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 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.