Plasma wall interaction study in the large helical device

Material probes were installed at the inner walls along the toroidal and poloidal directions in LHD from the sixth to eighth experimental campaign in LHD. The boronization was three times conducted in each campaign. Gas species, H2, He, Ar and Ne, were employed in both main and glow discharges during the campaign. After each campaign, gas retention and boron deposition behavior was examined to clarify the change of wall surfaces and plasma wall interactions. The amounts of retained gases were observed to be large in the vicinity of anodes used for the glow discharges. The amount of retained helium was observed to be relatively large, only one or two orders of magnitude smaller than that of hydrogen. The amounts of retained argon and neon were about two orders of magnitude smaller than that of helium. The thickness of deposited boron was large in the vicinity of both anodes and gas inlets. The toroidal uniformity of the boron deposition increased to about 70% in the eighth campaign by adjusting the gas flow rate. Understanding for the plasma wall interactions in LHD was deepened by the present study.