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Transition Behaviour between Erosion and Deposition on a Tungsten Surface Exposed to Deuterium Plasmas Containing Carbon Impurities

The erosion and deposition processes of tungsten (W) exposed to deuterium (D) plasmas containing a small amount (<10%) of carbon (C) ions are investigated by using a dynamic Monte Carlo simulation. The emphasis is placed on an abrupt transition from C deposition to W erosion as the plasma temperature is increased from 10 eV to 100 eV. The transition occurs as a result of little reflection (much deposition) of incident C ions from a C-rich layer and enhanced sputtering of deposited C atoms from a W–C mixed layer. When the deposited C thickness is increased just below the transition temperature, the C emission yield (i.e., the sum of the C reflection coefficient and the C sputtering yield) decreases so that a thick C-rich layer (2–8 µm or more), growing in proportion to the plasma fluence, is formed on W. When the deposited C thickness is decreased above the transition temperature, the C emission yield increases so that there is only a thin W–C mixed layer (100–300 nm), where the depth profile of deposited C is independent of the temperature, fluence and C-concentration of the plasma. The transition is strongly related to the depth profile of the deposited C in the W bulk, therefore, the transition temperature fluctuates according to the erosion and deposition histories and is strongly influenced by the chemical sputtering yield of the deposited C.