1. Measure against discharge and He/air separation in surrounding gas Manabu MORITSU ( Osaka University ) 8 Sep., 2015 COMET Collaboration Meeting 17 th @ Paris, France 1 COMET-CDC

2. Scene of the incident 2 COMET-CDC Prototype-4 A discharge problem was detected when we tested the prototype-4 chamber. The place of the discharge was outside the gas volume.

3. 3 Readout side Capacitor on tiny PCB Capacitor on tiny PCB Feedthrough Readout board RECBE The capacitor is installed for AC coupling between sense wire and RO board.

4. Discharge problem 4 Capacitor on tiny PCB Capacitor on tiny PCB Sense wire HV ~2000 V Readout cable Because of the discharge, HV was sometimes tripped with the current over 5 uA. Although the environment of the lab was not suitable to DC, detectors should be operated stably even in such a severe environment. Discharge !! 4.6 mm AVX 1808HC102KAT1A Capacitor = 1000 pF Tolerance = 3kV ~2000 V is applied at 4.6 mm. Temperature: 25 degC Humidity: 60—70%

5. Current status 5 RTV(Room Temperature Vulcanizing) silicone gum The discharge quietened down by molding the capacitor by RTV silicone gum.

6. Operation in He gas This capacitor is also used for the real CDC. In case of the real CDC, we should operate it in He gas filled in the detector solenoid magnet. Discharges are more likely to happen in He than air. 6 We can learn from MEG experiment. Helium can penetrate into silicone gum. They used epoxy instead of silicone gum. There is a precedent Thanks to advice by Nishiguchi-san.

7. Molding by epoxy We tried to mold by 3 types of epoxy, which are UV curable resins. Test in He gas is planned. 7 ThreeBond UV curable epoxy resins: 3057B, 3114, 3017E. Molded in a room Temperature: 24 degC Humidity: 52% [Note] We bought 5000 capacitor boards. It costs much to replace them to other one. Risk should be avoided in advance !!

8. By the way... 8 Let’s think about surrounding gas.

9. Surrounding gas in detector solenoid He gas is filled in the target region (inside CDC wall) to reduce the material thickness. In the outside of the region, surrounding gas is not necessarily Helium. 9

10. 10 This region should be filled with He, but others not. Surrounding gas in detector solenoid He

11. Plan of separation of He/Air 11 He  We plan to make gastight shield covers and flow dry air(N2).  Requirement of the gas tightness is not so tight.  It does not cost much and feasible. Al shield cover for RO circuit.

12. Plan of separation of He/Air 12 He  If we can separate the gases at more inner part, it also helps PMT of CTH.  We are discussing with CTH and muon stopping target group.

13. Summary 1.In order to fix the discharge on tiny capacitor board, we molded it with epoxy. Test in He gas will be done. 2.We plan to separate surrounding gas in electric shield cover from He gas and flow dry air (N2) into it. The risk of discharge will be avoided by the above double countermeasures. 13 Merci beaucoup

14. Backup 14

15. Paschen’s law 15 [Ref.] Lieberman and Lichtenberg, Principles of Plasma Discharges, Wiley 2005 Breakdown voltage: p: pressure d: distance a, b: parameters related to gas and electrodes # In case of ideal parallel plane electrodes 1 atm 0.5 cm

16. 16 「圧力が高い場合には電子の平均自由行程は小さくわずかの電子しか１ 平均自由行程で電離するのに十分なエネルギーを得ることができない。 このために大部分の電子は原子的励起あるいは分子的励起を起こす。そ れゆえ間隙間内で十分な電離を起こすには V B は高くなければならず、 p が 高いほど V B は高くなる。」 （補） Paschen curve の高圧側の振舞いについて [Ref.] A. von Engel （山本賢三、奥田孝美訳）, 「電離気体」, コロナ社, p.170 In our case of 1 atm (= 760 torr) and 0.5 cm, V B ≈ 5000 V for He V B ≈ 20000 V for N2~Air 実際には先が尖っていたりする効果でもっと低くなると思うと ヘリウムで 2000V くらいで放電し始めるのは reasonable かな。

17. 17 Belle-II CDC