Runaway current termination in JT-60U

Spontaneous termination of runaway current, generated at the plasma disruption, is found in JT-60U during the vertical plasma displacement event, where the safety factor at the plasma surface, qs decreases. For all shots with runaway electron generation, runaway current termination starts with the appearance of a spike in magnetic fluctuation and finishes at qs≥2. Growth rates of the spikes in the magnetic fluctuations decrease by an order of magnitude during the termination of runaway current. When each magnetic fluctuation with a slow growth rate appears, runaway current decays, and heat flux pulses are generated. Halo current during the runaway termination is small. Halo current is generated after the runaway termination, and reaches the maximum level at qs∼1. (author)