Study of H-mode physics in ASDEX Upgrade

H-mode physics in ASDEX Upgrade is studied with the help of fast diagnostics with high spatial resolution diagnosis the plasma edge. The power threshold for the L-H and H-L and H-L transitions is related to the edge plasma parameters. The edge temperature Te,edge at the transitions is the same for the L-H and H-L transitions at otherwise constant plasma parameters, indicating that the hysteresis in heating power is solely due to the reduced transport coefficients in the H-mode. The value of Te,edge necessary for the transition rises with increasing Bt and decreases with ne, inconsistent with the parameter dependence predicted by theories based on ion orbit loss as the dominant nonambipolar radial loss current. Measurements of the radial electric field Er reveal the well known large negative Er at the plasma edge. Using a new method with high temporal resolution, the authors find that there is no 'jump' of the radii electric field before the transition within >>100 ms. Er can jump by >>5 kV/m in less than 1 ms, but the full value of >>20-40 kV/m is established on a slower time-scale associated with the buildup of N-tilde p. Edge localized modes of type III and I are clearly distinguished by the MHD characteristics of their precursors. In operational space, they are separated by a critical Te,edge above which type III ELMs are stabilized whereas type I ELMs occur when the edge pressure gradient reaches the ideal ballooning limit. (author). 21 refs, 6 figs