X-point potential well formation in diverted tokamaks with unfavorable magnetic field direction

Fluid simulations of the boundary of fusion plasmas predict the formation of an electric potential well in the vicinity of the X-point in detached divertor conditions with B-t in the unfavorable direction for H-mode access. This potential well arises when the parallel current in the divertor is dominated by Pfirsch-Schluter currents and is closely related to previously reported potential hill formation in favorable B-t direction. A simple analytic model describes its dependence on plasma shape and divertor conditions. The poloidal particle transport in the divertor is dominated by the parallel flow, while cross-field particle transport in the vicinity of the separatrix is argued to be E x B-dominated, even in the presence of turbulence. With a potential well, the E x B-flow differs qualitatively from the classic drift pattern with the near-SOL poloidal E x B-flux enhanced and reversed while radially widening/compressing the outer/inner divertor leg, respectively.