K. Ohya et al., Simulation and experimental studies of impurity release from tungsten exposed to edge plasmas in TEXTOR-94, NUCL INST B, 153(1-4), 1999, pp. 354-360
Particle release from a W test limiter exposed to deuterium edge plasmas, c
ontaining impurities C and O, in the tokamak TEXTOR-94 is studied both expe
rimentally and theoretically by means of Monte-Carlo simulations. The simul
ation model combines the dynamic composition change in the surface layer wi
th the transport of released particles in the plasma. The release of W impu
rities from the limiter is strongly suppressed by prompt redeposition, wher
eas low-Z impurities (C and O) and D are much less influenced. Experimental
ly 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 li
miter is attributed to many low-energy (similar to 0.1 eV) atoms probably d
ue 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 e
mission indicates an important contribution of high-energy reflection to th
e release of D atoms from the W limiter, in addition to a large contributio
n of low-energy re-emission. (C) 1999 Elsevier Science B.V. All rights rese
rved.