Power and momentum removal in the Scrape-Off Layer of ASDEX Upgrade

Abstract

Dedicated experiments in ASDEX Upgrade (AUG) and interpretative studies of these experiments with the edge fluid code SOLPS have been carried out to investigate the relative importance of underlying physical processes of divertor transport and exhaust of momentum and power in tokamak plasmas in AUG. Analysis of pressure conservation in AUG H-mode plasmas shows that, at low electron target temperatures, the momentum losses in the scrape-off layer (SOL) near the separatrix are up to two orders of magnitude stronger compared to the momentum losses in the far SOL in flux tubes with the same target temperature. In L-mode-like and H-mode-like SOLPS 5.0 simulations, the momentum losses are up to two orders of magnitude weaker than experimentally observed at target temperatures below Te,t < 5 eV, and the dependence of the losses on the distance to the separatrix of the flux tube is only significant in the vicinity the separatrix. The L-mode-like simulations show that the impact of ion-atom charge-exchange on momentum transfer is larger than the impact of ion-molecule elastic scattering for the range of temperatures and plasma conditions observed in the SOL of AUG. Experiments and SOLPS-ITER simulations of the upper open divertor in AUG show that the net effect of increasing the core plasma density or the plasma current on the heat loads onto the targets strongly depends on the toroidal field direction. Within experimental and computational uncertainties, SOLPS-ITER predicted trends of the peak heat flux density at the targets with increasing core plasma density and plasma current are in quantitative agreement with the experimental data. Within the divertor region, these simulations show that the radial energy transport due to cross-field drifts is the primary component of the total radial transport contribution to energy transport. In L-mode-like simulations, the neutral model, the divertor closure, the boundary conditions and the SOLPS version do not significantly impact the target to upstream ratio of the total plasma pressure, ptot, t/ptot, u within the outer divertor, except for low temperatures (Te,t < 3 eV) in the vicinity of the separatrix. In the vicinity of the separatrix, drifts have a stronger impact on plasma momentum losses than the neutral model and divertor closure.