Modeling of erosion and deposition patterns on C–W and W–Ta twin limiters exposed to the TEXTOR edge plasmas

The erosion and deposition patterns on tungsten and tantalum test limiters exposed to the TEXTOR deuterium plasma containing a small amount of C impurity are simulated with the modified EDDY code. At the very top of the W and Ta limiters, there occurs neither erosion nor deposition, but the erosion proceeds slowly along the surface. When approaching the edge, the surface is covered by a thick C layer, which shows a very sharp boundary similar to the observation in surface measurements. In the erosion zone, the re-deposited carbon forms a W (Ta)–C mixed layer with small C concentration. Assumptions for chemical erosion yields of ∼0.01 for W and <0.005 for Ta fit the calculated widths of the deposition zone to the experimentally determined values. Possible reasons for the difference between W and Ta are discussed.