Experimental and Computational Studies of MHD Relaxation Generated by Coaxial Helicity Injection in the HIST Spherical Torus Plasmas

Coaxial Helicity Injection (CHI) experiments in the Helicity Injected Spherical Torus (HIST) device have produced discharges with peak plasma currents up to 150 kA and successfully sustained the plasma current for much longer than a resistive decay time. The flow measurements using a Mach probe have shown that the intermittent generation of the flow is correlated to the fluctuation seen on the electron density and current signals. The repetitive merging of the magnetized plasma jet ejected from the helicity injector may be responsible for the current sustainment in these discharges. This CHI current drive mechanism is supported by the 3D-MHD numerical simulation. The acceleration of ion flow up to ~50 km/s has been observed during the transition phase from the spherical torus (ST) to the flipped-ST relaxed state. We have found that the current self-reversal process involves the non-linear growth of the n=1 kink instability of the central column and the relevant magnetic reconnection flow.