Mixed-material coating formation on tungsten surfaces during plasma exposure in TEXTOR-94

A polished graphite sample with a tungsten layer on the plasma-facing surface was exposed to the scrape-off plasma of TEXTOR-94 for about 200 s of discharge time. The plasma exposure caused a contamination layer with a thickness up to 300 nm consisting mainly of the deposited plasma impurity carbon and the tungsten substrate material. The processes and conditions involved in the growth of such mixed material layers are examined. In accompanying experiments, the penetration of carbon into tungsten by diffusion and ion implantation at different temperatures has been measured. The influence of synergistic effects on the carbon penetration into tungsten by the simultaneous impact of carbon and hydrogen ions was studied by irradiation of tungsten with CH3 ions. The discussion also includes processes such as non-uniform deposition, preferential erosion and prompt local redeposition.