A new experimental classification of divertor detachment in ASDEX Upgrade

TLDR

The study introduces a new experimental classification of divertor detachment in ASDEX Upgrade and examines how an externally applied magnetic perturbation field influences the detachment process. The authors performed ohmic and L‑mode density ramp discharges across varied heating powers, magnetic field directions, and plasma species, measuring divertor electron density and volume recombination with spectroscopy, and investigated the impact of an external magnetic perturbation field. Detachment proceeds through three distinct states rather than a continuous evolution, with inner and outer divertor characteristics strongly coupled, and radiative fluctuations appear in the inner divertor near the X‑point before full detachment, as detected by fast diode bolometers.

Abstract

In this paper, a new experimental classification of divertor detachment in ASDEX Upgrade is presented. For this purpose, a series of ohmic and L-mode density ramp discharges at different heating powers, magnetic field directions and plasma species were carried out. For the first time at ASDEX Upgrade the electron density in the divertor volume and the occurrence of volume recombination were measured by means of spectroscopy. It is shown that detachment is not a continuously evolving process but rather undergoes three distinct states while the characteristics of the inner and outer divertor are strongly coupled. Before the complete detachment of the inner and outer divertor, radiative fluctuations occur in the inner divertor close to the X-point, observed for the first time via new fast diode bolometers. Finally, the effect of an externally applied magnetic perturbation field on the detachment process is investigated.