Material probe study for plasma facing wall of LHD

Impurity deposition and gas retention were investigated for material probes which were installed along toroidal and poloidal directions on plasma facing walls during the 4th experimental campaign in the large helical device (LHD). The carbon deposition was observed to be dominant on the surface of probes located far from the plasma. The retained amount of helium was also large in the probes located near the plasma, although that of hydrogen was large in the probe far from the plasma. The retained amounts of discharge gases and impurity gases were large near the anodes used for glow discharges, since the current density near the anode was large.