A conservative, implicit solver for 0D-2V multi-species nonlinear Fokker-Planck collision equations

TL;DR

This paper introduces a novel implicit algorithm for solving multi-species nonlinear Fokker-Planck equations that conserves physical quantities with high accuracy, suitable for modeling weakly anisotropic plasmas.

Contribution

The paper presents a new implicit solver using Legendre and King function expansions, ensuring conservation and high precision in solving complex collision equations.

Findings

High accuracy in weakly anisotropic plasma simulations

Convergence achieved with Legendre and King expansions

Superior performance compared to existing methods

Abstract

In this study, we present an optimal implicit algorithm specifically designed to accurately solve the multi-species nonlinear 0D-2V axisymmetric Fokker-Planck-Rosenbluth (FPR) collision equation while preserving mass, momentum, and energy. Our approach relies on the utilization of nonlinear Shkarofsky's formula of FPR (FPRS) collision operator in the spherical-polar coordinate. The key innovation lies in the introduction of a new function named King, with the adoption of the Legendre polynomial expansion for the angular coordinate and King function expansion for the speed coordinate. The Legendre polynomial expansion will converge exponentially and the King method, a moment convergence algorithm, could ensure the conservation with high precision in discrete form. Additionally, post-step projection onto manifolds is employed to exactly enforce symmetries of the collision operators. Through solving several typical problems across various nonequilibrium configurations, we demonstrate the high accuracy and superior performance of the presented algorithm for weakly anisotropic plasmas.