THE H-MODE IN THE ASDEX TOKAMAK

The L-H transition in the ASDEX tokamak was investigated. The level of fluctuations both in the scrape-off layer and in a narrow region inside the separatrix drops on a 100 mus timescale. Density turbulence coming from the edge and magnetic turbulence, as measured with magnetic probers, show a similar behavior. Before and during the transition no frequency shift in the fluctuation spectra could be detected which would indicate the rapid appearance of a radial E-field in the region of the H-mode barrier. Within milliseconds after the L-H transition new fluctuations propagating in the electron diamagnetic direction are destabilized which however exhibit no marked influence on across the barrier. In the bulk of the plasma fluctuation spectra show a frequency shift which is dominated by toroidal plasma rotation due to neutral beam injection. The initial transport barrier is deduced to lie inside the separatrix with a width of less-than-or-equal-to 3 cm.