Replacement of coupler and recovery of cavity for BEPCII Peng Sha (shapeng@ihep.ac.cn) on behalf of RF group Institute of High Energy Physics, CAS 6th, July, 2016 TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences

Institute of High Energy Physics, Chinese Academy of Sciences Outline Vacuum leak of coupler ceramic window Replacement of damaged coupler Recovery of west cavity Operation status present Summary TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences

Institute of High Energy Physics, Chinese Academy of Sciences Outline Vacuum leak of coupler ceramic window Replacement of damaged coupler Recovery of west cavity Operation status present Summary TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences

SC RF system of BEPCII storage ring There’re two separate SC RF systems in BEPCII storage tunnel. One (east cavity) is accelerating electron, the other (west cavity) is accelerating positron during collision mode. During synchrotron mode, east cavity is used, while west cavity is power off and detuned. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 4

SC cavity and coupler of BEPCII TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 5

Before the window vacuum leak (2015.11.18 2:10~2:37) The west window operated at 80kW, with Vc=1.5MV, e+ 600 mA. Then e+ beam lost happened, the vacuum pressure of west window changed from 8.2E-8Pa to 3.8E-7Pa. After vacuum recovery, the operator tried to increase the RF power. However, the vacuum increased from 7.6e-8 Pa to 4.5E-5 Pa at 18 kW. The vacuum slowly decreased to 8E-7 Pa after RF power off. Another important phenomena was that the temperature of the west window increased from 28 ℃ to 36 ℃ as the RF power increased from 0 kW to 17 kW. Then, the vacuum pressure increased from 8E-7 Pa to 2.1E-2 Pa as the RF power increased from 0 kW to 15 kW, and the temperature of the window is 32 ℃. Vacuum leak of window? TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 6

Before the window vacuum leak (2015.11.18 2:10~2:37) TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 7

Leak check of west window at 4.2K (1) Then, helium leak check of the west window was processed at 18th Nov 2015, with helium gas injected from the hole on the outer conductor of the window (airside). The vacuum pressure measured by gauges before and after He gas injection is as following. Condition West window vacuum (Pa) SBP vacuum (Pa) LBP vacuum (Pa) Before He gas injection 1.44E-3 8.2~8.6 E-7 7.1~7.2E-9 After He gas injection 1.07E-3 1.7E-5 1.2E-6 After 1 min of He gas injection 1.19E-3 9.6E-5 3.9E-5 After 3 min of He gas injection 1.22E-3 3.4E-5 TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 8

Leak check of west window at 4.2K (2) RGA data showed the maximum was helium gas after He injection: >1E-5 Torr (Helium). Meanwhile, the He current signal increased about 5 orders after He injection. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 9

Institute of High Energy Physics, Chinese Academy of Sciences Leak rate estimation Based on the leak check curve, the current signal increased about 5 orders; and the usual background value of leak rate is less than 1E-9 mbar.l/s. So the leak rate predicted is more than 1E-4 mbar.l/s. Also, considering the low temperature absorption effect. Giving the effective pumping speed of He is 1000 L/s, the leak rate is about 1E-2 mbar.l/s corresponding to the Helium pressure of 1E-5 Torr (measured by RGA). TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 10

Institute of High Energy Physics, Chinese Academy of Sciences Leak check result After cavity warming up (3 days), the leak checking of window and LBP was done at 21st Nov, 2015. The window leak rate: 3.0E-4 mbar.l/s (Since the air side of the window wasn’t filled with He gas, the real leak rate may be larger). LBP and feedthrough: no leak. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 11

Institute of High Energy Physics, Chinese Academy of Sciences Outline Vacuum leak of coupler ceramic window Replacement of damaged coupler Recovery of new window Operation status present Summary TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences

Coupler replacement at 22nd Nov, 2015 (1) 0:44 Vacuum pressure 4.8E-4 Pa (measured from the pump port), open the cavity valve slowly, vacuum pressure increased to 4.0E-2 Pa. 0:48 Leak check again: no leak around the LBP area, window leak rate : 3.0E-4 mbar.L /s. 0:57 Vacuum pressure 4.7E-3Pa, start to inflating high purity N2 into the cavity (purity: 99.9997%), gas flow 0.5L/min 07:00 Close the inflating valve (haven finished the inflating before) 08:00~11:00 forklift, clean-booth, class 100 clean room (outside the clean-booth), new coupler into place 11:00 preparation ready, start to replace the window TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 13

Coupler replacement at 22nd Nov, 2015 (2) 11:30 increase the inflating gas flow to 1.5L/min, close the decompression valve, disassemble the vacuum gauge which interfered with the clean-both 11:35 the vacuum gauge disassembling completed, decrease the inflating gas flow to 0.5L/min 11:38 increase the inflating gas flow to 1.5L/min again, start to loose the screws of the window-outer conductor connecting flange 11:55 screws disassembly finished; move the clean-booth right above the window; keep measuring the particle by particle counter. Pick up the window by clean-booth when the counter show zero. Cover the outer conductor port with a piece of PVC plate. Then move away the old window to the platform exit (always with clean-booth and class 100 clean room) 12:06 old window put into the storage container TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 14

Coupler replacement at 22nd Nov, 2015 (3) 12:19 new window with storage container moved just below the clean-booth 12:25 start to disassemble the screws of the window and the container connecting flange 12:35 screws disassembly finished ( the screws should be disassembled in advance to save time); measure the particle and assure it’s zero; then pick up the new window by clean-booth 12:42 new window with clean booth, removable class 100 clean room move to the place. Measure the particle and assure it’s zero. 12:53 new window put into the cavity slowly. 13:00 new window fall in place, start to assemble screws. Decrease the inflating gas flow to 0.5L/min and open the decompression valve 14:00 screws fasten finished TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 15

Coupler replacement at 22nd Nov, 2015 (4) 14:15 start to vacuum pump and leak check 15:40 leak check: no leak (background leak rate is 1.4E-9mbar.l/s) 15:57 leak check again: no leak (background leak rate is 1.2E-9mbar.L/s). Close the leak check device and open the turbo pump 16:33 vacuum pressure: 1.1E-4Pa; start to baking The cavity exposure time: 1 hour TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 16

Schematic of coupler replacement TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 17

Scene of coupler replacement (1) The coupler port covered with clean PVC plate (down) and plastic film (up) New coupler pick up from the container by clean booth Old coupler taken out TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 18

Scene of coupler replacement (2) New coupler move into place New coupler waiting for put down: particle measuring Air-side of the new coupler TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 19

Inspection of old coupler There are three snow figures on the outer conductor of the window vacuum side. Feel rough There is fish scale close on the ceramic. The ceramic color turns dark. The color close to the window–inner conductor EBW seam turns dark. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 20

Analysis of old coupler The snow figure formation reason guess: Serious discharging happened -- ions bombarding the outer conductor of the window -- snow figure formed The fish scale figure formation reason guess: Serious discharging happened -- ions bombarding the inner and outer conductor of the window and the inner choke -- copper sputtering and condensed on the ceramic -- after long period operation -- fish scale figure formed The fish scale figure is also the source of the excessive heating of the west window. The dark matter appeared on both the inner conductor and the ceramic is unknown. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 21

Institute of High Energy Physics, Chinese Academy of Sciences Outline Vacuum leak of coupler ceramic window Replacement of damaged coupler Recovery of west cavity Operation status present Summary TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences

Parameters of new coupler Test at KEK-2010 Test at IHEP-2013 Before replacement-2015 Leak rate < 4.2 E-10 Pa·m3/s <5E-10 mbar.l/s 5.0E-10 mbar.l/s Baking Time: ~24 hours Temperature:120 degree / Vacuum Before baking: 4E-6Pa Before conditioning:1E-6Pa After conditioning:6E-7Pa 2E-6Pa I/L status Vacuum I/L limit: 5E-5Pa First I/L power level: 41kW Vacuum I/L Number: 229 ARC I/L number: 1 Conditioning progress Severe outgasing points: 40~90kW Conditioning time from 0~300kW: 7 days Temperature near window 39.2 @300kW Power achieved 300kWPkly_power meter 150 kW(limited by klystron) TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 23

Baking before conditioning 145℃ baking for 63 hours. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 24

Institute of High Energy Physics, Chinese Academy of Sciences Conditioning (1) Interlock: Vacuum interlock: 2E-5 Pa (RT conditioning) 1E-6 Pa (cryogenic conditioning) Arc interlock: response time 120 us Temperature interlock, water flow and pressure interlock Room temperature conditioning (26.5 hours) Reach to 80 kW Without DC bias conditioning: 19.5 hours With DC bias conditioning: 7 hours Conditioning method: CW and pulse used alternately CW conditioning :17.2 hours pulse(10Hz-100us) conditioning: 9.3 hours Vacuum Interlock times: 24 times ARC interlock times: 8 times TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 25

Institute of High Energy Physics, Chinese Academy of Sciences Conditioning (2) Cryogenic conditioning Reach to 80 kW Detuned conditioning: 9.5 hours Without DC bias conditioning(0.5 hour) With DC bias conditioning (9 hours): +2000V/ -800 V ( step: 100V) Tuned conditioning: 3 hours TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 26

Q0 measurement of west cavity at 1st Dec, 2015 After replacement of west window, Q0 of west cavity decreased a lot. And radiation increased a lot (one order): 53mSv/h at Vc=1.3MV; 172mSv/h at Vc=1.4MV. Cavity has been polluted? It couldn’t run with Vc>1.1MV. Before window replacement TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 27

Institute of High Energy Physics, Chinese Academy of Sciences Hot N2 baking After sufficient discussion, hot N2 baking was adopted to improve the performance of west cavity. In order to protect inner surface of west cavity, hot N2 (< 90 ℃) was injected to the Helium vessel to bake the outer surface of the cavity, which was useful for outgassing. Besides, we also made the plan to replace the west cavity by spare cavity, which had completed horizontal test in 2011. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 28

Schematic of hot N2 baking Hot N2 injection Indium seal (<120℃) TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 29

Institute of High Energy Physics, Chinese Academy of Sciences Summary of hot N2 baking The baking lasted nearly one week (14th~20th Dec, 2015). The N2 temperature was kept < 90 ℃, the range of temperature in Helium vessel reduced from 17 ℃ to 5℃. The outgassing of west cavity was obvious. Coupler vacuum reached 3.6e-7 Pa at the end of baking, which improved by 0.5 order. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 30

Institute of High Energy Physics, Chinese Academy of Sciences After baking After baking, west cavity began conditioning and beam commissioning. Vc of west cavity was increased step by step, while radiation decreased greatly. before baking：172mSv/h at 1.4MV 1st Jan,2016: 133mSv/h at 1.72MV@1.89GeV/100mA 10th Jan,2016: 13.4mSv/h at 1.56MV@2.087GeV/330mA 18th Jan,2016: 9.1mSv/h at 1.54MV@2.087GeV/310mA 24th Jan,2016: 6.8mSv/h at 1.54MV@2.087GeV/360mA 2nd Feb,2016: 4.0mSv/h at 1.50MV 18th Feb,2016: 5.7mSv/h at 1.57MV TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 31

Q0 measurement of west cavity at 23rd Feb, 2016 Q0 increased obviously after baking. TTC Meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 32

Operation status present Till now, the west cavity has been running well. At 5th Apr 2016, the BEPCII luminosity reached the designed target 1*1033/cm^2/s. West cavity Injected power Reflected power Vacuum of west cavity Vacuum of west coupler TTC meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 33

Institute of High Energy Physics, Chinese Academy of Sciences Summary Since 2006, BEPCII SRF system have been running well and stably for 10 years. The replacement of coupler and recovery of cavity is the most severe challenge to RF system. The total process (replacement and recovery) lasted ~2 months (Nov 2015 ~ Jan 2016). The experience is very valuable for RF people. Fortunately, hot N2 baking was adopted and verified to be useful, which is creative and firstly used over the world. TTC meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences 34

Institute of High Energy Physics, Chinese Academy of Sciences Thanks for your attention！ TTC meeting at Saclay Institute of High Energy Physics, Chinese Academy of Sciences