Drift-kinetic studies of neoclassical poloidal rotation with finite orbits

A significant discrepancy of poloidal velocity from conventional theoretical predictions is found in global neoclassical drift-kinetic simulations of magnetic confinement fusion devices. The difference is identified as being due to the presence of large ion orbits. In the case of a large aspect ratio tokamak configuration with steep toroidal flow profiles, a novel heuristic model which estimates this nonlocal effect is presented and shown to be in good agreement with simulation results. The dominant nonlocal mechanisms captured by the model are associated with ion parallel flow modification due to the steep toroidal flow and radial electric field profiles. We compare simulation results with theoretical estimates based on the new model using profiles relevant for the National Spherical Torus Experiment. The carbon poloidal velocity observed in the simulation is in good agreement with the neoclassical theory modified by the newly identified nonlocal effects.