The Radiative Improved Mode in TEXTOR: Power Exhaust and Improved Confinement at High Density,” Fusion Sci

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 de-scribe experiments to study the impact of the radiating impurities on transport processes in the plasma bound-ary 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 convec-tive heat flux density onto the plasma-facing compo-nents. 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 ex-trapolation of the results obtained in TEXTOR, based on experiments in larger devices, is discussed