Physics and scaling of the H-mode transition in ASDEX Upgrade

Global and local edge parameters at the H-mode transition are investigated at edge densities below and near the H-mode density limit. Conditions for o nset of edge magnetohydrodynamic (MHD) instability (ELMs) and sustaining th e H-mode define an operational window for H-mode with a maximum edge densit y, which can only be reached with optimum adjustment of both heating and fu elling fluxes. Careful experiments are carried out to vary the edge density in a wide range and allow comparison with H-mode theories in various colli sionality regimes.