Energetic particle diffusion and convection interface in NUBEAM with initial value simulations

A numerical scheme to map 2D diffusion and convection coefficients for energetic particle transport in energy and minor radius to the constant of motion (COM) space is proposed for interfacing the NUBEAM guiding center simulations with different initial value models. Two possibilities are considered: in the first, the fine grid probability density function characterized by parameters that define diffusion and convection on each grid bin; in the second, a straightforward interpolation for the COM variables of interest, canonical momentum and energy, from the diffusion and convection values on a COM grid is proposed. Verification exercises using analytic representation of 2D diffusion coefficients and convective velocities are proposed for the NUBEAM code aiming at the time scales much longer than the characteristic precession motion times. The diffusion/convection kick steps are chosen to allow sufficient diffusion over the time, △t, much longer than their transit steps, τ_pr, and are examined in this work for the DIII-D test where △t » τ_pr≈0.5μsec.