The structure of the radial electric field in the vicinity of the separatrix and the L-H transition

The radial electric field in the vicinity of the separatrix and its shear is studied in detail by means of the B2-SOLPS5.0 transport code. Calculations are performed for various regimes of ASDEX Upgrade. The dependencies of the radial electric field on the local temperature, density, toroidal and poloidal magnetic fields and mean toroidal rotation are investigated. It is found that the radial electric field is of the order of the neoclassical electric field with the toroidal rotation contribution determined by the radial anomalous transport of the toroidal momentum. The parametric dependence of the calculated radial electric field coincides with that for the neoclassical electric field. A detailed analysis is performed of the fine structure of the electric field and of the parallel (toroidal) plasma flows in the separatrix vicinity. It is demonstrated that the parallel flow is transported from the SOL to the core and the distance where the flow pattern deviates from the neoclassical distribution is determined by the balance between the parallel viscosity and the force connected with the radial transport of the toroidal momentum. It is shown that in the case of the normal direction of the magnetic field, the shear of the electric field is larger than that for the case of the reversed magnetic field. This fact is explained by the different toroidal rotation contribution to the electric field. The obtained dependence of the electric field shear on the plasma parameters is consistent with the L-H transition threshold for ASDEX-Upgrade. The calculations predict a linear dependence of the heating power necessary to reach a given shear value in the separatrix vicinity, on the local density and the toroidal magnetic field. (orig.)