Plasma wall interaction and plasma edge properties with radiation cooling and improved confinement in TEXTOR-94

The Radiative Improved Mode obtained on the limiter tokamak TEXTOR-94 combines power exhaust by a radiating plasma boundary with improved energy confinement (as good as in the ELM-free H-mode in divertor tokamaks) at high plasma densities (equal to or even above the Greenwald density) in quasi-stationary discharges. Substantial changes of the plasma edge properties are observed with increasing radiation: a reduction of the plasma edge density and temperature, a reduction of particle transport out of the confined plasma volume and an increase of the penetration depth and of the fueling efficiency of neutrals. In addition to an increased inward drift this causes a steepening of the density profiles. For a given plasma current and heating scenario the transition to improved confinement takes place as soon as the density peaking reaches a critical threshold. With the injection of neon an increased sputtering yield of carbon and in addition photon induced desorption of oxygen from the wall are observed, leading to an increased dilution at the plasma edge, whereas the central dilution is only weakly affected.