Edge and divertor physics in ASDEX Upgrade with emphasis on density limit characteristics

A large SOL database covering a wide range of ASDEX Upgrade plasma parameters embodying data measured at the outer midplane and divertor has been built up to support the development of reliable scalings necessary for ITER. Clear dependencies between the neutral particle flux in the divertor and, e.g., the midplane separatrix density could be established. Additionally, the density operational space in the vicinity of the Greenwald limit has been successfully investigated. In contrast to the power independent Greenwald scaling, the disruptive L-mode density limit in terms of global parameters has been found to increase moderately with heating power: n-barDLe ∝ P0.3heat. In terms of local edge parameters the dependencies become significantly stronger. Additionally, the Greenwald limit could be exceeded by up to 40%. Since the H-mode density limit is defined by the non-disruptive H → L-mode transition, the transition threshold has been studied for radiative mantle and/or very high recycling discharges. For impurity seeded plasmas the generally accepted threshold scaling P ∝ n-bareBt with vanishing hysteresis is a good approximation. Very high recycling discharges are, however, characterized by a practically power independent threshold. In the vicinity of the Greenwald limit all discharges revert to the L-mode. In the gas refueled H-mode the Greenwald limit could not be exceeded. However, controlled pellet injection into H-mode plasmas yields phases with n-bare approx. 1.5 n-barGreenwalde. (author). 18 refs, 9 figs