Microscopic damage of materials exposed to glow discharge cleanings in LHD

Helium glow discharge cleaning (He-GDC) is frequently applied to reduce impurities like oxygen on the plasma facing materials in many plasma confinement devices, including large helical device (LHD). However, there are few reports which evaluated the wall conditioning effect from the point of view of the microstructure of materials. In the present study, therefore, microscopic damage to metals exposed to He-GDC in LHD were examined. Large numbers of dislocation loops and very high density of bubbles with sizes of 2-20 nm were formed by exposing to He-GDC. In addition, impurity deposits composed of Fe and Cr, which seemed to be re-deposited after sputtering of the first wall (SUS316L), were formed on all specimens. These serious surface modifications seem to have a larger impact on plasma fuel recycling under main plasma discharges. Instead of restoring the wall, significant damage by He-GDC creates gas trapping sites.