Nonlinear Simulation of Collapse Phenomenon in Helical Plasma with a Large Pressure Gradient

Abstract

Nonlinear magnetohydrodynamics (MHD) simulations are conducted for a helical plasma with a large pressure gradient to investigate the collapse event induced by MHD instabilities. The simulation results show that the ballooning-like instabilities on an intermediate spatial scale induce disordering of structures in the barrier region and a drop in the central pressure. It was revealed that the core pressure fall is related to disordering of the magnetic field structure. The simulation results are compared qualitatively with the experimental observations of the collapse events in the superdense core state of the Large Helical Device (LHD). Compared with our previous simulation studies of similar situations in the spherical tokamak (ST) plasma, the results for the helical cases show milder collapses than those in the ST case.