OPERATION EXPERIENCE OF CRAB CAVITY
9Brief history
9Crab cavity I/L system
9RF trip

9Crab voltage scanning
9Piezo breakdown
9Voltage drop of LER crab cavity
9Lower temperature operation
9Oscillation at high beam current with “Crab ON”
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9HOM load at HOM dampers

9Achieved parameters during beam commissioning
9SummaryKirk

BRIEF HISTORY
month/year

comments
Crab cavities were installed into KEKB.

Feb/2007Beam commissioning started with “Crab ON”.

Mar/2007

Cavity voltage of LER Crab dropped suddenly.

Jun/2007

Piezo actuator of LER Crab broke down for the first time.
Oscillation was observed at high beam current with crabbing collision.

Oct/2007

Physics run started with “Crab ON”.

May/2008

Cavity voltage of LER Crab was recovered slightly.

Oct/2008

Lower temperature operation was tried to recover cavity voltage of LER Crab.

Nov/2008

Beam study with “Crab ON” for LHC was tried.

Mar/2009

Lower temperature operation was retried.

Oct/2009

Oscillation was observed again, regardless of setting tuning phase offset and crab phase.

Dec/2009

Beam current with “Crab ON” achieved 1250mA for HER and 1700mA for LER.
Beam study with “Crab ON” for LHC was tried again.

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Jan/2007

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Crab cavities have been operated stably without any significant trouble for three years!

CRAB CAVITY I/L SYSTEM
Breakdown detector
RF output from pickup probes

Temperature

Several pickup probes are attached for monitoring
the transmit power and the RF output at
coaxial beam pipe.

Crab RF Off
Arc sensor
He pressure/level
Flow rate of cooling water

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VacuumBreakdown detector judges
whether Pkly, VC and φcrab (only VC for LER)
are normal, or not.
If not, Crab RF is switched off.
This is the most frequent cause of Crab RF trip.

Beam abort

Whenever Crab RF is switched off,the beam is aborted coincidently,
because of safety.
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RF TRIP OF CRAB CAVITY

Period 1 : Feb/2007~Jun/2007, Period 2 : Oct/2007~Dec/2007, Period 3 : Feb/208~Jun/2008
Period 4 : Oct/2008~Dec/2008, Period 5 : Apr/2009~Jun/2009, Period 6 : Oct/2009~Dec/2009

0.4/day
0.8/day

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2.8/day

0.1/day

558days

HER : 1.3/day
Total average
LER : 0.5/day

4

RF trip of LER Crab was very small last year.

HOM LOAD AT HOM DAMPERS| Crab cavity has two ferrite HOM dampers.
| There are two SiC dampers at downstream of LER Crab cavity.
yThey are also effective for damping HOM load generated in Crab cavity.

SiC dampers| SiC2 damper was introduced for reduction ofLoad (kW)Tmax (℃)

Flow (ℓ/min)k (V/pC)

LBP

HER 1250 mA
Coaxial

3.0

34.0

5.0

0.28LBP

6.9

40.0

7.0

0.74

Coaxial

2.5

31.3

5.0

0.13

LBP

12.4

47.2

7.5

0.68

SiC1

13.0

39.1

13.00.72

SiC2

12.2

41.5

10.3

0.66

Coax.

LER 1700 mA

Y. Morita

The 15th KEKB Accelerator Review
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HER

HOM load absorbed at SiC1.

LER

LBP

5Coax.

CRAB VOLTAGE SCANNING
Date

LER [MV] HER [MV]0.83 → 0.88

1.37 → 1.41

Mar/4/2008

0.88 → 0.88

1.37 → 1.38

Apr/5/2008

0.83 → 0.84

1.38 → 1.39

Apr/15/2008

0.84 → 0.85

1.39 → 1.40

Apr/25/2008

0.83 → 0.83–

Jun/4/2008

0.83 → 0.85

1.45 → 1.46

Jun/18/2008

0.83 → 0.83–

Oct/26/2008

0.85 → 0.83

1.37 → 1.34

Nov/22/2008

0.83 → 0.81

1.50 → 1.48

Dec/18/2008

0.81 → 0.84–

Apr/26/2009

0.85 → 0.83

1.35 → 1.32

May/20/2009

0.83 → 0.85

1.32 → 1.33

Jun/6/2009

0.85 → 0.86

1.33 → 1.37

Jun/18/2009

0.95 → 0.98

1.50 → 1.55

Nov/2/2009

0.87 → 0.90

1.29 → 1.34

Dec/12/2009

0.95 → 0.97

1.40 → 1.45

Voltage scanning is sometimes carried out
as part of beam tuning for higher luminosity.
During this operation, RF trip of Crab cavity
occurs frequently.

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Feb/29/2008

Very careful operation is necessary!

When Crab RF is switched off, RF conditioning
is sometimes done at local control room.

Max. operation voltage
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PIEZO ACTUATOR BREAKDOWN
ILER
PKly

Main
Position

φTunerLoad
VPiezoCavityComment

Jun/9/2007

LER

At RF recovery

Jun/23/2007

LER

At beam abort with RF trip

Oct/1/2007

HER

At RF recovery

Oct/4/2007

HER

At RF trip

Oct/16/2007

LER

At RF recovery

Oct/15/2008

HERAt RF trip

May/15/2009

LER

At beam abort with RF trip

Jun/11/2009

HER

At beam abort with RF trip

Oct/25/2009

LER

At beam abort with RF trip

Crab cavities have been operated
without Piezo actuator
during most of beam commissioning.
breakdown@10:53

Not significant!

Low level feedback system can control
Crab cavity without Piezo.

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Vkick

Date7

CAVITY VOLTAGE DROP OF LER CRAB
Cavity voltage of LER Crab was better than HER at horizontal test.
But, after installation to beam line, it dropped gradually.
And, it dropped suddenly from 1.3 to 1.0MV in Mar/17/2007.
Cavity voltage [MV] Comment

Dec/2006

1.9

horizontal test

Feb/19/2007

1.5

at beginning of beam commissioning

Feb/22/2007

1.3

maintenance day

1.3 → 1.0

suddenly dropped

Mar/23/2007

1.1

after thermal cycle

May/22/2008

1.2

slightly recovered

Jun/30/2008

1.3

after pulse conditioning

Dec/18/2009

1.3

not more recovered

black day Mar/17/2007

We hope cavity voltage of LER Crab is more recovered!
For example, at lower temperature, it can be operated?

The 15th KEKB Accelerator Review
CommitteeDate8LOWER TEMPERATURE OPERATION
Hosoyama-san’s group tried
lower temperature operation than 4.2K.

Vkick

First trial was done in autumn/2008.
It failed due to unexpected oil reductionof pumping system.

He Temp.

He Level
He press.
at cavity
He press.
at pumpThe 15th KEKB Accelerator Review
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PKly

Second trial was done in spring/2009.
Lower temperature operation was successful!
The operation was stable around 3.6K.
But, performance of LER Crab cavity
was not recovered.

9REMEDY FOR OSCILLATIONHER tuning offset:
φtun (HER)

oscillation
occurscrab phase : >+10°(HER/LER)

K. AkaistableTuning phase offset : +5°(HER)–8°(LER)The 15th KEKB Accelerator ReviewCommittee

LER tuning offset (degree)

Oscillation phenomenon was observed at high beam current
in beginning of beam commissioning in 2007.
Akai-san found remedy for suppression of this oscillation.

Oscillation is suppressed!

Both crab phase (degree)

But…

10OSCILLATION AT HIGH BEAM CURRENT
occurred again in autumn/2009,
WITH “CRAB ON” Oscillation
regardless of setting same values as previous.
VCPklystronφCrab

LER CrabThe 15th KEKB Accelerator Review
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Ibeam current

540Hz

Tuning phase offset : +15°(HER)–10°(LER)
crab phase : ~+19°(HER/LER)

Oscillation is suppressed again!
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HER Crab

ACHIEVED PARAMETERS
DURING BEAM COMMISSIONING
unit

LER

HER

Beam current (Crab ON)

mA

1700

1250

Crab voltage (operation)MV

0.8~1.0

1.3~1.6

1.1~1.5

1.7~1.8

Crab voltage (Max.)The 15th KEKB Accelerator Review
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content

Max. HOM PowerkW

2.5(coax)+12.4(LBP)+13.0(SiC1) +12.2(SiC2)

3.0(coax)+6.9(LBP)

Water temp. @HOM

deg.

31(coax) / 47(LBP) / 39(SiC1) / 42(SiC2)34(coax) / 40(LBP)# of total RF trips

count710

263# of RF trip / daycount

0.5 (0.4 / 0.1)

1.3 (2.8 / 0.8)

# of Piezo breakdown

count

5

4

operation temperatureK

3.6~4.2

3.6~4.2
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SUMMARY| Crab cavities have been operated stably without any

The 15th KEKB Accelerator Review
Committee

significant trouble.
| LER Crab cavity has small trip rate over one year.
| Trip rate of HER Crab cavity is decreasing.
| Lower temperature operation was tried and successful, but
performance of LER Crab was not recovered.
| HOM dampers are used above designed HOM power.
| Piezo breaks down frequently, but it is not significant.
| Cause is unknown for oscillation reappearance.

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VARIOUS COMPONENTS IN CRAB CAVITY
CCG @Coaxial Beam Pipe
Ion Pump

Helium Jacket

Cryostat
N2 Shield

CCG @Input Coupler
Input Coupler

Tuner RodCCG @Large Beam Pipe
HOM Damper

HOM Damper

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SUS Bellows

Gate Valve

Gate Valve
Coaxial Beam Pipe
Notch Filter
Coaxial Beam Pipe Side

Magnetic Shield
CCG @Cryostat
Stub Support
Crab Cavity

Helium Jacket for Coaxial Beam Pipe

Top View

Ion Pump

Large Beam Pipe Side

Pick-up Probe14