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

Full text: 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 the wall material (SUS 316L) 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 atomistic mechanism of GDC. A special feature of He-GDC was very heavy damage at the subsurface region; formation of dense nano-size bubbles, dislocation loops and cracks connecting the bubbles, in spite of low incidence energy. Helium trapping in the defective sub-surface region often disturbed quick start of hydrogen plasma experiments by desorbing slowly even at room temperature. In contrast to the He-GDC, radiation induced defects were scarcely formed by H-GDC and Ne-GDC. Based on the microscopic observation of the damage, GDC of LHD was successfully improved. Gas impurities in the LHD could be efficiently reduced by a combination of Ne-GDC and H-GDC. (author)