Calculation of impurity release from walls and its transport in edge plasmas

Computer simulation by the plasma-solid interaction code EDDY has been performed in order to investigate the erosion and impurity redeposition processes of both W and C materials under the mutual contamination, i.e., C deposition on W and W deposition on C. In the code, dynamic surface composition change during ion bombardment, ionization of released atoms in the plasma, and gyromotion of the ionized atoms in a magnetic field are included. The redeposition of sputtered atoms causes substantially lower effective sputtering yield and lower erosion rate of a W surface, and a decrease in low-energy component of the atoms finally penetrating into a core plasma. W impurities are deposited on a C surface due to the redeposition, whereas C impurities deposited on the W surface are sputtered by impact of most constituent D ions. The W (C) deposition on the C (W) surface substantially increases (slightly decreases) the reflection coefficient for D and impurity ions. (orig.)