Test particle modelling of ion collisional transport in tokamaks

A numerical method is presented for evaluation of the radial diffusion coefficient, based on the full orbit modelling of particle orbits in the tokamak geometry. The code solves the full orbit equations of motion for a set of test particles in an arbitrary equilibrium magnetic field. The effect of Coulomb collisions of test ions with background plasma particles is simulated by means of an equivalent Monte Carlo collision operator which scatters the pitch angle as well as the gyrophase of the particle. The radial diffusion coefficient is estimated by calculating the temporal dependence of the mean-square displacement of an ensemble of monoenergetic test particles. As an illustration of the method the effect of magnetic islands on the impurity collisional transport is studied. It is shown that in presence of m = 2, n = 1 resonant magnetic perturbation (RMP) the diffusion coefficient for the tungsten ions (W28+, E = 1 keV) can increase by a factor of 5-10.