Characterization of Surface Modifications of Plasma-Facing Components in LHD

The Large Helical Device (LHD) has been equipped with movable- and fixed-type material probe systems. Characterization studies of surface modifications on plasma-facing components (PFCs) have been actively progressing by using these probes. After exposure of the PFCs to the plasma, various kinds of surface analysis were conducted. The first walls and divertor tiles of LHD are made of stainless steel and isotropic graphite (IG-430U, Toyo Tanso Co., Ltd.), respectively. They are frequently exposed not only to high-power pulsed main discharges but also to wall-conditioning processes such as glow discharge cleaning (GDC). Thus, the surfaces of the PFCs are drastically changed due to sputtering erosion, impurity deposition, and melting damage. Graphite divertor tiles are eroded primarily during the main discharges; the eroded carbon migrates and deposits on the first-wall surfaces, particularly near the divertor array. First walls are eroded mainly during GDC, which significantly changes the condition of the PFCs. During the main discharges, the majority of incidence particles to the first wall are energetic neutrals (CX neutrals) generated by charge-exchange collisions. Studies of the material damage caused by CX neutrals also have been done. In this paper, the characteristics of surface modifications of PFCs by means of material probe experiments and subsequent surface analysis are summarized.