Behaviour of plasma discharges in TEXTOR with warm surrounding walls and preliminary observations made after a carbidization of the surface

Radiofrequency-assisted DC glow (RG) conditioning plasmas, electron cyclotron resonant (ECR) cleaning discharges and some 6.000 tokamak discharges have been made in TEXTOR with walls in the temperature range Tsub(w) = 25 - 3000C (4500C for the limiters). Whereas wall conditioning must be made in a warm vessel. Tsub(w) does not affect strongly the behaviour of the subsequent tokamak plasmas. The pulse duration (up to 2.8 and 2.4 s with toroidal currents of 340 and 500 kA), the voltage per turn, the recycling behaviour, the density and temperature profiles and their evolution are not significantly affected by Tsub(w) in a well conditioned system. This results probably from the fact that, even at 3000C, the impurity concentrations are very low (same order as in divertor experiments). In an attempt to reduce further the concentrations of the metal ions, the RG system has been used to implant 13 equivalent monolayers of carbon into the near surface layers at liner and limiter temperatures of 3000C and 4500C respectively. During following tokamak shots the radiation of the metal lines as measured by soft X-ray spectroscopy decreased by at least a factor of two to three as compared to the uncarburized case. (orig.)