Inspection of Arc Trails Formed in Stellarator/Heliotron Devices W7-X and LHD

Arc trails found in heliotron/stellarator devices Large Helical Device (LHD) and Wendelstein 7-X (W7-X) were inspected; arcing occurred on different locations at various situations. In LHD, a helium-plasma-induced tungsten nanostructure sample was installed and exposed to a LHD plasma. Many arc trails were formed only on the sample with nanostructures, suggesting an easy initiation of arcing compared to pristine tungsten. After the completion of annual campaign 2011 in LHD, arc trails appeared on a graphite divertor tile which was taken out for inspection. Because the arc trails had a linear shape, the arcing was likely caused by main discharge operation. In W7-X, some Langmuir-probes installed within the limiter tiles suffered severe damage with arc trails during the operational phase 1.1. After the operational phase 1.2b, in-vessel inspection was performed for the first time for all the plasma facing components focused on arcing. No arc trails appeared on graphite components including test divertor units. In turn, considerable number of the trails appeared on non-plasma exposed region. Most of the arc trails had no clear linearity, indicating that arcing initiated during glow discharge cleaning phase. Only a few trails seemed to be affected by the existence of an external magnetic field. A vast majority (80%) of arcs were initiated from the edge of surfaces, while half of arc trails starting on the interior region of surfaces were accompanied by deposition spots. Broad arc trails appeared on the surfaces of diagnostic ports and mirrors. (C) 2020 The Japan Society of Plasma Science and Nuclear Fusion Research