Radiated power and ionic effective charge during neon injection experiments on textor

A high fraction (γ) of the heat flow converted into radiation at the plasma periphery, a low edge temperature and a poloidally symmetrical boundary plasma are the essential features of a cold radiative plasma mantle. On TEXTOR, when neon is injected as an extrinsic impurity, the level of γ can be close to 1 and the edge electron and ion temperatures are seen to decrease significantly. If the working scenario is optimized, the incremental central plasma contamination turns out to be tolerable. As can be seen from a simple equation for the radiated power, the feasibility of such an optimization, which has been experimentally proved to occur with up to 4 MW of input power, is strictly related to the electron density, which must be as high as possible. When the electron density is raised, not only the energy radiated per impurity particle increases (for neon up to 30-40 keV), but the level of the intrinsic impurities can fall by as much as 30 to 40%. The ratio γ/Z_eff, which is a measure of the quality of the impurity cooling, can therefore reach values as high as 0.4 to 0.5. The improvement in the edge poloidal symmetry under radiative cooling conditions, predicted from simple theoretical considerations, has been experimentally observed.