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

D Galassi (1) , Patrick Tamain (2) , Hugo Bufferand (2) , G Ciraolo (2) , Philippe Ghendrih (2) , C Baudoin (2) , C Colin (1) , Nicolas Fedorczak (2) , N Nace (2) , E Serre (1)

1 M2P2 - Laboratoire de Mécanique, Modélisation et Procédés Propres
2 IRFM - Institut de Recherche sur la Fusion par confinement Magnétique

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.

Mots clés: Poloidal asymmetries; Turbulence; Divertor geometry; Large-scale flows

Domaines: Mécanique des fluides [physics.class-ph]; Physique des plasmas [physics.plasm-ph]

D Galassi, Patrick Tamain, Hugo Bufferand, G Ciraolo, Philippe Ghendrih, et al.. Drive of parallel flows by turbulence and large-scale E × B transverse transport in divertor geometry. 2016. ⟨hal-01398227⟩