Existence of a detachment cliff at ASDEX Upgrade

Abstract

The detachment cliff is a bifurcative transition to partial detachment recently discovered at the DIII-D tokamak. This work presents a database analysis of target parameters in L-mode and H-mode discharges to search for a detachment cliff at ASDEX Upgrade (AUG). Most of the transitions from attached to partially detached divertor conditions observed in H- and L-mode discharges in AUG show bifurcative-like characteristics that are consistent with the properties of the detachment cliff if the Bx∇B drift is directed towards the active X-point. In the operational space of power and density, the bifurcative transitions identified during an L-mode discharge occur at injected power and density higher than a threshold value Ptot > 0.7 MW and ne > 1.6 x 1019 m-3, respectively). Furthermore, the temperatures at which the transitions start are found to be insensitive to the injected impurity, to the injected power and to the value of the upstream density. Finally, the study of the evolution of the target parameters, of the intensity of the Dα line and of specific manometers and bolometer lines of sights shows that the physical process underlying the detachment cliff and the self-sustained divertor oscillations might be the same.