Physics Study of EC-Excited Current Generation via Current Jump in the Compact Plasma-Wall-Interaction Experimental Device

Non-inductive current start-up experiments by using electron cyclotron heating and current drive (ECH/ECCD) technique has been conducted in the Compact Plasma-wall-interaction experimental Device (CPD), which is a small spherical tokamak device. By applying 8.2 GHz RF under steady external fields, plasma currents are generated and closed flux surface configurations are formed spontaneously. Two discharge modes are found. The first one is the “current jump” discharge, in which the closed flux configuration is formed via the rapid increase of the plasma current under relatively high vertical magnetic field. The second one is a slow current increase discharge under lower vertical field, in which the closed flux surfaces are formed during a slow current increase with the slow increase of microwave power. Since the higher plasma currents under higher vertical fields can be obtained only via the current jumps, the current jump phenomena seems to be essential for the non-inductive start-up scenario based on ECH/ECCD technique. By comparing the co-directional O-mode injection and the normal X-mode injection in the current jump discharges, the current jump occurred at lower RF power in the co-directional O-mode than that in the normal X-mode. However, the values of plasma current just before and after current jumps did not depend on the injection mode.