Test particle simulation of non-ambipolar ion diffusion in tokamaks

The neoclassical return current and parallel viscosity due to the radial electric field (non-equilibrium poloidal rotation) in tokamaks are calculated using the 5-D (3-D in configuration space and 2-D in velocity space) Monte Carlo code ASCOT. Results are compared with the known neoclassical non-ambipolar return current expressions by Shaing [Phys. Fluids B 2 (1990) 2847] and Stringer [Nucl. Fusion 33 (1993) 1249] over a wide range of poloidal Mach numbers M p = E r /(v ti B θ ) and for various collisionalities. Here, v ti is the ion thermal velocity, E r is the radial electric field and B θ is the poloidal magnetic field. Quantitative differences exist due to a simpler collision operator and the various other approximations used in deriving the analytic expressions.