Ion reflection and sputtering at tungsten surface exposed to edge plasmas in TEXTOR

Ion reflection and sputtering at a W test limiter under simultaneous bombar dment with C and O ions, as well as D ions, in TEXTOR edge-plasmas are inve stigated using a Monte Carlo simulation model which combines dynamic compos ition change in the surface layer with transport of emitted particles in th e plasma. The main aim of this work is to discuss the prompt redeposition p rocess on the limiter and the impurity transport in the edge plasma. With increasing plasma density, the effective sputtering yields of W and de posited C considerably decrease due to the increase in the number of redepo sited particles, in addition to the decrease in the yields due to the simul taneous decrease in the plasma temperature. Due to the prompt redeposition, the energy distribution of sputtered W, depending on the plasma density (t emperature), substantially deviates from the well-known Thompson distributi on. The observation in TEXTOR that both the decrease in the intensity of WI line and the increase in the intensity of CII line with increasing plasma density are reproduced by our simulation. Nevertheless, dominant contributi on of much low-energy thermalized particles is found in the observed distri butions of D gamma and OII line spectra emissions. O 1998 Elsevier Science B,V. All rights reserved.