Resolving velocity space dynamics in continuum gyrokinetics

TL;DR

This paper introduces diagnostics for assessing velocity space resolution in gyrokinetic simulations, demonstrating that low collisionality allows accurate plasma modeling with fewer velocity grid points and proposing an adaptive collision frequency method.

Contribution

It presents new diagnostics for velocity space resolution and an adaptive collision frequency implementation in continuum gyrokinetics, enhancing simulation efficiency and accuracy.

Findings

Low collisionality enables fewer velocity grid points for accurate results

Diagnostics effectively monitor velocity space resolution in simulations

Adaptive collision frequency improves resolution in collisionless regimes

Abstract

Many plasmas of interest to the astrophysical and fusion communities are weakly collisional. In such plasmas, small scales can develop in the distribution of particle velocities, potentially affecting observable quantities such as turbulent fluxes. Consequently, it is necessary to monitor velocity space resolution in gyrokinetic simulations. In this paper, we present a set of computationally efficient diagnostics for measuring velocity space resolution in gyrokinetic simulations and apply them to a range of plasma physics phenomena using the continuum gyrokinetic code GS2. For the cases considered here, it is found that the use of a collisionality at or below experimental values allows for the resolution of plasma dynamics with relatively few velocity space grid points. Additionally, we describe implementation of an adaptive collision frequency which can be used to improve velocity space resolution in the collisionless regime, where results are expected to be independent of collision frequency.