An asymptotic-preserving 2D-2P relativistic Drift-Kinetic-Equation solver for runaway electron simulations in axisymmetric tokamaks

We propose an asymptotic-preserving (AP), uniformly convergent numerical scheme for the relativistic collisional Drift-Kinetic Equation (rDKE) to simulate runaway electrons in axisymmetric toroidal magnetic field geometries typical of tokamak devices. The approach is derived from exact Green's function solution with approximations quantifiable impact, and results a simple, two-step operator-split algorithm, consisting Eulerian step, Lagrangian orbit-integration step analytically prescribed kernels. AP character demonstrated by analysis dominant errors, as well experiments. demonstrate ability algorithm provide accurate answers regardless plasma collisionality on circular geometry.