Dynamic transition between erosion and deposition on a tungsten surface exposed to edge plasmas containing carbon impurities

By the exposure of high-fluence deuterium (D) plasmas (1023–1026 m−2) containing a small amount (<10%) of carbon (C), the dynamic transition from C deposition to the erosion of tungsten (W) with increasing plasma temperature from 10 eV to 100 eV is investigated using EDDY code. Above fluences of 1023 m−2, there appears to be a fluence-dependent change in the C deposition rate as a function of plasma temperature. At a fluence of more than 1025 m−2, a steady-state condition is reached where an abrupt transition is formed. This occurs due to temperature increases of only a few eV. The transition occurs as a result of small reflections in the incident C ions from a C-rich layer and the enhanced sputtering of the deposited C from a W–C mixed layer. This explains the very sharp boundary between the erosion and deposition zones observed on W test limiters exposed to TEXTOR edge plasmas.