Recent progress in understanding the L-H transition physics from ASDEX Upgrade

Abstract

Kinetic edge profiles are measured with high spatial and temporal resolution and their behaviour before and after the time of the L-H transition is analysed. It is shown that at low densities electron and ion channels are strongly decoupled and that the ion kinetic profiles determine the transition to H-mode. At medium densities, where electrons and ions are coupled, the H-L back-transition occurs at the same pressure gradients as the L-H transition. The threshold power for the H-L back-transition exhibits the same increase at low densities as the L-H transition. After the L-H transition the density development depends on the available neutral gas reservoir. Modelling of particle transport indicates that the temporal development of the density profiles can be reproduced assuming only a reduction in the edge diffusion coefficient. The influence of magnetic perturbation coils on the L-H transition power threshold strongly increases with density above a Greenwald factor of about 0.6.