Computational study of tokamak divertor recycling

A two-dimensional coupled plasma-neutral fluid transport computational model is used to examine divertor recycling. Realistic magnetic flux surface geometries and recycling between the edge and core plasma regions are included. Results are compared for several divertor cases to minimize the influence of uncertain anomalous radial transport. Time dependent studies show that a characteristic time to establish recycling is typically tens of milliseconds, determined by particle recycling across the separatrix. The results show that achieving high recycling solutions (high density and low divertor plasma temperature) requires two conditions: first, that xd > λ0, where xd is the target to separatrix distance and λ0 is the neutral mean free path; and second, that the ratio of the heat flux to particle flux across the separatrix (core boundary) Qbc/Γbc be below a critical value. The latter condition makes extrapolation to the fusion reactor regime appear difficult. High recycling solutions can drive parallel flow away from the target in large regions of the edge plasma, which means flow reversal. Poloidal flow is dominated by a large drift flux near the separatrix. Poloidal gradients in electrostatic potential are significant. It is speculated that the electric fields and currents combined with the large poloidal flow contribute to fluctuations observed in the edge plasma