Quantitative Analysis of Plasma Particles in Materials Exposed to LHD Divertor Plasmas

Surface modification and hydrogen retention of a LHD divertor tile used for plasma experiments for6 years were analyzed by using ion beam analytical techniques. Several clear divertor footprints were identified on the central area of the tile surface. This area is a so called erosion dominant area. While, on the both sides of the central area, surfaces were covered mainly with Fe and Ti, respectively. The amounts of hydrogen retention were strongly dependent on the chemical composition of the surface. In the Ti rich deposition layer appeared in the private side, very strong hydrogen trapping was detected. In contrast, Fe rich area in the opposite side, the retention was much lower than that of the normal carbon surfaces

Microscopic Modification of Wall Surface by Glow Discharge Cleaning and its Impact on Vacuum Properties of LHD

Abstract. Glow discharge cleaning (GDC) is a widely used technique for wall conditioning in fusion experimental devices. Though the cleaning effects of GDC are essentially related to the microscopic modification of the wall surface, there are few reports about it. In the present study, samples of wall materials were exposed to GDC plasma of hydrogen, helium and neon in the Large Helical Device (LHD) by using the retractable material probe transfer system and examined microscopic modification by transmission electron microscopy (TEM) to understand the underlying mechanism of GDC. Based on the results of the material probe experiments, GDC of LHD was successfully improved. Reduction of the impurities in the LHD vacuum vessel was drastically improved by using Ne-GDC. In the case of Ne-GDC, the specimen surface was covered with very thick re-deposited layer of Fe and Cr. Due to its high sputtering efficiency and very shallow penetration, it is likely that neon atoms effectively sputtered surface contamination without remaining serious damage and themselves in the sub-surface region. Retained Ne can be successfully removed by the following short H-GDC