Pellet injection with improved confinement in ASDEX

Tokamak discharges with repetitive pellet fueling were investigated in the ASDEX divertor device. The importance of sufficiently high divertor recycling for successful density build-up in the bulk plasma was demonstrated. In OH-heated discharges with high recycling an energy confinement time of 160 ms was reached, the normal values being 80 ms in the roll-over region. The peaked density profiles in this cases were accompanied by reduced or suppressed sawtooth activity and finally ended in a phase of strong central impurity accumulation. The particle transport was characterized by strong, non-classical inward drift, while the improved energy transport can be explained by two alternatives: (1) a local model which assumes neo-Alcator χe for the electrons and an χi = 3χneocl for the ions in the gas puff cases reduced to χi = χneocl for the optimum pellet cases; (2) a profile consistency picture were Te(a) determines the energy confinement. Low-power, NI-heated discharges with pellet fueling behave like ohmic discharges, while for high power in the L-mode no successful density build-up was possible, and τE was not improved. The H-regime was extended to anti ne = 1.2x1020 m-3 by the injection of pellets. In this case a density build-up takes place, but a further density profile peaking could not be observed. (orig.)