Spontaneous shift of divertor plasma footprints during a discharge in a helical-axis heliotron device

Abstract

A spontaneous shift of diverted plasma position during a discharge was investigated in a helical-axis heliotron device, Heliotron J. The shift of a few cm was observed for discharges with a non-inductive plasma current <3 kA and the plasma stored energy <3 kJ. The time dependence of the observed shift was related to that in the plasma current more closely than that in the stored energy. The most plausible mechanism for the observed shift is the change in the edge field topology caused by the plasma current. The three-dimensional finite-β equilibrium calculations assuming profiles of plasma pressure and plasma current density indicate that the effect of the plasma current on the field topology depends not only on the current flow direction but also on the current density profile. This experiment points out not only the importance of current control in fixing the divertor plasma position in a low-shear helical device but also the possibility of ‘divertor swing’ for the reduction of the divertor particle/heat load by controlling a small amount of plasma current within a tolerable influence on the plasma performance.