Coaxial helicity injection plasma start-up and magnetic reconnection on HIST

Abstract

The coaxial helicity injection (CHI) method is one of promising candidates for the non-inductive current drive in the spherical torus (ST) plasma concept. The transient-CHI (T-CHI) method for plasma start-up has been successfully applied to the NSTX device at PPPL for the start-up followed by inductive ramp-up1. The main purpose of the T-CHI experiments on the Helicity Injected Spherical Torus (HIST)2 at University of Hyogo is to understand the physics of the flux closure related to fast magnetic reconnection during the T-CHI start-up process. The recent MHD simulation on T-CHI for NSTX predicts the formation and breakup of an elongated Sweet-Parker (S-P) current sheet and a transient to plasmoid instability3. According to this simulation, the reconnection rate based on the plasmoid instability is faster than that by S-P model and becomes nearly independent of the Lundquist number S. This simulation has motivated us to seek reconnection phenomena observed in T-CHI start-up plasmas on HIST.