Collimator Issues
2022-12-14
The 26th KEKB Accelerator Review Committee
T. Ishibashi

Design2

KEKB type• KEKB type collimator has been used since KEKB era.
• It has a tapered beam-pipe and approaches the beam through 4 bellows chambers on both sides.
• These have been reused in 2 arc sections in HER.
• The tip is made of copper coated titanium with a length of 40 mm.
SuperKEKB type
• We referenced movable collimators for PEP-II in SLAC for the basic design of the SuperKEKB type.
• A collimator chamber has two movable jaws with RF-shields.
• Part of the movable jaws is hidden inside the antechambers to avoid trapped modes in gaps between the
movable jaw and the chamber. The chamber is also tapered to the center of the collimator in order to avoid
excitation of trapped-modes around the jaws.
• Materials at the tip of the jaws are tungsten (1st ver.), tantalum (2nd ver.), carbon (low-Z, special ver.), andhybrid (C+Ta, special ver.).

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

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

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

2

Location
2022abafter LS1

LER
- D06V1: primary collimator to suppress the injection BG- D02V1: collimator closest to interaction point (IP) and very important to suppress the detector BG.
- D06V1, D03V1: backups- D06H3: absorber against accidental firings of the injection kickers
During LS1- D03V1 will move to D05V1, that is a non-linear collimator.
- D06H1 will move to D06H4 that used for an absorber (countermeasure for accidental firings of injection kickers. D06H3 will be
used as the spoiler by installing carbon jaws in it).

3Material

Any materials should melt if high current beam hit the jaw.
Tungsten (W) was used for the material of the tip at first, but it was found that if it was damaged by the beam hit, it become embrittled andgenerate a lot of radioactive dusts.
• After that, tantalum (Ta) have been used as a default material of the tip.
• A special type of jaws named low-Z/carbon made from carbon fiber reinforced carbon (CFC) was developed for a more robust collimator, but
the impedance problem arose in 2020c and was removed (the radiation length of the carbon is long (~19.32 cm) compared with W/Ta (~0.35
cm/0.41 cm, and its conductivity is low (~3.0e5 S/m @ 5.044 GHz) ), so we need longer jaw to suppress the background).
• A special type of jaws named hybrid made from CFC and Ta was developed for the robust collimator.
Carbon jaw
This jaw has been used without being damaged and so on, if a high beam current induced in, it will
break, and we don't know what state it will be in if it breaks (In the worst case, cracks may beformed at the junction and discharges may occur.).
••Hybrid jaw (CFC+Ta)[T. Ishibashi][T. Sanami]

Maximum temperatures within 1 RL and melting
points for each material, induced by the positronbeam. The beam profile and energy are respectively
Gaussian and 4 GeV. Dx=y and the injected current
are 0.5 mm and 50 mA (3.12 × 1012 positrons),
respectively.

4

Investigation of Materials for more robust collimators• High-Z (Ta, W)• The radiation length is short (Ta: 0.41 cm, W: 0.35 cm), and we can decrease the length at the tip of the jaw to a few mm.
• The electric conductivity is acceptable (Ta: 7.61e6 S/m, W: 18.9e6 S/m). [Ref. Cu: 59.6e6 S/m]
• Easy to damage. Jaws with a short tip length of ~4 mm has been installed in the vertical collimators in the arc section for the robustness
because the scratch mark caused by the beam hit starts after one radiation length in the beam traveling direction. A tip length of 10 mm has
been installed in only D02V1 and D01V1.
• Large effect to raise the IR background when it’s damaged (This is derived from the mass density of the protrusions on the tip surface?)• Generate radioactive dusts when it’s damaged. Radiation Science Center has commented that the half-life time of the Ta may be longer
compared to that of W. However, it’s very easy to generate the dusts for W when it’s damaged…
• In the future, the radioactivity can limit the replacement work. We may have to wait the decay of the radioactivity and be not able to replace
during runs.

• Low-Z (Carbon)• The radiation length is long (C: 19.32 cm) . We need longer tip (>60 mm) to shield the halo, and this raise the geometrical impedance.
• The electric conductivity is very low (C: ~3.0e5 S/m @ 5.044 GHz), and the resistive wall also raises the impedance. However, we can avoid
this by adopting Cu plate on the carbon.
• The melting point is high.
• Very difficult to damage.

• Mid-Z (Ti)• The radiation length is intermediate (Ti: 3.56 cm), and we can adopt a dozen mm for the tip length.
• The electric conductivity is acceptable (Ti: 2.38e6 S/m). If we need Cu plating, we can do it as with the jaws of KEKB type collimators.
• Easy to damage from the experiences of KEKB type collimators in HER.
• Nevertheless, we’ve used damaged Ti jaws in HER without major impact on the IR backgrounds. (This is derived from the mass density?)(→ Titanium jaws may have good balance in terms of the durability and the impedance.)• Difficult to generate the radioactive dusts when it’s damaged from the experiences of KEKB type collimators in HER.

• We are developing carbon, hybrid (C+Ta), titanium, and MoGr jaws as collimators that are hard todamage or have little effect on the background even if they are damaged.
5

Major Issues – damage• The collimators have been frequently damaged due to beam hits.
• The source of beam instability that causes damage to the vertical collimators is still
unknown (sudden beam loss, H. Ikeda will report the details later).
• On the other hand, there are damaged events for known reasons, and that is thedamage to a horizontal collimator (D06H3) caused by accidental-firings of injection
kickers in LER. As countermeasures against this,- Carbon jaws will be installed in D06H3 and used as a spoiler for the uncontrollable beams.
- D06H1 will move to D06H4 and used as an absorber.

Damaged jaw of D06H3 in LER due to theaccidental-firings

Damaged jaw of D02V1 in LER due to the sudden
beam loss

Damaged tip of D09V1 (KEKB type) in HER
due to the sudden beam loss during 2022b.

6

Major Issues - damage (sudden beam loss)• Sudden beam loss has occurred within approximately 2-turn (~20 µs) before the beam abort.
• Beam orbit displacements in horizontal and vertical direction has been observe [H. Fukuma and S. Terui].
• No increase in the beam size has been observed by a fast beam size monitor [G. Mitsuka].
• This beam loss has never happened in single beam operation (because the short operation time with single beam?).
• It is more common in LER, but has also occurred in HER.
• Candidates of the cause [kick-off meeting of ITF sudden beam loss working group, https://kds.kek.jp/event/43499/]:- X-abort, dust, fire-ball in collimators, electron cloud in vertical collimators …
1st train 2nd train
a turnBunch oscillation recorder (BOR) and bunch current monitor (BCM), BOR signal is proportionalto (bunch displacement)×(bunch intensity).

7

Major Issues - damage (sudden beam loss)• Beam orbit displacements in horizontal and vertical direction has been observed.
Current [mA]

[H. Fukuma]

Horizontal [mm]Vertical [mm]Longitudinal [a.u.]8Major Issues - accidental firings of injection kickers

9• Accidental firings of injection kickers have happened in LER.
(It seems that the accidental firings tend to occur more frequently after the shutdown period somehow.)• When this event occurs, pressure burst near a horizontal collimator named D06H3 is observed due to the beamhit.
• In the current system, we cannot make the probability of the accidental firing zero, so some horizontal
collimators in each ring has been used to protect components against the accidentally kicked beams.
• As countermeasures against this,- Carbon jaws will be installed in D06H3 and used as a spoiler for the uncontrollable beams.
- D06H1 will move to D06H4 and used as an absorber.
Pressure burst near D06H3 collimator.

LS1

shutdown

10

shutdown

12

shutdown

14

shutdown

The number of the accidental firings on each run period.

LER current

CCG D06_L03

Horizontal position of the center ofgravity for each bunch measured with
BOR and the bucket number when an
accidental firing of an injection kicker
occurred.

8
6Damaged jaw in D06H3 by accidental firings.＿

4
2

2022b

2022a

2021c

2021b

2021a

2020c

2020b

0

Replacement Work (vertical collimator)• Before the replacement, we purge the beam pipes in the section where the collimator is
located with nitrogen and remove the drive mechanisms and cooling water pipes.
• It is also necessary to investigate the cooling water whether it’s radio activated.

Clearing out people

Remove the beam-pipes nearby, and rotate the chamber 90 degrees

10Replacement Work

Remove the racetrack type flanges in where the jaws are inserted.

Remove the damaged jaws.
(← this photo shows a cleaning work for D06H3. I was
not able to find the photo for D02V1.)

Clean the inside of the flange and chamber to remove deposits such as activated dust.

11Replacement Work

Attach RF-fingers to the spare jaws.

Insert them carefully so as not tobreak the fingers and so on.

Tighten the flanges.

• After that, we rotate the chamber -90 degrees andconnect the beam-pipes nearby and cooling water
pipes. Then, after the leak check pass, we re-installthe drive mechanisms and fine tune them.
• Align using a laser tracker with the help of Magnet
Group if necessary.
Drive mechanism

12

arc

Tsukubastraight

arc

Current condition – HER

Tsukubastraight

•
•
•
•

Name

Type

Tip Material(): longitudinal length in mm

Tip Condition

Drive Mechanism

D09H1
D09H2
D09H3
D09H4
D12H1
D12H2
D12H3
D12H4
D01H3
D01H4
D01H5

KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
SuperKEKBSuperKEKBSuperKEKB

Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)Ti (40)Cu coated Ti (40)Ti (40)Cu coated Ti (40)W (10)W (10)W (10)

damageddamageddamageddamagedhealthydamagedhealthyhealthyhealthyhealthyhealthy

-

D09V1

KEKB

Cu coated Ti (40)damaged

D09V2
D09V3
D09V4
D12V1
D12V2
D12V3
D12V4
D01V1

KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
KEKB
SuperKEKB

Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)
Cu coated Ti (40)W (10)

healthyhealthyhealthydamageddamagedhealthyhealthydamaged (only BTM)Work Plans during LS1

jaw replacement (Cu coated Ti (40)),drive mechanism upgrade?

upgraded
drive mechanism upgrade
upgraded-jaw replacement (Cu coated Ta (10))

D09V1 was damaged during 2022b and will be replaced with new one during LS1.
There was a problem with the displacement sensor in D12V3 during 2022ab, and the drive mechanism will be upgraded during LS1.
There is a request to upgrade the drive mechanism of D09V1 for the precise positioning.
We plan to coat spare jaws for D01V1 with Cu and install them as a countermeasure for fire-balls during LS1.

13

arc

Tsukubastraight

arc

Current condition- LER

Tsukubastraight

Name

Type

Tip Material(): longitudinal length in mm

Tip Condition

Work Plans during LS1

D06H1
D06H3
D03H1
D02H1
D02H2
D02H3
D02H4

SuperKEKBSuperKEKBSuperKEKBSuperKEKBSuperKEKBSuperKEKBSuperKEKBW (10), fully opened
OUT: Ta (10), IN: W (10)W (10)W (10)W (10)W (10)W (10)

healthydamagedhealthyhealthyhealthyhealthyhealthy

move to D06H4, jaw replacement (Ta (10?))
jaw replacement (Cu coated C (160))

D06V1
D06V2
D03V1
D02V1

SuperKEKBSuperKEKBSuperKEKBSuperKEKBTa (4)
Cu coated C-Ta hybrid (3)Ta (10)Ta (10)

damagedhealthyhealthydamagedjaw replacement (Cu coated Ti (10?))move to D05V1 (currently installed jaws will be spares)
jaw replacement (Cu coated Ta (10))• D06H1 will move to D06H4 and used as an absorber of uncontrollable beams due to accidental firings of injection kickers.
The jaws will also be replaced with new ones.
• Carbon jaws will be installed in D06H3 and used as a spoiler of the uncontrollable beams.
• Cu coated Ti jaws, which are one of candidates of durable jaws, will be installed in D06V1 and can be also used as a spoiler
for sudden beam loss (D05V1 can be used for the absorber) [A. Natochii, https://kds.kek.jp/event/42881/].
• D03V1 will move to D05V1 (non-linear collimator), and the jaws will also be replaced with new ones (Cu coated Ta, thelength depends on the radiation safety in OHO-section).
• We plan to coat spare jaws of the vertical collimators with Cu as a countermeasure for fire-balls and install them in D02V1
14
and D05V1.

Major Issues – impedance (summary of -1 mode instability in vertical direction)
In single-beam operation (no collision) in LER:
• Vertical beam size blow-ups have been observed at ~1.0 mA/bunch, and the threshold of the -1 mode instability
depends on the aperture setting of the vertical collimators, tuning of the vertical bunch-by-bunch feedback andso on.
• ITF TMCI working group (contact person: Mauro Migliorati, sub-contact person: me) had tackled to findcountermeasures against this and investigation of the mechanism through simulations and machine studies.
Ø ITF TMCI working group: https://kds.kek.jp/category/2247/
Ø ITF meetings: https://kds.kek.jp/category/2322/• A peak around νy-νs appears when the blow-ups occur (so we’ve called this “-1 mode instability in the group).
• Tune surveys indicate that this instability is not derived from a stop band of νy-νx-2νs=N.
• The dipole oscillation due to the -1 mode instability is spontaneously damped.
ü The growth time is 3.5 ms to 8 ms.

• The 0- and -1-mode seem to be coupled at around ~2.0 mA (ordinary TMCI) from a linear regression in ameasurement.
• We’ve found some effective countermeasures to the -1 mode instability.

1. Tuning of the vertical bunch-by-bunch feedback (BxB FB),
2. Increase the vertical tune,
3. Increase the vertical chromaticity,
4. Reducing the impedance in the vertical direction by opening vertical collimators.
ü The forth item is one of motivations to construct a non-linear collimator, and we’ve decided to construct it (A. Morita will reportabout the non-linear collimator later).

• K. Ohmi has reproduced this instability in simulations taking transverse wake and high gain feedback system witha multi-tap FIR filter in to account (K. Ohmi will report later).

15

Non-linear collimator[A. Natochii][example] collimator setting and βyky for βy*=1 mm optics.
Collimatorβy [m]

Half-aperture [mm] a)βyky [×1015, V/C] b)

D06V1

67.35

2.9

16.15

D06V2

20.57

2.6

5.64

D03V1

16.96

8.0

1.83

D02V1

11.89

1.1

11.76

D05V1

4.05 c)

4

0.69

a)
b)
c)

averaged value of top and bottom half-aperture at 2021-12-22
ky is a kick-factor obtaind by GdfidL simulations and a fit for σz=6 mm and Ta-jaw of 10 mm in
the longitudinal length.
sler_1707_80_1_cw80_NLC_04

Bunch current threshold of TMCI

𝐼!" =4𝜋𝜈# (𝐸/𝑒)
𝑇$ ∑% 𝛽&,% 𝑘&,%• D05V1 is a vertical collimator in the non-linear collimation system.
• D06V1 has been used to suppress the injection backgrounds (primary collimator) and has a large βyky because the βy is large,
and the aperture is narrow.
• D05V1 can have a small βy and almost the same performance of the background reduction as D06V1 with the wider aperture.
16→ It can decrease the βyky to approximately 1/20 if we can use D05V1 instead of D06V1.

Summary - Major Issues• Collimators have been frequently damaged due toØ accidental firings of injection kickers in LER,

• For a measure against this, we will develop and install carbon jaws in D06H3, and it will be used asa spoiler. Then, we will move D06H1 to D06H4 that used for an absorber for the uncontrollable
beams.

Ø sudden beam loss in the both rings.

• For a measure against the sudden beam loss,
1.
there is a possibility that the Cu coating on the tip of the jaw can avoid the sudden beamloss if they are triggered by fire-balls (T. Abe). We will coat the tip in all of the verticalcollimators with Cu.
2.
we will install short titanium jaws in D06V1 and use it as a spoiler, and use D05V1 (nonlinear collimator) as an absorber for the uncontrollable beams [A. Natochii, MDI taskforce meeting,https://kds.kek.jp/event/42881/].

3.

Ø Large βy of ~67 m at D06V1 is unwanted for the vertical impedance, but good for the spoiler.
Ø We have to set narrow aperture in D06V1, so the impedance situation remains unchanged.

If there is a possibility that electron cloud in the collimators can cause the sudden
beam loss (under investigating), we may put solenoid coils around them.

• Vertical beam size blow-ups in LER have been observed caused by the impedance and feedback in thevertical direction.
• This instability can be reproduced by simulations taking transverse wake and high gain feedback with a multi-tap
and FIR filter system into account [K. Ohmi].
• The impedance can be dramatically reduced by adopting a non-linear collimation system instead of D06V1, andthis will increase the threshold of the bunch current.
17Summary - Status and Future Plans• We’ve developed new jaws (C, hybrid, Ti, and MoGr) for a more robust system in terms of thebeam hits.

- However, even if these are realized, they are only tentative measures, because any materials should melt ifhigh current beam hit the jaw.
- It is essential to eliminate sources that causes the uncontrollable beam (sudden beam losses and accidentalfirings in injection kickers).

• In addition, several ideas have been proposed to make the collimator system more robust.

- alternate jaws to prevent deposition of metal on the opposing surface at tip (impedance increase andtrapped mode)- wider jaws to make room for the horizontal shift (impedance increase)- operation of double D02V1 in tandem (trapped mode)- vertical collimator at downstream of IP, that is D01 for example, to install jaws that have ultra-thin tips (BG
reduction)(): concerns
alternate jaws

wider jaws

vertical collimator in tandem

18

back-up

19

Spares that we have as of now
Tip Material(): longitudinal length in mm
HER vertical: W (10)HER vertical: Ta (10)HER vertical: Ta (10)HER vertical: Cu coated Ti (40)LER vertical: Ta (10）LER vertical: Ta (10）LER vertical: Ta (10）LER vertical: Ta (10）LER horizontal: Ta (10）Collimator to be installed (assumed)
D01V1
D01V1
D01V1
D09V1-V4, D12V1-V4
D02V1
D02V1
D02V1
D02V1
D01H3-H5, D02H1-H4, D06H4

Work Plans during LS1plan to coat with Cuplan to coat with Cuplan to coat with Cu and install on D02V1
plan to coat with Cu and install on D02V1
plan to coat with Cu and install on D05V1(?)
plan to coat with Cu and install on D05V1(?)

20

Scheduled to be manufactured (FY2022)
Tip Material(): longitudinal length in mm
LER vertical: Cu coated Ta (3.5-10）
LER vertical: Cu coated Ta (3.5-10）
LER vertical: Cu coated Ti (10?）
LER vertical: Cu coated Ti (10?）
LER horizontal: Ta (10?）
LER horizontal: Ta (10?）
LER horizontal: Cu coated C (160?）
LER horizontal: Cu coated C (160?）
HER vertical: Cu coated Ti (40）
HER vertical: Cu coated Ti (40）Collimator to be installed (assumed)

Remaks

D05V1
D05V1
D06V1
D06V1
D06H4
D06H4
D06H3
D06H3
D09V1-V4, D12V1-V4
D09V1-V4, D12V1-V4

longitudinal length depends on the radiation safety in OHO-section
longitudinal length depends on the radiation safety in OHO-section
can be used for spoiler against sudden beam loss (D05V1 can be the absorber)can be used for spoiler against sudden beam loss (D05V1 can be the absorber)
absorber for crazy beam due to accidental firings in inj. kickers
absorber for crazy beam due to accidental firings in inj. kickers
spoiler for crazy beam due to accidental firings in inj. kickers
spoiler for crazy beam due to accidental firings in inj. kickers
KEKB type jaw
KEKB type jaw• In addition to these, we plan to investigate MoGr [M. Pasquali et al., https://doi.org/10.1007/s40870-019-00210-1] as one of candidatesof the durable jaws,

21

Major Issues (impedance - βy* dependence)
D06V1

•
•
•

The collimator setting is based on that in physics run
during 2021c.
The aperture of the vertical collimators are scaled by√𝛽! at each collimator and √𝛽!∗ .
Each region of the beam-pipes in IR is below.
•

vertical collimator

IR

•
•
•
•

D05V1
(non-linear collimator)

üvertical collimator

IP chamber: X-shaped beam-pipe around the collision point
between “QC1RP505” and “QC1LP505”.
QC1LP: tapered beam-pipe including a BPM, bellows, andwelding-gap between “QC1LP515” and “QC1LP1375”.
QC2LP: tapered beam-pipe including a BPM and welding-gap
between “QC2LP1385” and “QC2LP2475”.
QC1RP: tapered beam-pipe including a BPM, bellows, andwelding-gap between “QC1RP1375” and “QC1RP485”.
QC2RP: tapered beam-pipe including a BPM and welding-gap
between “QC2RP2715” and “QC2RP1385”.
The resistive-wall impedance is included in this model.

IR22

Major Issues (impedance - βy* dependence)
Without NLC

𝐼#$ =4𝜋𝜈% (𝐸/𝑒)𝑇& ∑' 𝛽!,' 𝑘!,'𝜈! = 0.022, 𝐸/𝑒 = 4 GV, 𝑇" = 1e-5 s

With NLC

23