Research On Several Crucial Problems In Vacuum Operation Of EAST Plasma Vacuum Vessel

Tokamak,as one of Magnetic confinement fusion reactors,is the most prospective device to realize the utilization of fusion,which can supply renewable clean energy.Impurities release from the wall would lead to energy emission and pollution of the plasma.High concentration of impurity also results in many serious problems,such as the difficult to establish and maintain of plasma.These impurities would pollute the exhaust gas from vacuum.Therefore the fuel recovery and recycling become more difficult.In respect of the problems above,the impurity elimination,ultimate vacuum improvement and the ingredients of exhaust gas are intensive studied in EAST tokamak.The procedure of vacuum treatment steps including wall conditioning,before EAST 2015 winter campaigns are analyzed.After 745 hours baking(?200?)with 158.5 hours He-GDC and 52.5 hours D2-GDC,the ultimate vacuum reach 1.8 X 10-6Pa from 1.1×10-4Pa.In this process,131.6g H2O and 31.4g H2 have been removed.86.62g H2O have been removed only in baking,accounted for 65.82%of the total.And 30.2g H2 have been removed in GDC with baking,accounted for 96.18%of the total.It is also observed that H-removal rate is(7.2?94.0)× 10-4 atom/m2.s only in baking,and(1.8?5.2)× 1017 atom/m2.s in He-GDC,(4.1?13.0)× 1017 atom/m2.s in D2-GDC.This paper explores and summarizes vacuum treatments in Tokamak.It is beneficial to improve efficiency of wall conditioning.It also can shorten the experimental preparation time,and provides a reference operation for vacuum operation in future superconducting tokamaks.Lithium coating technology is routinely used in Experimental Advanced Superconducting Tokamak(EAST)to improve plasma performance.However,under the lithium coating wall condition,water leaking into EAST vacuum vessel would lead to serious problems,such as peeling off of lithium coating film and strongly increasing of impurity emission,which finally affect achievement of high performance plasma.The procedure of water leaking into EAST vacuum vessel under lithium coating wall in EAST 2016 winter campaigns analyzed.By quick leakage detection and analysis,the leakage location is founded,and the leakage problem is resolved in 5 hours,and then the plasma discharge is recovered soon.After 20 discharge shots(totally 182s plasmas),7.5 hours He-Ion Cyclotron Radio Frequency(He-ICRF)discharge cleaning(20 kW,0.05Pa)and 1.5 hours lithium coating experiment(about 15g)had been carried out,impurities level in vacuum are reduced dramatically.With continuous discharge operation,impurity level is decreased to low level as before leakage after 17 shots(totally 141s plasmas).During this leakage,totally 4.415g water has been leaked into EAST vacuum vessel.3.938g(?89.2%)has been reacted chemically with lithium film by producing 0.219g H2,87.7%of which(0.192g)was removed during wall conditionings.After wall conditionings,impurity level is recovered to the level before leakage.Quick leakage treatment and efficient wall conditionings would help the plasma discharge recover from water leak with lithium coating wall,and this study provides a reference operation for water leakage event in EAST tokamak.An Exhaust Analysis System(EAS)is designed and built with a wide range of inlet working pressure(10-3-105Pa),it's used to quantitatively analyze the ingredients of EAST exhaust gas.The results show that after an EAST run day with normal plasma operation,the total adsorbed gas by 10 external cryopumps is 1372.1 Pa.m3,91.5%of which is hydrogen and its isotope.Gas desorption during an external cryopump regeneration is also investigated by using the EAS,the results show that most desorbed gas(>99.9%)is hydrogen and its isotope when the temperature of cryopanels is lower than 69 K,and the other impurities would be desorbed mainly when the temperature is increased to higher than 150 K.Therefore a primary plan have been proposed,that utilize cryocondensation to fuel gas(hydrogen and its isotopes)separation from exhaust gas.The results provide a simple and feasible method for fuel separation and recycling for future fusion devices.