The Twenty-Third KEKB Accelerator Review Committee Report

10 July, 2019
Introduction
The Twenty-Third KEKB Accelerator Review Committee (ARC) meeting was held on July 810, 2019. Appendix A shows the present membership of the Committee. All committee
members attended the 23rd meeting. The meeting followed the standard format, with two daysof oral presentations by KEKB staff members, followed by discussions between the Committee
members.
The Committee welcomes the new Heads of KEK Accelerator Divisions III (Yusuke Suetsugu)
and IV (Makoto Tobiyama).
The Agenda for the meeting is shown in Appendix B. The slides of the presentations areavailable at http://accphys.kek.jp/indico/conferenceDisplay.py?confId=125. Appendix C
compares the required and achieved SuperKEKB beam parameters with those of KEKB.
Appendix D summarizes parameters of the injector complex.
Since the 22nd review, SuperKEKB has carried out the Phase 2 commissioning from March
to July 2018 and an initial Phase 3 running, with nearly the full Belle II detector installed, fromMarch to July 2019. The Committee examined the progress of the project and the present
challenges.
As always, the high standard of the presentations impressed the Committee. The Committee
was also pleased to see many presentations from newly hired KEKB staff. As highlighted in aprevious report, the next generation is important for the success of SuperKEKB operation overthe coming decades.
The ARC is concerned that the total number of KEKB accelerator staff continues to decrease.
International collaboration could be expanded, e.g. with CERN and IHEP, which may provide
additional expertise and resources.
The ARC also suggests organizing dedicated small reviews on specific topics more frequently
between the main ARC meetings, if necessary or helpful. These can be domestic orinternational, depending on the issue and situation.
The most important recommendations of the Committee were presented to the KEKB staff
members before the close of the meeting. The Committee wrote a draft report during themeeting that was then improved and finalized by e-mail among the Committee members. The
report is available at http://www-kekb.kek.jp/MAC/ .

Contents
A) Executive SummaryB) RecommendationsD) Findings and Comments
1.
Overview of SuperKEKB Status
2.
Present performance and plans
3.
Belle II status
4.
Beam-beam issues
5.
Optics analysis and issues
6.
Beam background (Belle II)

7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
A)

Beam background (Inj. Tuning)
Beam aborts status
QCS status and plans
Collision tuning (feedback, dithering)
MR Magnet System
Beam instrumentation at SuperKEKB
RF system status
Vacuum system status (collimators)
Fire at Nextef
Recovery of injector linac
Injector beam operation
Injector RF and LLRF
Injector beam monitors
RF gun, laser and electron beam commissioning
Positron source
Status of beam transport lines
Emittance preservation
Control system (timing system)

Executive Summary

SuperKEKB has carried out the Phase 2 commissioning from 19 March 2018 to 17 July 2018,
and already operated 3-4 months in Phase 3, from 11 March to 1 July 2019. During these two
running periods the vertical beta functions of both beams were squeezed in steps, down to 3
mm, and even to 2 mm with detector off, yielding peak luminosities of about 5x1033 cm-2s-1 and1.2x1034 cm-2s-1, respectively. The  y* of 2 mm is almost 3 times smaller than at KEKB andsets a new world record for storage-ring colliders. The commissioning time to reach aluminosity of 1034 cm-2s-1 was 5 times shorter than for the previous KEKB. In Phase 3 the BelleII detector is almost fully installed. An integrated luminosity of about 6/fb was delivered to BelleII during about two months from end of April to June 2019.
At present, the most important challenges are:1) fast beam losses leading to collimator damage, QCS quenches and beam showers hitting
the Belle II pixel detector;2) high detector background dominated by beam-gas scattering in the LER, which limits thebeam current and minimum beta*, and jeopardizes the integrity of the detector; and3) luminosity tuning with significant vertical emittance blow up and low beam-beam tune shift
in collision.
The ARC has formulated recommendations on how to address the above issues; it supports
an ambitious luminosity goal for the coming year.
B) Recommendations: The Committee has made recommendations throughout thedifferent sections below. The most significant of these recommendations and a few
more general recommendations are summarized here.
1. Develop priorities for the next year in collaboration with the Belle II detector group inorder to achieve an integrated luminosity of about 200/fb by July 2020. (R1.1)
2. The ARC endorses the proposed operational goals for Phase 3.1, toward y* of 0.64mm, 2 kAh, 200/fb, 3x1034cm-2s-1 by summer 2020. (R2.1)

3. By autumn 2019, evaluate different operating scenarios, select the one with the bestrisk-reward ratio, and establish a detailed commissioning plan for accelerator andbackground studies. (R2.2)
4. To avoid detector and QCS damage, separate the functions of machine protection anddetector background reduction. The first role can be fulfilled by a robust collimator(lower Z material) placed at an adequate location close to the abort system, far fromthe experiment. The second one is taken by the existing collimators closer to thedetector with a larger normalized gap. This approach implies the installation of
additional collimators in the two main rings. (R14.4)
5. Once the main rings are in a safe condition, i.e. after the installation of the additional
collimators, increase the beam current in steps and develop optimized beam-scrubbing
scenarios to accumulate, before summer 2020, approximately 2000 Ah at the
maximum possible beam current in accordance with the planning of the physics runs.
(R14.6)
6. Ensure that a power converter trip does not cause the QCS magnets to quench. (R9.1)
7. Perform a bakeout (T>100°C for several days) of vented vacuum sectors; evaluate theimpact of the in-situ bakeout on the mechanical integrity of the vacuum system(vacuum chamber expansion, bellow compressions, fixed points). (R14.1)
8. Allocate sufficient machine time for increasing the specific luminosity, including tuning
the linear, chromatic, and nonlinear IP aberrations of both beams, and for optimizing
their offsets, crossing angles, and betatron tunes, in order to increase the beam-beamtune shift and to reduce the vertical emittance blow up. (R5.6)
9. Develop a Crab-Waist lattice to mitigate the beam-beam blowup, even if it may notwork with the final design value of βy*. DAFNE experience clearly indicates that the
Crab-Waist optics can greatly improve the background, in addition to increasing the
maximum beam-beam tune shift. (R2.6 and R5.11)
10. Develop a new nomenclature for the names of beam runs, e.g. “Phase 3.14 etc.” or“Run20S”. (R2.7)
D) Findings and Comments
1. Overview of SuperKEKB Status
The goals defined for Phase-2 commissioning were: 1) to demonstrate positron injection
through the damping ring; 2) collision tuning with QCS (the final focusing quadrupoles); 3) todemonstrate “nano-beam” collision scheme; and 4) to confirm that the background is tolerable
for the vertex detector (VXD) to be installed in Phase-3. All of these objectives wereaccomplished in a rather short time given the complexity of the machine (~4 months).
For the first time, simultaneous beam injection into five rings was accomplished (HER, LER,
Damping Ring, PF and PF-AR). This means that all of the experimental programs at KEK canbe operated at the same time without interference, an achievement that will permit a lot ofscheduling flexibility in the years to come.
The damping ring commissioning was exceptionally fast, and the damping ring has been
operating satisfactorily ever since. A detailed optimization of the beam emittance and studies
of some other beam parameters remain to be done.

Collision tuning with the QCS magnets proceeded squeezing the beam sizes at the interaction
point until the first event could be recorded on 26 April 2018. The increase in luminosity
inversely proportional to the beta value at the interaction point demonstrated that the “nanobeam scheme” was working correctly. The nanobeam scheme had been proposed by
Pantaleo Raimondi at INFN-LNF for the Italian SuperB project around 2006, and tested
successfully at DAFNE with two different detectors. At SuperKEKB, it has now been
implemented, for the first time, with low-emittance beams on a high-energy collider. This is amajor success for the KEK laboratory that will be copied by other colliders in the world.
th

The QCS magnets worked well, meeting all the required specifications. These are extremelycomplex superconducting magnets with many correction coils so this is also a major
success. There were problems with magnet quenches, mostly triggered by beam loss,
possibly induced by beam-dust interaction in the beampipe.
The background was systematically studied and reduced to the point where the go-ahead wasgiven to install the delicate VXD detector, a difficult installation challenge which took eightmonths.
In Phase 2 a peak luminosity of 5.5x1033 cm-2s-1 was reached without Belle II pixel detector
and without Belle II CDC high voltage.
Phase-3 commissioning started in March 2019 and ended just before this ARC Review. The
goals for the Phase-3 were: 1) to start full-scale physics run with the complete VXD in Belle II;
and 2) accelerator and collision tunings with lower beta for higher luminosity.
Phase-3 was interrupted for three weeks by a fire in a building adjacent to the Injector Linac,
which spread carbon deposits on the Injector power supplies and modulators (see Section
15). After the recovery, the beta value at the interaction point was reduced to 3 mm andcontinuous injection in both rings was initiated. The luminosity was increased in parallel witha reduction in backgrounds so as to reach a luminosity of 5x1033 cm-2s-1 with acceptabledetector background.
In late June, the vertical beta value was reduced to 2 mm and a luminosity of 1.23x1034 cm-2s1
was recorded. However, for this luminosity peak, the Belle II detector was switched off due
to high backgrounds.
These results are a significant progress towards the goals of the Phase-3 commissioning
program. However, there are significant challenges for further increases in luminosity. The
beam-beam effect increases the vertical emittance, reducing the specific luminosity. Thiseffect was not expected and the underlying mechanism is still not clear; the blow up depends
on the betatron tunes. The background in the detector (primarily from the LER) is still highand, more importantly, there are background bursts from the stored beam which quench the
QCS magnets and have damaged the VXD detector.
At this time, the Committee sees no fundamental reason why the progress achieved so far will
not be continued in the next run. However, things will get harder and harder, so it is important
to manage expectations.
Recommendation:
R1.1 Develop priorities for the next year in collaboration with the Belle II detector group inorder to achieve an integrated luminosity of about 200/fb by July 2020.
R1.2 Strengthen communication and understanding among the accelerator groups including
linac, the Belle-II group, and foreign collaborators on commissioning procedures and strategy.
2. Present performance and plans
The run from March to July in 2019 was mostly dedicated to the goal of accumulating
integrated luminosity of 6 fb-1, as requested by Belle II. Despite the reduction of the run timedue to a fire in the modulator of a nearby X-band test facility (nextef), this goal was successfully
achieved. On the other hand, due to the short running time, the development of the machine
performance was not fully carried out. The achieved peak luminosity, 1.23x1034 cm-2s-1, wasthe goal of "Phase II". On a positive note, the running time required to reach a luminosity of1034 cm-2s-1 was 200 days, which is 1/5 of the time taken by the previous KEKB collider.
During this run β*x,y was set at (200/100, 3) mm for most of the time (LER/HER), and squeezed
to (80, 2) mm at the end of run. The luminosity performance at 2 mm looked acceptable, but
a good running condition for the detector has not been established yet.
The beam-beam parameter reached 0.036/0.020 (LER/HER) in the 2 mm optics. These values
are still much lower than the goal (∼0.07). Several issues are observed in the beam-beamphenomena:a) Blowup of the vertical emittance of the LER, even at a very low bunch current.
b) Slow degradation of specific luminosity with an increase of bunch currents.
Several causes are suspected for the blowup including higher order parameters such as
chromatic coupling, but not yet identified. Most first-order knobs have been scanned except
the vertical crossing angle, which has been left untouched until the last week of the run. A
scan of the vertical angle was indeed effective and improved the luminosity by 20%. Thus allother knobs must be rescanned again, and then again, to find the optimum.
For most of the time, the bunch current of positrons and electrons did not satisfy the energy
transparent condition, i.e., inverse of the beam energy ratio. Unlike for the previous KEKB,
there should not exist any a-priori constraints on the beam parameters, such as due to electron
cloud, so that the basic collision tuning should be performed starting from the energy
transparency condition.
The observed bunch lengthening in both rings introduces a significant drop of the specificluminosity, by about 20% for the typical bunch intensity of the last running period. Thus, thiseffect should be taken into account in the luminosity analysis.
A future plan has been proposed as an example including  y* squeeze, beam scrubbing dose,
peak and integrated luminosities. This plan shown in the second day of the review squeezes
the betay* down to 0.64 mm, adds 2 kAh of beam scrubbing dose, and accumulates anintegrated luminosity of 200/fb by the summer ; thus, it is more or less consistent with BelleII’s plan for the next year. We may define this accelerator commissioning phase as "Phase
3.1".
The collimation issue will be the most annoying issue for the beta squeeze. Currently the IP
upstream collimator of the LER (D02V1) has been set at about 60 σy, including the beambeam blowup. It is a little bit smaller than the physical aperture of QC1 quadrupole (70 σ y).
Since the optics between the collimator and the IP is fixed through the beta squeeze, the beam
size at the collimator increases as βy* is reduced. For the design βy*, the collimator aperture
becomes 22 σy, assuming the same level of the blowup as today.
The designation of Phase 1, 2 and 3 for accelerator and detector commissioning worked wellover the past several years separating the various work packages that needed to be done.
However, labeling the next 10 years of accelerator operation as part of Phase 3 will not be
descriptive enough to allow staff to efficiently plan for future work. Making a new nomenclature
would be good with a new set of names for future beam runs. As an example, each run could
be called by the calendar year and whether it will happen in the fall or spring (e.g. Run 20F or
Run 21S).

Recommendations:
R2.1: The ARC endorses the proposed operational goals for Phase 3.1, toward  y* of 0.64mm, 2 kAh, 200/fb, 3x1034cm-2s-1 by summer 2020.
R2.2: By autumn 2019, evaluate different operating scenarios, select the one with the bestrisk-reward ratio, and establish a detailed commissioning plan for accelerator and backgroundstudies.
R2.3: Do basic collision tuning around the energy-transparent condition. Confirm that eachscan covers a sufficient range for every parameter. Repeat each set of scans many times until
the luminosity saturates. Mitigate the limitations of knobs due to heating or capacity of power
supplies, if necessary.
R2.4: Develop a collimation scenario down to the design βy*. Try to close the D02V1 collimator
down to 22σy with the current βy*, to check the operability of such a narrow collimation. If it is
not operable, for instance due to tip-scattering, the project may need to prepare special
collimation sections in other places of the ring. Such additional collimators, separated from theIP, would also improve machine protection.
R2.5: The origin of the much larger than expected increase of bunch length with current should
be investigated. Measure bunch length versus current with collimators open and closed, andalso with and without collisions. Attempt to measure or indirectly infer the energy spread andlook for signs of microwave instability. The bunch lengthening must be taken into account inluminosity estimations.
R2.6: Develop a Crab-Waist lattice to mitigate the beam-beam blowup, even if it may not workwith the final design value of βy*.
R2.7: Develop a new nomenclature for the names of beam runs, e.g. “Phase 3.14 etc.” or“Run20S”.
3. Belle II status
The Belle II detector collaboration has worked very hard to get the detector ready for datataking. Over the last summer and fall the temporary detectors (Beast 2) have been removed
from the central region. The SVD and part of the PXD have been installed and are now
working. Only one of two layers of the PXD was ready at this time. Several problems in various
subsystems were uncovered during the Phase 2 running and these have been addressed witha much more robust and improved efficiency for the overall performance. There is still muchto do and more data will help to shake down the detector into a high performance mode.
The backgrounds in the detector are still higher than expected and at the highest currents andhighest luminosity the backgrounds were too high by about a factor of 3 for the detector totake data. It should be noted that the high luminosity reached near the end of the run was witha collimator-setting configuration not optimized for minimizing the backgrounds.
In addition, part of the PXD has been damaged by a very short, sharp radiation burst which
also quenched the QCS magnets. These events are so fast that the current abort system isunable to dump the beam in time to prevent either QCS quenches or high radiation bursts
going into the detector. The high background levels and the possibility of these intenseradiation bursts have made it more difficult to make progress toward improving the accelerator
performance.

Frequent injection background bursts, that are not catastrophic, but trip the CDC and makelarge backgrounds in the outer detector (TOP, ECL), seem to be due to "rogue pulses" in the
linac/BT. These injection background bursts are limiting the present Belle II operation to agreater extent than the steady state (DC) backgrounds, which are mostly from LER beam gas
scattering.
Recommendations:
R3.1: Continue to maintain close relations with the accelerator team. We strongly encourage
the continuation of a background liaison shifter in the control room.
R3.2: Assist the accelerator team in looking for ways of either creating an earlier signal toabort the beam and in coming up with a collimator configuration that can protect the detector
and QCS magnets.
R3.3: The accelerator team should closely cooperate with Belle II to develop countermeasuresagainst injection background bursts (e.g. additional BT collimators, injection phase control,
energy feedback, optics and orbit stabilization, etc.).
4. Beam-beam issues
At the end of June 2019, the SuperKEKB collider has reached a luminosity of 1.23x1034 cm2 -1
s with 1576 bunches and e+ and e- beam currents of 830 mA and 820 mA, respectively, witha large Piwinski angle of greater than 10. This luminosity was achieved during machine studies
without the Belle II detector taking data. In Phase 3, when Belle II was taking data, the peakluminosity achieved was 5.5x1033 cm-2s-1 with 1576 bunches and e+ (LER) and e- (HER) beam
currents of 617 mA and 644 mA, respectively. In Phases 2 and 3 the  y* was lowered from 8
mm to 3mm in several steps, and finally to 2 mm near the end of June 2019 during dedicatedmachine studies. During Phase 3 data taking, the x* was 100 mm in the HER and 200 mm in
the LER, but both horizontal beta functions were reduced to 80 mm at the end of June 2019.
The highest vertical beam-beam parameters for e+ and e- were 0.030 and 0.021, respectively,a very good start. The e- beam-beam parameter was limited by positron beam-beam verticalblow-up at higher currents. With future careful tuning the vertical beam-beam parameters will
increase towards the design value, which is about two or three times higher, for the LER and
HER, respectively.
Many of the accelerator tuning tools for increasing the luminosity have been developed andtested. Optics terms have been optimized including R1 through R4, IP dispersion, andcentering of the horizontal and vertical beam centroids. Feedbacks keep the colliding beams
centered. A factor of two in specific luminosity was achieved by x-y coupling correction (R2)
allowing the vertical IP spot sizes to be lowered from 1.25 micrometer to 0.33 micrometer. The
specific luminosity agrees with the geometrical calculated value at low currents. The R3
correction deals with the skew term in QC1 and needs to be corrected near the QC1 location.
The linear coupling parameters were well controlled. Vertical scans with dispersion corrections
were done at the end of June. The correction of the vertical crossing angle increased theluminosity by 20%. The measurement accuracy of the vertical angle is about 0.1 mrad andthis angle presently needs to be set to about 0.5 mrad for optimal luminosity improvement.
The specific luminosity falls faster with the product of the bunch charges than the ideal beambeam simulations suggest. Most (x2) of the drop in specific luminosity comes when the beam
bunch current product I+I- changes from 0 to 0.05 mA2 , while the desired final value will be
about 1.5 mA2. In detail, for the LER there are two blow-up regions 1) at bunch currents below
0.1 mA and 2) above 0.5 mA, while for the HER only a gradual blow up is seen over the samerange. The beam-beam simulations can match the observed specific luminosity fall-off if skew

sextupole and chromatic Twiss optics error terms are added. These terms will need to becorrected to increase the luminosity and illuminate the next layer of beam-beam issues.
The multi-bunch beam-beam effect was observed to be small.
If head-tail effects are observed in the future, then crab-waist controls may need to be added.
The study of the beam-beam effects in SuperKEKB has made a lot of progress but it is still
early days for many overall studies. Centering the colliding beams, IP angle corrections, andIR dispersion have been initially corrected. The  y* has been lowered systematically overPhase 2 and 3 to increase the specific luminosity. The specific luminosity goes down by abouta factor of 2 with only small bunch charges. In the longer term, the  y* needs to be lowered byanother order of magnitude (from 2-3 mm to 0.27 mm) which will require the beam-beamcorrection algorithms to work better and the IP beam measurements to be more precise. At agiven value of  y*, the specific luminosity needs to be increased by about a factor of two. These
efforts will likely couple directly to increased backgrounds in the detector.
There was some heating observed in the HER during the vertical angle beam scan. The
mechanism behind this heating should be discovered and mitigated.
Recommendations:
R4.1: Finish optimizing the luminosity at a  y* of 2.0 mm, concentrating on the skew sextupole
components and chromatic Twiss corrections.
R4.2: Make a detailed commissioning plan for the accelerator and backgrounds studies during
the next run needed for lowering the  y* from 2 mm to 1 mm and, shortly after, by summer
2020, to 0.64 mm.
R4.3: Identify the most efficient knobs for increasing luminosity, as for example found with thevertical IP angle.
R4.4: Establish a hierarchy of the expected most efficient knobs to be tried in operation forluminosity optimization, including the betatron tunes.
R4.5: Establish and implement a clear strategy how to always re-optimize the full performanceafter changes in a tuning knob (e.g. after the vertical IP angle is changed).
R4.6: Analyze the frequency content in the spiking luminosity jumps seen in the fast Belle-2
luminosity monitor. This could provide insight into the beam-beam processes and provide
further ideas on improving luminosity, avoiding emittance blow-up, and reducing background.
R4.7: Mitigate the beam heating observed during HER vertical angle scans.
R4.8: Include the bunch lengthening with beam current and, possibly, a model of the singlebunch transverse wake fields, in the beam-beam simulations.
R4.9: Study the possible impact of 22 Hz vertical IP orbit oscillations with 40 nm amplitude in
beam-beam simulations.
5. Optics analysis and issues
Optics control in modern colliders is fundamental to ensure machine safety and to maximize
peak luminosity.
Phase 3 optics global commissioning was very satisfactory with the excellent achievement of
reducing y* to 2 mm, a new world record for circular-collider IP beta functions.

Optics parameters at the IP are difficult to measure directly, and they are mainly optimized
with luminosity measurements. This approach shows some limitations such as a weak
response over the available tuning range of some corrector magnets. Stronger tuning magnets
might be needed, as it is being investigated.
Beam based alignment was not performed for Run 3. The vertical crossing angle, identified
and corrected during the run, could presumably have been detected at the start of the runusing beam-based alignment techniques.
Global optics deviations such as beta-beating, spurious dispersion and orbit distortion weresatisfactory, but slightly worse than in previous runs. For example, during the initial Phase 3
run, the vertical beta-beating was 7% in the LER and the horizontal rms dispersion error 20
mm in the HER, while in previous runs the largest beta-beating was 4% and largest horizontal
rms dispersion error 11 mm. The vertical rms orbit distortion has been steadily increasing
since Phase 1. Similarly, xy coupling has also slightly degraded. The permanent magnets
used for e-cloud mitigation do not seem to be the cause, as concluded from the tests
performed by changing configuration of permanent magnets.
HER vertical X-ray beam size monitor system has some smearing effects which limit theresolution at about 7 μm. The measured beam size was found to be independent of the beta
function at the source point for beta values in the range between 7 m and 28 m.
Chromatic and non-linear optics have not been corrected yet, and the first measurement
indicates a significant chromatic coupling.
Turn-by-turn beam position monitor data is not yet part of standard optics commissioning andit is used only in dedicated measurements.
Allocated time and human resources seemed to be not sufficient to address all linear and nonlinear optics commissioning.
Recommendations:
R5.1: Continue with the reduction of  y*.
R5.2: Next commissioning should start with applying beam-based alignment techniques
especially in the IR.
R5.3: Special attention should be paid to global optics deviations in order to reach a similar
optics performance as in Phase 2.
R5.4: Investigate possible new techniques that could help in the measurement of IP optics
parameters. Three examples are: (i) Measuring amplitude detuning, (ii) Measuring tuneshift
versus closed orbit bumps in the IR and (iii) Measuring resonance driving terms from turn-byturn BPM data around the ring.
R5.5: First steps towards chromatic and non-linear optics commissioning should be taken in
the fall 2019 run.
R5.6: Allocate sufficient machine time for increasing the specific luminosity, including tuning
the linear, chromatic, and nonlinear IP aberrations of both beams, and for optimizing theiroffsets, crossing angles, and betatron tunes, in order to increase the beam-beam tune shift
and to reduce the vertical emittance blow up.
R5.7: Exploit turn-by-turn BPM capabilities for optics and resonance driving terms
measurements.
R5.8: Measurements or estimates of momentum compaction factor and second orderdispersion might be useful to understand some issues of the machine, such as the anomalous

bunch length. Momentum compaction factor can be estimated from dispersion or fromsynchrotron tune versus RF voltage.
R5.9: Could vertical beta at HER X-ray source be increased beyond 28 m? At least for a test.
R5.10: If a crab-waist optics is considered for the future, first optics tests should be performed
to assess its difficulty, possibly after the installation of additional quadrupoles and BPMs.
R5.11: DAFNE experience clearly indicates that the crab-waist optics can greatly improve thebackground, in addition to increasing the maximum beam-beam tune shift.
R5.12: Explore collaborations with external laboratories to support the tasks mentioned inthese recommendations.
6. Beam background (Belle II)
The background team of Belle II has been very active in using machine time to separate out
the various background sources. They have been working together with the accelerator team
to better understand the various sources and to concentrate on mitigation of the backgrounds.
The team studied the large Touschek backgrounds seen in Phase 2 and came up with acollimator configuration (that includes a new collimator installed last summer) that wascompletely effective in suppressing the Touschek background for Belle II. This is a very
impressive achievement. This accomplishment indicates that an effective simulator for the
background is now in hand and this should help in giving the team confidence when they study
the next level of background suppression. The background team has concluded that the LER
beam-gas events are presently the dominant term in the Belle II backgrounds. They are now
studying this background source to see how either a new collimator or collimators, or adifferent collimator configuration with the present collimators can improve this background forBelle II.
The discovery of very fast beam loss events in which the QCS magnets quench, detector
collimator jaws are damaged and very high radiation levels hit the PXD detector has becomea major concern. Effort has been initiated to speed up the beam abort signal that comes fromthe diamonds located around the central beam pipe and just under the PXD detector. Thereis also a concerted effort in conjunction with the accelerator team to see if there are otherlocations around each ring in which sensors can be placed that can detect a very early signalthat the beam has become unstable, and allow, thereby, to abort the beam before it candamage any machine or detector component.
Recommendations:
R6.1: Establish scrubbing time to improve the vacuum in the LER. This should improve thecurrent primary beam-gas background. This means initially limited time for physics running in
the fall. However, the combination of more  y* squeezing and scrubbing should improve further
physics running.
R6.2: Care needs to be taken to try to minimize the number of QCS magnet quenches anddamage to collimators until an improved abort and/or protection system is in place (either more
collimators or collimator position changes and/or improved sensor positioning to detect a badbeam in order to induce an abort sooner). The possibility of a non-synchronized abort should
be studied.
R6.3: The background team will need to continue to study the background levels of various
background sources as the machine continues to improve and as lattice parameters change.
An old background may resurface or a new background source may appear. As the luminosityimproves, the background team will need to keep on the alert for the expected luminosity
induced backgrounds.
7. Beam background (Inj. Tuning)
Continuous horizontal injection was successfully set up for both beams. The stability of theinjected beam parameters (orbit, energy, energy spread, emittance, etc.) compared with
Phase 2 was, and is being, improved by a variety of measures and tools, such as energy
feedbacks for the J-ARC, DR and BT (beam transport). The actual injection efficiency is notknown due to large calibration errors of the beam current signals.
An injection tuning procedure was established which minimizes the detector backgroundcaused by the injected beam. The optimum injection phase suggests the presence of a long
longitudinal tail. Betatron tunes were optimized for minimum background during injection. It is
not obvious that these tunes are also the optimum tunes for beam-beam performance.
Injection into LER is extremely stable and does not cause any background concern.
Background from horizontal injection into the HER is also not expected to present any
showstopper for future operation at higher beam current or higher luminosity. However, atpresent, in the horizontal plane synchrotron-radiation photons may hit the detector after asingle reflection.
Beam aborts during injection are seen roughly once per day. 10% of these aborts are causedby sudden energy and orbit changes in the linac. For both LER and HER, new collimators will
be installed in the BT to intercept a low-energy beam. This should avoid beam aborts in caseof a linac klystron failure. The causes for the other 90% of injection beam aborts are still
unknown.
Recommendations:
R7.1: Continue the efforts to optimize and stabilize the beam injection.
R7.2: Improve the calibration of the beam current signals used for monitoring the injection
efficiency.
R7.3: Allocate more machine time for beam background studies, to maintain low beam
background conditions due to HER injection over longer periods of time.
R7.4: Compare injection efficiency and background with or without the other beam present.
R7.5: Simulate the beam background taking into account the experience from Phase 2 & earlyPhase 3 operation, and try to predict the beam background for the future running with higher
beam currents and lower beta function at IP
8. Beam aborts status
The SuperKEKB beam-abort system is presently designed to dump the beams within 37-48
micro-seconds from the interlock event to full extraction of all stored bunches. The exact
duration depends on the relative timing with respect to the beam abort gap and on the numberof bunches stored in the beam. The total dump execution time includes 2 micro-seconds forthe device request, 19 micro-seconds for the interlock processing time, 6-17 micro-seconds
wait time for the abort gap and up to 10 micro-seconds until all bunches are extracted. In
SuperKEKB there are 12 local instrumentation rooms that collect dump requests around thering. For each abort from an interlock a detailed abort analysis is performed.

In Phase 3 operation there have been 5 QCS quench aborts, triggered twice by power supply
trips and three times by beam loss events. The beam loss dump threshold had been loweredon June 5th. Beam losses during some of those QCS quench aborts were severe and havecaused damage not only to some collimators, but also to the vertex detector of Belle II. A
careful analysis of the sequence of events, signals recorded and damage induced waspresented. About 100 signals are connected to the beam abort system.
The team has worked out a list of measures that would reduce the time required for beamabort to 23-28 micro-seconds, 40% faster than presently. The proposed measures include a
speed-up of the quench detection, lower loss thresholds, shorter sampling times for theinterlock from the Belle II diamond loss detectors, optimized cable paths with shorter lengths
and the introduction of a second abort gap.
The experience with beam aborts illustrates the great importance of a well designed andcomplete machine protection and beam abort system. The team has performed a carefulanalysis of what happened and presented quite complete logging files of signals and events.
Several useful measures for system improvements have been worked out.
Recommendations:
R8.1: It is recommended that failure modes in the accelerator systems of SuperKEKB should
be reviewed for completeness and cases like an asynchronous beam dump, fast magnet
failures, ... should be added.
R8.2: It is recommended to install additional “protection” collimators away from the IR,
potentially close to the dump kickers, for defining the closest aperture, for intercepting beamlosses at an uncritical location as early as possible, for avoiding the generation of beam
background close to the IR and for fastest possible beam dump (see recommendations for
vacuum/collimators for more details).
R8.3: It is strongly supported that the Belle II detector establishes an independent interlock
with fast connection to the abort system for an additional layer of detector protection.
R8.4: It is recommended to continue the measures for improving the coverage and reaction
time of the machine protection and beam abort system. An international review might provide
useful input from other colliders with high stored beam energy.
9. QCS status and plans
The committee continues to be impressed by the compact design of the QCS superconducting
low-beta magnet system, and the fact that it is performing optically as expected. While themagnet system itself is reliable, its operation has been marred by numerous beam-induced,
and some power converter-induced, quenches, compounded by the long time required to recool the system due to limitation of the power of the refrigerator. Some of these problems arebeing addressed in collaboration with other sections of the SuperKEKB team. In particular the
beam-induced quenches are part of a global problem of beam-gas (or -dust) interaction andinadequate collimation that also leads to background problems. Some progress has been
made in going from Phase 2 to Phase 3, but as discussed elsewhere in the report there is stillroom for improvement – by introducing aperture limiting collimators, for example. Hardware
upgrades, such as installing tungsten radiation shields close to the QCS vacuum chamber,were suggested and should be considered to reduce the beam background. Could this bedone during the shutdown in 2020? Appropriate circuitry should be included to preclude
quenching due to power converter faults. Although the magnets have been designed such as
to withstand quenching, care should be taken to avoid damaging the compact (and complex)
magnet system.

An associated problem is the time it takes to re-cool the QCS after a quench. This has become
worse, particularly on the right side, presumably due to degraded insulation vacuum – which,
it is assumed, will be repaired during the summer shutdown. However, the installed coolingpower is barely sufficient, and it is proposed to improve the margin by replacing some of thepresent current leads with High Temperature Superconductor (HTS) lower sections cooled byliquid nitrogen at the warm end. This could be effective, the implication on the rest of thesystem (and the total cost) should first be fully evaluated, and compared with other ways of
increasing the cooling power (e.g. boosting by introducing LN2 cooling in the refrigerator
cooling cycle, to be used during the cooldown process).
Given the problem with the bellows connecting the QCS system to the central vacuum tube
consider improving the connection process. This should be done with special care to avoid
applying excessive force to the RF fingers. It would be advisable to set up a quality procedure,or have a check-list, associated with this process.
KEK proposes to study the possibility of replacing some sextupoles and correctors (presently
warm magnets) with stand-alone superconducting units cooled with cryo-coolers, using A15(Nb3Sn, Nb3Al), or HTS, superconductors to take advantage of their higher criticaltemperature. This would enable running at about 10 K at which temperature the cryocooler is
4 times more efficient than at the 5 K required for standard Nb-Ti superconductor. Thesematerials are however brittle and it is recommended that full account be taken of their strain
dependence, and the available strand sizes, before embarking on tests of samples that should
imperatively precede the manufacture of models. Present conductors also feature filament
diameters of at least 50 microns, which has an impact on stability when the conductors areused at the relatively low current density being considered for this application. It is noted that
presently KEK possesses neither expertise nor specialized tooling for the application of A15
materials, but it has built a sextupole using HTS.
Recommendations:
R9.1: Ensure that a power converter trip does not cause the QCS magnets to quench.
R9.2: Investigate the possibility of boosting the cooling power of the cryogenic system.
R9.3: As part of the long term strategy, explore the possibility of increasing the aperture of
QCS to mitigate the beam loss and quench issues in the quest for achieving smaller β y*. Thisshould be done without diverting significant resources from the present program.
R9.4: Determine with the help of energy-deposition simulations (FLUKA, Geant,...) the amountof local beam loss which quenches the QCS.
R9.5: Any future QCS upgrade should take full account of experience gained in using thepresent QCS, and the system optimization (performance, cost, risk) should be done in close
collaboration with the optics team.
10. Collision tuning (feedback, dithering)This year the committee heard a report on the collision tuning dithering system, which will be
required to keep the small beams in accurate alignment at the IP to maximize the luminosity.
The talk lists 17 contributors Who actually knows how this system works and how it is set-up?
What do the phase shifters etc. chassis do? The work is based on the system developed andoperated in PEP-II. It has numerous hardware elements and uses a commercial lock-in
amplifier as the heart of the detection of a luminosity signal. As presented, excitation coils areused to modulate the horizontal position of the LER beam in collision across the HER beam,
which results in a small modulation of the luminosity. A synchronous detection method is used
to provide an error signal for a feedback servo which adjusts the HER beam position tomaximize the luminosity. If the beams move at the IP this system acts to remove thedisturbance and keeps the beams at the IP aligned.
The general principle is clearly shown on slide 3, which highlights the use of a multiplier in thelock-in detection to process the luminosity signal, but does not show the entire signal
processing, especially the low pass filter required to separate the luminosity error signal fromthe higher frequency components.. The slide shows the rationale for the small modulation of
the LER beam. This small modulation acts to generate a detection signal that is the derivativeof the luminosity with respect to offset. The dither excitation excites the beam at a frequency
omega (79 Hz), but as seen in slide 3 and as explained on slide 16 the lock-in has to detectat 2*omega, so the reference signal used in the demodulator must be at 2*omega to detect
the modulation in luminosity at 2*omega.
This small modulation provides an error signal with the sign of error changing at the luminosity
maximum. The sign is critical for the position feedback as it provides a single-valued signal
which determines the direction to move the corrector. The derivative signal has a zero at the
maximum luminosity, and error switches sign on either side. The PI loop should take this error
signal and try to keep it at a reference level of zero.
If the modulation signal is so large as to move the beams completely out of collision, the error
signal is no longer the derivative. Instead, it will have the form of the luminosity vs. offset, so
the errors at offsets on either side have the same sign. In this case, the feedback position
servo does not know which direction to move, and the error signal is not single valued withoffset.
A single overview system block diagram was not provided, and it would be helpful inunderstanding the results. The reviewers were trying to understand the origins of themodulating signal, how the 2*omega term is generated (in the lock-in reference channel?).
There are numerous frequency dependent aspects, for example, the bandwidth and noise of
the luminosity monitor must be considered, the modulating frequency has to be slow comparedto the bandwidth and response of the luminosity monitor. The bandwidth of the lock-in
detection has to be appropriately selected to reduce the noise in the luminosity signal.
Similarly, the bandwidth of the PI servo loop has to be consistent with the bandwidth of thelock-in filter but fast enough to follow the beam motion and keep the luminosity at the
maximum. How noisy is the luminosity monitor at low currents or low luminosity? Do thevarious parameters change at low vs. high luminosity? None of these details were presented
to the reviewers.
The results presented looked at horizontal scans as well as at the impact of coupling between
horizontal dither and vertical offset. Slides 6 and 8 do not show the derivative shape of thedetected error signal. Instead, the magnitude shows a minimum at the luminosity peak,suggesting that something is not right in the signal processing. One explanation is that theexcitation was so large it moved the beams completely out of collision, another is that thedetection was done at omega and not at 2*omega.
A PI loop to position beam is mentioned, but what is the error signal to this loop? The detectedmagnitude? The detected phase? And the timescales and time responses look so slow as to
be of little use. The results show that the position does not settle in 10 minutes, which seems
a strange choice. The system seems to lose lock and the talk comments “The cause is not yet
understood, we are checking the algorithm”.
In summary, the system status is not yet operational, but it will be needed in the future.
We are confident this system will be in operation later during Phase 3 commissioning, andhope these comments are helpful in moving towards the next tests.

Recommendations:
R10.1: A block diagram showing the functions and the nominal signal levels for operation
would be very helpful in checking and operating the system. It should show the major signal
paths and have clear details of the signals from each of the various functions and chassis.
R10.2: A simple operating guide with set-up instructions and enough detail to allow operations
without a special expert is needed. A small group should be defined who can operate andconfigure the system, and this group should do the next beam tests.
R10.3: There are several nested feedback loops and signal processing functions which musthave consistent bandwidths and the excitation of the beam via the dither coils has to beappropriate for the beam size. It is necessary to have a table with important design choices oroperational specifications for commissioning as well as for future operations.
R10.4: The next system tests should be carefully structured to verify the proper small
modulation of the beam position, and to verify the expected derivative form of the detectedsignal before trying to close the PI loop. This requires care in setting up the system, as theexcitation level has to be carefully selected to match the beam cross section and kicker
strength. You need a small modulation, not a displacement so large that the beams go out ofcollision.

11. MR Magnet System
The mission of the magnet group includes not only normal-conducting magnets but alsomagnet power supplies including for QCS, and survey and alignment work for the entire
magnet system. It is recalled that a large portion of magnets of the Main Rings (MR) havebeen reused, and most of magnet power supplies are recycled units after refurbishment.
Despite such difficulties, the magnet group has managed to maintain the performance of theirsystems in a responsible manner. However, the occurrence of repeated problems with QC2LE
is a serious concern calling for a radical solution.
As was to be expected, decayed components of reused magnets such as flow sensors andswitches have shown failures and led to interruption of beam operation. While the flow
switches for HER dipole magnets were all replaced entirely, there are still many quadrupoles
and sextupoles equipped with old switches and sensors and it will take time and resources toreplace them entirely. Adequate planning may be needed to schedule this work so as tominimize the risk of trouble. A similar effort applies to the entire complement of rubber hoses.
We appreciate the clever way of adding coils for skew fields to enable a wider range in the IR
coupling knob scan, as well as the group’s effort to maintain their systems. Also with regard
to the nonlinear optics, it should be possible to correct the chromatic x-y coupling at the IP in
the LER using the rotation feature of the sextupoles to create skew components, but this has
not yet been tested. We expect this to be carried out in the near future.
Recommendations:
R11.1: Prepare adequate planning and resources for replacing old related components suchas switches, sensors, and rubber hoses not yet replaced.
R11.2: Carry out the test of correcting chromatic x-y coupling at the IP in the LER.
R11.3: QC2LE may need a radical approach to solve repeated problems.

12. Beam instrumentation at SuperKEKB
Several relevant R&D activities on beam diagnostics have been carried out during Phase 3
operation in order to maintain and improve the performance of existing tools, as well as todevelop new devices, aimed at optimizing beam parameters and collision setup, which are
expected to be necessary to achieve design luminosity.
Concerning the BPMs of the main rings, during the QCS cryostat insertion one out of the four
signal cables of the MQC1LE/LP beam position monitors in the final vertical focusing
quadrupole magnets got damaged. However, beam orbit measurements were still possible,
although with reduced accuracy, by exploiting the other three electrode signals. Since these
BPMs are important for beam position and optics measurement, it has been correctly decided
to restore their full functionality during the summer shutdown 2019. This operation is not simple
and not without risk, since it requires to disassembly the QCS cryostat located inside the BelleII detector.
The performance of the gated turn-by-turn beam-position monitor has been presented. This
module is based on FPGA, log-amps ADC and a fast gate switch. It has been mainly
developed for beam optics studies and used, so far, for beta and phase advancemeasurements. The new board being integrated in the EPICS control system can profit fromthe Analysis Server providing efficient FFTW and NAFF algorithms for data analysis.
Experimental measurements have shown that phase advance and beta function can be
determined with an accuracy of 0.07 x(2 Pi) and 17% respectively, when exciting the beam byusing the injection kicker.
Several beam size diagnostics tools serve the damping ring and the main rings.
Horizontal beam size in the damping ring is efficiently measured by a gated camera; fit of thedata as a function of the time returns a horizontal damping time consistent with the design
value.
The true resolution of the X-ray monitor was not measured in Phase 3, because the beta
function at the source point could not be sufficiently reduced. However, in Phase 2, a smearing
function on the order of 7 m was measured, which is smaller than any beam expected to be
seen at SuperKEKB.
A new kind of X-ray monitor based on Si-pixel sensor capable to return bunch-by-bunch beam
size has been presented. The monitor architecture integrates a spectrometer chip developed
at SLAC and is equipped with high speed readout electronics. The detector has been
preliminarily tested on the HER and it will be permanently installed during the next summer
shutdown.
Several presentations have shown how the beam trajectory optimization in the Transport Lines(TLs) is crucial for minimizing emittance growth in the TLs and injection background in thecollision rings. The BPMs installed along the TL are equipped with a rather old-fashioned
acquisition system based on obsolete components and working asynchronously. A detailed
plan has been presented to equip at least a subset of such BPMs with Libera acquisition
modules and to replace the acquisition of the remaining ones with a new generation Tektronix
12-bit ADC scope, which will deal with a stream of data merging input from different BPMs.
Large parts of the WS and LM transport line sections have been equipped with optical fiber
beam loss monitors. Such diagnostics tool has been successfully tested and used to verify the
impact of beam trajectory correction on the beam transport efficiency.

The longitudinal bunch-by-bunch feedback is operating stably with a damping time of less than
2 ms at a beam current of 500 mA in the LER. Studies in May 2019 showed the machine had
no unstable modes driven by HOMs, but motion at low modes from the fundamental cavity
impedances could be excited. The LLRF system has dedicated mode -1, -2 and -3 loops which
can be commissioned at higher currents.
The fast vertical IP orbit feedback is being tuned and commissioned. At present, it can
suppress oscillations below 30 Hz, but induces a large oscillation around 230 Hz.
Recommendations:
R12.1: Develop a comprehensive plan for the intervention aimed at restoring the damaged
cable of the MQC1LE/LP BPM in order to avoid other possible inconveniences and to restore
the proper alignment conditions of the components which must be removed.
R12.2: Consider a further development of the gated turn by turn monitor board in order to use
it for accurate single pass beam orbit measurements in the collider rings.
R12.3: Try to improve the X-ray monitor resolution by revising the optical setup and the kindof mask used by the detector.
R12.4: Test the long-term radiation resistance of the silicon-pixel detector of the new X-ray
monitor before installing it on the machine.
R12.5: Provide synchronization for the BPMs in the transfer lines, at least for the ones which
will run under the Libera module control.
R12.6: Investigate the origin of, and try to mitigate, the 230 Hz peak excited by the fast orbit
feedback.
R12.7 Explore the effect of the fast orbit feedback on beam-beam blow-up and luminosity
performance.
R12.8 Try to extract information of intrabeam scattering from the damping of horizontal andlongitudinal emittance of DR. This will provide additional information on vertical emittance, etc.
13. RF system status
The main ring RF systems are operating very well with very few trips and low down time. There
were a few trips of the ARES systems due to breakdown events in the coupling cavity. Theseshould be monitored in case they are a sign of aging, or become more frequent as the beam
current is increased. Two high power water load windows failed, leaking water into the
waveguide. These windows had operated reliably for many years. Failures are thought to bea result of unusual “water hammer” events. Water leak detectors have been installed to give
early warning of future failures. A secondary gas barrier could be considered to separate theload window from the rest of the waveguide, with the water leak detector in front of the window.
A modulation anode controller failed on one station, it was replaced with a fixed voltage divider.
This does not allow optimization of efficiency but otherwise did not prevent running. A newcontroller should be installed when convenient and other systems monitored in case of similar
failure.
The SRF systems are operating well with no reported degradation over time. The newbeamline HOM absorbers are effective and no signs of HOM-driven coupled bunch instabilities
have yet been detected by the transverse or longitudinal feedback systems. Low-modeoscillations have been observed and can be controlled by the dedicated feedback systems.
These oscillations swill become more challenging as currents increase and the feedback
controls must be flexible to accommodate parked cavities and other changes in the RF
system.
Failures of the piezo stacks reported at the last review have been traced to water absorption.
Drying with silica gel can stabilize the piezos in the tunnel. However running only mechanicaltuners without piezos was stable, it should be investigated whether piezos are reallynecessary.
Nine RF stations are running the new digital LLRF system with no issues. The remaining
stations are operating reliably on the old analog system. Plans should be made to phase out
the old systems and to replace them with new ones, as they become unsupportable.
The number of installed klystrons was adequate for Phase 2 and the beginning of Phase 3 but
the system has not been fully converted to the final configuration. Additional new klystrons willneed to be installed to reach the final 1-to-1 configuration. Given the high cost and long leadtime of the high power klystrons it would be prudent to buy ahead to support the planned
current increases.
No issues with the RF system beam loading transients were reported (R21.6 from the 2018
ARC report: Continue to study and understand the gap induced transients in the RF systems
and the effect on luminosity). RF overhead and control margin were adequate in the last run
but as currents increase this will become more difficult. For the next review it would be goodto have a presentation on the transient matching scheme and performance to date.
Recommendations:
R13.1: Monitor water load windows for signs of aging. Consider gas barriers to separate thewaveguide from the water windows.
R13.2: Given the high cost and long lead time of the high power klystrons plan ahead tosupport the planned current increases.
R13.3: Plan for the eventual phase out the old analog RF systems and replacement with newones as they become unsupportable.
R13.4: Implement drying and protection of the piezos and investigate whether these piezos
are really necessary.
R13.5: For the next review prepare a presentation on the transient matching scheme andperformance to date.
14. Vacuum system status (collimators)
In this initial part of Phase 3, the SuperKEKB vacuum team has obtained three crucial results.
First of all, the dynamic pressure rise due to the circulating beam is decreasing as expected
for the LER and it is much lower in the HER, thanks to the accumulated photon dose in the
KEKB era. Then, the electron cloud effects are not present anymore after the installation of
permanent magnets in most of the drift vacuum chambers. Finally, the vacuum system of thedamping ring perfectly fulfils the requirements.
Despite these important achievements, the pressure around the interaction region (IR) is toohigh to ensure a safe running of the detector. The high background, most probably generated
by beam-gas scattering, prevents the detector from achieving its ideal running performanceand spoils its discovery capability.

An attempt to understand the correlation between gas density in different locations anddetector background has been made by heating the NEG pumps: hydrogen released from the
NEG material increases the pressure locally and a simultaneous growth of the detector
background is recorded. This elegant measurements should be coupled with beam-gas
simulations to better understand the detector behaviour.
The pressure profile around the IR indicates that the highest pressures are in the LER at thelocation of the collimators that were changed after Phase 2 2, i.e. D02V1 and D02H1. Even if
all precautions were taken to preserve the surface conditioning, the installation of unscrubbed
components resulted in inevitable pressure bumps at the position of the new equipment. In
the future, the effect of the installation of new components could be mitigated by ex-situ prebaking. After baking, the components should be kept under vacuum and installed in the rings
in a flow of dry nitrogen.
To attain safe conditions for the detector at nominal beam parameters, the dynamic pressurearound the IR should be 10 times lower than the present one. This objective can be tackled in
two different ways. Firstly by baking as much as possible of the two sectors where the recently
installed collimator are located. If the heating is carried out at a temperature higher than 100°C
for several days, an average pressure decrease by at least a factor of three seems to beachievable. However, the local bakeout cannot fully avoid additional beam scrubbing and,possibly, a dedicated beam run. At the present trend, additional 2000 Ah seem to benecessary, obtainable, e.g., by about 2 to 3 months of running at the present maximum beam
current in the LER. More time will be needed if the selected current is lower or if the scrubbing
is performed parasitically during physics runs and low-beta tests.
Another important component that can affect the gas density and the vacuum system integrity
is a bellows of the interaction-point vacuum chamber (HER side). It has been found that theexternal wall temperature rises up to 60°C at the present highest current; a much higher
temperature is expected in the RF shield. These problems have been attributed to a lack ofelectrical contact in one of the fingers. The plan is to change the affected bellows. This will
provoke a loss of the beam pipe conditioning that could be recovered during a dedicatedbeam-scrubbing run.
The most worrying event that affected the collimators system is the beam impact on the jawsof three collimators, two in Phase 2 (D02V1 and D01V1) and another one in Phase 3 (D02V1),
and the subsequent quenching of the QCS magnet and a pressure bump in the position of thedamaged collimator. The beam instabilities developed extremely fast, only in a few turns. After
Phase 2, the scratched jaws were replaced; those damaged in Phase 3 will be replaced in thisyear’s summer or winter.
A possible cause of the beam instability is a falling dust particle, most probably made ofaluminium. The detailed mechanism leading to fast instability after collision with dust is notknown at present. It deserves more attention in view of the large impact it could have on the
future runs at higher beam current.
The consequences of such disrupting instabilities on the machine and detector safety triggereda discussion about the need for additional collimators, the change of their configurations andthe need for a faster beam abort system.
The first action should be to separate, in the collimation system, the role of machine protection
and detector background reduction. In detail, this suggests to add a robust collimator as a firstline of defence, designed for intercepting the beam in dangerous situations, and equipped witha robust jaw material of lower Z. This first-line collimator has to be placed at N phase advancefrom the triplet magnets to protect them; possibly two phases for better phase space coverage:N, (N+0.5), ideally close to beam dump kicker for fast beam extraction. The protection
collimators has to be put deepest in beam sigma, e.g. 15, and be equipped with fast-reacting
beam loss monitors that trigger beam dump in case of excessive losses. The collimators close
to the IP will be kept at a wider setting, e.g. larger than 20, to ensure the protection of thedetector. The setting strategy has to be studied in detail and adapted to the squeeze.
Recommendations:
R14.1: Perform a bakeout (T>100°C for several days) of vented vacuum sectors; evaluate theimpact of the in-situ bakeout on the mechanical integrity of the vacuum system (vacuum
chamber expansion, bellow compressions, fixed points).
R14.2: To avoid excessive dynamic pressure rise, carry out ex-situ bakeout of newcomponents to be installed in the rings and store them under vacuum or dry nitrogen.
R14.3: Investigate a possible change in the design of the RF shields in the IP, so that
impedance-related heating is avoided. For example, analyse the design of the new RF shieldproposed for the HL-LHC project in the triplet magnets area. Check for HOM heating.
R14.4: To avoid detector and QCS damage, separate the functions of machine protection anddetector background reduction. The first role can be fulfilled by a robust collimator (lower Z
material) placed at an adequate location close to the abort system, far from the experiment.
The second one is taken by the existing collimators closer to the detector with a larger
normalized gap. This approach implies the installation of additional collimators in the two mainrings.
R14.5: Remeasure the collimator impedance with the beam, and estimate the total impedancewith the additional collimators.
R14.6: Once the main rings are in a safe condition, i.e. after the installation of the additional
collimators, increase the beam current in steps and develop optimized beam-scrubbing
scenarios to accumulate, before summer 2020, approximately 2000 Ah at the maximum
possible beam current in accordance with the planning of the physics runs .
R14.7: If necessary, repeat the dust removal operation to reduce beam-dust interactions andthe the associated risk of damage for the collimator jaws.
R14.8: Determine with the help of energy-deposition simulations (FLUKA, Geant,...) theamount of local beam loss which may damage the various collimators.
R14.9: Critically revise the mechanical design of the collimators in order to exclude possible
sources of significant HOM heating or discharge events.
15. Fire at Nextef
Around 10:00pm in the evening of 3 April 2019, there was a serious fire at the Nextef facility,
which is adjacent to (but separate from) the Injector Linac. Nextef is dedicated to testing highpower RF structures and has two RF modulators. It has been in operation for over ten years
and there is only a local operator during the day shift. This meant that the fire, which waspreceded by two, rather common faults, was already out of control by the time the fire alarm
was triggered. The cause was identified to be a capacitor encased in a plastic case whichpunctured. This capacitor ignited its nearby neighbor capacitors to make the fire larger. This
fire caused soot to be deposited over a large area of the linac. This kind of capacitor has sincebeen improved by the manufacturer and the new version was installed at the ATF. This kindof capacitor is not used in the B-Factory Injector.
Nextef has initiated a series of corrective actions to prevent a reccurrence. These include:
replacing the plastic-cased capacitors with ceramic cased capacitors (the kind installed in the
SuperKEKB linac modulators), reducing the voltage gradient in the capacitors, performing
routine checks of the capacitors, installing remote visual monitoring via TV cameras, andputting the fire detectors and fire suppression equipment inside the modulators.
The first equipment to be re-installed will be the S-band test stand that is required for the
replacement of accelerating modules in the SuperKEKB injector.
Recommendations:
R15.1: Implement the proposed fire suppression improvements.
R15.2: Prepare the S-band test station as soon as possible.
16. Recovery of injector linac
The impact of the Nextef fire on the SuperKEKB commissioning program wasconsiderable. While there were no injuries or direct damage to the injector equipment, carbon
soot infiltrated the injector equipment room, covering everything. The soot is liable to cause
discharge of the high-power pulsed modulators and could short-circuit the instrumentation. It
was decided to clean all of the components as quickly as possible, but some equipment wasleft un-energized until the summer shutdown in order to speed up the recovery.
The clean-up was extremely well organized and proceeded rapidly with first beam out of theinjector after only three weeks. The linac energy was lowered from 1.5 GeV to 1.35 GeV, but
this did not have much impact on the injection at this stage of the commissioning.
The state of the injector equipment protection was re-evaluated after the fire. Many of thefactors that affected Nextef are not present in the SuperKEKB injector. However, additional
TV cameras will be installed.
Recommendations:
R16.1: Move the fire detection and suppression equipment inside the modulators.
R16.2: High voltage cables outside the cabinet are also flammable and should be protected.
R16.3: Consider providing isolation barriers between sections of the equipment gallery to limit
the damage in case of a fire.
17. Injector beam operation
The injector can perform the stable simultaneous top-up injection to HER, LER, PF, and PFAR with the thermionic gun, RF gun, pulsed magnets and moveable girder in Phase 3, which
increased the integrated luminosity by 237% compared with the normal injection. The HER
injection with RF gun worked well enough for the whole run without any significant problem.
The committee congratulates the SuperKEKB team for these great accomplishments. The
stability of the stored beam current during simultaneous top-up injection is better than 0.5%
in all the rings, and even as small as 0.02% in the PF ring. Energy feedback, beam orbit
feedback, dispersion measurement and correction, and emittance measurement are allcontributing to the good performance of the injector, with an energy jitter at the BT smaller
than 0.025% and a correction of the ~1 mm orbit drift down to ~0.1 mm.
The vertical emittance of the positron beam increases by a factor of 3 after the damping ring ,which could be caused by insufficiently optimized DR extraction.
Recommendation:
R17.1: Try to understand the dilution of vertical emittance of the positron beam after the DR,
by optimizing the beam transport and optics parameters, during and after extraction.
R17.2: Develop a detailed plan for realizing the final goal of the injector performance. The
heating issue is the first priority for increasing the repetition rate of the thermionic gun. Moredetailed studies are necessary.
18. Injector RF and LLRF
The injector and linac RF systems are operating well, supporting simultaneous injection into
four machines plus the damping ring, and fast top-up of LER and HER. The phase control
between the main ring and the linac was improved and many sources of RF drift and jitter
were identified and corrected including intermittent cable connections and pulsed magnet
jitter. Some slow drifts remain, leading to emittance increase over time and injection losses.
Further improvements in timing stabilization are planned for the summer, including the subharmonic buncher.
The damping ring RF operated reliably with two cavities installed. No issues were reported.
Damping ring synchronization and bunch selection timing are working as planned. The beamgate rate was increased without issues allowing more frequent top-up.
Some linac RF equipment was affected by smoke and soot from the nearby fire at Nextef andwas off-line for 3 weeks for clean up. Thanks to a great effort by many people the linac
operation was restored and ran reliably for the rest of the run. The energy of the first part ofthe linac was reduced as a precaution and some diagnostics were not available, but overall
injector performance was good.
Normal top-up injection does not seem to produce unacceptable backgrounds however offnormal pulses can cause spikes in losses. Energy collimation in the transfer line may help.
The team should investigate whether fast enough detection is possible to inhibit injection of
off-normal pulses. Hunting significant sources of jitter in the injector and linac should continue.
New RF monitoring systems are installed on all RF sources and can capture waveforms on apulse by pulse basis and save the last pulse after an interlock. This will be very useful indiagnosing problems and sources of jitter.
Detection of off-normal pulses may be automated to collect statistics or flag systems for expert
intervention. The team should consider developing a machine learning algorithm to filter thislarge data set as is being considered in other places.
Recommendations:
R18.1: Investigate whether fast enough detection is possible to inhibit injection of off-normal
pulses.
R18.2: Continue hunting sources of jitter in the injector and linac.

R18.3: Consider developing a machine learning algorithm to process captured RF waveforms
and other profiles.
19. Injector Beam Monitors
An upgraded injector Beam Position Monitor (BPM) readout system provides position
resolution below the 10 m requirement of SuperKEKB. The system is gated andsynchronized to distinguish the position of the four (or five) independent beams, which ofcourse is exceptionally useful.
Each BPM can be calibrated without beam, and using this feature, it was found thatapproximately 30% of the BPMs exhibit instability (at a small level). This issue will be further
investigated.
The beam size at dispersive regions is related to the beam energy spread. A clever KEK-era
technique based on an 8-channel BPM was re-applied using the new BPM readout system tomonitor the quadrupole moment of the beam, which is related to beam size, and provides a
non-invasive measurement of the relative energy spread of the beam. The technique cannot
distinguish beam size from beam distribution, but nonetheless provides a valuable relative
measurement of energy spread. The method was validated using a nearby view screen.
YAG view screens and improved optical imaging provide higher resolution for beam profile
monitoring compared to older Chromox view screens.
Recommendation:
R19.1: Investigate if sufficient synchrotron light is available at the end of the J-ARC to provide
real-time non-invasive monitoring of the beam energy spread.
20. RF Gun, Laser and Electron Beam Commissioning
To meet SuperKEKB luminosity goals, the biggest challenges imposed on the RF gun were
succinctly stated: the photogun must provide 4 nC bunches with 10 m beam emittance andit must operate reliably. These are very demanding requirements.
It seems the three gun approach is a good design, with the highly reliable thermionic gun
providing beams for the photon factories and the LER, the Quasi-Travelling Wave RF gun
providing beam to the HER, and the third gun (the Cut Disc Structure RF gun) serving as aspare. Both the RF and thermionic guns are operating reliably, but have not yet been required
to operate continuously at full bunch charge. The back-up RF gun is available in case ofproblems with the primary gun.
There remain many choices for RF gun designs, drive lasers and photocathodes, but the
primary QTW RF gun with Ir7Ce2 photocathode, driven with the “simple” Nd/Yb hybrid laser
system proved to be very reliable and fulfilled all the demands of the recent run periods,
exhibiting over four months of continuous operation – a remarkable achievement.
Noteworthy accomplishments that served to improve reliability included the implementation ofswitchable master oscillator oscillators, higher dopant Nd:YAG amplifier crystals to reduce
pump diode currents, and a 3x improvement in QE from the Ir7Ce2 photocathode using a novel
heating plug.
A two-laser beam approach could be implemented to provide some temporal control of thelaser pulses (and thereby explore space charge effects) but most operation to date was
performed with one laser beam.

It seems that adequate diagnostic systems and controls were implemented.
Beam emittance at ~1 nC operating condition was sufficiently good.
The group remains very ambitious and enthusiastic about implementing improvements, withno shortage of ideas.
Recommendation:
R20.1: Proceed as planned, preparing for operation at higher bunch charge.
R20.2: Fully test the Cut Disc Structure RF gun located at the 90degree position at A1 before
replacing it with another RF gun design.
21. Positron source
The flux concentrator (FC) presently installed at the positron source was successfully benchtested at the design current of 12 kA but has only been operated at 3.5 kA during Phases 2
and 3. This was sufficient for recent runs but higher current operation will be required once
Belle II background rates are reduced. There is still concern of high voltage discharge at theflux concentrator. So far this has not limited injection or top-up. The snubber circuit is effective
in reducing the peak voltage. The new flux concentrator was made from non-work hardened
copper. New stronger alloys are under consideration. A detailed mechanical analysis is inprogress and should be completed. The fatigue limit of the materials should be compared, notjust the initial yield strength.
A number of steps are being taken to ensure rugged and reliable operation of the FC at 12
kA: uniform gap between coil windings, gap optimization, snubber circuit to reduce the voltage
during the current pulse. A new FC made of copper alloy will arrive in October.
Additional steering magnets in the area of the solenoids should help address orbit issues.
The steps being taken seem reasonable; however, this work lacks manpower.
Recommendation:
R21.1: Complete the mechanical analysis of the flux concentrator and the evaluation ofalternative materials, including fatigue limit. Continue to study the solid insulation in the gap
and a reliable work-hardening technique.
22. Status of beam transport line (BT)In preparation for Phase 3, some important modifications were done in the Beam Transport
Lines (BTL):
1) To prevent septum field leakage in the MR ducts, the beam pipe was replaced by awhole carbon-steel section; as a result the measured leakage was suppressed by afactor of more than 20, both its dipole and quadrupole components. This has been
confirmed by measuring the longitudinal field distribution and by a horizontal scan. The
leakage field acting on the circulating beam was reduced to BL_max=1.35 e-5
Tm/fringe , B’L_max = 6e-4 T/fringe. It is worth noticing that the same conclusions
could have been obtained, with much less effort, by measuring the variation of thebeam parameters when switching the septum on and off.
2) Wire scanners were improved: a 3 times thinner wire is used, and a 45 degree rotation
is applied.

3) Optical fibers were installed in the BT tunnels for both wire scanners and loss monitors.
They are ready for 2-bunch, 25 Hz, 2 nC/bunch injection.
Beam status is presently marked by a beam loss in the final section of BTL, to be cured throughcareful orbit steering. In Phase 3 operation a few problems were found:
1) The DR extraction septum suffered a malfunction of its main capacitor voltage control
unit, to be followed up during the summer maintenance.
2) The BPM system does not meet the measurement accuracy goals; electronics andsoftware need upgrade.
A collimator will be relocated to intercept energy-modulated beams caused by linac single
pulse faults.
There are future plans for modification of the radiation safety system and the construction ofa beam diagnostics beamline within the BT.
Recommendations:
R22.1: Continue to go forward toward zero beam loss in the BT lines.
R22.2: Refine the design of the diagnostics line.
23. Emittance Preservation
For the NanoBeam collision concept, the quality of the injected beam is a critical issue. So far,the SuperKEKB injection team is working on meeting the injection requirements.
The beam emittance growth is a key concern here. Closely following the recommendations of
the previous Review, a serious work is carried out to identify the emittance growth sources, toanalyse the possible cures, and to implement needed improvements.
The target emittances are (horizontal/vertical) 100/15 μm LER, 40/20 μm HER, with the energy
spread of 0.16% and 0.07%, respectively.
Five potential sources of emittance growth in Linac and BTL were under study:a) Residual dispersion that transforms energy spread into beam size. Indeed, its correctionin the J-Arc resulted in 187-47=140 μm, or almost −50 μm emittance reduction. The KEK teambelieves that in addition to bends, strong quadrupoles and sextupoles give a serious
contribution to residual dispersion, and conclude that orbit feedback is necessary to preserve
emittance values at the linac end. Its operation is successfully tested. In BT the dispersion
correction is not yet done, insufficient BPM resolution is one of the problems. An upgrade of
relevant BPMs is planned.
b) Beam phase space jitter contributes a lot to the linac emittance; dispersion and betafunction mismatch need to be corrected. A few hardware-malfunction sources of jitter(“binarization” of a pulse magnet, RF phase jitter caused by bad cable connections) have beenfound and eliminated.
c) Wakefield in accelerating structure is a reason for bunch-tail oscillations that blow-up the
projection emittance. The effect will be especially serious with 2x2 nC bunches from
Thermoionic Gun in the future. Wake-free steering, to avoid beam orbit offset in theaccelerating structures, will be needed in the next run.

d) Dispersion in accelerating cavities converts energy offset into betatron emittance.
Experimental evidence from ECS on/off is given where horizontal positron emittance showeda 5-fold blowup when the ECS was on. This effect is attributed to residual dispersion at the
ECS RF cavity plus jitter effect.
e) Emittance excitation by quantum fluctuations in BT bends is also addressed; theestimated effect is reported not to be serious.
First results of emittance measurement showed serious degradation at the end of June 2019,
when the linac recovery after the fire accident was presumably not yet complete. In the end of
the BT the measured electron beam emittance is 400/150 μm with a 1 nC bunch; quantitative
understanding of these numbers is still underway. Another planned improvement is realigning
the bending magnets and installing quadrupole magnets in the ECS to cancel the quadrupolar
component of the bending magnets, and. thereby, to correct the residual dispersion at the
ECS accelerating cavities.
The Committee is impressed by the considerable effort paid to understanding the emittance
growth problem and encourages further studies, as were outlined in the recommendations
from the previous ARC review.
Recommendations:
R23.1: Pursue the work on analyzing and understanding the results of emittancemeasurements.
R23.2: Compare the normalized jitter amplitude and its evolution to the beam loss locations
and an aperture model.
R23.3: Determine the frequency contents of the beam position jitter. In case that dominantfrequencies in the position jitter are found, correlate them to possible technical sources.
R23.4: Perform a careful and rigorous study on the timing jitter of the RF trigger and the gun
trigger, which might explain the observed energy jitter.
R23.5: Develop robust convergent algorithms for monitoring and/or minimizing the emittance
growth in the linac and BT lines, suitable for everyday operation.
24. Control system (timing system)
The committee was very impressed by the presentation which highlighted the major progressin the control and timing system for SuperKEKB. This is a major and critical technical function
which is integral with operation of the Linac, the damping ring, the HER and LER rings as well
as operation of the Photon factory. Some of the technical functions are also integral to the
ATF facility. The basic control system provides control/monitoring of over 10,000 discrete
components through more than 150 IOC devices, while the flexible timing system coordinates
every timed function and synchronizes thousands of pulsed devices.
The successful results in Phase 1, Phase 2 and Phase 3 show the strength of the timing andcontrol functions implemented. This is a challenging job, which requires integration ofnumerous legacy modules and operating modes with the necessary functions for the newmachines. The reviewers greatly respect the skill and care the team has brought to the tasks.
They can be proud that every success in the commissioning rests on the core control andtiming functions they have developed.

This review highlighted recent progress in the web-based monitoring systems, theimplementation of a site-wide synchronized timestamp function, and the development ofsynchronizing beam gate and permission functions to control timed elements.
The rapid commissioning of the top-up injection was possible only because the timing andcontrol functions were there and ready. Similarly, the expanded software and hardware
functionality allows more complex damping ring operations and synchronization of the linac,
damping ring and HER/LER rings for future high current operations. The efficiency of the topup injection is seen to be an important result of the timing system flexibility.
Several new archiver software functions have been developed which are shown to be fasterand more efficient than the existing CSS system. These important software functions will benecessary in fault file and system diagnostics for the operating machine.
The design team has made excellent use of modular hardware and software functions thatare shared with other labs and projects. This is an excellent way to leverage the unique skills
at KEK and take advantage of the investments made by the larger community. The use of
EPICS and the modular EVG and EVR hardware functions allow the team to keep the complex
system operational and allow flexible upgrading without lots of interventions and reengineering.
Recommendations:
R24.1: Continue to develop the synchronization and timing functions to support flexible
operations in SuperKEKB.
R24.2: Consider developing a machine learning algorithm or other automated processing toprocess captured RF waveforms and other fault file profiles.
R24.3: Continue to collaborate with partner labs and coordinate on shared software andhardware projects.
R24.4: The presentation highlighted 11 publications in progress, and this contribution to thefield and literature is to be praised and encouraged going forward.

Appendix A

KEKB Accelerator Review Committee Members

Frank Zimmermann, Chair

CERN

Ralph Assmann

DESY

Paolo Chiggiato

CERN

John Fox

Stanford University

Andrew Hutton

JLab

In Soo Ko

POSTECH

Catia Milardi

INFN-LNF

Evgeny Perevedentsev

BINP

Matt Poelker

JLab

Katsunobu Oide

CERN and KEK (ret.)

Qing QinIHEP

Bob Rimmer

JLabJohn Seeman

SLAC

Michael Sullivan

SLAC

Tom Taylor

CERN (ret.)

Rogelio Tomas

CERN

Seiya Yamaguchi

KEK, Director of Acc. Laboratory, Ex Officio Member

Yusuke Suetsugu

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

Makoto Tobiyama

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

Kazuro Furukawa

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

Appendix B

Agenda of the 23rd KEKB Accelerator Review Committee

08:30 - 09:00
09:00 - 09:05
09:05 - 09:20
09:20 - 09:50
09:50 - 10:20
10:40 - 11:10
11:10 - 11:30
11:30 - 11:50
11:50 - 12:10
13:30 - 13:50
13:50 - 14:20
14:20 - 14:40
14:40 - 15:00
16:10 - 16:25
16:30 - 16:49
16:55 - 17:17
08:30 - 09:00
09:00 - 09:22
09:30 - 09:52
10:20 - 11:20
11:35 - 11:57
13:30 - 13:42
13:45 - 13:57
14:00 - 14:22
14:30 - 14:45
14:50 - 15:20
15:30 - 15:40
15:40 - 20:00
08:30 - 11:00
11:00 - 12:00

July 8 (Monday)
Executive Session
WelcomeOverview of SuperKEKB Status
Present Performance and Plans
Belle II Status
Beam-Beam Issues
Optics Analysis and Issues
Beam Background (Belle II)
Beam Background (Inj. Tuning)
Beam Aborts Status
QCS Status and Plans
Collision Tuning (Feedback, Dithering)
MR Magnet System
Beam monitors (XRM, SRM, Loss mon., FB)
RF System Status
Vacuum System Status (collimators)
July 9 (Tuesday)
Executive Session
Fire at Nextef
Recovery of Injector Linac
Injector Beam Operation
Injector RF and LLRF
Injector Beam Monitors
RF GunPositron Source
DR Injection and Extraction Beam Line
Emittance Preservation
Control System (Timing System)
Report writing / Executive Session
July 10 (Wednesday)
Executive Session / Report Writing
Close-out

M. Yamauchi
Y. Suetsugu
Y. Ohnishi
T. Iijima
K. Ohmi
H. Sugimoto
H. Nakayama
N. IidaH. Ikeda
N. Ohuchi
R. UekiM. Masuzawa
G. Mitsuka
T. Kobayashi
T. Ishibashi

T. AbeK. Furukawa
M. SatohT. Miura
F. Miyahara
R. Zhang
Y. Enomoto
T. MoriY. Seimiya(presented by N. Iida)
H. KajiAppendix C

Required and achieved SuperKEKB parameters and comparison with KEKB

parameter

KEKB w BelleSKB Phase 3
w Belle II

SKB Phase 3
w/o Belle II

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

200

100

80

80

32

25

y* (mm)

5.9

5.9

3

3

2

2

0.27

0.30

x (nm)

18

24

2

3.8

2

3.8

3.2

4.6

y (pm)

150

150

?

?

88

61

8.6

12.9

I (mA)

1640

1190

495

496

800

822

3600

2600

nb

1584
0.75

1576
0.314

0.317

1576
0.51

0.52

2500

Ib (mA)

1.04

1.44

1.04

y *

0.098 0.059 0.0333 0.0189 0.0355 0.0197 0.069 0.060

Lsp (1030cm-2s-1 mA-2)

17.1

30.7

29.5

214

L (1034cm-2s-1)

2.11

0.48

1.23

80

*The beam-beam parameter is computed without hourglass factor or any geometric factors.

Appendix D

Required and achieved parameters in the injector complex

KEKB
Achievement

Phase-2
Achievement

Beam

e+

e–

e+

e–

e+

Energy(GeV)Stored current
(A)

3.5

8.

4.

7.

4.

1.6

1.1

1

1

150

200

50

e- 10
→1

1

1400

Stage

Life time (min.)Phase-3summer’19
Achievement

Phase-3
1st Year Plan

Phase-3 Final
Requirement

e–

e+

e–

e+

e–

7.

4

7

4

7

0.83

0.94

3.6

2.6

3.6

2.6

100

20 (typ)

–

–

6

6

1.6

3.6

1.35

70
(typ)
3.5

e- 10
2-3

2-3

e- 10
→4

4

310

200/
5

200/
40

120/6

54/67

100/
15
(H/V)

40/2
0
(H/)

100/
15
(H/V)

40/20
(H/V)

0.13
%

0.13%

n/a

n/a

n/a

n/a

0.16
%

0.07
%

0.16
%

0.07%

2

2

2

2

1

1

2

2

2

2

Bunch charge
(nC)
Norm.
Emittance() (m)Energy spread
Bunch / Pulse
Repetition rate
(Hz)

50

Simultaneous
top-upinjection(PPM)

3 rings(LER, HER, PF)

25

No Top-up

50(LER+PF+PF-AR <25 Hz)
4+1 rings
rings (LER, HER,
DR, PF, PF-AR)

50

50

4+1 rings (LER,
HER, DR, PF,
PF-AR)

4+1 rings (LER,
HER, DR, PF, PFAR)