Vacuum System Status
T. Ishibashi, KEK
on behalf of KEKB Vacuum Group
Contents
・Updates of vacuum system before Phase-3
・Present status- vacuum scrubbing- electron cloud effect in LER- problems
・Summary and plans
2019/7/8

23rd KEKB Review1

Updates of vacuum system - ECE• Countermeasures for Electron Cloud Effect(ECE)• Approximately 91% of the drift spaces of LER were covered by magnetic field in thebeam direction with permanent magnets or solenoid coils before Phase-3.
• In Phase-2, approximately 86% of that were covered as presented the previous KEKB Review.
Type-1 unit

Magnetic field of Type-1 unit
Typical strength ~ 60 G

Magnetic free
1/2500/2RF, 3.6 A
dmax=1.2

4.5x1011 m-3(r<8 mm)

Type-1 unit
1/2500/2RF, 3.6 A
dmax=1.2

4x1010 m-3(r<8 mm)

Permanentmagnets

2019/7/8

Iron yoke(plate)

23rd KEKB Review2

Updates of vacuum system - collimators• Installation of new beam collimators• LER: D02H1, D02H2, D03H1, D06V1• HER: D01H3

H: horizontal
V: verticalCollimator location• Moved a collimator from D06H4 to D06H1 in LER
D02H1 collimator

Shematic drawing of thehorizontal collimator

2019/7/8

23rd KEKB Review

3Updates of vacuum system - collimators• Trouble: water leakage from a movable jaw of D02H1 collimator.
• Changed to a backup jaw.

2019/7/8

23rd KEKB
Review
Investigation by sniffer
schemeand Snoop®

4Updates of vacuum system - dusts
R14.2: Prepare a plan to mitigate the pressure bursts in case it turns out to be a limitation for theoperation in Phase III.
• Countermeasures for dust events with pressure bursts• Beam losses or aborts accompanied by local pressure bursts have been observed in LER since Phase-1.
• Beam pipes, which had caused the events frequently, were knocked in order to drop dusts from the top of
the beam channel.
• In some regions that caused the event frequently, the dusts were removed by a vacuum cleaner.
•BLB3RP, BLX4RP.2, BLX4RP.1

Cleaned region

2019/7/8

Collected dusts fromvacuum cleaner

23rd KEKB Review5

Updates of vacuum system ‒ jaw damage• LER: A huge beam loss and pressure burst happened near D02V1 with a QCS quench at
11:20 June 25th, 2018 in Phase-2. Stored current was 728 mA. The tungsten tips of thejaws were damaged. After that we were not able to run the luminosity mode because ofhigh backgrounds in this condition. → move the collimator• HER: A huge beam loss and pressure burst happened near D01V1 with a QCS quench at
11:20 June 9th in Phase-2, 2018. Stored current was 766 mA. Jaws were damaged. Afterthat we were able to run the luminosity mode in this condition.

Pressure bursts near D02V1

Photo of a D02V2 bottom jaw.

Photo of a D02V2 top jaw.

beambeam
2019/7/8

23rd KEKB Review

6Updates of vacuum system ‒ jaw damage• Jaws of D02V1 top, bottom and D01V1 top were changed to newones in the presence of Radiation Science Center.
• D02V1: ~10 µSv/h• D01V1: ~30 µSv/h• The damaged tungsten is likely to be crumbly by the embrittlement.
→ plan to change the material from tungsten to tantalum
Photo of a D01V1 top jaw.

2019/7/8

23rd KEKB Review

7Updates of vacuum system• others• Installation of longitudinal feedback kicker in LER.
• Changed septum beam pipes to reduce the leakage field into MR.
• Installation of bellows chambers with SR mask in HER downstream of IP
because temperature rise was observed in a stainless chamber caused bySR in QCS.
• Abort system by pressure burst besides collimators to avoid the jaw damage.
• etc.

2019/7/8

23rd KEKB Review8

Present Status ‒ vacuum scrubbing• MR• Maximum stored beam current in Phase-3: HER 940 mA, LER 830 mA• Beam dose in Phase-3: HER 536.3 Ah, LER 500.3 Ah

2019/7/8

23rd KEKB Review

9Present Status ‒ vacuum scrubbing (LER)• The dP/dI in MR has been smoothly decrease by the vacuumscrubbing.
• In LER, the dP/dI curve in Phase-3 is similar with that in Phase-2 becausesome regions were exposed to the atmosphere for vacuum works. ThedP/dI at the end of Phase-3 is below that of Phase-2.
Cu chamber in KEKB LER

2019/7/8

23rd KEKB Review

10Present Status ‒ vacuum scrubbing (HER)• The dP/dI in MR has been smoothly decrease by the vacuumscrubbing.
• In HER, the dP/dI in SuperKEKB is below that in KEKB at the final stage by
the historical effect.
Cu chamber in KEKB HER

2019/7/8

23rd KEKB Review

11Present Status ‒ vacuum scrubbing (DR)• DR
• Beam dose: 3.9 Ah.
• No trouble.

2019/7/8

23rd KEKB Review

12Present Status ‒ ECE
R14.1: Continue the experimental investigation of electron cloud and the conditioning of the TiN
coating. Implement the proposed test with permanent magnets on the antechamber of the drift
beampipes.
• In Phase-1, vertical beam size blow up and nonlinear behavior of pressures from Id~0.1 by ECE
were observed in LER.
• The current linear density (Id) is the bunch current divided by the bunch spacing.

• The threshold was relaxed to Id~0.2 by an installation of permanent magnets on bellowschambers.
• A threshold in KEKB without solenoids was approximately Id=0.04.
ECE in Phase-1 with/without permanent magnets in bellows chambers

2019/7/8

23rd KEKB Review

Permanent magnets and iron yoke on abellows chamber

13

Present Status ‒ ECE• Permanent magnets were installed on ~86% of the drift spaces before Phase-2.
• The blowup was not observed by Id=0.4 in Phase-2.
• Approximately 91% of the drift spaces were covered with them before Phase-3.

• The blowup was not observed by Id=0.55 in Phase-3.
• The design value is Id=0.72 (3.6 A, 2500 bunches, 2 RF-bucket spacing)
ECE study in Phase-2

ECE study in Phase-3 (preliminary).

courtesy of H. Fukuma
2019/7/8

23rd KEKB Review

14

Present Status ‒ problems• Collimators are quite effective to reduce BG.
• Collimator damage• LER: A huge beam loss and pressure burst happened near D02V1 with a QCS
quench at 22:11 June 9th, 2019 in Phase-3. Stored current was 660 mA. Thetungsten tips of the jaws were damaged. After that we were not able to run thephysics mode because of high backgrounds in this condition.
• This is a similar situation as a QCS quench event in Phase-2, and we need thedetailed analysis about this incident. One of the candidates that caused thisincident is a dust event.
Photo of a bottom jaw in D02V1 takenfrom a view port

Photo of a top jaw in D02V1 taken froma view port

Pressure bursts near D02V1 collimatorbeam

2019/7/8

beam

23rd KEKB Review

15

Present Status ‒ problems• Collimator damage• At the same time, pressure bursts were observed in a beam pipe with aclearing electrode. → a dust event caused the beam energy loss?
• We are plan to knock the beam pipes in wiggler section by next
commissioning.
• Any materials would melt if whole beams hit the jaw.
→ Need fast abort to avoid the damage!
Maximum temperature induced by beam hit

Pressure bursts in NIKKO wiggler section

Temperature [℃](1×1012 e-/pulse (16 mA), effective bunch radius: 50 μm, R.L. = 0.5)

2019/7/8

23rd KEKB Review

Melting point
Max. temp

16

Present Status ‒ problems• Dust events• The number of dust events has been decreasing with the operation time.
• Aging effect by low current operation• Quick abort before a burst occurs• Dust drop by a knocker• However, it can cause beam energy loss.
→ drop dusts using the knocker before next commissioning→ proposed an aging operation with high current before a physics run

2019/7/8

23rd KEKB Review

17

Present Status ‒ problems• Temperature of IP bellows chamber in HER in Phase-3, which is between a QCS
beam pipe and an IP chamber. ~60℃ （850 mA, 3.06RF, 1578 bunches）. This
happened in 1 of the 4 bellows chambers.
• The similar heating on an another one had also been seen in Phase-2. It waschanged to new one before Phase-3.
• The RF shield is a conventional fingers type. Fault of the electrical contact?
→ We have a plan to replace it with new one in this summer shutdown.

Temperature of IP bellows chamber
3.06sp, 1576 bunch

6.12sp, 789 bunch

Detached IP bellows chamber after Phase-2

Discharged mark2019/7/8

・This finger was off after the installation
23rd
KEKBPhase-2.
Reviewand put it backbefore・This happened in 1 of the 4 chambers.

18

Present Status ‒ problems• Pressure upstream of IP in LER.
• Vacuum component is dominant for BG in LER.
• Especially, pressures near D02V1 and D02H1 are high.
• D02H1 collimator had a water leakage trouble before Phase-3, so it could
raise the pressure.

→ try baking these collimators and beam pipes in tunnel
IRD02V1

D02H2

D03H1

D02H1 (water leakage)

D06V2

D06H1

2019/7/8

23rd KEKB Review

19

Present Status ‒ problems• Pressure upstream of IP in LER.
• Vacuum bump study using NEG pump heating in Phase-3• D02, D06, D12 seem to be sensitive to BG.
courtesy of H. Nakayama

2019/7/8

23rd KEKB Review

20Summary and plans• Vacuum scrubbing is going well.
• Countermeasures for ECE in LER are quite effective.
• Problems:• Collimator damage• Dust events• BG in derived from vacuum• IP bellows chamber heating• Near Future Plans:• Change the damaged collimator(D02V1 bottom) in the summer/winter.
• Knock beam pipes in LER.
• Try baking collimators in tunnel.
• Change IP bellows chamber in this summer shutdown.
• Add a vertical collimator at D06V1 in LER in the winter shutdown.
2019/7/8

23rd KEKB Review21