Modeling of Material Mixing Effects on Plasma Surface Interactions in Magnetic Fusion Devices

The material mixing effects on the erosion and deposition processes of W under simultaneous bombardment with deuterium (D) ions and a small amount of carbon (C) ions are investigated by using a dynamic Monte Carlo simulation code, EDDY. The transition from deposition to erosion occurs abruptly when the plasma temperature is increased. Below the transition temperature, a thick C layer appears as a result of the low reflection coefficient of the deposited C so that the W is perfectly protected against sputter erosion. Above the transition temperature, a balance between the incident and released C on the W surface is reached where the surface is eroded only due to the reduced physical sputtering of the W-C mixed layer. The erosion and deposition patterns observed on the W side of the W-C twin test limiter exposed to TEXTOR edge plasmas are reproduced by using the modified EDDY code. On most parts of the surface, the enhanced physical sputtering of the deposited C suppresses the formation of a thick C layer, whereas near the limiter edge the C deposition dominates where the thickness increases with increasing C fluence. The extent of the deposited area which is broader on Ta than on W is simulated due to higher surface temperature (stronger diffusion) or lower chemical sputtering yield of the deposited C.