ICRF long-pulse discharge and interaction with a chamber wall and antennas in LHD

Abstract During long-pulse discharges by minority ion cyclotron range of frequencies (ICRF) heating (majority: He ++ and minority: H + ) in the large helical device (LHD), phenomena of hot spots on antenna side protectors, localized divertor plate temperature increase and sparks from inner side of torus were observed. Plasma duration time was limited by the gradual or sudden density increase. The gradual density increase was due to outgassing from the graphite divertor plates or antenna side protectors with hot spots. The spark resulted in sudden density increase. It is thought that the localized divertor plate temperature increase and sparks are attributed to the high-energy ions accelerated in front of ICRF antennas. By suppressing these phenomena the world largest input energy was achieved.