20120221
MAC2012
Yoshiyuki MORITA

Improvement of superconducting cavities
Handling of large HOM powers
Recovery of degraded cavities

Machine parameters and HOM power••

Machine parameter of SuperKEKB/HER– Beam Current ：2.6A/2500 bunches– Bunch length：5mm
HOM power– Cavity loss factor：1.2V/pC @σ=5mm– Expected HOM power：31kW– LBP damper load：26kW• Damper loss=0.26 V/pC
– SBP damper load: 20kW
• Damper loss=0.30 V/pC
– Total heat load per cavity：46kWBeam pipe: 150Φ

SBP

LBP

Cavity loss factorParameters of HER
Maximum current: 2.6A ←1.35A
Bunch length: 5 mm
←6-7mm# of bunches: 2500
←1600

Loss factor
1.2V/pC

Issues related to present ferrite HOM damper system••••

Ferrite material is sintered on a copper base pipe by hot
isostatic press (HIP) method.
Ferrite– Thickness: 4 mm
– Ferrite surface temperature ：190℃ @2.6 A– Gas release from ferrite triggers high voltage breakdown
Cooling water– Water temperature＜60℃• To prevent air bubbles• Deaeration system >60℃
Chiller unit
– Total heat load：46 kW per cavity– Cooling capacity （with a safety margin）：60kW– Cooling capacity of present chiller: 27 kW

Beam current

SBP load

LBP load

(A)

(kW)

(kW)

21215

2.62026

HIPped ferrite （Thickness：4mm）

Cupper pipe (3/8”) for cooling channel
Flange: Stainless steel

Development of new HOM dampers for SuperKEKB••

HIPped ferrite

To suppress gas release from ferrite material– Suppress temperature rise of ferrite– Use 3 mm thick ferrite (KEKB damper: 4 mm)• For better thermal conduction• Damper loss factor is expected to be reduced– 1/3 of the total load is damper self loss
To suppress temperature rise of cooling water– Introduce double cooling structure

Thickness :4→3 mmTaper section

Prototype damper with 3 mm thick ferrite
Two cooling channels

Single cooling structure

A-type coolingB-type cooling
Double cooling structure

Ferrite thickness: 3 mm

High power test results
3mm thick dampers with double cooling structure
Absorbed power

RF power flow

SBP damper

LBP damper
Expected heat loads were absorbed in dampers butsurface temperatures decreased only 20℃cc1 cc2 cc3 cc4 cc5 cc6Temperature distribution at 14 kWSCS

Double cooling structure suppressed
temperature rise of the copper base

Thermo-couple numberCracks after HPT

DCS

Cracks were found after HPT
3mm thick ferrite might not have enough strengthOther methods to handle large HOM power••

Enlarge beam pipe diameter– 150φ→200φ– Decrease loss factor– Decrease HOM power
Additional dampers– Add SiC dampers on the beam pipe
– To reduce ferrite damper load
– Studies for SiC damper

Beam pipe structureLoss factor
＠σ＝5mm
（V/pC）

Modifications
to be madePresent BP (150φ)

1.2

200φ BP

0.8

GV dia.
Bore dia. (Q-mag.)

200φ BP
Taper (200φ→150φ)

0.95

GV dia.

200φ LBP only
Taper (200φ→150φ)

1.05

LBP-GV dia.

Present beam pipe: 150Φ

SiC

SiC

Recovery of degraded cavities
Vacuum leakage
•••

•

•

Vacuum leakage happened at cooldown in May 2010
– Indium sealed flange was suspected as a cause
The cavity was removed from KEKB ring
The cryomodule opened– Bolts tightening flanges were loose (60kgｆcm)– Thermal cycles might loosen those volts– Those bolts were retorqued (150kgｆcm)
Cooled and high power tested– No vacuum leakage• Loose bolts were cause of the leakage– Degraded with strong field emissions• Voltage reached 1.3 MV• Limited by the radiation safety rule
• Cavity surface was probably contaminated with dust
particles
Q factors of other cavities– D10A, D10C, and D11C degraded with field emissions at highfields– Still acceptable for SuperKEKB operation– Recovery of degraded cavity is desirable for long time operation

10%

Cold cathode gauge at BPLiquid level was
kept at 10 %Metal vacuum gauge at HPC

Pirani gauge at vacuum insulation

Qo measured from cryogenic heat loads

ProcessedHorizontal high pressure rinsing•••High pressure water rinsing (HPR) is effective to recover degradedcavities contaminated by dust particles
If we can apply HPR to cavities in cryomodule,–
Saves a lot of time and costs
R&D is ongoing– Ultrapure water jet (8MPa) from a stainless steel nozzle– Wasted water was collected by an aspirator at the equator area
– Evacuated with residual water

Stainless steel nozzle

HHPR in our clean roomRF performance after HHPR•••••••••

2009.3.30 EPII (10μm)
2009.4.16 Baked at 120 ℃
2009.5.21 Cold test after EPII
2009.6.17 HHPR(1st trial)– Evacuated in vertical position– Residual water dropped on BPF
2009.6.25 Cold test after 1st HHPR
– Without bake2010.2.8 HHPR(2nd trial)– Evacuated in horizontal position– Residual water stayed in EQ
2010.2.25 Cold test after 2nd HHPR
– Without bake
Processed once but no degradation
Applicable for cavity rinsing in a cryomodule

After 1st HHPR

After 2nd HHPR

After EPII

Plan of the next three years••

HOM power handling– Stop development of 3 mm thick ferrite dampers– Develop 4 mm thick ferrite dampers with a double cooling structure– Assemble 200ΦBP and install to the recovered cavity– Study SiC dampers to lower ferrite damper loads
Recovery of degraded cavities– Try HHPR in a cryomodule for performance recovery– Exchange the recovered cavity with one of degraded cavities

FY2012
HOM damper
development

FY2013

FY2014

If not recover
HHPR in clean roomCold test
EPII (if necessary)Cold test

Install in SuperKEKB
Cool-down test

High power test for 4mm
thick dampers with double
cooling structure
Study for SiC dampers

Recovery of degradedcavity (CA-B03)
and200Φ beam pipe

HHPR in cryomodule
High power test
Assembly of 200Φ BPmodules

Attach 200Φ BP for low LF
High power test

backupDamper loss factor•Damper loss factors– Use CST-PS– Calculate 1 to 4 mm thick ferrite dampers– Not reduced as expected (8%)σ=5 mmSummary of leaked cavities and Q0 degradations
DateOperation phase

Installed place

Cavity ID

Leaked at

1998/10/23

Cool-down

D11B

CA-B03

He vessel, no influence on cavity vacuum pressure
Repaired in April, 1999

1999/05/20

Cool-down

D11A

CA-B02

He vessel, no influence on cavity vacuum pressureJan. 2001

Warm-up

D10A

CA-B06

Indium seal of LBP, resealed in clean room

Sep. 2001

Cool-down

D11C

CA-B04

Indium seal of LBP, resealed in clean room

2004/01/09

Cool-down

D10C

CA-B08

Indium seal of LBP, resealed in clean room

2004/12/25

In beam operation

D11A

CA-B02

HOM damper flange, HOM damper exchanged
Exchanged with CA-B01 in June 2006

2005/07/02

Warm-up

D10A

CA-B06

Indium seal of SBP or LBP, retorqued

2008/07/07

Summer shutdown

D11A

CA-B01

Ion pump connector, retorqued

2010/05/01

Cool-down

D11B

CA-B03

Indium seal of SBP or LBP, retorqued
Exchanged with CA-B02 in Nov. 2010
CA-B02
Resealed with 0.5 mm thick indium in Oct. 2006
Metal gaskets exchanged in Dec. 2006
Installed in KEKB in Nov. 2010

Q0 degradation
at Vc=2 MV
HPC exchanged

LeakLeakHigh power input couplers exchanged
during summer shutdown in 2005