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 diagnosing the plasma edge. The power threshold for the LK and H-L transitions is related to the edge plasma parameters. The edge temperature T-e,T-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 T-e,T-edge necessary for the transition rises with increasing B-t and decreases with n(e), 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 E-r reveal the well known large negative E-r at the plasma edge. Using a new method with high temporal resolution, the authors find that there is no 'jump' of the radial electric field before the transition within much greater than 100 ms. E-r can jump by much greater than 5 kV/m in less than 1 ms, but the full value of much greater than 20-40 kV/m is established on a slower time-scale associated with the buildup of (N) over 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 T-e,T-edge above which type III ELMs are stabilized whereas type I ELMs occur when the edge pressure gradient reaches the ideal ballooning limit.