THE INFLUENCE OF PLASMA-EDGE PROPERTIES ON HIGH CONFINEMENT DISCHARGES WITH A RADIATING PLASMA MANTLE AT THE TOKAMAK TEXTOR-94

The radiative improved mode obtained on the limiter tokamak TEXTOR-94 combines the possibility of power exhaust by a radiating plasma bounda ry (with a fraction gamma of the radiated power with respect to the to tal input power up to 90% with neon or argon cooling) with improved en ergy confinement (as good as in the ELM-free H-mode in divertor tokama ks) at high plasma densities (line-averaged central-electron density e qual to or even above the Greenwald density limit n(GW)) in quasi-stat ionary discharges. An overview is given of the substantial changes in plasma-edge properties occurring at high radiated power levels gamma, These changes are characterized by a reduction of the plasma-edge dens ity and temperature, a reduction of particle transport out of the conf ined plasma volume and an increase of the penetration depth of deuteri um and impurity atoms. As a consequence, the particle confinement time increases and the electron-density profiles steepen. The transition t o improved confinement take's place as soon as the density peaking rea ches a critical threshold. An internal transport barrier is observed i n the bulk of RI-mode plasmas (at r/a less than or equal to 0.6) chara cterized by an increase Of the pressure gradient and Of the shear of t he toroidal velocity compared to discharges without additional impurit y seeding. The dilution at the plasma boundary is strongly increased b y the seeded impurities whereas the central dilution is only weakly af fected.