1. Replacement of coupler and recovery of cavity for BEPCII
Peng Sha
on behalf of RF group
Institute of High Energy Physics, CAS
6th, July, 2016
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Institute of High Energy Physics, Chinese Academy of Sciences

2. 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

3. 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

4. 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.
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5. SC cavity and coupler of BEPCII
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6. 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?
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7. Before the window vacuum leak (2015.11.18 2:10~2:37)
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8. 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
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9. 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.
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10. 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).
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11. 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.
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12. 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
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Institute of High Energy Physics, Chinese Academy of Sciences

13. 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: %), 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
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14. 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
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15. 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
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16. 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
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17. Schematic of coupler replacement
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18. 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
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19. Scene of coupler replacement (2)
New coupler move into place
New coupler waiting for put down: particle measuring
Air-side of the new coupler
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20. 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.
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21. 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.
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22. 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

23. 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
Power achieved
300kWPkly_power meter
150 kW(limited by klystron)
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24. Baking before conditioning
145℃ baking for 63 hours.
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25. 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
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26. 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
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27. 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
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28. 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.
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29. Schematic of hot N2 baking
Hot N2 injection
Indium seal (<120℃)
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30. 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.
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31. 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
10th Jan,2016: 13.4mSv/h at
18th Jan,2016: 9.1mSv/h at
24th Jan,2016: 6.8mSv/h at
2nd Feb,2016: 4.0mSv/h at 1.50MV
18th Feb,2016: 5.7mSv/h at 1.57MV
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32. Q0 measurement of west cavity at 23rd Feb, 2016
Q0 increased obviously after baking.
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33. 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
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34. 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.
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35. Institute of High Energy Physics, Chinese Academy of Sciences
Thanks for your attention！
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