Particle collisionality in scaled kinetic plasma simulations

TL;DR

This paper introduces a scaling method for particle collisionality in kinetic plasma simulations that use artificial parameters, enabling more accurate modeling of collisional effects without increasing computational costs.

Contribution

The authors develop a species-dependent collisionality scaling technique for PIC simulations that preserves key transport and relaxation properties of weakly collisional plasmas.

Findings

Accurately reproduces relaxation rates compared to theoretical predictions

Maintains electron and ion transport properties in scaled simulations

Easily integrable into existing PIC frameworks

Abstract

Kinetic plasma processes, such as magnetic reconnection, collisionless shocks, and turbulence, are fundamental to the dynamics of astrophysical and laboratory plasmas. Simulating these processes often requires particle-in-cell (PIC) methods, but the computational cost of fully kinetic simulations can necessitate the use of artificial parameters, such as a reduced speed of light and ion-to-electron mass ratio, to decrease expense. While these approximations can preserve overall dynamics under specific conditions, they introduce nontrivial impacts on particle collisionality that are not yet well understood. In this work, we develop a method to scale particle collisionality in simulations employing such approximations. By introducing species-dependent scaling factors, we independently adjust inter- and intra-species collision rates to better replicate the collisional properties of the physical system. Our approach maintains the fidelity of electron and ion transport properties while preserving critical relaxation rates, such as energy exchange timescales, within the limits of weakly collisional plasma theory. We demonstrate the accuracy of this scaling method through benchmarking tests against theoretical relaxation rates and connecting to fluid theory, highlighting its ability to retain key transport properties. Existing collisional PIC implementations can be easily modified to include this scaling, which will enable deeper insights into the behavior of marginally collisional plasmas across various contexts.