DETERMINATION OF PARTICLE-TRANSPORT COEFFICIENTS IN ASDEX BY GAS MODULATION

Particle transport coefficients have been obtained for a wide range of conditions and confinement regimes in ASDEX by analysis of the density perturbations induced by modulation of the gas feed. The data are sufficiently precise to distinguish diffusion and convective velocity in the central and the outer parts of the plasma. A strong inward convective velocity, especially in the outer part of the plasma, is found in all cases. Particle diffusion in the outer region is often found to be closely related with energy confinement. Diffusion is lower in deuterium than in hydrogen and decreases strongly with density at densities for which the energy confinement improves with density. Diffusion decreases in the transition from saturated Ohmic confinement to improved Ohmic confinement. In normal L-modes driven by neutral beam heating, diffusion is greatly increased compared with that in an Ohmic discharge, but diffusion decreases with increasing plasma current. The H-mode is distinguished from the L-mode principally by an increase in the inward convective velocity in the outer part of the plasma.