Modification of the helical divertor footprint induced by the net toroidal plasma current in the LHD heliotron

Abstract

In this study, the modification of the helical divertor footprint induced by the net toroidal plasma current was investigated in the Large Helical Device (LHD). The divertor footprint shifted a few centimetres when the net toroidal plasma current, as measured by Rogowski coils, was induced via the neutral-beam injection. The shift direction was related to the plasma current direction, and the shift width showed almost linear dependence on the plasma current divided by the toroidal field strength. Magnetic field line tracing calculation considering the plasma current within a simple model revealed that the modification of the edge field line structure causes the footprint shift. The predicted and experimental shift widths agreed qualitatively.