The Experimental Study Of Gas Electron Multiplier Diagnostic System On EAST Tokamak

Gas Electron Multiplier is widely used in ionizing radiation device owing to the advantages of it’s compact structure,spatial resolution and good time resolution.The ionizing radiation of tokamak is various during plasma operation.GEM detector has good radiation resistance under neutron and gamma background and is widely used in various experimental tokamak for soft X-ray(SXR)research.The GEM camera is designed and installed on EAST tokamak in 2018 which is used for macroscopic MHD instability and plasma disruption research under high stored energy operation.The GEM camera has three advantages comparing with existing system:1,the gasous detector has better radiation resistance than semiconductor;2,the integrated electronic system has smaller size and easy to be installed and maintained;3,the 2D detector only occupy a window to realize 2D imaging diagnosis which save diagnostic window resources on tokamak.The GEM camera has obtained the SXR distribution and MHD signal and the main research of this paper are as follows:Firstly,Design and build the GEM camera system on EAST tokamak.The photons in range of 3.5-30 keV from the vacuum chamber is observed by 2D-GEM during plasma operation.The camera contains 12×12 channels and the maximum time resolution is 2 kHz.SXR emission in polodial section is obtained from GEM data based on Tikhonov algorithm.Secondly,the NTM excitation mechanism research is conducted with the novelty GEM camera system.The n=1 RMP field with fixed phase is penetrate into the core of the plasma as the seed island to trigger NTM.The parameters of the background plasma are the same but the study shows that the NTM cann’t be always triggerred successful.The successful excitaion is related to two successive GEM crash near q=1 surface.The 2nd crash amplitude is exponential decay with βp and there is threshold existed whenβp>0.76.no successful experiment is observed.Statistics and analysis indicate that the m/n=2/1 is not forcibly driven by the n=1 RMP,but is probably due to electron heat transport between the q=1 and the q=2 resonant surface.The NTM trigger process and observed results is described detailly in the paper which have implications for further investigation of NTM locking and disruptions.Thirdly,the GEM update is conducted which aims at improve time resolution.The maximum time resolution is increased from 2 kHz to 100 kHz.The new imaging module enable GEM diagnostic system observes high frequency MHD and disruption.The disruption caused by MHD instability is preliminarily studied.The upgraded system can identify the high-frequency MHD successfully and conducting imaging diagnosis,which indicates that the system has the ability to study physical process with shorter time scales.The tudies shows that GEM camera has limitations in response to high-energy photons.Supply the high-energy photon data to further disruption study is necessary.Finally,based on the domestic 2D CZT array,a hard X-ray spectrometer(HXS2D)with an integrated electronic system which is base on FPGA to study HXR direct from plasma was developed.The system contains 128 channels and is sensitive to photons in the energy range of 0.1-1MeV.The energy spectrum resolution is about～6.8%(662 keV),and can obtain the spectrum in～ms.It has the advantages of small size(diameter 10 cm,length～50 cm)and high time resolution(30 microseconds).The system has been used to study disruption and successfully obtain the kinetic information of the redistribution of high energy electron spectrum caused by MHD during plasma disruption.In conclusion,this paper has applied the GEM camera in EAST Tokamak for the first time,and focused on the NTM instability,an important physical factor that causes the disruption of high stored-energy plasma.The generation of seed island have been studied for the first time with GEM camera;complete the GEM camera upgrade,and study the nonlinear process of magnetic island and plasma disruption;and successfully develop a compact 2-D Hard X-ray Spectrometer(HXS-2D)based on GEM camera,further carry out the study of plasma disruption caused by MHD,and obtain the physical information related to the energy redistribution of high-energy electrons.