Evaluation of Plasma-Wall Interactions Using Material Probes in the Large Helical Device ∗)

Control of plasma-wall interactions (PWIs) and understanding their mechanism are essential issues for realization of fusion reactor. In this study, plasma-wall interactions during FY2010 and FY2011 experiments in the Large Helical Device (LHD) were analyzed by using material probes. We installed material probes at the positions expected to have different deposits of materials such as boron and titanium and investigated the correlation between this impurity deposition and gas retention. The desorption behavior of hydrogen and helium depended on the level of impurity deposition. We observed a new peak for hydrogen desorption at the probe with large boron deposits. The amount of retained hydrogen was large in that probe; however the amount of retained helium was small in the same probe, which is opposite what we observe in the probe with titanium deposits and the probe with a thin deposition layer. c