The SuperKEKB Has Broken the World
Record of the Luminosity
Yoshihiro Funakoshi (yoshihiro.funakoshi@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;

Circumference = ~3016m• An upgrade of KEKB B-factory (KEKB).
• High-luminosity electron-positron collider toseek out new physics beyond standard model• Main ring (MR) is composed of
Low Energy Ring (LER);
4.0 GeV Positron, 3.6 A(design)
High Energy Ring (HER);
7.0 GeV electron, 2.6 A(design)

HER

Main Ring

LER• Positron damping ring: newly constructed• Design Luminosity : 8.0´1035 cm-2×s-1

ü 40 times maximum luminosity of KEKBü Twice beam current of KEKB (´2)
ü Squeezing by* with nano-beam collision scheme(´20)
ü Over a period of 10 years, a 50-fold increase inintegrated luminosity relative to the original KEKB
is expected.

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SuperKEKBSuperKEKB
Y. Funakoshi

2SuperKEKB project history• Phase1 operation (2016.Feb. ~ June);• Phase3 operation (2019.March~);• Vacuum scrubbing, low emittance beam tuning,and background study for Belle II detector
installation• w/o final focusing system (QCS) and Belle IIdetector• Physics run with fully instrumented detector.
•

Top-up injection in both rings• Phase3 2019ab (2019.3~7)•• Phase2 operation (2018.Mar. ~ July);• Damping ring for positron was introduced.
• 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.

“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~)•“Highlight from SuperKEKB Beam Commissioning” by K. Shibata @IPAC2020 (2020 May)ü New nomenclature 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

SuperKEKB project history

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3

History of beam operation

IPAC2020

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4Comparison of machine parameters
KEKB
achieved
Ibeam [A]IPAC2020
K. Shibata
SuperKEKB
2020 May 1st

IPAC2022
at present
SuperKEKB
2022 June 8th

SuperKEKB
design

LER

HER

LER

HER

LER

HER

LER

HER

1.637

1.188

0.438

0.517

1.321

1.099

3.6

2.6

# of
bunches

1585

783

2249

2500

Ibunch [mA]1.033

0.74950.55930.66030.58730.4887

1.440

1.040

by* [mm]

5.9

5.9

1.0

1.0

1.0

1.0

0.27

0.30

0.129

0.090

0.02360.02190.0407(0.0565)a0.0279(0.0434)a0.08810.0807xy
Luminosity[1034cm-2s-1]

2.11

1.57

Integrated
Luminosity [ab-1]

1.04

0.03

doubled

4.65

80

0.40

50

a) High bunch current collision study
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5Comparison of Luminosity(design goal)(achieved)

The SuperKEKB Has Broken the World Record of the Luminosity!
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6Machine Parameters of SuperKEKB
LER
4.0

Beam Energy
Circumference
Crossing angle

80
1.321

Crab waist ratio
Beam current @Maximum Luminosity
Number of bunches

0.58739.12-0.0233
5.69
2.98E-4

Bunch current @Maximum Luminosity
Total RF voltage Vc
Synchrotron tune ns
Bunch length sz
Momentum compaction ac
Betatron tune nx / ny

2249

40
1.099
0.488714.2-0.0258
6.03
4.54E-4

GeV
m
mrad
%
A
mA
MV
mm

44.524/46.592

45.532/43.575

80/1
0.224

60/1
0.224

mm
µm

0.040780.027924

min.

Beta function at IP bx* / by*
Measured vertical beam size (XRM)@IP sy*Vertical beam-beam parameters xy
Beam lifetime
Luminosity (Belle 2 CsI)
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3016
83

HER
7.0

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4.65

Y. Funakoshi

Touschek dominant1034 cm-2 s-1
7

Luminosity improvement [1/4] Crab waist• Introduction of crab waist at SuperKEKB• Motivations• The beam-beam performance was poor in spite of all of knob tunings for
improving it. It was limited by beam-beam resonances which can besuppressed by crab waist.

• Method• FCC-ee type scheme: use of imbalance sextupoles in the vertical local
chromaticity correction section.

• Time table• 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%)

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8Specific luminosity w/ and w/o crab waist
Strong-Strong Beam-Beam simulation

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9Summary of crab waist scheme• Benefits of use of crab waist scheme• Suppression of beam-beam blowup• Specific luminosity was improved.

• Increase of the bunch currents of both beams• Without crab waist, beam injections was limited due to beam blowup.

• Beam lifetime issue• Dynamic aperture shrinks w/ crab waist and the lifetime decrease w/ crab
waist was expected.
• But in by*= 1mm case, no lifetime decrease was observed in LER and HER.
• The narrow physical apertures at collimators determine the lifetime.

• In the case of lower by*, simulations showed the lifetime w/ crab waist will set a strong
limit.

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10Luminosity improvement [2/4] Higher beam currents• We have been increasing beam currents
with fighting with obstacles• Obstacles• Hardware damages due to fast beam losses• Frequency hardware troubles on collimators (andBelle II sub-detectors) happened when the bunch
current in LER is larger than 0.7 mA. The recent
increase in beam currents was achieved byincreasing the number of bunches while respecting
the limit from bunch current limit (<~0.7mA/bunch).
(to be addressed later)• Detector beam background• Beam aborts• Beam instability• Beam injection

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Current background level in Belle II
One of the most vulnerable sub-detectors is
the Time of Propagation (TOP) particle ID system●

Based on K. Kojima slides, Nagoya Univ., Jan 2022

Current background rates in Belle II are acceptable andwell below limits●

Belle II did not limit beam currents in 2021 and 2022○

It will limit SuperKEKB beam currents eventually,
without further background mitigation●

To reach the design luminosity an upgrade of crucial
detector components is foreseen (e.g. TOP short
lifetime conventional PMTs)
In view of replacement of a vulnerable part of PMTs in LS1(Long Shutdown 1), the BG limit of TOP PMT was raised to
5 MHz in 2022.
Beam Gas BG in LER is expected to be lowered in theprocess of vacuum scrubbing. We also expect that BG will
be lowered by IR radiation shield reinforcement done in
LS1. On the other hand, luminosity related BG12will increase
with a higher luminosity.

Current background limit
for the TOP PMT rate

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12Luminosity improvement [3/4] Bunch-by-bunch feedback gain
• In May 2021, the luminosity increased by lowering gain of the bunch-by-bunch
feedback system in HER.
• Noise mixed in 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.

HER beam current

sy* HERsy* LER

Beam sizes

LER beam current
Luminosity

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Specific luminosity

The luminosity increased by lowering HER vertical FB gain by 4dB + 4dB.
The increase IPAC2022
in the luminosity
was ~25%.
SuperKEKB Y. Funakoshi

13Luminosity improvement [4/4] by*
In 2022b run, we tried by*=0.8mm. The specific luminosity
was higher than by*=1mm case. We could not store higher
beam currents due to poor injection efficiency. We will re-try
soon.
by*=0.8mm

Achieved (0.8mm)
Design goal (0.27mm, 0.30mm)

In the data in cyan and blue, estimated valuesof by* in HER were less than 1mm(setting value)
due to horizontal orbit change in SLY depending
on total beam current.
SLY: Sextupoles at local chromaticity correction

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14Performance limiting issues [1/4] Fast & large beam loss
• Observations• Fast and large beam loss (< 3 turns) (particularly in LER)
• The loss causes damage of collimators and Belle II inner sensors, and QCS quench• Empirical rule: Bunch current must not exceed 0.7mA.

• Obstacle to machine operation• We have been conservative in increasing beam currents (particularly bunch currents).
• This issue determines the speed of increasing beam currents and then slows down increase ofluminosity.

• Mechanism of fast & large beam loss
• Still not understood well

• A hypothesis was proposed by T. Abe.
• A microparticle heated by the beam-induced field causes a macroscopic vacuum arc.
• We will continue to study this hypothesis• A joint Belle2-SuperKEKB team has been working to identify the original places of fast beamlosses. Recent progress shows collimators near the injection region are the most possible
candidates.
(https://kds.kek.jp/event/41394/contributions/209334/attachments/154298/195935/16aA56103.pdf)• Investigations are ongoing to fully understand this issue and countermeasures are being sought.
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Typical large beam loss events (LER)
Bunch oscillation recorder (BOR)
Horizontal oscillation
Bunch oscillation recorder (BOR)
Vertical oscillation
BOR amplitude= oscillation amplitude × bunch current
Bunch current monitor (BCM)
Amount of beam loss (from BCM)

1 turnBeam was aborted here by beamabort system based on informationfrom beam loss monitors.

• Very fast beam loss: within 3 turns• No bunch (dipole) oscillations were observed before beam loss.

– In some cases, beam oscillation in the previous turn of beam loss was observed.

• No beam size blowup is observed before beam loss.
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History of large beam loss events 2022

K. Matsuoka

The three big accidents of LER beam loss in 2022 happed at Ib>~0.7mA/bunch within a day after increasing the beam
current at the three different Nbunch -> Empirical rule: we must not exceed 0.7mA/bunch.
In the case of a small number of bunches (Nbuch = 793, 61, 31), we haven’t observed the large beam loss with a higher
bunch currents.
Occasionally, large beam loss in LER happened with bunch currents lower than 0.7 mA but the total current was high(For example, on June 3rd, Ib=~0.62mA/bunch with a high total current (1325mA)).
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Severe damage on LER vertical collimator• After a huge beam loss event on June 6th in 2021, LER BG increase significantly.
• D02V1 collimator jaws were severely damaged (deep scar on the bottom jaw).
• Typically, collimator replacement work and the baking runs take 3~4 days.

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18Performance limiting issues [2/4] Beam injection• SuperKEKB injection scheme

K. Furukawa et al, Poster, THPOST011• Injector Linac provides e+ and e- beams. (e+: thermionic gun, DR, e-: RF gun)• Synchronization between injector and rings allows 1-bunch or 2-bunch injection per pulse.
• Top-up injection is achieved for e+ and e- beams at 50Hz at maximum(sum of e- and e+).

• Beam current limitation• The maximum stored beam currents in the rings are determined by the balance between
the charge sent from Linac and the charge loss due to beam lifetime.
• Increasing linac charge is important.

• The shorter beam lifetime at smaller by* (dynamic aperture) requires a more powerfulinjection. Conversely, injection sets a limit on the achievable by*.
• Machine operation with the optics of by* = 0.8mm is being tried in this run.

• The injection efficiency is also a very important issue.
• Depends on by*, bunch currents, machine tuning, collimator setting…
• Typical values of injection efficiency with by*=1mm: ~50%(LER), ~40%(HER)• Emittance preservation in Linac and BT is important.
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Linac bunch charge history
ElectronPositron(from rf gun)(BT)

2020a,b2020c

2021a,b2021c
2020a,blarge
discharge
event

2021a,b
2020c・New FC
・
Solenoidcurrent↑

Target (design) valuese+e-Charge / bunch [nC]4

4
IPAC20222021c

・beam tuning

・Steering,BPM
in Unit 1-5, 1-6
・beam tuning

DOE
for 2nd
line
laser

DOE
for 1stline
laser(after e+ target)(before DR)
Linac end
(BT)

SuperKEKB
20

・Solenoid current (SL_16_)optimization・beam tuning (primary e- bunch charge↑)

Y. Funakoshi

Linac, BT Emittance measurement using wire
scanners (2021c)

PositronError bars:
Blue: RMS of multiple measurements
Red: average of error of individual measurement

Linac

Target (design) values

BTe+ElectronError bars:
Blue: RMS of multiple measurements
red: average of error of individual measurement

Linac

BT

e-

Normalized emittance (H/V) [µm]100/1540/20
For better injection efficiency, suppression of emittance growth in BT lines is important.
The emittance growth is bunchcharge SuperKEKB
dependentY.and
may be caused by CSR effect.
IPAC2022
Funakoshi
2022/June/12th21Performance limiting issues [3/4] Beam-beam performance• Observed luminosity performance is much lower than simulations with BBSS (Beam-BeamStrong-Strong). This has been and will be a challenge at SuperKEKB.
D. Zhou, et al, Poster, WEPOPT064• Candidates of causes• Machine imperfections: Non-zero linear and chromatic coupling (M. Masuzawa, Contributed Oral,TUOZSP2) and dispersions at IP, beam-current dependent optics distortion due to orbit change at QCS*
and SLY*, etc.
• Imperfect crab waist scheme; Interplay of beam-beam interaction and beam coupling impedance.
• Beam oscillation excited by injection kickers at LER causes luminosity loss by ~10%.

Short time trial

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Beam-beam issues [contn’d]• Ways to better beam-beam performance• Beam-beam simulations predict better beam-beam performance with••

Smaller vertical emittance in single beam (matter of optics corrections)
Higher crab waist ratio in HER (strength)• Identification of causes of discrepancy between simulations and experiments• Better working points• Beam-beam parameters• Achieved values in physics runs: : xy(LER) =0.0392 , xy(HER) = 0.0269• Achieved values in high bunch collision study: xy(LER) =0.0565 , xy(HER) = 0.0434• By increasing bunch currents in physics run, higher xy and then a higher luminosity is expected.
Pink: Nb=31, March 28th 2022
Blown: Nb = 61, April 5th 2022
Orange: Nb = 393, April 5th 2022

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23Performance limiting issues [4/4] Impedance related issues• Single bunch effect• LER TMCI (Transverse Mode Coupling Instability)• The apertures of vertical collimators scale as βy*, TMCI will set a limit on the bunch current. Extensive machinestudies have been done on this issue.
• With the use of 2 vertical collimators and taking into account the impedance from the high-β region around final focus
quadrupoles, the TMCI threshold will be lower than the design bunch current of 1.44mA when by*<0.6mm.
• By introducing a nonlinear collimator (NLC), we can use more vertical collimators and meanwhile reduce Belle II BG.

• Single bunch beam blowup in LER (-1 mode instability)• Beam blowup has been observed with a threshold ~0.8mA/bunch, .
• This blowup has been intensively studied. The interplay of the feedback system and vertical impedance was
identified to be the main source of beam blowup. Fine-tuning of FB system helped suppress the blowup.

• Multi-bunch (coupled bunch) instability• Low-frequency resistive wall (RW) impedance gives the fastest growth time (1.6ms@600mA in HER,
3.6ms@600mA in LER). This instability has been well suppressed by the bunch-by-bunch feedback
system so far.
• The longitudinal coupled bunch instability caused by fundamental mode impedanceof RF cavities has been well suppressed by -1 mode dumpers in both rings.

• Electron clouds• 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 effect has been observed in LER.

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Study on TMCI and -1 mode blowup• We’ve observed vertical beam-size blow-ups around 0.8 mA/bunch in LER with single-beam operations, and this 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 νy-νs appears (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 vertical bunch-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 under investigation (S. Terui et al., Poster, WEPOTK050), but we’ve found two ways todeal with this instability.
1.
Tuning of the vertical bunch-by-bunch feedback
2.
Reducing the impedance in the vertical direction by opening vertical collimators
ü The second point is one of motivations to introduce the nonlinear collimator.

Peaks in a tune spectrum and a Bunch
Oscillation Recorder (BOR). The expectedbunch current when the 0- and -1-mode
couples is 2 mA/bunch or above.

νy

νy-νs
[H. Fukuma]

T. Ishibashi
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Upgrade plan
• Long Shutdown 1 (LS1): July 2022 – September 2023
• Belle II: additional VXD detector installation, TOP counter PMTs replacement• SuperKEKB: Upgrade works in this opportunity• Medium term plan for increasing luminosity• We will aim at the luminosity of 1 x 1035 cm-2 s-1 within 1 or 2 years after LS1
with by* = 0.8mm.
• The operation with by* = 0.6mm will also be tried.

• Long term plan for luminosity upgrade• To squeeze by* down to design values (0.27mm in LER and 0.30mm 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 Long
Shutdown 2 (LS2) in around 2027. The upgrade plan is being studied.
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Major upgrade items during LS1
K. Nakamura• IR radiation shield modification

H.
Yamaoka• For BG reduction

Additional PE andconcrete shields around
Belle II

New heavy metal shield on IP
bellows• New heavy metal shields around IP bellows• Additional concrete & polyethylene shields around Belle II• Material change from W to SUS of QCS cryostat front plate• Non-linear collimator (LER)

K. Oide• For impedance and BG reduction••••• Robust collimator head (LER)

~38 m
S.
Nakamura• As countermeasure against kicker-pulser misfiring and resulting
destruction of collimator

Construction site of non-linear collimator
Beam channel for injection
beam

Carbon collimatorhead

New collimation scheme less likely to cause TMCI
Removal of 50 wiggler magnets
Installation of 2 skew sextupole and 5 quadrupole magnets
Installation of new vertical collimator with wider aperture

T. Ishibashi• Replacement with carbon head of horizontal collimator D06H3• New beam pipes with wider aperture at HER injection pointY.
Suetsugu• For improvement of injection efficiency• New beam pipes with wider aperture• New BPM for precise measurement of injected beam

Beam pipe at
HER injection point

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Example of parameters for L= 1 x1035cm-2s-1
LER

•

HER

# of bunches

2345+1

Luminosity

1.0 x 1035 cm-2s-1

Itotal2.35 A

1.64 A

Ibunch

1.0mA

0.7mA

by*

0.8mm

0.8mm

This parameter list was made based on a high bunch current collision study.
– We will need higher bunch currents.

••

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We will aim to achieve the parameter list.
In the process of aiming at the parameter set, we will need to study various
issues and aim at the luminosity with solving issues found and with modifying
the parameter set.
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International Task force for SuperKEKB upgrade
Mission• Bring ideas and exchange notes to solve various problems we face as aluminosity frontier machine, to achieve SuperKEKB design luminosity.
• Short term
• Working together on a to-do list with priority for LS1 to achieve luminosity of the order of 1035 cm-2 s-1
without any large-scale modification of accelerator components.
• Longer-term• Searching for ideas to achieve the design luminosity.

Four working groups (sub-groups) organized••••

OpticsBeam-beam
TMCI
Linac

History•••••

Started with the Initial members recommended by ARC members
The first kick-off meeting was held in July, 2021
More people joined us.
There have been 6 ITF general meetings many more sub-group meetings held so far.
ITF working in close collaboration with KEKB commissioning team.

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International Task force for SuperKEKB upgrade
Examples of activities• Lattice translation and repository for SuperKEKB; Optics
optimization and simulations with independent codes.
• Dynamic aperture optimization, new optics design.
• Beam-beam simulation, impedance calculation, instability theories.
• Deep discussions on the simulation results and new ideas.
• Proposed many machine study items and discussion on the results.
You are welcome to join us!

Hiroyasu Ego
Mika Masuzawa

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We will continue to make every efforts to improve SuperKEKB performance toward design goal.

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)Spare slides

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直接入射路ラティス設計の進捗状況
PF-AR直接入射路全体（案）35Large beam loss event (LER) March 23rd 2022

D06V1 collimator was damaged.

36

What is cause of large beam loss?
• Usual beam instability may not be the cause.
• Too fast beam loss (< 3 turns)• Dipole oscillation?
• Almost no dipole beam oscillation was observed before previous turns of beam loss by BOR (bunch
oscillation recorder).
• In some cases, some dipole oscillation was observed. Before the dipole oscillations are observed by
BOR, the oscillated beam particles are lost?

• Energy loss?
• Beam loss is not significant at horizontal collimators where the dispersion is large.
• No large orbit change was observed at Libera monitor where the dispersion is large.
• In the simulations on collision with dust particles, the main cause of beam loss is energy loss.

• Beam size blowup?
• Beam size blowup has not been measured with fast beam size monitor yet.
• In the simulations on collision with dust particles, the beam size blowup due to multiple scattering issmall.

• Dipole kick associated with some vacuum arc events in beam pipe? (by T. Abe)
• Damaged collimator head can be the source of the event.

• Observation Tools• More number of BORs will be helpful to understand the cause.
• Identifying where the beam loss starts is helpful to understand the phenomena.

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H. Nakayama

April ~ June 2021

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Nano-beam collision scheme
•

•

KEKB

Proposed by P. Raimondi at INFN-LNF for the ItalianSuperB project around 2006 and tested successfully at
DAFNE.

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
𝐿≈

100 µm

𝛾± 𝐼±𝜉#±∗2𝑒𝑟" 𝛽#±∗𝜉!± ≈ 𝑟# 𝑁∓⁄2𝜋𝛾±𝜎%,'((

2022/June/12th𝐿∝𝛽!∗ ⁄𝜀!

constant

𝐼±
∗
𝛽#±

𝐿%& ≡

𝐿
𝑛' 𝐼'(𝐼')IPAC2022

∝

1
∗
𝛽#±

SuperKEKB• Beam bunces collideü With sufficiently small sx*.
ü At a large horizontal crossing angle.
• The longitudinal size of the overlap between colliding
bunches is much shorter than bunch length.

• It is possible to squeeze by* much smaller than bunch
length.
• Luminosity increases in proportion to 1/by*.

Y. Funakoshi

392022/June/12th

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40Machine parameters2022/June/12th

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noi
se

Betatrontune
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43Specific luminosity & beam-beam parameters

xyLER = 0.0565
xyHER = 0.0434

Luminosity/bunch = 5.86 x 1031 cm-2s-1

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47SuperKEKB Transverse FB systems

2022/June/12th

Collaborating SLAC(US-Japan) and INFN-LNF(KEK-LNF)
IPAC2022

SuperKEKB

Y. Funakoshi

M. Tobiyama

482022/June/12th

IPAC2022

SuperKEKB

Y. Funakoshi

492022/June/12th

IPAC2022

SuperKEKB

Y. Funakoshi

502022/June/12th

IPAC2022

SuperKEKB

Y. Funakoshi

51

Bunch length (measurement by streak camera;
H. Ikeda)

Vacuum Scrubbing - LER

53

Vacuum Scrubbing - HER

54

2022/June/12th

IPAC2022

SuperKEKB

Y. Funakoshi

55

w/ crab waist by*=0.27mm

2022/June/12th

IPAC2022

SuperKEKB

Y. Funakoshi

56

Vertical collimators setting (LER)• Setting in physics run– D06V1: primary collimator• Most tightly closed• Suppressing injection BG– D02V1: second collimator• Very important for reducing BG– D06V2, D03V1• backup: not so tightly closed•

D06V1 collimator is planned to be
replaced by nonlinear collimator(NLC).
– 𝑘! 𝛽" of nonlinear collimator will bemuch less than usual collimators.

57

Scheme of NLC
−𝐼

𝜋
∆𝜇" =
2

K. Oide58

Beam kick by skew-sextupole• Multipole expansion&

𝜙 𝑟, 𝜃 = . 𝑎# 𝑟 # 𝑐𝑜𝑠 𝑛𝜃 + 𝑏# 𝑟 # sin(𝑛𝜃)#$%𝜙',) 𝑟, 𝜃 = 𝑎' 𝑟 ' cos 3𝜃 = 𝑎' 𝑥 ' − 3𝑥𝑦 * (skew-sextupole)• Magnetic field of skew-sextupole,-,-

𝐵+ = − ,+ = -3𝑎' 𝑥 * − 𝑦 * , 𝐵" = − ," = 6𝑎' 𝑥𝑦• SK2

V-kick does not depend on sign of v-position.
Only one side vertical collimator is needed (top or bottom).
𝐿 𝜕 * 𝐵+
𝐿
𝑆𝐾* =
=
6𝑎
𝐵𝜌 𝜕𝑦 *𝐵𝜌 'Horizontal offset reduces v-kick.

• Beam kick by skew-sextupole
Δ𝑝" =

., /
.0

=

12*

𝑦 * − 𝑥 * , Δ𝑝+ =../.0

A particle with both x and y offset
receives horizontal kick also.

= 𝑆𝐾* 𝑥𝑦59

NLC uses nonlinear kick of skew sextupoles. by* at the skew sextupoles is set to be large.
à Required collimation capability can be obtained with a wide aperture of collimator.

Collimator settings (by*=1mm case)βy* = 1 mm
Dec. 22nd 2021

without NLC

with NLCβy [m]

Collimator
Half aperture[mm]

βyky [V/C]

NLC

2.9

10.0

1.036e+145.607e+15

D06V2

20.6

2.677

5.607e+157.98558.597e+14

D03V1

17.0

7.98558.597e+14

1.069

1.332e+16

D02V1

11.9

1.069

1.332e+16βy [m]

Collimator
Half aperture[mm]

βyky [V/C]

D06V1

67.3

3.107

1.248e+16

D06V2

20.6

2.677

D03V1

17.0

D02V1

11.9

• With NLC, the impedance (by * ky) from D06V1 can be reduced by anorder of 2.

60

T. Ishibashi

Impedance Model and Predictions
In case of two vertical collimators in use
Without NLC

𝐼*+ =4𝜋𝜈% (𝐸/𝑒)𝑇, ∑- 𝛽#,- 𝑘#,-𝜈! = 0.022, 𝐸/𝑒 = 4 GV, 𝑇" = 1e-5 s

With NLC

w/o NLCw/ NLC

by* →
[mm]

1.0

0.8

0.6

0.27

Sby・ky [V/C]

4.70e16

5.79e16

6.86e16

1.37e17

Ith TMCI [mA]

2.41

1.96

1.65

0.82

Sby・ky [V/C]

3.37e16

4.19e16

4.98e16

1.07e17

Ith TMCI [mA]

3.37

2.71

2.28

1.06

61

Benefits•

Benefit of nonlinear collimator– We can decrease transverse ring impedance• We can raise the threshold of TMCI. But the threshold of TMCI may be higher thandesign bunch current of 1.44mA with by*=0.6mm optics in use of 2 collimators.
– What about QCS chamber impedance with by*=0.6mm optics?

• The single-beam blowup of -1 mode (single bunch effect) is observed at>~1mA(1.2mA) with present by*=1mm optics and this blowup is suppressed by
opening D06V1 collimator. This means that the blowup is also suppressed by using NLC.
• In high bunch current collision study showed that the LER blowup is well suppressed in
the collision condition and there was no significant change when we opened D06V1
collimator. Then we couldn’t show that using NLC will directly bring higher luminosity.
– What about situation with higher impedance with lower by*? (experiences with carbon head collimator)• The resistive wall impedance of D06V1 collimator can be reduced drastically. But its
effect to the instability is small.
• We can use one more vertical collimator seriously to suppress beam background.
– Andrii’s simulation shows that BG can be significantly reduced with by*=0.6mm optics using NLC.

– NLC collimator is more irrefrangible?
• Beam hit– If an abnormal beam comes from outside of skew-sextupole pair, the (nearly) same amount of beam
particles hit the NLC collimator as the case of usual collimator.

• Discharge at NLC collimator• Effect of damages surface of collimator head on beam

62

Bad experience with carbon head collimator

Ib,th = ~1.55 mA/bunch (with C1= 4p)

T. Ishibashi

63

Single Bunch Tune Shift in LER

Bunch current was limited at around 0.8 mA.

Dec. 17th 2020

H: -0.0019
V: -0.0229
vert. tune

12/17

single bunch
current

Beam current can not be stored larger than 0.8 mA/bunch.

(mm)

11/5

12/17

D02V1

1.52 / -1.20

1.70 / -1.23

D03V1

2.00 / -1.98

0.68 / -1.38

D06V1

3.25 / -3.23

2.69 / -1.21

D06V2

2.22/ -1.89

2.28 / -1.83

H: -0.0018
V: -0.0131

11/5

6
4

Sbyky= 1.14e17 V/C

Y. Ohnishi

Dec. 8th 2020
Beam Size at High Bunch Current Study
Luminosity meas.
260 mA

270 mAnb = 393 bunches
280 mA

320 mA
290 mA

300 mA

We stoped the luminosity measurement above 270 mA
since we observed "TMCI ?".

HER vert. beam size is oscillating at higher LER current.

⑤ Single vertical beam size in LER is
smaller than 0.2 μm (nominal).
④ The dipole motion is suppressedby increasing BxB FB gain (0.81 mA/bunch).
However, "beam size" (dipole motion) was still large.

① LER Beam-Beam blowup decreases ② Beam size blowup③ BxB FB gain +12 dBmdue to dipole motion
as decreasing HER current
0.73 mA/bunch

We gave up high bunch current collision study due to unexpected blowup in LER.

65

Transverse coupled bunch instability due to resistive
wall impedance (LER)
by*=1mm, Itot=600mA
βy [m]

by*=0.6mm, Itot=3600mA

Collimator
Growth Time
Half aperture Length[mm]
[ms]
[mm]

βy [m]

D06V1

67.3

3.34

4

234

D06V2

20.6

3.08

4

600

20.6

D03V1

17

8.0

10

5104

D02V1

11.9

1.0

10

Normalchambers

19.0

R=45

~3016mTotal(Calc.)
Mes.

Collimator Half
Length[mm]aperture [mm]67.3(2.9*) 2.2(5.48*)

Growth Time [ms]411.1(3997*)

1.89

4

23.1

17

5.65

10

299.7

14

16.5

0.89

10

1.19

8.85

19.0

R=45

~3016m

1.48

5.2

0.61(0.64*)

3.6*) w/ NLC
2022/3/28
2021/12/22

66