Change of Edge Plasma Distribution during a Discharge in Heliotron J

In order to monitor the diverted plasma flux distribution for various field configurations in Heliotron J, a new edge-plasma monitor system was installed. During a single discharge, a high-speed video camera (250-500 fps) records a stack of two-dimensional images of visible light emission (mainly from recycling neutrals) near a movable carbon target, which is inserted to the peripheral region of the core plasma. A spontaneous shift of the brightest spot of the light emission along the target was clearly observed. This shift is qualitatively consistent with a poloidal shift of the diverted plasma position which was observed in the diverted plasma flux measurement with a Langmuir probe-array near the wall at a different toroidal position. These findings indicate the global shift of the edge plasma position during a discharge. The most plausible mechanism for the observed shift is the change of the edge field topology caused by a non-inductive plasma current of a few kA.