Simulation and experimental studies of impurity release from tungsten exposed to edge plasmas in TEXTOR-94

Particle release from a W test limiter exposed to deuterium edge plasmas, containing impurities C and O, in the tokamak TEXTOR-94 is studied both experimentally and theoretically by means of Monte-Carlo simulations. The simulation model combines the dynamic composition change in the surface layer with the transport of released particles in the plasma. The release of W impurities from the limiter is strongly suppressed by prompt redeposition, whereas low-Z impurities (C and O) and D are much less influenced. Experimentally observed radial distributions of WI and CII line intensities in front of the limiter can be explained by physical sputtering of W and deposited C, respectively, in addition to high-energy reflection of impurity C ions. On the other hand, the CII line emission observed in front of graphite as a limiter is attributed to many low-energy (similar to 0.1 eV) atoms probably due to chemical sputtering by impact of D ions, which is strongly suppressed by deposition of high-Z impurities, such as W. The observed D gamma line emission indicates an important contribution of high-energy reflection to the release of D atoms from the W limiter, in addition to a large contribution of low-energy re-emission. (C) 1999 Elsevier Science B.V. All rights reserved.