Linearized model Fokker-Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests

TL;DR

This paper introduces a numerically implemented gyrokinetic collision operator that accurately models collisional effects across all regimes while preserving key physical properties, enhancing simulation fidelity.

Contribution

It presents a new collision operator based on the linearized test-particle model with conservation laws, implemented in GS2, capturing physics across all collisionalities without artificial dissipation.

Findings

Operator preserves conservation laws and H-Theorem.

Accurately models physics from collisionless to collisional regimes.

Implementation in GS2 is fully implicit and numerically stable.

Abstract

A set of key properties for an ideal dissipation scheme in gyrokinetic simulations is proposed, and implementation of a model collision operator satisfying these properties is described. This operator is based on the exact linearized test-particle collision operator, with approximations to the field-particle terms that preserve conservation laws and an H-Theorem. It includes energy diffusion, pitch-angle scattering, and finite Larmor radius effects corresponding to classical (real-space) diffusion. The numerical implementation in the continuum gyrokinetic code GS2 is fully implicit and guarantees exact satisfaction of conservation properties. Numerical results are presented showing that the correct physics is captured over the entire range of collisionalities, from the collisionless to the strongly collisional regimes, without recourse to artificial dissipation.