Characterization of the fluctuating detachment state in ASDEX Upgrade

In this article we present experimental investigations of divertor detachment in ASDEX Upgrade. For this purpose we performed a series of ohmic and L-Mode density ramp discharges. The electron density in the divertor volume is determined with a spectroscopic measurement of the Stark broadened D-epsilon line shape. These measurements are combined with several other diagnostics, yielding a consistent picture of detachment. It will be shown that detachment in ASDEX Upgrade is not a continuously evolving process but can be divided into three different states. Within these states, the conditions in the inner and outer divertor are strongly coupled. Radiative fluctuations, measured with fast diode bolometers, combined with a region of high electron density appear at the X-point during one of these states. We show how N-2 seeding during this state alters the divertor conditions. Finally an unstable situation was found, where the divertor plasma oscillates between two detachment states. (C) 2013 Elsevier B.V. All rights reserved.