Performance of the Ti doped graphite RG-Ti-91 at the divertor of the Tokamak ASDEX Upgrade

During the 1994/1995 discharge period two graphite tiles at the outer and the inner lower divertor plates of the tokamak ASDEX-Upgrade have been replaced by tiles of RG-Ti-91 which is a graphite, doped with about 2 at% of Ti and subsequently treated at high temperatures and pressures to achieve a high thermal conductivity perpendicular to one direction. The RG-Ti-91 tiles have been installed with the directions of high thermal conductivity normal to the plasma exposed surface. The plates have been exposed to about 670 tokamak discharges each lasting about 2 to 3 s. The performance of the discharges has not been influenced by the presence of the RG-Ti-91 tiles, i.e., no additional impurities, such as Ti. have been observed in the plasma. The temperature increase of the RG-Ti-91 tiles during high power loads was about a factor 2 lower than the temperature increase of fine grain graphite plates. The surfaces of the tiles showed preferential erosion of the carbon. and TiC crystallites staying out on the surfaces. This is similar to the surface structure which develops at D-ion bombarded surfaces of this RG-Ti-91 material in the laboratory.