Material probe analysis for plasma facing surface in the large helical device

Material probes have been installed at the inner walls along the poloidal direction in large helical device (LHD) from the first experimental campaign. After each campaign, the impurity deposition and the gas retention have been examined to study the plasma surface interaction and the degree of wall cleaning. In the 2nd campaign, the entire wall was thoroughly cleaned by glow discharge conditioning and the number of main discharge shots increased. For the 3rd and 4th campaigns, graphite tiles were installed over the entire divertor strike region, and then the wall condition was significantly changed compared with the case of a stainless steel (SS) wall. It was seen that graphite tiles in the divertor were eroded mainly during main discharges, and the SS first wall mainly during glow discharges. During main discharges the eroded carbon was deposited on the entire wall. A reduction of metal impurities in the plasma was observed, which corresponds to the carbonized wall. The deposition thickness was great at the wall far from the plasma. Since the entire wall was carbonized, the amount of discharge gases retained such as H and He became large. In particular, helium retention was large at a position close to the anodes used for helium glow discharge cleanings. One characteristic of the LHD wall is a large retention of helium since the wall temperature is limited to below 368 K. In order to reduce the recycling of the discharge gas, wall heating is necessary. (author)