Status and Operation Experiences of BEPCII SRF System
Status and Operation Experiences of BEPCII SRF System Jianping Dai Institute of High Energy Physics, CAS 2017/09/14 jpdai@ihep. ac. cn
Overview of BEPCII One machine / Two purpose: 1 SCC per ring for colliding mode; 2 SCC & outside ring for dedicated SR users 2
Parameters of BII SCRF System Parameter Collision mode SR mode E Ge. V 1. 89 (1. 0~2. 1) 2. 5 C m 237. 5 241. 1 αp 0. 033 U 0 ke. V 123. 4 377. 8 Uk ke. V ~22 396 402 h Designed Achieved Frf MHz 499. 800 499. 8 0. 06 499. 800 I m. A 910 250 Vc MV 1. 5 or “>” 2*1. 0 1*1. 8 Pb k. W 132 143 2*50 107 Vc stability 1% 1% or “<” Phase stability 1 o ~ 0. 5 o 0~720 o -180~+180 o Range of Phase Tuning 3
ü A new SC cavity was developed as a spare cavity in 2011; ü The spare cavity was moved into the tunnel in this August for beam test. Similar design West cavity(@2006) East cavity(@2006) Q 0@2 MV 0. 99 E+09 0. 55 E+09 Spare cavity (@2017) 1. 02 E+09 4
1) Operation with DC bias voltage on couplers l Generally , for KEKB-type cavities, bias voltage is only used for coupler conditioning, NOT operation. × In 2007, when the beam current exceeds 100 m. A in BER, the SC cavity tripped frequently due to its arc interlock of window and following vacuum pressure raised quickly. l After several months of hard beam conditioning, a DC bias voltage (1. 5 k. V) was used on the coupler for beam operation, for both cavities. ü This proved to be effective and crucial. The current increased fast after that. No negative effect was found. 5
2) Baking of the WEST window × In Nov. 2013, the temperature of the window of the west coupler was found to be excessively high after an arc interlock event. (Increased to 50 o. C in 5 min. @3 k. W). (Overheating NOT found in EAST coupler) × Characteristics of the overheating l Sensitive to the tuning angle of the cavity l Not uniform (One side was 10 o. C higher than the opposite side) l Usually became more serious after the big outgassing happened. l After 3 times 150 o. C baking, the overheating relieved much 6
2) Baking of the WEST window (Cont’d) Ø Analyzed reasons of window overheating l Bad vacuum resulted in some serious discharging, then the discharging produced a large number of ions, which bombarded the copper surface. (5. 5 E-8 Pa for EAST coupler while 7. 5 E-8 for WEST coupler) l The sputtered copper reacted with the main residual gasses and formed some kind of copper compounds of Cu. C 2 or Cu. H. l The copper compounds deposited on the ceramic surface, which resulted in the ceramic overheating. l Both Cu. C 2 and Cu. H are thermolabile. Their thermal decomposition temperatures (Cu. C 2: 100 o. C , Cu. H: 60 o. C ) are below the baking temperature of 150 o. C , which can well explain why the overheating can be alleviated by 150 o. C baking. Unfortunately, after two years operation, the window of WEST coupler cracked on Nov. 18, 2015, during operation. 7
3) Replacement of the WEST window Nov. 18, 2015 8
Leak check by helium mass-spectrometer detecting system l After cavity warming up (3 days), the leak checking of window and LBP was done at 21 st Nov, 2015. l The window leak rate: 3. 0 E-4 mbar. l/s l LBP and feedthrough: no leak. The cracked window was decided to be replaced by a new home-made window on line 9
WEST power coupler replacement on 22 nd Nov, 2015 Ceramic of the new window The replacement took about 7 hours, and the cavity exposure time is 1 hour 10
Inspection of the cracked window 11
Analysis of the cracked window Ø The snow figure: Serious discharging happened => ions bombarding the outer conductor of the window =>snow figure formed Ø The fish scale figure: 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 Ø The fish scale figure should also be the source of the overheating of the WEST window. Ø The dark matter appeared on both the inner conductor and the ceramic is unknown. 12
4) Hot N 2 baking of the WEST cavity × After replacement of the coupler, Q 0 of west cavity decreased a lot. And radiation increased a lot (one order): 53 m. Sv/h at Vc=1. 3 MV; 172 m. Sv/h at Vc=1. 4 MV. Cavity has been polluted? It couldn’t run with Vc>1. 1 MV. 13
l After sufficient discussion, hot N 2 baking was adopted to improve the performance of west cavity. l Hot N 2 (~90 ℃) was injected to the Helium vessel to bake the outer surface of the cavity, which was useful for outgassing. 14
ü After one week of hot N 2 baking, the window vacuum improved from 1. 1 E-6 Pa to 3. 6 E-7 Pa at the end of baking. ü Radiation and Q 0 improved much. Q 0 from 3. 91 E 8 at 1. 3 MV to 9. 52 E 8 at 1. 5 MV ü Up to now, the west cavity operates well. 15
Summary Ø BEPCII SRF system has been running well and stably for ~10 years. Ø The crack of the west coupler was the most severe challenge to BEPCII SRF system. Ø Hot N 2 baking was adopted to improve the Q value of the cavity and verified to be useful. This experience might be valuable for RF people. Thanks for your attention! 16
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