Two-dimensional dynamics of divertor detachment at ASDEX Upgrade

A newly installed thermal helium beam diagnostic in the outer divertor of ASDEX Upgrade (Hörmann et al 2024 Rev. Sci. Instrum. 95 113507) was used for the first time to analyse two-dimensional (2D) electron temperature, density and pressure profiles. This diagnostic measures profiles with unprecedented spatial and temporal resolution, enabling the analysis of detachment transitions from onset state to fluctuating state in L-mode, including smooth transitions, detachment cliffs, and self-sustained divertor oscillations (SSDOs). This is a novelty for detachment studies, as such high-resolution 2D measurements have been lacking up to now, especially during the transitions between different divertor states. We focus on the pressure conservation and the behaviour of ne and Te along and perpendicular to the flux surfaces in the different phases of the transition. In addition, the propagation direction and velocity of the temperature collapse during SSDOs are investigated for the first time. They travel in the direction from the near scrape-off layer towards the far scrape-off layer and from the target towards the X-point at a velocity of a few tens of meters per second in the poloidal plane. This measurement is compared with the current models of SSDOs and detachment cliff generation.