X-ray Imaging Study Of The Disruption Instability In Tokamak Plasma

Plasma disruption is a kind of macroscopic instability which affects the stable operation of tokamak.High parameter plasma disruption will pose a serious threat to the safety of experimental device.It is helpful to understand the active control method of disruption instability during the study of plasma disruption.The paper is based on advanced X-ray band GEM camera(Gas Electron Multiplier)and fast energy spectrometer imaging system,the evolution characteristics of X-ray radiation from plasma were observed in the tangential direction of EAST.Through the tomgraphy of the 3-D spatial distribution of X-ray radiation to get the emission spatial distribution of X-ray on the cross section of EAST.The X-ray radiation distribution images of locked-mode of magnetic island,vertical displacement event and X-ray radiation distribution in different energy segments during plasma disruption are studied:It is found that low-energy fast electrons(3.5～30keV)are produced at the beginning of locked-mode of magnetic island,which limit the energy and flux of fast electrons and triggers the vertical displacement event(VDE).The growth rate of VDE was studied based on X-ray imaging system.The growth rate of VDE increased from-6 to-1 in logarithmic coordinates,which was slightly lower than the magnetic measurement results,and had the same variation trend.Based on fast spectrometer imaging system,runaway electrons during disruption are studied.The energy spectrum evolution and spatial distribution of the runaway electrons during the current annihilation stage are studied.The hard X-ray spectra will move during the runaway discharge process,and the spatial distribution of X-ray radiation varies with different energy intervals during this process.The main research contents of this paper are as follows:Firstly,the experimental research methods of disruption instability are summarized,the magnetic topological structure and X-ray radiation characteristics caused by the hydromagnetic instability are studied based on numerical simulation,and the technical route of plasma disruption research based on plasma X-ray radiation is clear.Then,the key diagnostic parameters and tomgraphy of the GEM camera are described,and X-ray radiation images under various disruption conditions are analyzed.In the process of locked-mode disruption,according to the GEM prototype data,the disturbance structure changes before and after locked-mode are obviously observed,and the rotation of the disturbance structure is observed in the process of high sampling rate acquisition(the most highest sampling rate could reach 100kHz),which has a certain relationship with the rotation of the magnetic island of the tearing mode.The GEM counting rate was compared with the negative peak value of the loop voltage in the process of MGI disruption.It was confirmed that the change of GEM counting rate was positively correlated with the change of the loop voltage,and the negative peak value of the loop voltage was closely related to radiation cooling.In the vertical displacement disruption,the movement of plasma core in the view field of GEM camera can be clearly observed,which corresponds well with the vertical displacement signal,confirming that GEM camera has the ability to study plasma vertical displacement event.According to the experimental observation,a high peak value of GEM counts was found in the latter process of disruption,which could not be analyzed by the GEM tomgraphy at this time,but the occurrence of this peak value was in good correspondence with the hard X-ray system.According to this feature,a set of hard X-ray spectrometer system was built based on the characteristics and layout of the GEM system.The effective data is obtained initialy in the experiment.Finally,based on the HXS-2D system,the runaway electrons dynamics during the disruption process is studied,The X-ray radiation spectra of different energy segments were reconstructed,and it was found that the energy spectra moved on the EAST cross section,revealing the spatiotemporal evolution characteristics of runaway electrons with different energies after plasma disruption,and providing a reference database for the development of active runaway electron suppression methods.