THE SUPERKEKB HAS BROKEN THE WORLD RECORD OF THE
LUMINOSITY
Y. Funakoshi∗ , T. Abe, K. Akai, Y. Arimoto, K. Egawa, S. Enomoto, H. Fukuma, K. Furukawa,
N. Iida, H. Ikeda, T. Ishibashi, S. Iwabuchi, H. Kaji, T. Kamitani, T. Kawamoto, M. Kikuchi,
T. Kobayashi, K. Kodama, H. Koiso, M. Masuzawa, K. Matsuoka, T. Mimashi, G. Mitsuka,
F. Miyahara, T. Miyajima, T. Mori, A. Morita, S. Nakamura, T. Nakamura, K. Nakanishi,
H. Nakayama, A. Natochii, M. Nishiwaki, S. Ogasawara, K. Ohmi, Y. Ohnishi, N. Ohuchi,
K. Oide, T. Okada, T. Oki, M. A. Rehman, Y. Seimiya, K. Shibata, Y. Suetsugu, H. Sugimoto,H. Sugimura, M. Tawada, S. Terui, M. Tobiyama, R. Ueki, X. Wang, K. Watanabe, R. Yang,
K. Yoshihara, S. Yoshimoto, T. Yoshimoto, D. Zhou, X. Zhou, and Z. Zong,
KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract
The SuperKEKB broke the world record of the luminosity
in June 2020 in the Phase 3 operation. The luminosity has
been increasing since then and the present highest luminosity
is 4.65 × 1034 cm−2 s−1 with 𝛽∗𝑦 of 1 mm. The increase of theluminosity was brought with an application of crab waist,by increasing beam currents and by other improvements inthe specific luminosity. In this paper, we describe what wehave achieved and what we are struggling with. Finally, we
mention a future plan briefly.

INTRODUCTION
The purpose of SuperKEKB is to search for a new physics
beyond the standard model of the particle physics in the B
meson regime. SuperKEKB consists of the injector Linac,
a damping ring for the positron beam, two main rings; i.e.
the low energy ring (LER) for positrons and the high energy
ring (HER) for electrons and the physics detector named
Belle II. The beam energies of LER and HER are 4 GeV
and 7 GeV, respectively. The design beam currents of LER
and HER are 3.6 A and 2.6 A, respectively. The design
luminosity is 8 × 1035 cm−2 s−1 . More detailed parametersof SuperKEKB is described elsewhere [1]. The Phase 3
beam operation started in March 2019 and has continued
until now. An initial report on the Phase 3 operation is shownelsewhere [2]. In this report, we summarize the progress ofSuperKEKB after IPAC2020.

OPERATION HISTORY
The history of machine operation in Phase 3 is shownin Fig. 1. In the figure shown are the history of the HER
beam current, the LER beam current, the peak luminosity
and the total integrated luminosity (delivered and recorded
values) from the top to the bottom, respectively. Both in
the beam currents and the luminosity, there has been a great
progress since IPAC2020 held in May 2020. Table 1 shows acomparison of machine parameters in 4 cases. In comparison
between the parameters at present (June 8th 2022) with thoseon May 1st 2020, the peak luminosity has increased about∗ email: yoshihiro.funakoshi@kek.jp

Figure 1: Operation history in Phase 3.

by a factor 3. This increase of the luminosity has been
mainly brought by the increase in the beam currents and theimprovement in the specific luminosity. In both cases 𝛽∗𝑦 is
the same value, 1 mm. With the similar bunch currents, thevertical beam-beam parameters have been improved. This
indicates that the specific luminosity has been improved. In
comparison between the parameters at present with thoseachieved by KEKB, the peak luminosity at present is morethan twice higher than the achieved value at KEKB. But if
we compare the present beam performance with the design
values of SuperKEKB, we are still at an early stage of the
project.
A comparison of the peak luminosity of various colliders
as the function of the CMS (center-of-mass system) energy isshown in Fig. 2. On June 17th 2009, KEKB set an luminosity
record of 2.11 × 1034 cm−2 s−1 . This record was bettered by
LHC in May 2018. On June 15th 2020, the luminosity ofSuperKEKB broke the world records which had been set by
LHC. The highest luminosity accomplished by SuperKEKB
on June 8th 2022 is 4.65 × 1034 cm−2 s−1 .

Table 1: Comparison of machine parameters.

Ibeam [A]# of bunches
Ibunch [mA]𝛽∗𝑦 [mm]𝜉𝑦Luminosity [1034 cm−2 s−1 ]Integrate luminosity [ab−1 ]*) values in high bunch current study

KEKB
Achieved
LER
HER
1.637 1.188
1585
1.033 0.7495
5.9
5.9
0.129 0.090

SuperKEKB
2020 May 1st
LER
HER
0.438
0.517
783
0.5593 0.6603
1.0
1.0
0.0236 0.0219

2.11
1.04

1.57
0.03

SuperKEKB
2022 June 8th
LER
HER
1.321
1.099
2249
0.58730.48871.0
1.0
0.04070.02790.0565∗ 0.0434∗
4.65
0.41

SuperKEKB
Design
LER
HER
3.6
2.6
2500
1.440
1.040
0.27
0.30
0.0881 0.0807
80
50June 1st 2020, the same setting of the crab waist sextupoles
has been used up to now. Effectiveness of the crab waist

Figure 2: Comparison of luminosity of various machines.

LUMINOSITY IMPROVEMENT
Crab waist scheme
In March 2020, we decided to introduce the crab waist
scheme, which was an option in the design of SuperKEKB.
The motivations of the introduction were in the following.
The beam-beam performance was poor in spite of all of knob
tunings for improving it and it was limited by beam-beamresonances which can be suppressed by the crab waist. Thisis the second application of the crab waist scheme following
DAΦNE [3] for actual collider machines. The crab waist
scheme was realized by making an intentional imbalanceof strength of sextupole magnets in the vertical local chromaticity correction section. The crab waist scheme wasintroduced by following the steps described blow.
• 2020 March 16th : LER crab waist (40%)• 2020 March 24th : LER crab waist (60%)• 2020 April 24th : HER crab waist (40%)• 2020 June 1st : LER crab waist (80%)
Here, the strength of the crab waist (crab waist ratio) is also
shown. The strength (imbalance) of the crab waist sextupoles
which brings the complete crab waist is 100%. The lower
crab waist ratio means the weaker crab waist sextupoles(weaker imbalance). Since the setting in the final step on

Figure 3: Comparison of specific luminosity of different
crab waist settings.
is shown in Fig. 3. In the figure, the green dots show thespecific luminosity without the crab waist. The others showthat with crab waist and the pink dots correspond to thatafter the final step. Here, the specific luminosity is definedas the total luminosity divided by the number of bunches andby the bunch current product. As is seen in the comparison
between the green dots (w/o crab waist) and the pink dots
(w/ crab waist), the specific luminosity was improved withthe crab waist and the improvement is higher as the bunch
currents increase. In addition, the bunch currents could be
increased with the crab waist. Without the crab waist, thebunch current product was limited at around 0.38mA2 dueto the beam-beam blowup. With the crab waist, we could
increase the bunch current product up to over 0.5mA2 . Thisis also a benefit of the crab waist. As a side effect of the crab
waist, it was expected that dynamic aperture shrinks and thelifetime decreases. In the case of 𝛽y∗ = 1mm, however, no
lifetime decrease was observed in both LER and HER. Thiswas because the narrow physical apertures at collimators

determine the lifetime. In the case of lower 𝛽y∗ , simulations
showed the lifetime with crab waist will set a strong limit.
The experimental result that the crab waist improves the specific luminosity is supported by the beam-beam simulations
as is shown in Fig. 4. While the green line in the graph shows

currents eventually, without further background mitigation.
To reach the design luminosity, an upgrade of crucial detector components is foreseen (e.g. short lifetime conventional
PMTs for TOP (Time of Propagation) counter). The beamgas BG in LER is expected to be lowered in the process of
vacuum scrubbing. We also expect that BG will be loweredby IR radiation shield reinforcement. On the other hand, theluminosity related BG will increase with a higher luminosity.

Bunch-by-bunch feedback gain
In May 2021, the luminosity increased by lowering gain of
the bunch-by-bunch feedback system in HER. The feedback
system has two loops and the feedback gains of the bothloops in the vertical direction were lowered by 4dB. As aresult of this gain change, the luminosity increased by ∼ 25%.
Noise mixed in the FB system affected the luminosity. The
noise was caused by a troubled module. Since the noise
frequency was near the betatron tune, its effect was large.

Squeezing 𝛽∗𝑦Figure 4: Beam-beam simulations without the crab waist
and with the crab waist. Experimental data taken in 2021
are also shown.
the result of the strong-strong beam-beam simulation without the crab waist, the black line shows that with crab waist(LER:80% and HER:40%). In both cases, the longitudinal
impedance was considered in the simulations. Effectivenessof the crab waist scheme is clearly demonstrated in the figure.
Other data in the figure are experimental data taken in 2021
with the crab waist. If the simulation reproduced the experimental data correctly, the experimental data would agreewith the black line. In reality, however, there is a large discrepancy. As for this disagreement, we will mention again
in the following section.

Increasing beam currents
Increasing the beam currents has been one of the maincauses of the luminosity improvement. As shown in Fig. 1,
we have been increasing beam currents gradually with fighting with several obstacles which are listed in the following.
• Hardware damages due to fast and large beam losses• Detector beam background• Beam injection• Beam aborts• Beam instability
Of those obstacles, the fast and large beam loss has put us themost serious restriction. As is addressed in the following section, frequent hardware troubles on collimators (and BelleII sub-detectors) happened when the bunch current in LER
is larger than 0.7 mA. The recent increase in beam currents
was achieved by increasing the number of bunches while respecting the limit from bunch current limit (< 0.7mA/bunch).
Current beam background (BG) rates in Belle II are acceptable and well below limits and Belle II did not limit beam
currents in 2021 and 2022. It will limit SuperKEKB beam

Figure 5 shows a history of 𝛽∗𝑦 in various machines. Also
as for squeezing 𝛽∗𝑦 , SuperKEKB is the front runner in theworld. In usual physics run, the machine is operated with 𝛽∗𝑦of 1 mm. In 2020 and 2022, we tried to squeeze 𝛽∗𝑦 down
to 0.8mm. Figure 6 shows the specific luminosity achievedin 2021 and 2022 as function of the bunch current product.
The orange dots show the specific luminosity with 𝛽∗𝑦 of
0.8 mm and others are data with 𝛽∗𝑦 of 1mm. The specificluminosity with 𝛽∗𝑦 of 0.8 mm was higher than those with𝛽∗𝑦 of 1 mm. The operation with 𝛽∗𝑦 of 0.8 mm was a shorttime trial and we could not store higher bunch currents due
to poor injection efficiency. We will retry it again and expect
a higher luminosity than that with 𝛽∗𝑦 of 1 mm in near futurewith improving the injection efficiency. In the data in cyan
and blue, estimated values of 𝛽∗𝑦 in HER were less than 1
mm, 0.8 mm or lower, with the setting value of 𝛽∗𝑦 of 1 mmdue to horizontal orbit change in SLY’s depending on the
total beam current. Here, SLY means the sextupoles for thevertical local chromaticity corrections near IP (Interaction
Point). We achieved an unexpectedly higher luminosity withthis unexpectedly low 𝛽∗𝑦 .

PERFORMANCE LIMITING ISSUES
Fast and large beam loss
We have encountered frequently events where the beam islost very fast and largely. The events occur in both rings but
the LER beam loss is more serious. Figure 7 shows a typical
data of the large beam loss event. As is seen in the figure,more than a half of the beam current was lost within 3 turns.
Almost no beam oscillations were observed in both horizontal and vertical directions before the beam loss, althoughsome vertical oscillation was observed in some other events.
No beam size blowup was observed using the turn-by-turnbeam size monitor before the beam loss. The large losses
often cause damages of the vertical collimators and the damage brought increase of detector beam background. In somemost possible candidates. Investigations are ongoing to fully
understand this issue and countermeasures are being sought.

Figure 5: History of the IP vertical beta function (𝛽∗𝑦 ) in
various machines.

Figure 7: An observed event of an LER large and fast beamloss as function of time. The top row: horizontal oscillationfrom Bunch Oscillation Recorder (BOR). The second row:vertical oscillation from BOR. The third row: data of bunch
current monitor (BCM). The bottom row: amount of beamloss (from BCM). The BOR amplitude is the product ofoscillation amplitudes and bunch currents.

Beam injection

Figure 6: Specific luminosity achieved in 2021 and 2022 asfunction of the bunch current product.

cases, the loss causes a QCS (superconducting magnets nearIP) quench. In other cases, the loss causes a damage of BelleII sub-detectors. The frequency of these events has been
increasing as the total beam current increases. Based on
experiences of the events which occurred during the period
from March to mid-May 2022, we worked out an empirical rule to prevent the events that the bunch current must
not exceed 0.7 mA per bunch. The recent increase in beam
currents was achieved by increasing the number of bunches
while respecting this rule. We have been very conservative inincreasing beam currents, particularly bunch currents. Thisissue limits the speed of increasing beam currents and thenslows down increase of luminosity. The mechanism of thefast beam loss has not been understood well. A hypothesis
was proposed to try to explain the event in our team. In the
hypothesis, a microparticle heated by the beam-induced field
causes a macroscopic vacuum arc and the beam is kickedby the vacuum arc. We will continue to study this hypothesis. A joint Belle II-SuperKEKB team has been working
to identify the original places of fast beam losses. Recent
progress shows collimators near the injection region are theWe summarize the SuperKEKB injection scheme below. The injector Linac provides the e+ and e- beams (e+:thermionic gun, e-: RF gun). The DR is used for the e+
beam. Synchronization between the injector and the rings
allows 1-bunch or 2-bunch injection per pulse.Top-up injection is achieved for e+ and e- beams at 50 Hz at maximum(sum of e- and e+). We will face a serious problem that the
maximum beam currents in the rings are determined by thebalance between the charge sent from Linac and the charge
loss due to beam lifetime. The shorter beam lifetime atsmaller 𝛽∗𝑦 due to the decrease of dynamic aperture requiresa more powerful injection. Conversely, injection sets a limiton the achievable 𝛽∗𝑦 . This issue will become more serious
in future with smaller 𝛽∗𝑦 . However, even now with 𝛽∗𝑦 = 1
mm, the beam currents are limited by the beam injection
sometimes when the injection efficiency becomes low. Typical values of injection efficiency with 𝛽∗𝑦 = 1 mm are 50 %and 40 % for LER and HER, respectively and it changes
largely depending on machine conditions and tuning. The
injection efficiency depends on various parameters; machine
tuning conditions, collimator setting, bunch currents, 𝛽∗𝑦and so on. Improving the injection efficiency will be an
importance subject hereafter. One of the important issues
on the injection efficiency is emittance preservation in Linac
and the beam transport (BT) lines. The design values of normalized emittance of the e- beam are 40 and 20𝜇m, in thehorizontal and vertical directions, respectively. They are 100
and 15𝜇m, for the e+ beam. From Linac to the beginning ofBT, the design emittances are almost realised. However, atthe end of BT, the emittances get larger by a factor of 3 or
5 in both beams and in the both directions. The emittancegrowth depends on the bunch charge. The effect of CSR(Coherent Synchrotron Radiation) is suspected as the cause.
We need to study further and to find countermeasures. In
future, we also need to increase the charge of the Linac beam.
The achieved values of the charge of the e- and e+ beams at
the end of BT are 1.5 nC and 3 nC per bunch, respectively
as a result of continuous efforts. The design values are 4 nC
per bunch for both beams.

Beam-beam performance
As shown in Fig. 4, observed luminosity performanceis much lower than simulations. This has been and will
be a challenge at SuperKEKB. Candidates of causes of thediscrepancy are machine imperfections such as non-zerolinear and chromatic coupling and dispersions at IP, beamcurrent dependent optics distortion due to orbit change at
QCS and SLY’s and so on. Imperfect crab waist scheme
and interplay of beam-beam interaction and beam coupling
impedance could be the causes. The recent observationthat luminosity degrades by ∼ 4% during LER injection
can explain a part of the discrepancy. For improving beambeam performance, beam-beam simulations predict better
performance with smaller vertical emittance in the single
beam, which is a challenge of optics corrections, and a higher
crab waist ratio in HER. Other possibilities are identificationof causes of the discrepancy between the simulations and theexperiments and finding better working points. As for the
beam-beam parameters, achieved values of 𝜉 𝑦 ’s in physics
runs are 0.0392 and 0.0269 in LER and HER, respectively.
Achieved values in high bunch current collision study are
0.0565 and 0.0434. By increasing bunch currents in physics
run, higher 𝜉 𝑦 ’s and then a higher luminosity are expected.

Impedance related issues
In SuperKEKB, the apertures of vertical collimators are
set very close to the beams. The half aperture of the verticalcollimators is set about 2 mm or narrower and its impedance
would cause the TMCI (Transverse Mode Coupling Instability) particularly in LER. We have intensively studies their effects. We have observed vertical beam-size blow-ups around
0.8 mA/bunch in LER with single-beam operations, andthis value is about 50% or more lower than an expected
TMCI threshold. When the beam-size blow-ups have been
observed, a peak corresponding to 𝜈 𝑦 − 𝜈𝑠 appears and so
we call this “-1 mode instability”. The impedance in vertical collimators contributes to this instability, and opening
apertures of them can increase the threshold. The verticalbunch-by-bunch feedback system with a standard setting
enhances this instability, and its tunings can suppress theinstability. The mechanism of the -1 mode instability is still
under investigation, but we have found two ways to deal withthis instability; (1) Tuning of the vertical bunch-by-bunch
feedback, (2) Reducing the impedance in the vertical direction by opening vertical collimators. The second point is oneof motivations to introduce the nonlinear collimator. The
apertures of vertical collimators scale as 𝛽∗𝑦 , TMCI will set
a limit on the bunch current. Results of the machine study

on TMCI in LER are summarized below. With the use of 2
vertical collimators and taking into account the impedancefrom the high-𝛽 region around final focus quadrupoles, the
TMCI threshold will be lower than the design bunch currentof 1.44 mA when 𝛽∗𝑦 < 0.6 mm. By introducing a nonlinear
collimator, we can raise the threshold or use more vertical collimators and meanwhile reduce Belle II BG. Coupled
bunch instability from the resistive wall impedance and fromthe electron clouds has been well suppressed by the bunchby-bunch feedback so far. The longitudinal coupled bunch
instability caused by fundamental mode impedance of RF
cavities has been well suppressed by -1 mode dampers inboth rings. In the current beam condition (4 or 6 ns bunch
spacing, < 0.7 mA/bunch), no significant beam size blowup
due to the electron clouds effects has been observed in LER.

FUTURE PLANS
SuperKEKB will be shut down from July 2022 to September 2023. We call this shutdown as Long-Shutdown 1 (LS1).
The main purpose of LS1 is to install additional VXD’s (vertex detectors) and to replace a vulnerable part of PMTs of
the TOP counters. In this opportunity, the following works
will also be done on the accelerator side.
• IR radiation shield reinforcement for BG reduction• Installation of a nonlinear collimator for impedance
and BG reduction• Replace collimator heads with robust ones in LER• New beam pipes with wider aperture at HER injection
point for improvement of injection efficiency• others
Within 1 or 2 years after LS1, we will aim at the luminosityof 1 × 1035 cm−2 s−1 with 𝛽∗𝑦 = 0.8 mm. We will also try tosqueeze 𝛽∗𝑦 down to 0.6 mm.
To squeeze 𝛽∗𝑦 down to design values (0.27 mm in LER
and 0.30 mm in HER), further upgrade works will be required, including an extensive IR upgrade to improve beam
lifetime. We have a plan to do those upgrade works in LongShutdown 2 (LS2) in around 2027. The upgrade plan isbeing studied. The international task force for SuperKEKB
upgrade has been organized and in action since July 2021.
On important issues at SuperKEKB, the SuperKEKB team isworking together with the task force in four working groups;
optics, beam-beam, TMCI and injector Linac. The missionof the task force is to bring ideas and exchange notes to solve
various problems at SuperKEKB as a luminosity frontiermachine, to achieve SuperKEKB design luminosity.

REFERENCES[1] T. Abe et al., “Technical Design Report of SuperKEKB”,
https://kds.kek.jp/indico/event/15914/.
[2] A. Morita, “Status of early SuperKEKB Phase-3 commissioning”, in Proc. IPAC’19, Melbourne, Australia, May 2019, pp.
2255-2257. doi:10.18429/JACoW-IPAC2019-WEYYPLM1[3] C. Milardi, “DAΦNE setup and operation with the crab-waist
collision”, in Proc. EPAC’08, Genoa, Italy, June 2008, pp.
2599-2601.