The Development And Experimental Investigation Of Lithium Coating With Tungsten First Wall In EAST

Tungsten as a high Z metal with high melting point,high thermal conductivity and low retention of hydrogen isotopes,has been selected as the solid divertor material for EAST and future ITER fusion devices.However,tungsten wall probably results in serious high Z impurity accumulation in plasma core due to strong plasma and wall interactions,and it leads to serious radiation power loss.It has to control the concentration of tungsten impurity to be lower than 10-5 in fusion devices operation.Therefore,it is very key to study how to decrease tungsten impurity level in future high power and long pulse discharges operation in EAST and future fusion devices.This paper will present the results of different lithium wall conditionings under tungsten divertor in EAST.This paper studies the influence of three kinds of lithium coatings under tungsten divertor on tungsten impurity,hydrogen recycling and plasma performance in EAST.Also,it further analyzes the mechanisms of the impurity and recycling reduction with various lithium coating technologies.In addition,this paper discusses the advantages and disadvantages various lithium coating technologies.These investigations are beneficial for guiding EAST future steady operation under tungsten divertor.We develop three kinds of lithium coating technologies,including lithium evaporation coating assisted by He-ICRF before running day,inter-shot lithium coating conditioning and real-time lithium powder injection during plasma operation.With the upgrade of lithium coating system,the lithium coverage on EAST first wall surface was progressively increased.The recent results show that about 95%of the first wall was effectively covered by lithium film.With the help of the uniform Li coverage,it leads to more effectively control of impurity and recycling in plasma,and longer lifetime of lithium coating film.A delicated inter-shot lithium coating system was deleveloped and successfully applied on EAST,and it shows good compatibility with plasma operation.The real-time lithium powder injection could effectively suppress the tungsten impurity accumulation in plasma core,reduce the divertor recycling,and suppress the ELMs.At present,the lithium coatings are the routine wall conditioning technologies in EAST.Lithium evaporation coating under tungsten divertor shows that it can effectively reduce the tungsten impurity sources and decrease the tungsten impurity concentration in plasma core.Meanwhile,the lithium coating also can effectively decrease the low Z impurity in plasma,such as carbon and oxygen impurities,and can reduce the hydrogen recycling and H/(H+D)ratio to be lower than 10%.With the help of the lithium coating,a new record of 101.2s H-mode discharge was realized in EAST.The in-shot lithium coating under tungsten divertor indicates that the new designed system worked well in EAST,and it didn't produce any negative influences on plasma operation.This new system can provide a fresh lithium coating for every discharge in EAST,especially for future long pulse plasma operation.Simultaneously,this system can operate during the discharges to real-time inject lithium vapor into plasma,which is probable to suppress impurity accumulation or ELMs.The inter-shot lithium coating also can effectively reduce the impurity concentration,and control recycling,which is similar as Li coating before plasma running day.The real-time lithium powder injection under tungsten divertor can effectively reduce tungsten impurity sources,and suppress tungsten impurity accumulation in the plasma core both in L-mode and H-mode discharges.A significant reduction of tungsten impurity concenctration in over 30 s long pulse H-mode discharges by the real-time lithium powder injection was successively achieved.The main mechanism of tungsten impurity suppression is obviously reduced divertor electron temperature by powder injection,and it results in reduced surface tungsten impurity sputtering.In addition,the real-time lithium powder injection can effectively reduce the divertor recycling,which indicates lithium powder can effectively flow to divertor region and coat on the divertor tiles to effectively reduce hydrogen release.Furthermore,it is the first time to confirm that the real-time lithium powder injection effectively suppresses the ELMs under tungsten divertor,and the H98 confinement factor maintains at?1.2.All of the above results of lithium coating under tungsten divertor prove that the lithium coating can effectively reduce the high Z tungsten impurity concentration,and suppress its accumulation in the plasma core.Simultaneously,the lithium coating can reduce the low Z impurity,such as carbon and oxygen,and control hydrogen recycling.The lithium coating provides pretty wall conditioning for high power and long pulse plasma operation under tungsten divertor.The research provides a valuable reference to tungsten impurity suppression for long pulse steady-state operation in future fusion device under tungsten first wall.