Analysis of impurities deposited on a material probe placed in large helical device (LHD). Effect of a carbon divertor on impurity deposition

The third experimental campaign in large helical device (LHD) was carried out from June to December in 1999. In this campaign, graphite tiles were placed in the divertor region. To investigate the effects of the graphite divertor on plasma wall interactions, material probes were placed at the inner wall of vacuum vessel along the poloidal direction. As impurity species, Fe, Ni, Cr, C and O were observed in every sample. On the surface of the samples placed close to the plasma, Fe concentration was observed as high as 25 at.%. The deposition amount of Fe per one plasma discharge shot became approximately a half of that of second campaign. Carbon was largely deposited on the samples placed near to the graphite divertor and at the port. The thickness of deposited carbon was large, approximately 800 nm. Helium was retained in every sample. The amount of retained He strongly depended on the position of material probe in the LHD, i.e. the amount of He in the sample placed near plasma is larger than that on the port. It is indicated that the helium retention is mainly due to the helium glow discharge cleaning and during main discharge plasma. In the third campaign, metal impurities in the plasma were observed to be largely reduced by the use of graphite divertor. The plasma stored energy also increased by the reduction of metal impurity emission. In the present study, plasma material interaction based upon material probes contributed to a good plasma discharge performance. (author)