Developments of new treatment techniques for SRF cavities at KEK HORIZONTAL HPR FOR PERFORMANCE RECOVERY Yoshiyuki MORITA Motivation of developing HHPR SRF cavities Performance degradation R&D for HHPR using test cavity Application to degraded cavity HPT results LHC-CC13 Yoshiyuki Morita

Motivation of developing HHPR RF Performance of SRF cavities degraded in the long term operation at KEKB – Qo of several cavities significantly degraded at ~2MV with FE – Due to particle contamination during repair of vacuum leakage coupler gasket exchange – Present degradations are still acceptable for SuperKEKB (1.5MV) – Further degradations make the operation difficult – Performance recovery is desirable HPR is effective to clean the particle contamination – If we can apply HPR to the cavity in the cryomodule, We can save time and costs; no need for cavity disassembly We can reduce risk of leakage; no need for re-sealing at the indium joint HHPR was developed for performance recovery – Horizontal insertion of the HP water nozzle – Water evacuation by the aspirator pump – Applicable to cryomodule

SRF accelerating cavities 8 modules in HER Belle Ⅱ DR ParameterKEKBSuperKEKB RF Voltage (MV/cav)1.2~21.5 Beam Power (KW/cav)350~ Beam Energy (GeV)87 Beam Current (A) Bunch Charge (nC)10 Bunch Length (mm)65 HOM power (kW/cav) MHz SC cavity  Single cell  Large beam pipe  Ferrite HOM damper  Coaxial input coupler The first four cavities were installed in1998 and commissioned. The other four were installed in Eight SC cavities were operated for more than 10 years at KEKB. KEKB machine operation was shut down in June 2010 to upgrade. Those SRF cavities are to be operated at SuperKEKB. The first beam operation will start in January 2015.

Vacuum leakage, repair and degradation Degraded Other degraded cavities 10% Leakage at indium sealed flange Vacuum leakage happened at cooldown in May 2010 – Leakage at indium-sealed flange? Opened the cavity – Bolts of the flange were loose (60kgf*cm) – Those bolts were retightened (150kgf*cm) High power tested – No vacuum leakage – Q degradation with field emission Voltage reached 1.3 MV Degraded Qo factor with FE Limited by the radiation safety rule at test stand Particle contamination Other degraded cavities – Degraded Q factors at ~2MV D10A, D10C, and D11C – Still acceptable for SuperKEKB operation – Performance recovery is desirable R&D for HHPR started in 2009.

Horizontal HPR R&D using prototype test cavity HHPR parameters (manual operation) Water Pressure6 MPa NozzleStainless steal Φ0.54mm x 6 Driving speed0.33 mm/sec (cell) 0.66 mm/sec (BP) Rotation speed12 0 /sec Rinsing time20 min Test cavity Horizontal insertion of HP water pipe Water evacuation pipe (connected to aspirator) Small BP Large BP High pressure rinsing Manually operated Horizontal HPR was applied to our test cavity in the clean room

Cold test results after HHPR Before HHPR, EPII (10μm) Baked at 120 o C Cold test after EPII 1 st HHPR – Rinsed cavity cell, iris and BP – Evacuated in vertical position – Residual water dropped on BP flange Cold test after 1 st HHPR w/o baking – Observed processing once – No degradation 2 nd HHPR – Evacuated in horizontal position – Residual water remained in cavity cell Cold test after 2 nd HHPR w/o baking – No degradation We confirmed that HHPR was applicable to our cavity 123 Vc(MV)

Improvements of HHPR for application to cryomodule Developed automatic nozzle driving system Water evacuation through the same BP Installed new ultrapure water system at assembly area After HHPR – Evacuate residual water in pickup port Stainless steel nozzle Teflon tube to aspirate water in pickup port HHPR in clean booth at assembly area HHPR parameters (automatic operation) Water Pressure7 MPa NozzleΦ0.54mm x 6 Driving speed1 mm/sec Rotation speed6 0 /sec Rinsing time10-15 min

Horizontal HPR in clean booth Before HHPR, EPII (10μm) once again baked at 120 o C Cold test 3 rd HHPR – HHPR in clean booth at assembly area – New HHPR system – Rinsed cell and iris areas for 10 min. Cold test after 3 rd HHPR w/o baking – Fields reached to 8 MV/m (2MV) – Fields degraded to 6 MV/m (1.5MV) with FE Inspected in clean room – Found a small pit <1 mm near LBP iris HHPR(4th HHPR) – Rinsed cell and iris area for 15 min. Wider SBP iris area Cold test after 4th HHPR w/o baking – Qo recovered – w/o FE Degraded Effectiveness of HHPR was demonstrated Recovered 123 Vc(MV)

HHPR application to the degraded spare cavity Taking out the cavity from test standMoving to the assembly area To the 4 th floor underground Landing After the success of the HHPR to the test cavity, we applied HHPR to the performance degraded spare cavity. The cavity was moved to the assembly area in the KEKB tunnel.

HHPR application to the spare module Taking out the inner conductor of the HPC End beam pipes and HOM dampers were dismounted Setting the HHPR apparatus Before we applied HHPR, HPC and HOM dampers were dismounted in a clean booth. Then the HHPR driver was set to the cavity.

HHPR application to the spare module Opening the dummy flange Water jets Automatic drive High pressure water pumpAspirator pumps Evacuation after HHPR The degraded cavity was HHPR’ed with a new automatic drive and water evacuation system

HPT results: Qo measurements HHPR’ed cavity was HP tested at 4.4 K. The cavity voltage reached 2MV and degraded Q factors recovered. The Qo value at 1.5 MV was 1.6x10 9. This recovery is sufficient for the operation at SuperKEKB.

HHPR to another degraded cavity After the successful performance recovery, we tried HHPR to another degraded cavity. Performance recovered cavity was installed in the HER ring. On the other hand, degraded cavity was took out of the ring, then HHPR’ed in the assembly area. This cavity was installed in the test stand. The high power tests of this cavity will be conducted in February Recovered cavity installed into the HER ringHHPR setup for another degraded cavity HHPR’ed cavity in the test stand cavity

Summary We have developed HHPR using the test cavity – Horizontal insertion of the HP water nozzle – Water evacuation by the aspirator pump We applied HHPR to the degraded spare cavity module – RF performance recovered successfully – The recovery is sufficient for the operation at SuperKEKB – However, the recovery is not perfect Further optimization (rinsing time/area)? Re-contamination? Recovered cavity: Installed in the HER ring Another degraded cavity: HHP rinsed – to be HP tested in Feb. 2014

Horizontal HPR HHPR parameters (automatic operation) Water Pressure7 MPa NozzleStainless steal Φ0.54mm x 6 Driving speed1 mm/sec Rotation speed6 0 /sec Rinsing time15 min

HHPR application to our spare cavity module Moved the cavity to assembly area Dismounted end parts and HPC Set HHPR apparatus in clean booth High pressure water jet in cavity cell Water jet nozzle and aspirator Evacuate with residual water

Summary of leaked cavities and Q 0 degradations DateOperation phase Installed place Cavity IDLeaked at 1998/10/23Cool-downD11BCA-B03He vessel, no influence on cavity vacuum pressure Repaired in April, /05/20Cool-downD11ACA-B02He vessel, no influence on cavity vacuum pressure Jan. 2001Warm-upD10ACA-B06Indium seal of LBP, resealed in clean room Sep. 2001Cool-downD11CCA-B04Indium seal of LBP, resealed in clean room 2004/01/09Cool-downD10CCA-B08Indium seal of LBP, resealed in clean room 2004/12/25In beam operationD11ACA-B02HOM damper flange, HOM damper exchanged Exchanged with CA-B01 in June /07/02Warm-upD10ACA-B06Indium seal of SBP or LBP, retorqued 2008/07/07Summer shutdownD11ACA-B01Ion pump connector, retorqued 2010/05/01Cool-downD11BCA-B03Indium seal of SBP or LBP, retorqued Exchanged with CA-B02 in Nov Q 0 degradation at Vc=2 MV CA-B02 Resealed with 0.5 mm thick indium in Oct Metal gaskets exchanged in Dec Installed in KEKB in Nov HPC exchanged Leak installed Leak installed

Input coupler conditioning before cool-down The input coupler has to be conditioned with fully reflected RF powers up to 300 kW before cool-down. This conditioning took longer time than usual. RF trips occurred many times below 120 kW by the vacuum pressure rise. Conditioning with minus biasing, especially at -600V, reduced the vacuum pressure rise and increased input RF powers. Multipacting at the outer conductor would be the cause of the vacuum pressure rise. x:Outer +:Inner o:Inner-outer Conditioning history KEKB input coupler MP map