An experimental investigation on the high density transition of the Scrape-off Layer transport in ASDEX Upgrade

TL;DR

This study investigates the high-density transition in the Scrape-off Layer of ASDEX Upgrade, revealing a regime change in filament behavior and transport mechanisms that are crucial for future fusion reactors like ITER and DEMO.

Contribution

It provides experimental evidence linking filament dynamics and transport regime changes to the density shoulder formation in the SOL, supported by multiple diagnostic techniques.

Findings

Filament speed and size increase at the transition.

Flattening of SOL density and heat flux profiles.

Transition from conduction to convection-dominated transport.

Abstract

A multidiagnostic approach, utilizing Langmuir probes in the midplane, X-point and divertor walls, along with Lithium beam and infrared measurements is employed to evaluate the evolution of the Scrape-off Layer (SOL) of ASDEX Upgrade across the L-mode density transition leading to the formation of a density shoulder. The flattening of the SOL density profiles is linked to a regime change of filaments, which become faster and larger, and to a similar flattening of the $q_{\parallel}$ profile. This transition is related to the beginning of outer divertor detachment and leads to the onset of a velocity shear layer in the SOL. Experimental measurements are in good agreement with several filament models which describe the process as a transition from conduction to convection-dominated SOL perpendicular transport caused by an increase of parallel collisionality. These results could be of great relevance since both ITER and DEMO will feature detached divertors and densities largely over the transition values, and might therefore exhibit convective transport levels different to those observed typically in present-day devices.