Operational limits of ASDEX Upgrade H mode discharges in the new closed Divertor II configuration

Systematic investigations of gas fuelled, high density H mode discharges with the new closed Divertor II of ASDEX Upgrade are reported. The focus is on operational limits (H-L transition, density limit) over a wide range of externally controllable plasma parameters. It is found that the H mode threshold power dramatically exceeds the generally accepted prediction Pthreshheat µ [`n]eBt when high densities are approached. Highest densities (density limit) are achieved close to the H-L mode back-transition and are generally associated with complete divertor detachment. The scaling of the density limit is assessed and discussed with a view to detachment based models and the Greenwald limit.