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

Ion reflection and sputtering at a W test limiter under simultaneous bombardment with C and O ions, as well as D ions, in TEXTOR edge-plasmas are investigated using a Monte Carlo simulation model which combines dynamic composition change in the surface layer with transport of emitted particles in the plasma. The main aim of this work is to discuss the prompt redeposition process on the limiter and the impurity transport in the edge plasma. With increasing plasma density, the effective sputtering yields of W and deposited C considerably decrease due to the increase in the number of redeposited particles, in addition to the decrease in the yields due to the simultaneous decrease in the plasma temperature. Due to the prompt redeposition, the energy distribution of sputtered W, depending on the plasma density (temperature), substantially deviates from the well-known Thompson distribution. 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 contribution of much low-energy thermalized particles is found in the observed distributions of Dγ and OII line spectra emissions. (orig.)