitude of any longitudinal bunch oscillations.

Beam-beamFindings
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Simulations show that specific luminosity can be reduced by slow vertical emittance
blowup for the HER beam and by bunch lengthening due to longitudinal wakefields
Is the crab waist scheme as effective as predicted (could observations be explained by
not applying 100% crab waist strength)?
Vertical emittance blowup: beam-beam instability (insufficient. crab waist strength,
imperfect crab waist?) —> crab waist strength is shown in simulation to work with 80%(LER), while 40% strength (HER) is not good enough, but limits arise from dynamic
aperture.
Extra decay in specific luminosity is suspected to be possibly due to higher-thanexpected longitudinal impedance and extra bunch lengthening (confirmed by streakcamera?) or due to microwave instability driven by longitudinal Impedance
Vertical emittance blowup could be caused by chromatic coupling, by transverse beamcoupling, or by noise. A correction of the chromatic R1 parameter was successfully
demonstrated in the LER by tuning rotatable sextupoles; a correction of the LER
chromatic R2 will be attempted in Run 2022a; the correction of the chromatic coupling in
the HER using skew-sextupoles has not been tried yet.
The horizontal emittance blowup is significant in the LER. This was improved throughhorizontal tune adjustments.
Strong-strong BB simulations face CPU time issues (2 months for plot presented) andsome physics issues (non-Gaussian beams, beam coupling impedances, full lattice,chromatic coupling at IP, realistic IR model including realistic crab waist). Probably still to
be done is a full optimisation of crab waist strength & betatron tunes for realistic machineconditions.
Mitigations considered:○ Increase strength of HER crab waist○ Refine chromatic coupling○ Extended tune scans○ Reduce impedance (open Coll)○ Increase momentum compaction○ Squeeze beta y functions at IPComments:● Congratulations for the very thorough and detailed work. This is impressive.

Recommendations:● The high momentum compaction optics (when/once available) should be triedin operation before the shutdown.
● Clarify the performance reach of the crab waist scheme, its operational range
in usable strength and its impact on reachable specific luminosity and on
dynamic aperture.
● Consider using supercomputer facilities or an upgrade of CPU power for theimportant and very insightful strong-strong beam-beam simulations.
● Complete the inclusion of both longitudinal and transverse beam coupling
impedance model in the beam-beam simulation tools● Study the effect of measured magnitude of longitudinal bunch oscillations (ifany) on the simulated beam-beam performance.
● Simulate the specific luminosity versus bunch-current product including thespace charge. Carry out simulations for different crab waist strengths, otherthan the actual settings (80%/40%).
● From an operations point of view, refine chromatic coupling optimization
which could give a rather easy gain in terms of vertical blowup control.

Injection
The linac subgroup has been quite active, but only with participation from within KEK. Outreachto international partners should be strongly encouraged, e.g. CSR modeling is an active topic in
the light source community.
The straight injection line using the PF-AR injection tunnel has a great potential to improve theinjection efficiency drastically, by reducing the regular synchrotron radiation and the R56
coefficient by an order of magnitude. The beam line itself is a conventional transport line and notdifficult compared to other fancy devices for the linac. A design study must be started
immediately.
Simultaneous top up injection into 4 rings has been operating smoothly. Various linac upgrades
including replacing old linac structures have been carried out. The RF gun is operating stably
after recommissioning following the arcing event. The diffractive optical element is in routine use
and provides a more uniform beam profile. Emittance of the second bunch in 2-bunch injection
is still a problem. Degradation of the laser window was noted and will be replaced. New pulsed
quads will be installed before/after the J-ARC section for optics matching of each beam mode.
This may help to decrease beam loss at J-ARC. The new flux concentrator for positrons isworking well. 3 nC charge has been achieved. Emittance growth after BT1 is still a problem. A
new transport line with much less bending may be helpful.

Findings:●●●

2nC, 20um must be extended to 4nC with 40/20 um (H/V)
Gradual decrease of bunch charge during run - The reason was traced to laser window
deterioration. Excellent that this was identified. Any other possible effects like this?
Operation of structures at 20 MV/m instead of 8 MV/m. Problems with high field emission
and discharges - 4 year plan for new acc. structures, 4 installed and 12 to go.

Recommendations.
● Identify ways to reduce the 50% loss of positron intensity in linac. Fullsupport for large aperture pulsed quad for up to 10 nC. Will it also reduce thefractional loss and increase transmission rate? What is the expectation?
● Do we have CSR calculations? What are the predictions now and for the newbend chamber?
● Look into the design of a straighter injection line and examine its
performance.
● Try to involve additional collaborators and experts from both inside andoutside KEK.