Observation of termination process of long pulse plasma discharges using stereoscopic fast framing cameras in the Large Helical Device

Abstract

Long pulse discharges in the Large Helical Device (LHD) were interrupted by the emission of large amounts of carbon dusts released from a closed helical divertor in the previous experimental campaign (FY2013). In order to control the dust emission, the configuration of the closed divertor near lower and upper ports was modified in the last experimental campaign (FY2014). While the modification of the divertor configuration has successfully controlled the release of dusts, long pulse discharges were often terminated with abrupt increase in iron ion emission. The termination process of a long pulse plasma discharge was observed with a stereoscopic fast framing camera which can identify the three-dimensional positions of dusts. It proved that the dusts penetrated into the main plasma through the plasma periphery, which could induce the radiation collapse in the plasma discharge.