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

Summary form only given. 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.So far, the formation of the closed flux surfaces in the TCHI start-up plasma on HIST has been confirmed by using internal magnetic probes4. Also, the toroidal current density profile has been significantly modified with variations in the bias poloidal coil flux. In this conference, we report that the existence of reconnecting plasmoid in the toroidal current density profile during the start-up phase has been found for the first time. Experimental observation shows that two or three plasmoids are generated in the elongated current sheet. For the stable discharges, the plasmoid merges and develops to a large-scale flux structure due to inward current diffusion during the decay phase. For unstable discharges with the low bias flux, the magnetic configuration is distorted due to the n =1 kink instability during the current rise phase. However, as the plasma starts to decay resistively, it relaxes back to an axisymmetric state. During the relaxation, the current conversion from toroidal to poloidal direction occurs, leading to the generation of the large closed flux with a selfgenerated toroidal field. These findings could verify that the plasmoid instability in the elongated current layer in the presence of the strong guide field allows the formation of an X-point and the fast flux closure via magnetic reconnection in the T-CHI start-up.