Study On Simulation Of The Toroidal Radiation Asymmetry By Massive Gas Injection

With the rapidly development of the economy,energy is considered as the material basis of human activities and its demand will rapidly increase.The Global Energy Outlook indicates that clean alternatives and energy substitution will become the basic trends in changes in energy supply structures and end-use energy structures respectively.So far,Tokamak is still considered to be the most hopeful scheme to achieve controlled nuclear fusion which can solve the global energy crisis.However,there are still many instabilities in the tokamak discharge.The most serious case is the occurrence of the plasma major disruption.With the happen of the plasma major disruption,the first wall material and the divertor target will bear huge heat load.Meanwhile,the powerful electromagnetic force produced in the vacuum wall will cause damage to the internal structure of the device and the runaway current formed by a large number of runaway electrons with high energy will cause more harm to the device.Therefore,in the International Thermonuclear Experimental Reactor(ITER)construction,we must have corresponding plans to avoid the occurrence of the plasma major disruption.If the plasma disruption is unavoidable,we need to take actions to mitigate the plasma disruption,thus reducing the damage caused by the disruption.Massive gas injection is to artificially inject a large amount of impurity gas into the thermal plasma to effectively reduce the thermal deposition and electromagnetic forces the first wall material and the diverter target through physical processes such as ionization,radiation and recombination when the plasma disruption happens.Recently,experimental and theoretical studies have shown that disruption mitigation of massive gas injection is divided into two different stages.In the pre-thermal quench phase,the mixture of impurity gases and local plasma in the edge generates cooling and radiation and the radiation at this phase is usually localized.In the thermal quench phase,the particles are transported along the magnetic flux in the toroidal direction,resulting in the toroidal widening of the radiation.Massive gas injection induces MHD instability and.Experiments and numerical simulations on the DIII-D have confirmed the existence of large instability modes.The instability modes can produce a significant heat convection between the plasma in the core and the impurity in the edge under certain conditions.Finally,it will lead to the enhanced thermal radiation in the three-dimensional space and the impurity generates convection in the radial direction to realize the plasma disruption mitigation.Absorption and mixing of impurity into the plasma can enhance the density in the core to a certain degree,thereby suppressing the generation of runaway electrons.Massive gas injections will produce a non-uniform distribution in the poloidal and toroidal directions,and then generate a toroidal radiation asymmetry.This thesis shows the simulation results of radiation asymmetry of disruption mitigation using massive gas injection.First,massive gas injection is divided into three phases: the pre-thermal quench phase,the thermal quench phase,and the current quench phase.Then we will explain the physical processes in the three phases by analyzing the evolutions of various variables.In the pre-thermal quench phase,Ar impurities diffuse across the separatrix and are ionized after collisions with plasma.The diffusion tendency of the impurity is from the upper valve to the high field side with the influence of the magnetic field.In the thermal quench,the diffusion tendency of the impurity is from the high field side to the low field side.Then we focus on the mixing and assimilation efficiency and the radiation asymmetry of disruption mitigation by massive gas injection.Impurity assimilation efficiency can only reach 3% in the thermal quench phase,and the thermal radiation toroidal peaking factor reaches 1.74.Finally,we simply discuss the radiation asymmetry above compared with the experiments on J-TEXT tokamak.