Beam collimators(focusing on 2020c, 2021ab mainly)
25th KEKB Review
2021-09-01
T. Ishibashi

1

Contents• Beam collimators in SuperKEKB main ring
• Design• Basic design• RF shield• Impedance• Issue• Damage• Low-Z collimator• Beam size blowup in LER (TMCI)• Accidental firing of injection kickers• Plan• Replacement of the damaged jaws, installation of hybrid type jaws
• Relocation of D02V1 collimator to match the betatron phase between this and
QC1RP• Non-linear collimator at LER OHO section (during Long Shutdown-1 in 2022)• Summary2Beam collimators in SuperKEKB• In HER, 8 horizontal and 8 vertical KEKB type collimators have been reused at the same location as KEKB
era. 3 horizontal and 1 vertical SuperKEKB type collimators have been installed in Tsukuba straightsection.
• In LER, 7 horizontal and 4 vertical SuperKEKB typecollimators have been installed.

KEKB type[Y. Suetsugu et al., NIM A 513, 465 (2003)]

SuperKEKB type
[T. Ishibashi et al., PRAB 23, 053501 (2020)]

Location of collimators in MR

3

Design•••••

We referenced movable collimators for PEP-II in SLAC for the basic design.
A collimator chamber has two movable jaws, which are placed the horizontal/vertical direction.
Part of the movable jaws is hidden inside the antechambers to avoid a trapped mode in gaps between the
movable jaw and the chamber.
The chamber is tapered to the center of the collimator in order to avoid excitation of trapped-modes.
Materials at the tip of the jaws are tungsten (1st ver.), tantalum (2nd ver.), and carbon (low-Z, special ver.).

Collimator in PEP-II[S. DeBarger et al., SLAC-PUB-11752]

Vertical direction
Horizontal direction
SuperKEKB type collimator

4Design – RF shield• Contact finger-type RF shield is attached on the side wall of each jaw.

• The fingers are made of silver-plated INCONEL.
• The contact surface on the chamber is made of rhodium-plated stainless steel.

• Contact-less comb-type RF shield is adopted between the longitudinal end of the jaw
and the facing surface on the chamber.

Jaw with RF-shield5

Design – Impedance• Impedance in the collimators is calculated with GdfidL.
• The loss and kick factors of the SuperKEKB collimators are less than those of the KEKB
ones except for loss factors in the aperture of 15 mm or wider for a horizontal
collimator of KEKB.

[T. Ishibashi et al., PRAB 23, 053501 (2020)]

Loss factor (bunch length: 6 mm)

6Kick factor (bunch length: 6 mm)

Issue – Damage
•
•
•
•
•

* short tantalum tip which has 5 mm.

Jaws of the collimators were damaged by beam hit.
• LER: D02V1 (2021ab), D06V1(2021ab)*, D06V1 (2020ab)*, D06V2 (2019c), D02V1 (2019ab), D02V1 (Phase-2)• HER: D01V1 (Phase-2)
A huge beam loss and pressure burst happened near them with a QCS quench .
The damaged tungsten tip embrittled. For the tantalum tip, it did not embrittle.
The cause is unknown. A candidate is an interaction between the beam and a dust in the beam pipes.
The damaged jaws of D02V1 were replaced once during 2020c and 2021b because of the high BG level.
[S. Terui]

Before damage

After damage

D02V1 bottom side (38 μSv/h)taken on 2021-06-08

D02V1 top side (95 μSv/h)7

Issue – Damage•••

In case of a damaged event in 2021-06-02 20:13 (2021b run).
Pressure near CCG D02_L18 and D06_L12, which are placed near D02V1 and D06V1
collimator respectively, was increased instantaneously (beam hit the jaws probably).
QCS quench happened (QC1LP, QC1RP in LER).
Status of QCS

Pressure and beam current
LER Current

CCG D02̲L18

8

Loss monitors in the rings•••Issue – Damage

Sudden beam loss happened within 2-turn before the beam abort.
Dose on VXD diamond sensors of Belle II were extremely high and the signal saturated (usual beam loss abort: ~50 mRad or
less).
The cause of the huge beam loss is unknown so far. One of the candidates is the interaction between the beam and a dust in
the beam pipe.

•

Results of simulations indicate that the beam loses at not the vertical collimators but the horizontal collimators mainly [Y. Funakoshi].
a turnBOR(LER H)

1st train
2nd train

BOR(LER V)BCM

BCM loss

Bunch oscillation recorder (BOR) and bunch current monitor (BCM),
BOR signal is proportional to (bunch displacement)×(bunch intensity).

Dose in VXD diamond sensors of Belle II9Issue – Damage• Question from Frank Zimmermann• Can we perhaps get a thermomechanical analysis of the effect of beam loss at a collimator and alsoa calculation of deformation mode frequencies of a collimator?

• Answer• We’ve started to investigate the displacement and the deformation mode frequencies.
• The deformation during the operation is dozens of microns by our calculations and measurements,
probably.
• We’ve observed a deformation mode frequency around 10 Hz and the amplitude in the verticaldirection is ~5e-8 m.

10Issue – Damage• Temperature at the tip• We have used a special jaw, which has a thermocouple at the tip, in D02V1 during 2019b（βy*=2 mm）
Half aperture of D02V1• The temperature does not depend on the aperture of the collimator.
D02V1TOP temp2.3mm[ °]• The temperature depends on the beam current.
[S. Terui]
D02V1TOP temp4.5mm[ °]70Thermo couple

Temp[℃]

60

50

40

30

20

0

200

400

600

LER beam current [mA]

800

1000
11

Issue – Damage• The heat input at the tip was roughly estimated using CST particle studio for a condition of ILER=800 mA.
• The temperature at the embedded temperature sensor is ~65℃ in ANSYS, and this is consistent with it
during the operation.
• In this thermomechanical analysis using ANSYS, the displacement is ~20 μm.
[S. Terui]
Maximum displacement at tungsten: ~20 μm

Parameter︓
5W/mm2
Cooling water 0.02 W/mm2/ ℃

Temp 65℃

12Issue – Damage[S. Terui et al.]• We measured the displacement at the tip by the heating.
• The displacement is less than 20 µm in this setup.
Displacement meter(capacitor type)

nary
i
m
i
l
pre

Collimator jaw (Cu)Collimator head flange
(SUS)
Temp
[℃]

(without cooling water)

100

fix

80

Collimator jaw (W)70

Temp [℃]

Attached a ceramic heater and thermometeron the tip.

90

60

50

40

30

20
0

0.002

0.004

0.006

0.008

0.01

0.012

13

Displacement value [mm]

0.014

0.016

Issue – Damage• Mode frequencies measurement• We measured the deformation mode frequencies in the actual machine (D02V1 collimator)
with the water flow on/off.
Jaw’s axis

Measurement for the jaw (+ chamber)
Measurement for the chamber

14Issue – Damage• Mode frequencies measurement• There is an eigenmode around 10 Hz, and the maximum amplitude to the beam(vertical direction) is ~5e-8 m for the jaw with the cooling water flow.
Amplitude[H. Yamaoka et al.]

D02V1

Floor

15

Issue – Damage• Mode frequencies measurement[H. Yamaoka et al.]Integrated power spectrum density regarding to the frequency

D02V1

Floor

16

Issue – Low-Z collimator• Materials with a short radiation length is very effective as a beam tail shield, however the
beam loss is localized and the temperature of that exceeds the melting point.
• In order to protect the collimators for BG suppression from abnormal beams, wedeveloped a collimator with carbon*.
* Glass like carbon coated and impregnated C/Ccomposite (GCX2002-U_GP2B, Toyo Tanso Co.,Ltd.).

•

Tested: bonding test, tensile test, impregnation/coating test for dust and outgassing reduction,
RF absorbing test(2.45 GHz, 5.04 GHz), radiation degradation and so on• Installed in a existing collimator (D06V1) during 2020 summer
Maximum temperatures within 1 RL and meltin
shutdown and worked for BG suppression during 2020c,
however removed during 2021 winter shutdown because of the points (e+, 4 GeV. D: 0.5 mm, 50 mA, FLUKA).
impedance problem.

Carbon (L=60 mm)
Carbon jaw installed in D06V1.

17

Issue – Beam size blowup in LER (TMCI)• First high bunch current study with the single-bunch operation to observe the threshold.
D06V1 (Carbon, L=60 mm) survey on 2020-12-02
In the study, the maximum bunch current was ~1.04 mA/bunch (We were not able to inject above this).

L: length of the flat region at the tip

Calculated value:

D06V2 (Ta, L=10 mm) survey on 2020-12-02
We were able to accumulate ~1.5 mA/bunch at least (We were able to inject above this).

Calculated value:• In this study, the maximum bunch current looks consistent with the calculated value, but we didn’t measure the beam
size because of the single-bunch operation.
• After this study, a vertical beam size blowup derived from collimators’ impedance has observed for the multi-bunch
18
operation, and the threshold is 0.7-0.8 mA/bunch (much lower than the calculated value).

Issue – Beam size blowup in LER (TMCI)• The impedance of D06V1 is larger than that of D03V1.
• The kick factor of D06V1 (carbon, L=60 mm) is about twice larger than that of D03V1（tantalum, L=10 mm）by GdfidL.
→ Replaced the low-Z jaws with tantalum jaws before 2021a.
[Y. Ohnishi]

1

2

3

4

5

6

Carbon jaw installed in D06V1.

1.
2.
3.
4.
5.
6.

vertical beam size blowup with increasing bunch current
changed vertical tune
adjusted BxB FB gain
D03V1 open（18:09-18:14, ±1mm → ± 2 mm）
D03V1 close（18:16, ±2 mm → ±1mm）
D06V1 open（18:22, ±1.95 mm → ±2.25mm）
19Issue – Beam size blowup in LER (TMCI)• We can accumulate ~1.5 mA/bunch at least in this study.
• We‘ve not observed the coupling of the modes 0 and -1 yet.
• We‘ve observed the vertical beam size blowup before the coupling.

2021-02-17
βy* = 8 mm, single-bunch operation
Tune shift: ~0.008 mA-1
νy ~ 0.595 @ 1.6 mA/bunch
We were not able to inject up to ~1.7 mA/bunch (rep. rate 1 Hz).
TMCI threshold (calc.): ~1.77 mA/bunch
Δ~0.009

D02V1: 2.17 mm, -1.8 mm, D03V1: ±9 mm,
D06V1: ±2 mm, D06V2: ±9 mm

20Issue – Beam size blowup in LER (TMCI)[K. Ohmi et al.]21Issue – Beam size blowup in LER (TMCI)[K. Ohmi et al.]22

Issue – Beam size blowup in LER (TMCI)[K. Ohmi et al.]23Issue – Beam size blowup in LER (TMCI)
Change chromaticity

Change chromaticity
and collimator’s aperture[S. Terui et al.]Change chromaticity and FB gain

• The beam size blowup can be suppressed by increasing the vertical chromaticity.
• The threshold of the blowup increases with the negative chromaticity.
24• The blowup can be suppressed by increasing the feedback gain.

Issue – Beam size blowup in LER (TMCI)[K. Ohmi]25Issue – Beam size blowup in LER (TMCI)• Summary of TMCI in LER• We can accumulate the bunch current up to a calculated values using a broad impedance model,
however we’ve observed the vertical beam size blowup around ~60% of the calculated values.
• We’ve not observed the coupling of the modes 0 and -1, so K. Ohmi has suggested that we shouldobserve this, which is an "ordinary" TMCI, in a higher vertical tune (νy > 0.6).
• The beam size blowup has been observe for the single-beam operation (no collision). We’ve observed abeam size blowup in LER with the collision. It has been caused by the beam-beam, because the beam
size depends on not the collimators’ aperture but the current balance between LER and HER. We've notbeen able to accumulate the bunch current till the TMCI threshold with the collision so far.
• The bunch current threshold about the beam size blowup depends on the tune and the chromaticity.
• Proper vertical tune and/or higher chromaticity can increase the threshold.
• The threshold of the blowup may increase at least up to ~0.9-1.0 mA with present collimator setting bychoosing the vertical tune and/or the vertical chromaticity.
• For higher beam current operation that is assumed for L=1×1035 cm-2 s-1, a bunch current more than
1.0 mA will be needed for LER, so this beam size blowup may be more serious.
• There is a possibility that the tune to avoid the blowup is not good for the luminosity performance.
• D. Zhou‘s strong-strong beam-beam simulation has predicted that the luminosity will increase by
10% by moving from the present tune to the designed tune (νx, νy)=(.53, .57).
26

Issue – Beam size blowup in LER (TMCI)• (Example) parameter sets for L=1×1035 cm-2 s-1.
[Y. Funakoshi]

LER

HER

LER

HER

# of bunches

1564+12345+1

Luminosity

4.79 x 1034 cm-2s-1

1.0 x 1035 cm-2s-1

Itotal1.41 A

0.986 A

2.35 A

1.64 A

Ibunch

0.898 mA

0.630 mA

1.0 mA

0.7 mA

by*

1 mm

1 mm

0.8 mm

0.8 mm27

Issue – Accidental firing of injection kickers• Accidental firing of injection kickers in LER has happened.
• Example:• On 2021-05-14 00:35, a both ring abort happened.
• QCS quench (QC1RP a1, a2, b1).
• A pressure burst at D06H3 collimator was observed.
loss

LER current

BOR(LER H)

Pressure burst(CCG D06_L03)
BOR(LER V)BCM

BCM loss28

Issue – Accidental firing of injection kickers• D06H3 collimator has used for the protection from the accidental firing, and it was severely damaged.
• We replaced them with healthy ones during this summer shutdown.
• We don’t have the spare jaws for D06H3 collimator. Thus, we removed jaws of D06H1 and installed them into D06H3.
• We installed new jaws into D06H1 instead, however the length of the jaws is short for this collimator. The minimum
aperture of D06H1 is ±14 mm.
• We’re staring to manufacture the spare jaws made of tantalum, and it’ll be delivered till the beginning of next Jan.

• Plan to replace all of the thyratrons with another ones, which have higher breakdown voltage, during
this summer shutdown [T. Mimashi].

beam

beam
D06H3 IN (~700 μSv/h)

D06H3 OUT (~320 μSv/h)29

Plan – Replacement of the damaged jaws, installation of hybrid type jaws
• We plan to replace the damaged jaws during this summer shutdown.
• LER: D06V1 (Ta, L=4 mm), D06V2 (Ta-C hybrid, L=4 mm)• HER: D09V1 (Cu-plated Ti, L=40 mm), D09V2 (Cu-plated Ti, L=40 mm)(): type of new jaws
D06V2 Top (2020-12-18)

D09V2 (2021-07-09)

D06V1 Top (2021-06-08)
beam
scar

D06V1 Bottom (2021-06-08)
beam

D06V2 Bottom (2020-12-18)
D09V2 (2020-07-10)30

Plan – Replacement of the damaged jaws, installation of hybrid type jaws
• We plan to replace the damaged jaws of D06V2 with hybrid type jaws, which has carbon and tantalumat the tip in order to improve the robustness regarding to the beam hit, for the test purpose.
Model

ActualTantalum

Copper

CFC (Carbon-fiber-composited Carbon)
Tantalum

CFC
Cu

Cu plate31Plan – Replacement of the damaged jaws, installation of hybrid type jaws
Positron Flux

[Positron Beam]・Beam Size : σy = 100um, σx = 300um
property・Beam Energy : ４GeV・No energy spread・Lower half of the beam hit the jaw
・Beam current (assuming 99.4kHz)：
1 mA, 5 mA, 10 mA, 30 mA, 50 mA[Y. Morikawa, S. Terui]Temperature Rise(1mA)-Hybrid typetantalum

Temperature Rise(1mA)-All Ta

Carbon32

Plan – Replacement of the damaged jaws, installation of hybrid type jaws
Temperature Rise(1mA)-Hybrid type

[Y. Morikawa, S. Terui]Temperature Rise(1mA)-All Ta

When 30 mA beam hits the jaw, the volume of themelting tantalum is 68 % less compared to the standard
jaw. This might relax the bad effect on the background(durable jaw?).

33

Plan – Relocation of D02V1 collimator• Phase matching between D02V1 collimator and QC1RP in LER as much as possible by moving the collimator.
• We moved D02V1 3247 mm by the reconstruction of the existing components.

D02 building(2021-05-26)

Tsukuba
experimental hall
(Belle II)

e+ beam(2021-08-27)
3247 mm34

Plan – Relocation of D02V1 collimator• For the βy*=1 mm optics, the target of the moving length is 3000 mm,
however the actual length is 3247 mm to relocate during this shutdown.
• Simulations by A. Natochii indicate that even this configuration (3247 mm
shift) can reduce the detector background.
[H. Koiso]
βy* [mm]

Length of move [m]Phase difference
before relocation

βx/βy/ηx before
relocation [m]Phase difference βx/βy/ηx after
after relocation relocation [m]

1

2.964

1.463

10.89/13.95/0.1380

1.500

24.53/12.00/0.3569

0.8

3.871

1.469

13.01/18.40/0.1362

1.500

34.17/21.10/0.4209

0.5

2.074

1.478

16.31/14.32/0.1382

1.500

32.01/16.27/0.2908

0.27

3.235

1.495

25.22/98.09/0.1289

1.500

61.43/48.25/0.3640• We plan to bake around the D02V1 after the relocation work.
35

Plan – Relocation of D02V1 collimator[A. Natochii]

36

Plan – Non-linear collimator at LER OHO section• Present collimator setting (2021ab)• D06V1: primary collimator: most tightly closed and suppresses the injection BG.
• D02V1: second collimator: closest vertical collimator to IP and very important to suppress BG• D06V2, D03V1: backup: not so tightly closed. D03V1 can be used for a backup of D02V1.

• If we can replace D06V1 collimator with
non-linear collimator (NLC) for example,
the Σ𝛽! 𝑘! dramatically decreases.
NLC

37

Plan – Non-linear collimator at LER OHO section• Collimator setting during physics run in 2021b (βy*=1 mm)

D06V1 top

byc [m]2021-06-30 [mm]2021-07-02 [mm]

67.3

3.06

3.84

-2.65

-2.65

2.27

2.25

-2.26

-2.24

8.00

7.99

-8.00

-7.99

1.30

1.71

-1.14

-1.35

5.7

5.7

D06V1 bottom
D06V2 top

20.6

D06V2 bottom
D03V1 top

17.0

D03V1 bottom
D02V1 top

13.9

D02V1 bottom
NLC@OHO

2.9• The βy at the D06V1 is large, and the aperture is narrow.
• The βy at the NLC is small, and the aperture is wide.
→ can dramatically decrease the Σ𝛽! 𝑘! if we can replace D06V1 with NLC.

38

Plan – Non-linear collimator at LER OHO section• Scheme

−𝐼

∆𝜇! =

𝜋
2

[K. Oide]39

Plan – Non-linear collimator at LER OHO section[K. Oide]40

Plan – Non-linear collimator at LER OHO section[A. Natochii]

41

Plan – Non-linear collimator at LER OHO section[A. Natochii]42

Summary• Collimators are indispensable to reduce the detector BG and avoid the
QCS quench. However, there are issues:• Damage• TMCI• Relocation of D02V1 during this summer could relax the TMCI bunch
current limitation because we can open the aperture when we keep thesame BG level during 2021ab.
• Non-linear collimator at LER OHO section could relax the TMCI. We haveplans to construct this during Long Shutdown-1 (LS1), 2022.

43

backup44

Design – Impedance
L=10 mm

L=5 mmCarbon, L=60 mm

Non-linear slope (geometry to reduce the impedance)• K. Yokoya, Impedance of Slowly Tapered Structures, CERN SL/90-88(AP).

In our case, a~2 mm, b=45 mm, R=22.5.
The reduction factor is ~0.47.
46

Non-linear slope (geometry to reduce the impedance)
Elliptical jaw

Exponential jaw

Elliptical jaw + Elliptical pipe

Exponential jaw + Exponential pipe

47Non-linear slope (geometry to reduce the impedance)
Linear jaw

Elliptical jaw

Exponential jaw

48

Non-linear slope (geometry to reduce the impedance)
Current design vs exponential vs elliptical

Cut test for aluminum plate using wire-cutting machine49

Non-linear slope (geometry to reduce the impedance)

The exponential jaw is the best to reduce the kick factor for the narrow aperture.
The kick factor of the exponential one is smaller than that of the linear one by ~10% for H.A.=2 mm.

50

Non-linear slope (geometry to reduce the impedance)

The elliptical tapered pipe is the best to reduce the kick factor.
The kick factor of the elliptical one is smaller than that of the linear one by ~4% for H.A.=2 mm.

51

Non-linear slope (geometry to reduce the impedance)

The combination of the exponential jaw and the elliptical tapered pipe is the best to reduce the kick factor.
The kick factor of this combination is smaller than that of the current design by ~22% for H.A.=2 mm.

52

Non-linear slope (geometry to reduce the impedance)

53

Non-linear slope (geometry to reduce the impedance)

54Vertical tune shift

55

Horizontal tune shift

56Vertical tune scan

57

Beam size blowup

Tune scan

58

Beam size blowup
Chromaticity scan

59

Beam size blowup

60

Beam length measurements for collimator apertures[H. Ikeda]

We measured the bunch length in LER in the collimators’ impedance study simultaneously.
No correlation for the collimators’ apertures.
However, it’s longer than expected.

61