Nonlinear MHD simulation of magnetic relaxation during multi-pulsed helicity injection in spherical torus plasmas

摘要

Coaxial helicity injection (CHI) with a magnetizedi coaxial plasma gun (MCPG) is an attractive sustainment method which has been used as non-inductive current drive and plasma startup in spheromak and spherical torus (ST). However, a critical issue of the CHI is the evidence that during the relaxation, the central open jflux column (COFC) around the central_ post develops a helical distortion due to the nonlinear growth of the n=1 kink instability and the magnetic reconnection destroys the closed flux surfaces. In order to improve this defect of the CHI, the multi-pulsed operation of CHI (M-GHI) that causes a stepwise buildup of the magnetic field of the configuration by pulsing a MCPG has been proposed to achieve quasi-steady state plasma with increase of the current flowing in closed flux surfaces region. The M-CHI has been for the first time demonstrated. in the SSPX gun-spheromak device [1] and performed in the HIST spherical torus device [2] to obtain both improved energy confinement and sustainment. The purpose of this study is to investigate the mechanism to, rebuild the magnetic fields by using resistive nonlinear 3D-MHD simulations [3]. We focus our attention on the effects of the M-CHI on dynamics of the low-q ST.