Experimental Investigation Of Disruption Mitigation Based On Massive Gas Injection On J-TEXT Tokamak

The fast loss of thermal and magnetic energy during an unmitigated disruption in International Thermonuclear Experimental Reactor(ITER)can lead to heat fluxes exceeding melt thresholds for plasma-facing components(PFCs),generate high electromagnetic loads close to the design limits and potentially cause the generation of high energy runaway electrons(REs).The Disruption Mitigation System(DMS)is one of the most important systems in ITER,it will inject a large amount of impurity to increase the radiation and shutdown the plasma rapidly.The ITER DMS has to fulfill three aims:Reducing the thermal loads on the PFCs during the thermal quench(TQ);Reducing the electromagnetic loads during the current quench(CQ);Suppressing or mitigating the generation of REs.The successful and effective disruption mitigation will ensure the safe operation of ITER.Therefore,it is necessary to study the characteristics of plasma response and the related physical mechanisms during disruption mitigation.The study of disruption mitigation,which is based on the Massive Gas Injection(MGI),has been performed on J-TEXT tokamak.To understand the physical properties associated with MGI disruption mitigation,especially the toroidal radiation asymmetry and REs,several related diagnostics have been developed.The toroidal radiation arrays have been developed to study the radiation response and evaluate the toroidal radiation peaking factor(TPF)during the disruption.The synchrotron radiation camera has been developed to study the generation and loss of high energetic REs,which is confined in the core region.The main research contents of the thesis are as follows:Firstly,the characteristics of plasma response during MGI shutdown have been studied,including the evolutions of electron temperature and density,Magnetohydrodynamic(MHD)activities,radiations and the generation of REs.The MGI rapid shutdown can be divided into three phases:the pre-thermal quench(pre-TQ)phase,the TQ phase,and the CQ phase.During pre-TQ phase,n=1 dominant MHD instability will be destabilized due to the injection of impurities.The initial phase of the mode is determined by the MGI position.And the spreading of impurities is three-dimensional.The inward propagation of the impurities can be blocked and accumulate around q=2 surface when the impurities reach the vicinity of q=2 surface.The radiation cooling mechanism will drive the 2/1 tearing mode to grow rapidly and trigger the disruption eventually.The TPF can be understood as the result of impurity distribution and n=1 mode.Statistical results indicate that increasing the amount of impurity injected or lowering the edge safety factor will accelerate the shutdown process.The RE plateau can be stably formed by injecting an appropriate amount of argon impurity,which is considered to be the result of a combination of primary and hot tail mechanisms.The regime of RE generation has been systematically addressed on J-TEXT tokamak,which depends on the plasma configuration,pre-disruption electron density,toroidal field,edge safety factor,and MHD activities.Secondly,the disruption mitigation with locked modes has been studied.The results show that both of the phase and the width of 2/1 mode have effects on MGI shutdown dynamics and the generation of RE current.The 3D effect between the injected impurities and the 2/1 locked mode mainly affects the pre-TQ and TQ phase,but there is no significant effect during the CQ phase.When the mode is larger than a critical width(W_T≈5 cm),the effect of mode phase on MGI shutdown dynamics satisfies the sine relationship of n=1.The penetration depth and assimilation of impurities can be enhanced during pre-TQ,leading to a faster TQ and well-developed stochastization,if the relative phase between the O-point of 2/1 mode and the MGI valve is?=+90°.Conversely,the penetration depth and the assimilation of impurities are suppressed,leading to a slower TQ and a lower degree of stochastization.The vortex-like E×B flow around a larger island may be the main reason.The toroidal radiation asymmetry is worse in discharges with locked modes.Based on the experimental results,the effect of 2/1 mode on the generation of RE current has been studied.The results show that the RE current can be completely suppressed with?=+90°,while there is no obvious suppression effect with the opposite phase.Finally,numerical modelings based on 3D extended MHD code NIMROD have been carried out for J-TEXT plasmas with stationary,pre-existing 2/1 mode.The simulation results show that the pre-existing island will influence the MGI shutdown dynamics significantly,including impurity mixing,particle/heat transport,and MHD evolutions.The larger degree of stochastization in the plasma will be beneficial to the loss of REs when MGI is injected at a particular mode phase.The simulation results are consistent with the experimental results.The research work in this thesis lays a foundation for the successful operation of the ITER DMS and has important reference value for understanding the ITER disruption mitigation dynamics and optimizing the disruption mitigation strategy.