Observation of plasma termination processes using stereoscopic fast framing cameras in the Large Helical Device

The Large Helical Device has performed long pulse discharge experiments for more than one decade. Long pulse discharges in the previous experimental campaign in FY2013 were often terminated by the large amounts of carbon-rich dusts released from closed divertor regions, which were caused by exfoliation of mixed-material deposition layers. The plasma termination by the release of dusts was reduced by modification of the closed divertor configuration. Some long pulse discharges in the last experimental campaign in FY2014 were interrupted by iron-rich dusts released from stainless steel armor tiles on helical coil cans. Stereoscopic observations of the three-dimensional trajectories of the dusts revealed that the dusts penetrate into the main plasma through the ergodic layer and the Last Closed Flux Surface. (author)