Neoclassical transport in strong gradient regions

Regions of strong gradients in tokamaks such as the pedestal and internal transport barriers form because turbulence is somehow suppressed. As a result, neoclassical transport becomes important but standard approaches to neoclassical theory are not valid if the gradients are sufficiently large. We have previously presented a neoclassical ion formulation that allows for gradient length scales of density, temperature, and electric potential of the order of the ion poloidal gyroradius, but simultaneously keeps orbit widths small in inverse aspect ratio (Trinczek et al. 2023). The theory captures finite poloidal gyroradius effects such as strong poloidal variation. We have extended the formulation to electrons. We have also studied how the parallel flow is determined. In the limit of interest where the ion neoclassical particle flux is small, asymmetries in phase space introduced by the strong radial electric field lead to a new mechanism of parallel momentum damping that sets the mean flow. The radial electric field can then be determined self-consistently via quasineutrality.