Numerical Simulation of MHD Relaxation during Non-Inductive Startup of Spherical Tokamaks

Nonlinear resistive MHD computation with zero-beta closure is used to investigate the relaxation of helical current filaments into tokamak-like plasmas for non-inductive startup of spherical tokamaks.~ A localized, volumetric current source has been added to the NIMROD~code (nimrodteam.org) to model miniature washer-gun current sources in the lower divertor region of the Pegasus Toroidal Experiment at the Univ. of Wisconsin.~ When the induced magnetic field is smaller than the vacuum field, a helical filamentary current channel forms on open field lines, comparable to experimental results in similar conditions.~ Relaxation into tokamak-like plasmas has been demonstrated in experiment with induced fields that exceed the vacuum field [N. W. Eidietis, et al., J. Fusion Energy 26, 43 (2007)], and simulation results in these conditions with comparisons to experiment are presented.~ We discuss the relaxation process including the roles of diffusion, JxB forces, and magnetic reconnection in the evolution of the magnetic topology.