K. H. Lee, H. Y. Lee, Young-Gi Kim, J. Yang, S. M. Yang, K.J. Chung, Y.S. Na and Y. S. Hwang Residual Gas analysis during Glow discharge cleaning, Baking and UV treatment in VEST Nuplex / A3 summer school / 2015.07.20

4. Conclusion & Future Work 1. Introduction Contents 2. Experimental Setup - impurity monitoring system 4. Conclusion & Future Work 1. Introduction 3. Experiment - Residual gas analysis in VEST - GDC - Baking wall conditioning - UV treatment

Introduction Motivation The early plasma current quenching even with sufficient loop voltage has been observed in VEST It is considered to be resulted from the radiation cooling due to the oxygen impurity influx during a discharge limited by the tungsten limiter. And the oxygen exist in water form Impurities inside fusion devices can affect plasma performance Confirmation of Impurity control effect focusing on water outgassing by wall conditioning techniques Impurity control Impurity diagnostics Monitoring system

Introduction Motivation Residual Gas Analyzer Source Quadrupole Detector      𝑓 2 𝑟 0 2      𝑓 2 𝑟 0 2 m : ion mass f : frequency (MHz) ro : radius between poles

Experimental setup Impurity Monitoring System Design Gauge PFEIFFER full range gauge Differential Pumping Chamber Angle valve MDC KGV-1500C Gauge PFEIFFER full range gauge Hot cathode filament gauge Gate valve MDC KGV-1500C QMS 200 PFEIFFER full range gauge Valve Gate valve – CF 6” Roughing valve/ foreline valve Vent valve Pump Oil rotary pump (D20A) - NW40 ->pumping speed:380l/m ->ultimate pressure:5× 10 −3 Torr PFEIFFER TMP 521P - CF 6” ->pumping speed:290l/m( 𝑁 2 ) ->ultimate pressure:5× 10 −10 Torr Differential pumping RGA operating pressure: ~7.5×10 −5 𝑡𝑜𝑟𝑟 Pressure difference between VEST main chamber and RGA system can be made by controlling gate valve in front of RGA Real time Impurity monitoring during high pressure such as VEST shot operation, Glow discharge cleaning is possible

Experiment Residual Gas Analysis of VEST VEST device has main 8 peaks ->1, 2, 14, 16, 17, 18, 28, 32,44 Cascade shape of 16, 17, 18 line Indicates water exist Mass16 : Mass17 : Mass18 = 1 : 22 : 77 Gas type and partial pressure can be calculated from peak current data ->major gas : Hydrogen, Methane , Water Air(Nitrogen, Oxygen, Argon) Carbon dioxide Every partial pressure which is taken from RGA signal changes with total pressure -> use water/air(Ar) ratio to evaluate air is choosed because it has enough high signal and it is not related directly with wall conditioning effect VEST full spectrum data (2015-06-29 10:22)

Experimental Setup - discharge current : 1.5A - cleaning time: 6hours Experiment GDC Glow discharge plasma H+ ion Water, oxygen Before 4/21 5:54 분 After 4/21 10:21분 4/21 12:31분 Glow discharge cleaning in VEST Experimental Setup - discharge current : 1.5A - cleaning time: 6hours Ions are accelerated by DC, AC or RF voltages toward the wall. And this wall surface is sputter cleaned.

Partial pressure of gases and water/air ratio Experiment GDC Partial pressure of gases and water/air ratio 6hours of 𝑯 𝟐 1.5A GDC When GDC is started, partial pressure of all gases increase by hydrogen gas injection through gas line which have slight other gases After discharge cleaning, partial pressure of water goes down and then return to before state for 4hours but it need more research to know its cause Water to air(argon) ratio help to evaluate wall conditioning effect because it is not affected by pressure changes. Water to air ratio shows that water comes off of the wall at first. when GDC is finished, detached water get out of the chamber through the pump or go to wall again. It appears water ratio decreasing Before 4/21 5:54 분 After 4/21 10:21분 4/21 12:31분

Experiment Baking effect Experimental setup Heating tool : Heat tape, band heater Heating time : 1h, 4h, 12h Temperature : 180°C Heating area : 1649 𝒄𝒎 𝟐 Heating part : cryo pump duct whole chamber baking would be more effective than partial baking. But in VEST, because of complicated structure, we should take partial baking method. We think that cryo pump duct is one of the most effective parts because it is the path which the water goes out.

Experiment Baking effect Partial pressure of gases and water/air ratio Heater on During 12hours baking treatment, 𝑯 𝟐 𝟎 partial pressure decreasing is observed. Water to air ratio changes from 1.94 to 1.80 When heater turns on, water ratio increase slightly. And then water decrease steadily RGA에 찍힌 pressure와 total pressure는 다를 수 있음 RGA에는 after baking이 reference보다 조금 낮에 측정됨

Experiment UV treatment Experimental setup Light source: UV lamp through quartz window(SANKYO DENKI) in-vacuum lamp UVB-100(rbd instrument, movable) Source power : 36W(outer lamp) 20W(in-vacuum lamp) Wave length : 253.7nm(outer lamp) 245nm(70%),185nm(30%) Treatment time: 8hours

Experiment UV treatment Partial pressure of gases and water/air ratio UV lamp on UV treatment using two lamps shows water partial pressure growth Water to air ratio changes from 1.3 to 1.4 it means that water detached from wall remain in chamber. Two peaks of pressure and other signals is observed. But is can not understand yet. RGA에 찍힌 pressure와 total pressure는 다를 수 있음 RGA에는 after baking이 reference보다 조금 낮에 측정됨

Experiment UV with Baking Comparison UV treatment only case with UV + Baking RGA에 찍힌 pressure와 total pressure는 다를 수 있음 RGA에는 after baking이 reference보다 조금 낮에 측정됨 UV treatment only UV treatment with baking In UV treatment case, water detached from wall can not get out from chamber. It shows water ratio increasing Baking of cryo pumping duct helps to release water. In UV with baking case, water ratio which is increased in early stage is eliminated with time. Finally, water ratio is reduced after treatment finished steadily. Water ratio changes from 1.42 to 1.38 for 16hours

Experiment comparison with reference Reference case that any treatment is not conducted also shows water ratio changes from 1.98 to 1.94 for 12 hours All wall conditioning method that we are taken shows water removing effect. GDC : 1.83 to 1.41 (8h) Baking: 1.94 to 1.80 (12h) UV + Baking : 1.42 to 1.38 (16h) Although UV with baking treatment shows lower reduction effect than other methods or reference, it seems to be because starting point is already low. Reference GDC RGA에 찍힌 pressure와 total pressure는 다를 수 있음 RGA에는 after baking이 reference보다 조금 낮에 측정됨 Baking UV + Baking

Conclusion & Future work - Find the optimum of GDC, Baking and UV treatment condition such as heating area, time, temperature etc. - Research how to hold the GDC effect - Comparison between VEST operations before and after wall conditioning Conclusion - In order for impurity monitoring in VEST, RGA with differential pumping system is developed. - After glow discharge cleaning, water ratio decrease significantly in VEST - Baking treatment can help to decrease water in VEST wall. But In VEST, it can not cover whole chamber because of complicate outside structure - In this situations, water detaching effect of UV treatment is another solution. Even though, it needs baking to release water, UV with baking seems to be able to replace whole baking

Partial pressure calculation Experiment Backup Partial pressure calculation 𝑃𝑃 𝐴 = 𝐼 𝐴𝐵 × 𝐹𝐹 𝑁28 𝐹𝐹 𝐴𝐵 × 𝑋𝐹 𝐴 × 𝑇𝐹 𝐵 ×𝐷𝐹×𝑆 ≈ 𝐼 𝐴𝐵 𝐹𝐹 𝐴𝐵 × 𝑋𝐹 𝐴 ×𝑆 𝑃𝑃 𝐴 = partial pressure of Gas A 𝐼 𝐴𝐵 = Current of the peak at Mass B from Gas A 𝐹𝐹 𝑁28 = Fragmentation Factor for Nitrogen Mass 28 (usually taken as 1.0) 𝑋𝐹 𝐴 = Ionization Probability of A 𝑇𝐹 𝐵 = Transmission factor for Mass B 𝐷𝐹 = Detection Factor (usually taken as 1.0) 𝑆 = Sensitivity for Nitrogen at Mass 28 in Amps/Torr RGA에 찍힌 pressure와 total pressure는 다를 수 있음 RGA에는 after baking이 reference보다 조금 낮에 측정됨 -> 이 식으로 부터, 각 gas들의 partial pressure 값을 절대량으로 알 수 있음 다만, 질소에 대한 RGA 자체의 sensitivity를 몰라 상수인 S를 제외하고 계산함 단위가 pressure로 나타나진 않겠지만 각 gas의 절대량을 알고 비교할 수 있음 -> 그러나, partial pressure는 압력이 감소하면 따라서 감소하므로 다른 압력일 때 비교하기 어려움. 위의 그래프에서와 같이 모든 mass의 current의 총합이 pressure를 따라 가므로 이를 나누어 주어 gas별 분율로 계산후 비교함