RADIATED POWER AND IONIC EFFECTIVE CHARGE DURING NEON INJECTION EXPERIMENTS ON TEXTOR

A high fraction (gamma) of the heat how converted into radiation at th e plasma periphery, a low edge temperature and a poloidally symmetrica l boundary plasma are the essential features of a cold radiative plasm a mantle. On TEXTOR, when neon is injected as an extrinsic impurity, t he level of gamma can be close to 1 and the edge electron and ion temp eratures are seen to decrease significantly. If the working scenario i s 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 experim entally proved to occur with up to 4 MW of input. power, is strictly r elated to the electron density, which must be as high as possible. Whe n the electron density is raised, not only the energy radiated per imp urity 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 g amma/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, predic ted from simple theoretical considerations, has been experimentally ob served.