Highlight fromSuperKEKB Beam Commissioning
Kyo Shibata (kyo.shibata@kek.jp)
On behalf of SuperKEKB Commissioning GroupInter-University Research Institute Corporation High Energy Accelerator Research Organization (KEK)
大学共同利用機関法人 高エネルギー加速器研究機構 (KEK)SuperKEKB• SuperKEKB;• An upgrade of KEKB B-factory (KEKB).
• High-luminosity electron-positron collider toseek out new physics hidden in subatomic
particles.
• Main ring (MR) is composed of
Low Energy Ring (LER);
4.0 GeV Positron, 3.6 A
High Energy Ring (HER);
7.0 GeV electron, 2.6 A

HER

Main Ring

LER• Design Luminosity : 8.0×1035 cm−2⋅s−1

 40 times maximum luminosity of KEKB Twice beam current of KEKB (×2)
 Smaller emittance for nano-beam collision
scheme (×20)
 Over a period of 10 years, a 50-fold increase inintegrated luminosity relative to the original KEKB
is expected.

2020/May

IPAC'20 Virtual Presentation

SuperKEKB
2

Nano-beam collision scheme• Proposed by P. Raimondi at INFN-LNF for the Italian SuperB
project around 2006 and tested successfully at DAΦNE.

KEKB
Crossing angle : 22 mrad

• Implemented at SuperKEKB, for the first time, with lowemittance beams on a high-energy collider.
1 µmSuperKEKB

100 µmCrossing angle : 83 mrad
5 mm
1 µm

• To avoid Hourglass effect:

50 nm
5 mm
𝛾𝛾± 𝐼𝐼± 𝜉𝜉𝑦𝑦±
𝐿𝐿 ≈∗2𝑒𝑒𝑟𝑟𝑒𝑒 𝛽𝛽𝑦𝑦±𝜉𝜉𝑦𝑦± ≈∗
𝑟𝑟𝑒𝑒 𝑁𝑁∓ ⁄2𝜋𝜋𝛾𝛾± 𝜎𝜎𝑥𝑥,eff

2020/May𝛽𝛽𝑦𝑦∗ ⁄𝜀𝜀𝑦𝑦• Beam bunces collide With sufficiently small σx*.
 At a large horizontal crossing angle.
• The longitudinal size of the overlap between colliding
bunches is much shorter than bunch length.

100 µm𝐿𝐿 ∝constant

𝐼𝐼±
∗
𝛽𝛽𝑦𝑦±

𝐿𝐿

1
𝐿𝐿𝑠𝑠𝑠𝑠 ≡∝ ∗𝑛𝑛𝑏𝑏 𝐼𝐼𝑏𝑏− 𝐼𝐼𝑏𝑏+ 𝛽𝛽𝑦𝑦±• It is possible to squeeze βy* much smaller thanbunch length.

IPAC'20 Virtual Presentation• Luminosity increases in proportion to 1/βy*.

3Machine parameters

2020/May

IPAC'20 Virtual Presentation

4SuperKEKB project history• Phase1 operation (2016.Feb. ∼ June);• Vacuum scrubbing, low emittance beam tuning,and background study for Belle II detector
installation• w/o final focusing system (QCS) and Belle IIdetector• Phase2 operation (2018.Mar. ∼ July);• Pilot run of SuperKEKB and Belle II w/o pixel vertex
detector (PXD)• Demonstration of nano-beam collision scheme• Study on background larger than at KEKB due tomuch lower beta functions at IP.

• Phase3 operation (2019.March∼);• Physics run with fully instrumented detector.
• Phase3 2019ab (2019.3∼7)• “Status of Early SuperKEKB Phase-3 Commissioning” by
A.Morita (WEYYPLM1) @ IPAC’19 (2019.5.22)• Phase3 2019c (2019.10∼12)• Phase3 2020ab (2020.2∼)
 New nomenclator of each run of Phase3“Phase3 YYYYxx”
Calendar year

a : Winter shutdown - Marchb : April - Summer shutdown
ab : Winter shutdown – Summer shutdown
c : Summer shutdown – Winter shutdown
April is beginning of Japanese fiscal year.

SuperKEKB project history

First hadronic event in the Phase3 2019ab
2020/May

IPAC'20 Virtual Presentation

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Phase3 2019ab (2019.3∼7)
βy* = 8 mm, βx* = 200 mm• Mission of Phase3 2019ab operation;(For high current study)• Starting full-scale physics run with fully
instrumented Belle II detector.
• Accelerator and collision tunings withlower βy* for higher luminosity.

βy* = 2 mm
βx* = 80 mm

βy* = 3 mm, βx* = 200 mm/100 mm (LER/HER)

continuous injection

 βy* was already squeezed to 3 mm during
Phase2.

• Overview of Phase3 2019ab• From 11/Mar. to 1/July.
• Started with vacuum scrubbing.
• Collision tuning started on 25/Mar.
• 3-week interruption due to fire accident in
Linac building (3/Apr. – 26/Apr.)• Commencement of continuous injection inboth rings from May.

collision started on
25th March

1.23×1034 cm−2⋅s−1
0.55×1034 cm−2⋅s−1• Increased the integrated luminosity.

vacuum scrubbing
2020/May

IPAC'20 Virtual Presentation

fire in injector Linac building
6

Phase3 2019ab (2019.3∼7)
βy* = 8 mm, βx* = 200 mm• Overview of Phase3 2019ab (2019.3∼7) (cont’d)(For high current study)

βy* = 2 mm
βx* = 80 mm

βy* = 3 mm, βx* = 200 mm/100 mm (LER/HER)• Beam current:• For physics run : ∼660 mA(HER)&∼620 mA(LER)• Maximum : 830 mA (HER) & 940 mA (LER)

continuous injection• Beta squeezing:• Physics run was mainly done with
βy* = 3 mm
βx* = 100 mm(HER) & 200 mm(LER)• βx*/βy* were squeezed to 80 mm/2 mm at last.

• Luminosity :• Peak Luminosity w/o Belle II detector taking
data : 1.23×1034 cm−2⋅s−1• Peak Luminosity w/ Belle II detector taking
data : 0.55×1034 cm−2⋅s−1• Total Integrated luminosity : 6.1 fb−1

collision started on
25th Marchvacuum scrubbing
2020/May

IPAC'20 Virtual Presentation

1.23×1034 cm−2⋅s−1
0.55×1034 cm−2⋅s−1

fire in injector Linac building
7

Luminosity in Phase3 2019ab
Specific luminosity as a function of βy*• Luminosity and beta squeezing;• Collision tuning was mainly done with
βy* = 3 mm & βx* = 200 mm/100 mm (LER/HER)• Peak luminosity = 0.55×1034 cm−2⋅s−1• at ∼600 mA• with 3.06 RF bucket sp./1 train/1576 bunches

𝐿𝐿𝑠𝑠𝑠𝑠 ∝Y. Suetsugu

1
∗
𝛽𝛽𝑦𝑦±

• βx*/βy* were squeezed to 80 mm/2 mm on 21/June.
• Peak Luminosity = 1.23×1034 cm−2⋅s−1• at 830 mA (HER) & 820 mA (LER)• with 3.06 RF bucket sp./1 train/1576 bunches• Belle II could not take data due to high background noise.
• Running time required to reach 1.0×1034 cm−2⋅s−1 was 200 days

Specific luminosity as a function of bunch current products
Y. Ohnishi• 1/5 of the time taken by KEKB.

• Specific Luminosity (Lsp) Increased in proportion to 1/βy*• Nano-beam scheme collision worked well with βy* = 2 mm !!
• Beam-beam parameter can be kept even though βy* was squeezed.

• Slow degradation of specific luminosity with a increase of bunch
currents.
• Beam-beam effect increases the vertical emittance,
reducing the specific luminosity.
2020/May

IPAC'20 Virtual Presentation

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Integrated luminosity in Phase3 2019ab• Integrated luminosity during Phase3 2019ab was 6.1 fb−1

Integrated luminosity history
Delivered• ∼63 % of delivered luminosity was recorded.

63%• Continuous injection helped to increase integrated
luminosity.

Recorded
6.1 fb−1• Continuous injection in both rings commenced from May.
• Increased the integrated luminosity by 237 % compared with thenormal injection.

Normal injection mode

Continuous injection mode

Integrated luminosity
237 % up !!

2020/May

IPAC'20 Virtual Presentation9

Major Issues of Phase3 2019ab• Fast beam losses leading to collimator damage;

T. Ishibashi

vertical collimator• “High radiation dose to Belle II” and “QCS quench” due tosubsequent beam showers hitting the Belle II and QCS.
• The beam instabilities developed extremely fast, only in a few
turns.
 A possible cause of the beam instability is a falling dust particle in LER.
 Detailed mechanism is not known.

 Faster beam abort system and additional robust collimatorwith lower Z material are required.

• High detector background• Dominated by beam-gas scattering in the LER.

collimator head damaged by beam

 Limits the beam current and minimum βy*.
 Jeopardizes the integrity of the detector.

• Frequent injection background bursts

 Seemed to be due to “rogue pulses” in the Linac/BT.
 Limited the Belle II operation to a greater extent than the steady state(DC) backgrounds, which are mostly from LER beam gas scattering.

 More vacuum scrubbing (LER), Linac tuning and injection
tuning are required.
2020/May

IPAC'20 Virtual Presentation

beam
10

Major Issues of Phase3 2019ab• Abnormal RF-shielded bellows of IP vacuum chamber;• External wall temperature of one of four IP-bellows rose up to morethan 60 °C at high current.
• As a result of internal check with a fiberscope after Phase3 2019ab
operation, it was found that 3 RF fingers were out of normal positions.
• During Phase3 2019ab operation, It was also known that one BPM cable
was disconnected to BPM electrode.

Temperature [°C]

Temperature of abnormal bellows .
3.06sp, 1576 bunch

6.12sp, 789 bunch

HER current [mA]

Belle II
image observed through the fiber scope

fiberscope was pushed from here.
BPM

bellows
IP bellows chamber
2020/May

IPAC'20 Virtual Presentation11

Major works after Phase3 2019ab• Replacement work of IP bellows and BPM cables;• QCS was pulled back from Belle II detector.
• Bellows chamber and damaged BPM cables were replaced.
• After replacement work, soundness checks of bellows and BPM
cables were performed.

• RF-fingers may have come out from their normal positions during installation
work before Phase3 2019ab.
• BPM cables may have been damaged during installation work before Phase3
2019ab.

Bellows

BPM cable

QCS was pulled back from Belle II detector.

Replacement work
Bellows chamber
2020/May

IPAC'20 Virtual Presentation12

Major works after Phase3 2019ab• Replacement of damaged collimator head (D2V1) and in-situ baking;• Detector background was dominated by beam-gas scattering in the LER.
• In-situ baking was a countermeasure against high background noise.

• Installation of new BPM just before injection point (HER).
• More precise injection tuning became possible.

Collimators were baked by hot-air circular to reduce pressure.

T. Ishibashi

collimator head replacement work

2020/May

IPAC'20 Virtual Presentation

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Phase3 2019c (2019.10∼12)
βy* = 1.2 mm, βx* = 80 mm• Mission of Phase3 2019c operation;βy* = 1.5 mm, βx* = 80 mm
βy* = 2 mm, βx* = 80 mm• Machine developments

βy* = 1 mm, βx* = 80 mm

βy* = 1 mm,
βx* = 80 mm/60 mm(LER/HER) βy* squeezing down to less than 1 mm.
 Background studies• Physics run on weekends and 0:00-9:00 (owl
shift).

• Overview of Phase3 2019c (2019.10∼12)• From 15/Oct. to 12/Dec.
• Started with vacuum scrubbing.
• 1-week interruption due to BT(e+) magnet
trouble and DR ext. septum trouble.
• Collision tuning started on 29/Oct.

BT(e+)
magnettroubleDR ext.
septumtrouble1.88×1034 cm−2⋅s−1
1.14×1034 cm−2⋅s−1

vacuum scrubbing
2020/May

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Phase3 2019c (2019.10∼12)• Overview of Phase3 2019c (2019.10∼12) (cont’d)• Beam current:

βy* = 1.2 mm, βx* = 80 mm

βy* = 1.5 mm, βx* = 80 mm
βy* = 2 mm, βx* = 80 mm

βy* = 1 mm, βx* = 80 mm

βy* = 1 mm,
βx* = 80 mm/60 mm(LER/HER)• Maximum : 700 mA (HER) & 880 mA (LER)• Mostly low current (200∼300 mA) for machinedevelopments• Beta squeezing:• Started with βy* = 2 mm & βx* = 80 mm• βy* was squeezed down to 1 mm via 1.5 mm and1.2 mm.
• βx* of HER was reduced to 60 mm at last.

• Luminosity :• Peak Luminosity w/o Belle II detector taking data :1.88×1034 cm−2⋅s−1• Peak Luminosity w/ Belle II detector taking data :1.14×1034 cm−2⋅s−1• Integrated luminosity (Phase3 2019c) : 4.0 fb−1
• Total integrated luminosity : 10.1 fb−1

BT(e+)
magnettroubleDR ext.
septumtrouble1.88×1034 cm−2⋅s−1
1.14×1034 cm−2⋅s−1

vacuum scrubbing
2020/May

IPAC'20 Virtual Presentation15βy* squeezing in Phase3 2019c• βy* squeezing history;

Y. Suetsugu• Now we are operating with βy* = 1 mm.
 Smallest value in the world!!
 Lower than the bunch length of 6 mm.

2020/May

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Luminosity in Phase3 2019cSpecific luminosity as a function of 1/βy*• Luminosity;•

Peak Luminosity w/o DAQ = 1.88×1034 cm−2⋅s−1••••

𝐿𝐿𝑠𝑠𝑠𝑠 ∝at 650 mA (HER) & 850 mA (LER)
with 3.27 RF bucket sp./2 train (2 abort gaps)/1476 bunches
βy* = 1 mm & βx* = 80(LER)/60(HER) mm
Belle II could not take data due to high background noise.

• Peak Luminosity w/ DAQ = 1.14×1034 cm−2⋅s−1• at 360 mA (HER) & 500 mA (LER)• with 6.12 RF bucket sp./2 train (2 abort gaps)/783 bunches• βy* = 2 mm & βx* = 80 mm

1
∗
𝛽𝛽𝑦𝑦±

Y. Suetsugu

Specific luminosity as a function of bunch current products• Integrated luminosity : 4.0 fb−1

• Specific luminosity;• Specific Luminosity (Lsp) Increased almost in proportion to 1/βy*• Nano-beam scheme collision worked well with βy* = 1 mm !!

Y. Ohnishi• Slow degradation of specific luminosity with a increase of bunch currents.
• Beam-beam effect increases the vertical emittance, reducing the specificluminosity.
• Key issue in the future commissioning.
2020/May

IPAC'20 Virtual Presentation17

Major issues of Phase3 2019c
Trend of beam parameters• Unstable modes at high bunch current;• Flip-flop phenomena between HER and LER beam sizes
were observed at high bunch currents.
 Seems to be sensitive to betatron tune.
 Coherent beam-beam head-tail instability?

Flip-flop phenomena

Current
Bunch
current

βx*

80 mm

βy*

1 mm
HER

180 mALER

310 mA

HER

0.458 mA/bunch

LER

0.788 mA/bunch

Current products

0.36 mA2

LuminosityΣy* Understanding the mechanism is required.

σy* (HER)

Currents

σy* (HER)
σy* (LER)
σx* (HER)
σx* (LER)
νx (LER)σy* (LER)
σx* (HER)

νy (LER)νx (HER)νy (HER)σx* (LER)
Y. Ohnishi

2020/May

IPAC'20 Virtual Presentation18

Major issues of Phase3 2019c• Optics issues;

x-y coupling measurement• Some difficulties were gradually revealed as
decreasing βy*.
1. Difficulty in x-y coupling correction;2.

 σy increased even though the coupling correction
seemed to work well.
 x-y coupling correction with skew Q coils of
sextupole magnets in Local Chromaticity
Correction section may cause this issue.
 Not yet understood well.

Difficulty in more precise measurement of
optics;

Before x-y coupling correction
x-y coupling correction

σy measurement by X-ray monitor

x-y coupling correction
seems to work well.

• We have to deal with 10 µm or less beam positionchange to evaluate optics parameters, and thenthe residual of the orbit correction is 20∼30 µm in
RMS.
• Estimated beam position changes in this orders at
sextupoles have non-negligible impact onemittance.
 Method for more precise measurement of optics
should be established.
2020/May

IPAC'20 Virtual Presentation

After x-y coupling correction

19Improvement in beam abort system• For faster beam abort to avoid damages to collimators and BelleII detector;

New bunch filling pattern : 2 Trains, 2 Abort Gaps

• Improvement in abort trigger delivery time on accelerator side.
 21 ∼ 39 µs (2019b) → 17∼30 µs (2019c) (reduced by −0∼9 µs !!)abort gap

• More to come from detector side. (by ∼7.5 µs)

New abort gap to
reduce waiting time
for kicker firing.

Time chart of beam abort trigger delivery

Phase32019ab> ∼2 µs2∼10 µs• Faster triggers from QCS
and Loss Monitors

Phase3
2019c

∼2 µs

2020/May

2∼10 µs

Phase3 2019ab

Beam dumping system
waiting for beam gap

Beam interlock
system processt1

abort gap

H. Ikeda
Kickerfired

All bunches
are aborted

t2

LER:7∼17 µs
HER:6∼16 µs

t3max. 10 µs

t4# of Beam Abort

Devicerequests
Abort

Signalsent
to Kicker• Improvement in abort timing
synchronization (by 4 µs)• Reduction of waiting time forabort gap (by 0∼5 µs)

3∼8 µsmax. 10 µs

Phase3 2019c

Mean:50.5μs
Med:48.6μs
Min:39.9μs

Trigger time [s]# of Beam Abort

Beam
Abort
requesta failure is detected…

Y. Suetsugu

Mean:39.2μs
Med:37.1μs
Min:33.5μs

Trigger time [s]• Beam abort trigger issued faster by ∼10 µs.
 However, one collimator was damaged again.

IPAC'20 Virtual Presentation20

Detector background of Phase3 2019c• Storage beam BG

Vacuum scrubbing in LER• Major source was still beam-gas scattering in LER.
• Even though βy* squeezing (3 mm → 1mm) should increase theloss rate, BG was improved by a factor of 2∼3 compared to 2019ab.
 Tighter collimator settings, which can be available thanks to morestable injection with smaller emittance growth in beam transport line.
 Progress of vacuum scrubbing, which was helped by in-situ baking ofcollimators.

• Injection BG;

Y. Suetsugu

Vacuum scrubbing at
collimator location in LER
T. Ishibashi

2019bbaking2019c• LER injection BG on detector leak current

 Large leak current caused DAQ deadtime and limited maximum beam
currents for physics run.

• Long duration of LER injection BG

Example of long-duration injection background of LER
T. Koga ∼13 % dead time @ 12.5 Hz injection.
 Injection BG duration should be suppressed to increase integrated
luminosity.

• Synchrotron radiation BG on PXD

 PXD started to see significant SR increase after HER βx* squeezing (80mm → 60 mm).
 Rotation of HER horizontal orbit reduced SR by ½.
 SR hit was correlated with injections. Why?

 Stable injection with low emittance is a key challenge in thenext run.
2020/May

IPAC'20 Virtual Presentation21

Major works after Phase3 2019c• Replacement of damaged collimator head;

T. Ishibashi• One of LER collimator head was damaged during Phase3 2019c.
• 4 collimator heads were damaged in total so far.
 LER : D06V2 (Phase3 2019c), D02V1 (Phase3 2019ab), D02V1 (Phase2)
 HER : D01V1 (Phase2)

 Robust collimator is required in order to increase beam current.
• R&D of Low-Z collimator is undergoing.
• Short Ta head was set to D06V2 in a chain of low-Z collimator R&D.

• Installation of new vertical collimator (D06V1) in LER.

 Reduction in BG by a factor of 2.5 is expected by a simulation.
Damaged head

2020/May

Head opposite the damaged headNew vertical collimator in LER

IPAC'20 Virtual Presentation22

Major works after Phase3 2019c• Cabling work for crab-waist scheme for LER• Beta function waist of one beam is oriented along thecentral trajectory of the other one.

Y. Ohnishi• The vertical beta-function rotation is provided by sextupole
magnets placed on both sides of the IP.

• Simulation indicated the improvement of luminosity.
• Reduction of beam-beam blowup, beam-tail, then beam
background.
• Effective down to βy* ∼ 0.5 mm.

 We decided to try crab-waist scheme in SuperKEKB
last December.
• Try to LER first, then try to HER.
• Cabling work for crab-waist for LER was completed during
winter shutdown.

K. Oide2020/May

IPAC'20 Virtual Presentation

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Phase3 2020ab (2020.02∼07)• Mission of Phase3 2020ab operation;

LER crab-waist 40 % LER crab-waist 60 %• Physics run toward more than 100 fb−1 by the end of βy* = 2 mm, βx* = 80 mm
June.
• Demonstration of crab-waist scheme

HER&LERcrab-waist

βy* = 1 mm, βx* = 80(LER) mm/60(HER) mm• Overview of Phase3 2020ab (2020.Feb.∼20/April)• From 25/Feb. to 1/July.

• This talk covers things only up to 20/April (before HER crabwaist ).

• Started only with LER for vacuum scrubbing.
• HER beam on 2/March• Beta squeezing:

1.57×1034 cm−2⋅s−1• Keeping βy* = 1 mm & βx* = 60(HER)/80(LER) mm• More time is required for Linac tuning and Inj. tuning.

• Crab-waist:••••

LER : 0 % → 40 % on 16/March
LER : 40 % → 60 % on 24/March
Cabling work for HER crab-waist (20-23/April)
Machine operation was resumed on 23/April.

2020/Mayvacuum scrubbing

IPAC'20 Virtual Presentation

Cabling work for
HER crab-waist
24

Phase3 2020ab (2020.02∼07)• Overview of Phase3 2020ab (2020.Feb.∼20/April) (cont’d)• Beam current:

LER crab-waist 40 % LER crab-waist 60 %βy* = 2 mm, βx* = 80 mm

HER&LERcrab-waist

βy* = 1 mm, βx* = 80(LER) mm/60(HER) mm• Maximum : 520 mA (HER) & 500 mA (LER)• Luminosity (up to 20/April):• Peak Luminosity w/ Belle II detector taking data :1.57×1034 cm−2⋅s−1• Integrated luminosity : 13.9 fb−1 (up to 20/April)

1.57×1034 cm−2⋅s−1

We are here now.
(∼20/April)vacuum scrubbing
2020/May

IPAC'20 Virtual Presentation

Cabling work for
HER crab-waist
25

Phase3 2020ab (2020.02∼07)• Specific luminosity;

Specific luminosity as a function of bunch current products• Specific luminosity was kept high at highbunch current products.
• LER Beam-beam blowup at high bunch currents
was suppressed by LER crab-waist.
• Contrary to the energy transparency condition,
HER current can be increased more than LER
current to push up luminosity .

Y. Ohnishi• IHER (520 mA) > ILER (440 mA)• EHER (7 GeV) > ELER (4 GeV)HER current
LER current
Luminosity

σy* (HER)
σy* (LER)

2020/May

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Luminosity performance with LER crab waist
K. Oide2020/May

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Summary• Phase3 2019ab (2019.3∼7) & 2019c (2019.10∼12);• Physics run with fully instrumented detector has finally started.
• “Nano-beam scheme” collision was demonstrated up to βy* = 1mm.
• βy*/βx* have been squeezed to 1 mm/60 mm.

• Peak Luminosity w/o Belle II detector taking data : 1.88×1034 cm−2⋅s−1• Peak Luminosity w/ Belle II detector taking data : 1.14×1034 cm−2⋅s−1• Integrated luminosity up to end of Phase3 2019c : 10. 1 fb−1

• Continuous injections increased integrated luminosity by 237 % compared with the normal injection.

• Key challenges were;
•
•
•
•
•

Slow degradation of specific luminosity with a increase of bunch currents.
Optics difficulties revealed as squeezing βy*.
Fast beam losses leading to collimator damage, QCS quench, and detector damage.
Large detector backgroundAnd so on.

• Phase3 2020ab (2020.2∼);• Started on 25/Feb. and will end on 1/July.

• This talk covered things only up to 20/April (before HER crab-waist).

• LER crab-waist (60%) pushed up peak Luminosity w/ Belle II detector taking data to 1.57×1034 cm−2⋅s−1.
• Cabling work for HER crab waist has just finished on 23/April.
• Effect of HER crab-waist will be reported at IPAC’21!!

2020/May

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Fin.
Thank you for your attention.

Inter-University Research Institute Corporation High Energy Accelerator Research Organization (KEK)
大学共同利用機関法人 高エネルギー加速器研究機構 (KEK)