Study of plasma contamination during ICRF heating under boronized wall conditions in TEXTOR

The impurity content of the plasma during the steady state phase of ICRF minority heating in TEXTOR is found to be, for boronized wall conditions, a function of the thermal load on the limiter only. This new experimental result, which is to be related to the RF power absorption characteristics of the TEXTOR plasmas, has been established by comparing, at different RF power levels, the plasma effective charge with the thermal deuteron flux to the limiter. During the transient phase of the RF pulse, however, relevant RF-specific effects in the boundary plasma have been observed. These effects, which are accompanied by a release of impurities, are suppressed almost completely if a high energy RF induced tail of minority ions is previously built up in the target plasma. This is due to a large increase of the damping of the fast wave. The RF pre-heating at the moderate power level of 0.8 MW has been found experimentally to be very effective, in agreement with the predictions of a full wave Fokker-Planck simulation code. Comparison of Z_eff with spectra in the UV, VUV and SXR ranges leads to the conclusion that the carbon influx dominates the plasma contamination.