1

Collimators and TMCI
15th annual Belle PAC Review meeting
Machine, MDI, Beam background, Operation plan
T. Ishibashi

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Table of Contents• Introduction: Beam collimators in SuperKEKB main ring
• Topics- Low-Z (Carbon) collimator in D06V1 collimator- Trial of new LER optics in which the betatron phase matches verticalcollimators• Troubles- Damage of D02V1 collimatorʼs jaws
- Vacuum leak in D02V1 collimator• Issue- TMCI in LER derived from collimatorsʼ impedance• Collimators during 2021ab• Study Plans for 2021ab

Introduction: Beam collimators in SuperKEKB main ring
• LER: 4 vertical（D06V1, D06V2, D03V1, D02V1） and 7 horizontal（D06H1, D06H3, D03H1, D02H1, D02H2-H4）• HER: 9 vertical（D01V1, D12V1-V4, D09V1-V4） and 11 horizontal（D01H3-H5, D12H1-H4, D09H1-H4）
damage and vacuum leak (2020c)••

carbon (2020c)
tantalum (2021ab)

blue：SuperKEKB typered：KEKB typeFeatured collimators in 2020c- D06V1：carbon jaws (installed during 2020 summer shutdown)- D02V1：damaged and replacement work during 2020c. vacuum leak.
- D03V1：new collimator (installed during 2020 summer shutdown)
Featured collimators in 2021ab- D06V1：changed to tantalum jaws

KEKB type

SuperKEKB type[Y. Suetsugu et al., NIM A 513, 465 (2003)][T. Ishibashi et al., PRAB 23, 053501 (2020)]

3

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Topics: Low-Z (Carbon) collimator in D06V1 collimator• Materials with a short radiation length is very e ective as a beam tail shield, however the beam loss islocalized and the temperature of that exceeds the melting point.
• In order to protect the collimators for BG suppression from abnormal beams, we developed a collimator* Glass like carbon coated and impregnated C/C composite
with carbon* and installed it in D06V1 during 2020 summer shutdown.
(CX-2002U̲GP2B, Toyo Tanso Co.,Ltd.).
• In 2020c, no abnormal pressure rise and heating are observed, however its impedance lower the bunch
current threshold.
H. Nakayama

D06V1 aperture

Carbon (L=60 mm)

ff

backgroundTopics: Trial of phase matched optics between collimators in LER• In its

nal stage (Dec. 14th), LER optics was changed to one where the phase advance between D02V1-D03V1 and

D02V1-D06V1 are 3.5 and 16, respectively.

- Protecting D02V1 by the two collimators.
• By closing the D03V1 aperture to

1 mm、the storage BG in TOP was decreased by 30-40%. However, injection BG in

VXD was increased and the e ect of collimatorsʼ impedance became prominent (talk later).

ff

fi

Hit rates of TOP and CDC

H. Nakayama

56

Troubles: Damage of D02V1 collimatorʼs jaws
2020-11-15 13:01:13, beam abort with QCS quench- LER: 509 mA, HER: 469.6 mA- Pressure burst was observed in D02V1. The other pressure burst in D05 section was also observed (dust event?).
- After that, the BG levels were higher by a factor of 2 and limited the beam currents .
* D02V1 was shifted ~3.5 mm to outside of the ring to avoid scars, but the BG level was still high.
* Horizontal oscillation related to the injection seems larger than expected.
- The damaged jaws were replaced to spears from Nov. 18th to 21st.
QC1RP

LER current
Pressure

Coil-2

Ring outside

Coil-3Coil-1Ring
insideCoil-4

7

Troubles: Damage of D02V1 collimatorʼs jaws
• Why D06V1 was not able to protect D02V1?

- D06V1 had a vertical o set of ~ -300 μm to the beam. If D02V1 also had a vertical o set of +200 μm, D06V1
could be wider than D02V1.

* The o set study for D02V1 on Dec. 15th shows that it has almost no o set, however this was after thejawʼs replacement work.

- If the dust event in the D05 section between D02V1 and D06V1 happened, the strayed beam hits D02V1?
D03V1 had been wider than D02V1.
H. Nakayama

Collimator setting (Nov. 15th)
Collimatorβy [m]

νyaperture [mm](jaw̲pos - BPMy)

D06V1TOP

67.3

28.86

2.21

58.8

D06V1BTM

67.3

28.86

-2.99

79.5

D03V1TOP

17

41.44

2.34

123.9

D03V1BTM

17

41.44

-1.66

87.86

D02V1TOP

13.9

44.88

1.54

90.13

D02V1BTM

13.9

44.88

-1.16

67.9

Optics: sler̲1706̲80̲1.sad
Assuming εy = 21 pm

ff

ff

ff

ff

ff

ff

Taking vertical o sets into consideration. →V-o set [mm]D06V1: -0.3
D03V1: 0.4
D02V1: 0.2# of σy8Troubles: Vacuum leak in D02V1 collimator• On Nov. 23rd 0:28 (LER: 599.7 mA), a pressure burst happened in D02V1, and beam was aborted by it.
• Vacuum leakage was detected at the bottom side ange. A scar on the seal surface was found.
→ replaced to the spare one.

• On Nov. 24th, replacement work done and vacuum scrubbing resumed.
• On Nov. 27th, physics run resumed (LER βy* = 2 mm).
• The outgassing from the spare ange had been high, thus we baked-out D02V1 in-situ during this winter shutdown.
dP/dI in LERLER current and pressure (D02)
LER current
NEG activ.

D02V1 work
NEG activ.

scar mark

fl

fl

D02

Issue: TMCI in LER derived from collimatorsʼ impedance
C1 fsE/e
Ib,th =
D06V1 (C, 60 mm) survey (2020-12-02)∑i βikT,i(σz)
The maximum bunch current was ~1.04 mA/bunch limited by an instability in the collimator settings.
Collimator
D06V1
D06V2
D03V1
D02V1

βy [m]
67.3
20.6
17
13.9

aperture [mm] kT [V/pC/m] a)
2.0
841b)
3
237
3
237
3
237

Ib,th ≈ 0.99 mA/bunch[Handbook of Accelerator Physics andEngineering 3rd Printing (2009)]

C1 ≈ 8, fs = 2.13 [kHz], E/e = 4 [GV]

552 c)

a) Kick factors are calculated by Gd dL（σz: 6 mm）.
b) including lossy metal (Gd dL 2020-07-23, T. Ishibashi).
c) loss-free (Gd dL 2013-10-15, T. Ishibashi)This study is conducted taking the beam orbit and(1.49 mA/bunch)

the D06V1, D03V1 vertical o set into consideration.
B-PosY [mm]V-o set [mm]D06V1: 0.44

D06V1: -0.3

D06V2: 0.22

D06V2: 0

D06V2 (Ta, 10 mm) survey (2020-12-04)
D03V1: 0.04
D03V1: 0.4
D02V1: 0.16
D02V1: 0
We were able to accumulate ~1.5 mA/bunch at least.
However, we were not able to measure the vertical tune accurately because the main peak and side band were coupled.
Collimator
D06V1
D06V2
D03V1
D02V1

βy [m]
67.3
20.6
17
13.9

aperture [mm] kT [V/pC/m] a)
4.0
249 b)
1.8
490
2.0
430
1.0
1287

205 c)

Ib,th ≈ 1.31 mA/bunch• In this study, we were not able to measure the beam size because of the single-bunch operation.
• After this study, a vertical beam size blowup due to a dipole mode was observed for the multifi

ff

fi

fi

ff

bunch operation, and the threshold is 0.7-0.8 mA/bunch (lower than the calculated values).

Issue: TMCI in LER derived from collimatorsʼ impedance
1. vertical beam size blowup with increasing bunch current• The impedance of D06V1 is larger than that of D03V1.

2. changed vertical tune

- The kick factor of D06V1 (carbon, L=60 mm) is about twice
larger than that of D03V1（tantalum, L=10 mm）by Gd dL.

3. adjusted BxB FB gain
4. D03V1 open（18:09-18:14, 1mm →2 mm）

5. D03V1 close（18:16, 2 mm → 1mm）
6. D06V1 open（18:22, 1.95 mm → 2.25mm）

Tune spectrum(12/17, LER 0.478 mA/bunch)

1

2

3

4

5

6

fi

Y. Ohnishi

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Issue: TMCI in LER derived from collimatorsʼ impedance• Vertical tune shift was large. -0.0229 mA-1（2020-12-17）, -0.0131 mA-1（2020-11-5）.
• In terms of the bunch current, itʼs necessary to manage the collimatorsʼ aperture limiting with the tune shift.
- For example, if we try to suppress it ~0.012 mA-1, the collimator setting is on June, 2020（The BG was about twice larger
than that in Dec., 2020）。
• Carbon jaws had contributed on the impedance in LER, we decided to replace it with tantalum jaws with 5 mm length during thiswinter shutdown.
Y. Ohnishi

H. Nakayama
Theoretical formulaJun. 30th
Dec. 17(carbon)Dec. 17(if D6V1 is Ta)

Issue: TMCI in LER derived from collimatorsʼ impedance
Reuse damaged tantalum jaws with short tip in D06V1 for 2021ab• We found the damage when we opened the collimator chamber for a carbon jawsʼ installation workin 2020 summer.
• Thereʼs a possibility that the jaws was damaged on 2020-06-02 15:54 (2020b).
- Beam abort with pressure burst near D06V1 (LER: 546.8 mA, ~4 10-6 Pa).
- Small pressure burst happened in D02V1 (~1.7 10-7 Pa) and D06V2 (~5 10-7 Pa).
- VXD dose: ~119 mRad
Tantalum jaw with 5 mm length

top side jaw

Pressure

D06V1

beam
bottom side jaw

beam

D06V2

VXD dose1213Collimators during 2021ab
LER

•
•

Carbon jaws in D06V1 collimator were replaced with tantalum to reduce the impedance.
In situ baking of D02V1 and D06V1 collimators to reduce the base pressure.

HER•

Damaged jaw in D09V3 has been replaced with new one,LER D02V1 in-situ baking

which is copper coated titanium.

•

Drive mechanism of D12V1 has been replaced with new
one for precise positioning.

Others•

Prepared PVs to interactively monitor the kick factors
and ∑ βikT,i . (H. Nakayama, Y. Ohnishi)i

HER: Removed jaw from D09V3

HER: D12V1 and drive mechanism - before

after

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Study Plans
Machine studies in LER during 2021ab• Tune shift measurement (see gure on lower left)• High bunch current study (see gure on lower right)• remeasure them in the current situation (D06V1: Ta-5 mm)[K. Ohmi]• Chromaticity scan study• measure the beam size and the threshold of the blowup with changing the chromaticity• ESRF, SOLEIL, NSLS-II operate with large positive chromaticity (ξ>~5) and result in higher threshold (1mA→ >10 mA) after 2000.
and so on (studies related to the injection)
S. TeruiBeam size[μm]@SumB*k=36.2E15V/C

Threshold of the beam size is~1 mA/bunch.

Preliminary

Beam size[μm]@SumB*k=32.4E15V/C
180
Beam size[μm]@SumB*k=32E15V/C_ξyNegative160

βy* = 2 mm, 30-bunch operation

Tune shift: 0.008 mA-1
We were not able to inject up to ~1.7 mA/bunch.
side band

Beam size[μm]

TMCI threshold (calc.): ~1.77 mA/bunch
Design: 1.44 mA/bunch

140
120
100
80

βy* = 8 mm, single-bunch operation

Preliminary

D02V1: 2.17 mm, -1.8 mm, D03V1: 9 mm,

60

D06V1: 2 mm, D06V2: 9 mm
40
26

27

28

29

fi

fi

Beam current[mA]

30

31

32

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backupTopics: Bunch length measurements for collimator apertures• We measured the bunch length in LER in the collimatorsʼ impedance study simultaneously.
• No correlation for the collimatorsʼ apertures.
• However, itʼs longer than expected.
H. Ikeda

16

Topics: Damage of D06V2 (found after 2020c)• After 2020c, a damage of D06V2 (Ta, L: 10 mm) was found.
• Thereʼs a possibility that the jaws was damaged on 2020-06-08 15:01(2020b).
- Beam abort with pressure burst near D06V2 (LER: 580 mA, ~7 10-7 Pa).
- Pressure bursts except for D06V2 were not observed.
- VXD dose: ~225 mrad.
• This damage is probably an answer for a mystery of the strange response on BG in D06V2.
D06V2 TOP
beam

D06V2 Bottom
beam

17

Topics: Damage of D06V2 (found after 2020c)

18• Weʼve prepared records that inform the operators to have doubts about the damage.
- Example: VAHCCG:D01̲H03:WORRY̲CLM(D01̲H03 is a name of a CCG near D01H5 collimator)- The records issue the alarm when a pressure near collimators is larger than 5 10-7 Pa for a moment
and VXD:Rad:MaxDoseLastAbort is larger than 100 mrad within 5 min after the pressure burst.

• It was decided that D06V2 jaws were not replaced during this winter shutdown as a result of discussions.
- We have only two sets of spare jaws for the vertical collimators. These should be kept for 2021ab.
- D06V2 has not much contributed to reduce BG .
Pressure bursts on 2020-06-08 15:01

5E-7 PaVXD dose> 100 mradIssues: Horizontal oscillation related to injection in LER• After D02V1 had been damaged in 2020c, we shifted it ~3.5 mm to outside of the ring to avoid scars, but
the injection BG level was still high.

• Why was the BG level high?
- The scar or the protrusions are widely spread? → These are localized when we observed the removed
jaws.

- Horizontal oscillation related to injection is large and particles went through the jaws?
D02V1 Top Side

D02V1 Bottom Side

beam

beam

beforedamageafter
damage19

20

Issues: Horizontal oscillation related to injection in LER• We observed horizontal oscillations for the injected beams and for storage beams, which is caused by animperfection of a cancellation between the injection kickers (and mis-match between MR and BT?).

- Adjusted the kickersʼ timing and monitored the oscillation using turn by turn monitors.
• The horizontal oscillation for the injection beam at D02V1 is ~8.8 mm (peak-peak, ideally 4.3).
• The horizontal oscillation for the storage beam at D02V1 is ~4.3 mm (peak-peak, ideally 0 mm).
• There is a possibility that particles go through the jaws by the horizontal oscillations.
- Jawʼs full width: 12 mm
S. Terui, G. Mitsuka

568th bucket from the injected bucket
BOR(imperfection of the cancellation between kickers)

Storage beam oscillation by the imperfection
near D02V1

Injection beam oscillation@QC2

2.6 μs

Turns

Turns

Issues: Horizontal oscillation related to injection in LER• We also checked an e ect on the BG using a horizontal orbit bump at the vertical collimators.
- When the beam orbit had been shifted to the inside of the ring, the loss at D06V1 hadbeen decreased. The VXD BG had been increased.

- Particles can go through the jaws because of the horizontal oscillation.
neutron monitor at D06V1

ref. line

VXD BG

ref. line

horizontal beam position at D06V1

ff

inside of ring

S. Terui, H. Koiso

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Issues: Horizontal oscillation related to injection in LER• Countermeasures for the horizontal oscillation- Develop a vertical collimator with wider jaws.
* increase the impedance* can secure a space for the horizontal shift of the collimator when the jaws are damaged.
* can be useful to reduce HER storage BG by adopting this structure on D01V1.
- Perfectly cancel the waves between kickers using new kicker correctors(?) [T. Mimashi].
- Correct the mis-match in the injection region(?).
• If we can reduce the injection BG without using the collimators, we can open the apertures.
→ longer beam life time, higher (bunch) current

L: longitudinal tip length of jaws
d: half aperture
σz：bunch lengthff

dy: vertical beam o set

N. Andrii
MDI meeting2020-07-09

23

Others• 2020-12-14 0:55, beam abort (VXD diamond) with pressure burst at D06H3.
• LER: 480 mA, HER: 449.9 mA• A part of the bunches was suddenly kicked (caused by a mis re in a kicker?)• There are no scars on the tip of D06H3 (tungsten) using a ber scope after 2020c, but a part of the tip at the edge
seems to be peeled o .

S. Terui

LER current
CCG D06̲L03

kicker mis re?

top
edge
side

fi

fi

fi

ff

fi

We have 2 sets of spear jaws (Ta, 10 mm) for the vertical collimators.
We try to machining the damaged tip to x the scar or protrusions under a supervision of KEK Radiation Science Center.
ff••peeled o ?

Future plans (personal opinion)

LER• In the near future (next 1-2 years), what should we update the collimators in LER?
• Observed bunch current limit derived from the collimatorsʼ impedance.
• Damages of jaws.
→ adopt jaws with short length at the tip to reduce impedance with avoiding the damage by beam hit.
Name
D02V1
D03V1
D06V2
D06V1

Jawʼs type
standard

OthersTa (L=10 mm) is currently installed.

（Ta, L=10 mm）
short length tip

Option：wider jaw, short length tip etc.
Ta (L=10 mm) is currently installed.

（Ta, L=1.5-2 mm）
short length tip

Option：[wider jaw+short length tip] etc.
Ta (L=10 mm) is currently installed.

（Ta, L=1.5-2 mm）
short length tip

Option：[wider jaw+short length tip] etc.
Damaged Ta (L=5 mm) is currently installed.

（Ta, L=1.5-2 mm）

Option：[wider jaw+short length tip] etc.

HER

• In the near future (next 2-3 years), what should we update the collimators in HER?
• The impedance can also limit the bunch current in HER in the future (see page. 24).
Name
D01V1

KEKB type V

Jawʼs typewider jaw
（Ta, L=10 mm, width=22 mm）

Others
We donʼt need re-design the collimator chamber until the width is 22 mm.
The βx at D01V1 decreases with βy* squeezing.
Option：[wider+short length tip] etc
Need re-design the collimator and the bellows and beam pipes beside it.

SuperKEKB type with short tip jaw

KEKB type jaws are made of Ti with 40 mm (1.12 R.L.) + Cu coating（Ta, L=4 mm(？)）

Integrate two KEKB types with phase advance ~π to a SuperKEKB type.
(D09V1-D09V3, D12V3-D12V4 etc)24

Short tip jaw - How shorten the length?

25• Carbon with 60 mm length has almost same performance for the BG reduction as tungsten with 10 mm length.
• Carbon with 60 mm (~0.31 R.L.) is equivalent to tantalum with ~1.3 mm by the scaling.
• Is tantalum with 1.5-2 mm (0.36-0.5 R.L.) su cient to reduce the BG? → need simulations
MDI meeting2020-12-03
N. Andriiffi(C: 1R.L. = 193.2 mm)(Ta: 1R.L. = 4.094 mm)

Short tip jaw - kick factor• The kick factor of the 2 mm tip is ~7% smaller than that of the 10 mm tip.
• The kick factor of the 2 mm tip is ~30% smaller than that of the 50 mm tip.

26

TMCI rough estimation
Ib,th ≈ 0.59 mA/bunch

LER (Design: 1.44 mA/bunch)

Collimatorβy [m]

aperture [mm]# of σykT [V/pC/m]a)

D06V1
D06V2
D03V1
D02V1
QC1(1.12 m)

61.44
19.24
16.96
111.75
2686

1.4
1.3
1.0
2.4
13.5

61
101
83
77
89

706
764
1294
310

βy [m]
15.47
19.44
15.47
16.74
16.74
15.47
15.47
19.44
153.194390

aperture [mm]
-0.8
-1.5
-0.9
-1.7
2.0
-1.1
0.9
-1.5
2.0
13.5

# of σy
57
95
64
116
136
78
64
95
45
57

kT [V/pC/m]a)
1670
826
1463
718
598
1168
1463
826
412

27
fi

a) Kick factors were calculated with Gd dL (σz = 6 mm).
Collimator model:
D06V1-V2, D03V1, D02V1, D01V1:
SuperKEKB type (L: 5 mm, W: 12)
D09V1-V4, D12V1-V4:
KEKB type (L: 40 mm, W: 50 mm)

Ib,th ≈ 0.76 mA/bunch

HER (Design: 1.04 mA/bunch)

Collimator
D09V1
D09V2
D09V3
D09V4
D12V1
D12V2
D12V3
D12V4
D01V1
QC1(-1.16 m)

Optics: sler̲1704（βy* = 0.27 mm, εy = 8.64 pm)
sher̲5780（βy* = 0.30 mm, εy = 12.9 pm)

Note that the design bunch length in HER is not 6 mm
but 5 mm.

kT(0.005)/kT(0.006) ≈ 1.2
Ib,th ≈ 0.63 mA/bunch

C1 fsE/e
Ib,th =∑i βikT,i(σz)[Handbook of Accelerator Physics andEngineering 3rd Printing (2009)]

LER: C1 ≈ 8, fs = 2.13 [kHz], E/e = 4 [GV]HER: C1 ≈ 8, fs = 2.8 [kHz], E/e = 7 [GV]