The radiative improved mode in TEXTOR: Power exhaust and improved confinement at high density

The concept of a cold radiating plasma boundary has been proposed as a solution to the problem of power exhaust in magnetically confined fusion plasmas. We describe experiments to study the impact of the radiating impurities on transport processes in the plasma boundary and the plasma core in the tokamak TEXTOR. The injection of impurities (neon, silicon, or argon) leads to the formation of a radiating plasma boundary where up to 90% of the input power can be distributed to large wall areas, thereby strongly reducing the convective heat flux density onto the plasma-facing components. At high plasma densities the impurity seeding leads to a transition to an improved confinement state termed the radiative improved mode. This operational scenario combines high density and high confinement with power exhaust by radiation under quasi-stationary discharge conditions. The confinement improvement can be explained by a reduction of transport caused by the ion temperature gradient mode. This reduction is initiated by the impurity content and amplified by a characteristic steepening of the density profiles of the background plasma. The extrapolation of the results obtained in TEXTOR, based on experiments in larger devices, is discussed.