Divertor plasma flow near the lower x-point in ASDEX Upgrade

A reciprocating probe in the lower divertor of ASDEX Upgrade, capable of accessing the low-field (LFS) and high-field side (HFS) scrape-off layers (SOLs) as well as the private flux region, was equipped with a Mach probe and used to measure flows in the vicinity of the lower x-point. We report on our measurements from ohmic and low-power H-mode discharges with ion B × ∇B drift towards the bottom x-point, and discuss their relevance to the current SOL/divertor flow understanding. In ohmic discharges, we present the evolution of divertor SOL and private flux flow profiles for increasing central 〈n_e〉. We show that the private flux flow is mainly directed from the HFS to the LFS at low densities. At medium-high densities the flow profile becomes more symmetric, and at very high densities the flow direction reverses on the LFS separatrix, having a LFS to HFS direction inside the private flux. We discuss the possible mechanisms that could affect divertor flows and produce such behaviour and conclude that pressure asymmetry between the two divertor legs combined with an E × B drift towards the inner divertor is a likely driving mechanism. At the HFS SOL, very large Mach numbers (typically exceeding M ≤ 1) were observed in most cases. In low-power H-mode discharges inter-ELM flows were observed to be very similar to ohmic ones.