Simulation Of Runaway Electron Generation And Diffusion During Major Disruptions In The HL-2A Tokamak

The electric field in the toroidal direction, which is induced when there is a plasma major disruption in tokamak, will accelerate thermal electrons whose velocities beyond the critical value, to runaway electrons (REs). The energy of these REs are always tens or hundreds MeV, which will have a serious damage to the first wall of the devices. Thus, the research on the runaway electron behavior during the confinement major disruptions in tokamak has become one of the important tasks in the fusion field both at home and overseas.The generation and also the diffusion of REs during a major disruption in tokamaks have been studied in this work. The parameters are used the experimental parameters of the No.15335discharge in the HL-2A tokamak, and the model couples the primary generation rate equation, the secondary generation equation, or the diffusion term resulted from magnetic perturbation and the Maxwellian equations.First, without considering the magnetic perturbation, we modeled and analyzed the impacts of the toroidal effects and the effective-ion charge on the generation of REs during the major disruption in the HL-2A tokamak theoretically. The results show that, the toroidal effects (the neo-classical correction to the conductivity and the correction to the secondary generation mechanism) have a small effect on the REs. After the theoretical analysis, we can find that the REs increase as the increase of the effective-ion charge, which induces into an increase about the conversion rate of Ohmic current to the runaway current.Then, we analyzed the influence of magnetic perturbation on the REs during the major disruption in the HL-2A tokamak (applying the diffusion term induced by the magnetic disturbation to the RE generation rate equation). The result shows that the stronger magnetic perturbation will’restrain’ the RE generation. The longer-duration magnetic perturbation can also have the same effect as the former. In conclusion, we obtain that the stronger and longer-duration magnetic perturbation can inhibit the RE generation. In addition, we compared the spatio-temporal distribution of the RE density including magnetic perturbation with that no magnetic perturbation and found that the radial distribution of the RE density outwards and the corresponding runaway current increases after the integration. Meanwhile, we found that the spatio-temporal distributions of the toroidal electric field are different between the HL-2A tokamak experiment and the JET tokamak experiment. In the HL-2A tokamak, the parallel electric field does not have the central hollow phenomena as that arising in the JET tokamak.