Turbulence in the near scrape-off layer towards the L-mode density limit in ASDEX-Upgrade

Abstract

The L-mode density limit in ASDEX Upgrade is investigated with focus on the scrape-off layer. We observe a chain of events that involves the collapse of the E×B outer shear layer (just outside the separatrix), widening of the electron pressure fall-off length, outer divertor detachment, density shoulder formation, reduction and flattening of the parallel Mach number, and enhanced turbulent cross-field transport. The latter effect was associated with higher amplitude density fluctuations at low frequencies, where the turbulent cross-phase between density and velocity fluctuations pointing radially outward was favourable.

Keywords

• Turbulence
• Scrape-off layer
• Electrostatic probes
• Tokamaks
• Density limit
• Detachment