Microscopic modification of wall surface by glow discharge cleaning and its impact on vacuum properties of LHD

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, specimens of wall materials were exposed to GDC plasma of helium, hydrogen and neon in the Large Helical Device (LHD) by using the retractable material probe transfer system and irradiation damage was examined by transmission electron microscopy to understand the underlying microscopic mechanism of GDC. In the case of Ne-GDC, the specimen surface was covered with a thick deposited layer of Fe and Cr but no radiation induced defects were observed. Due to the high sputtering efficiency and very shallow penetration, it is likely that neon atoms effectively sputter the surface contamination without leaving serious damage or remaining in the sub-surface region. After the Ne-GDC phase, retained Ne can be successfully removed by the following short hydrogen GDC. It was shown that a two-step GDC with Ne and H is very effective to clean the metallic surface of the LHD.