Drive of parallel flows by turbulence and large-scale E × B transverse transport in divertor geometry

Poloidal asymmetries of parallel flows in edge plasmas are investigated by the 3D fluid turbulence code TOKAM3X. A diverted COMPASS-like magnetic equilibrium is used for simulations. Measurements and simulations of parallel Mach numbers are compared and exhibit good qualitative agreement. Small-scale turbulent transport is observed to dominate near the Low Field Side midplane, even though co-existing with significant large-scale cross-field fluxes. Despite the turbulent nature of the plasma in the divertor region, simulations show a low effectiveness of turbulence for the cross-field transport towards the Private Flux region. Nevertheless, a complex pattern of fluxes associated to the average field components, are found to cross the separatrix in the divertor region. Large-scale and small-scale turbulent E × B transport, along with the grad(B) drift, drive the asymmetries in parallel flows. A semi-analytical model based on mass and parallel momentum balances allows one to evaluate the poloidal drifts effects on the asymmetry pattern. As in experiments, a reversed B_T simulation provides one a way to separate self-consistently the effects of turbulent transport and the large-scale flows, that must reverse for a reversed field. The large-scale contribution is found to be responsible for typically the 50% of the effect on the Mach number, evaluated at the top of the machine. The presented picture shows a complex interplay between drifts and turbulence, underlining the necessity of a global approach in the edge plasma modelling including the self-consistent turbulence description.