Simulations of the effects of pre-seeded magnetic islands on the generation of runaway current during disruption on J-TEXT

Abstract

Simulations of argon (Ar) massive gas injection (MGI) into J-TEXT plasmas with 2/1 mode magnetic islands (mode penetration) are performed with the 3D magnetohydrodynamic (MHD) code NIMROD. In order to study the effect of the magnetic island phase on the loss of runaway electrons (REs) in disruption, four different phases of the pre-existing 2/1 magnetic island have been implemented. It is found that the RE confinement is drastically affected by the magnetic island phase during the thermal quench (TQ) phase. Simulation results show that the curve of the remaining RE ratio vs relative toroidal phase between the preseeded m/n = 2/1 islands and the MGI valve approximates a sinelike function dependence. The optimized phase difference for runaway suppression is predicted to be toroidal 90° (Δϕ=ϕMGI−ϕn=1). It is verified that the trajectories of low energy REs follow magnetic field lines strictly. A discrepancy in the evolution of the flux surface among different toroidal phases of 2/1 islands has been found, which greatly depends on the magnetic perturbations induced in disruption. A stronger low-order MHD activity might contribute to the accelerated processes of impurity assimilation and the TQ phase in the optimized phase. These simulations suggest that the relative phase between the MGI and 2/1 islands is important for RE suppression in future tokamaks.