PELLET INJECTION WITH IMPROVED CONFINEMENT IN ASDEX
Tokamak discharges with repetitive pellet fuelling were investigated in the ASDEX divertor device. The importance of sufficiently high divertor recycling for a high density at the separatrix and for successful density buildup in the bulk plasma was demonstrated. In contrast to low recycling discharges where no permanent improvement of the energy confinement time was achieved, in OH-heated discharges with high recycling an energy confinement time of 160 ms was reached, the normal value being 80 ms in the rollover region. The peaked density profiles in this case 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 the following alternatives: (1a) a local model which assumes neo-Alcator chi-"rm e" for the electrons and chi-"rm i" = 3chi-"rm neocl" for the ions in the gas puff cases, reducing to chi-"rm i" = chi-"rm neocl" for the optimum pellet cases; (1b) the assumption chi-"rm i" = chi-"rm neocl" under all conditions and an electron energy confinement worse than neo-Alcator in the follover region in gas-puff- discharges; (2) a profile consistency picture where T-"rm e"(a) determines the energy confinement. Low power, NI heated discharges with pellet fuelling behave like Ohmic discharges, while for high power in the L-mode no successful density buildup was possible, and tau-"rm E" was not improved. The H-regime was extended from a density maximum =n-"rm e" = 1.2 times 10"20" m"-3" by the injection of pellets. In this case a density buildup takes place, but further density profile peaking could not be observed.
Bibliographic Reference: NUCLEAR FUSION, VOL. 28 (1988), NO. 5, PP. 827-848
Record Number: 1989126104800 / Last updated on: 1989-06-01
Available languages: en