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 insi de the separatrix drops on a 100 mus timescale. Density turbulence com ing from the edge and magnetic turbulence, as measured with magnetic p robers, show a similar behavior. Before and during the transition no f requency shift in the fluctuation spectra could be detected which woul d indicate the rapid appearance of a radial E-field in the region of t he H-mode barrier. Within milliseconds after the L-H transition new fl uctuations propagating in the electron diamagnetic direction are desta bilized which however exhibit no marked influence on across the barrie r. In the bulk of the plasma fluctuation spectra show a frequency shif t which is dominated by toroidal plasma rotation due to neutral beam i njection. The initial transport barrier is deduced to lie inside the s eparatrix with a width of less-than-or-equal-to 3 cm.