Effect of surface roughness and substrate material on carbon erosion and deposition in the TEXTOR tokamak

The technique of (CH4)-C-13 tracer injection through test limiters was applied to study the influence of surface roughness and substrate material on the local C-13 deposition. Spherically shaped graphite limiters with integrated gas injection were prepared with two different grades of surface roughness, similar to 0.1 and similar to 1 mu m. In addition, tungsten limiters with a roughness of similar to 0.1 mu m were used to study the substrate material effect. The limiters were exposed to the SOL plasma for two discharge scenarios, Ohmic and neutral beam heated. The C-13 deposition efficiency-the ratio of the locally deposited to the injected amount of C-13-and the deposition pattern were evaluated by post-mortem surface analysis. Surface roughness has a pronounced effect and increases the C-13 deposition efficiency on graphite with rougher surface by a factor of 3-5 compared with smoother graphite. On tungsten a factor of 2-4 less carbon is deposited than on graphite with similar surface roughness. A systematically higher amount of C-13 (by a factor 1.5-2.5) was deposited on limiters exposed to Ohmic compared with neutral beam heated plasmas.