On the accuracy of the binary-collision algorithm in particle-in-cell simulations of magnetically confined fusion plasmas

TL;DR

This paper investigates how the accuracy of binary-collision algorithms in particle-in-cell simulations of magnetically confined fusion plasmas is affected by collision cell size and field fluctuations, proposing potential improvements.

Contribution

It provides a numerical analysis of collision algorithm accuracy under large cell sizes and fluctuating fields, and suggests minor modifications for better conservation.

Findings

Accuracy decreases with larger collision cells.

Field fluctuations impact conservation properties.

Minor algorithm modifications can improve accuracy.

Abstract

Ideally, binary-collision algorithms conserve kinetic momentum and energy. In practice, the finite size of collision cells and the finite difference in the particle locations affect the conservation properties. In the present work, we investigate numerically how the accuracy of these algorithms is affected when the size of collision cells is large compared with gradient scale length of the background plasma, a parameter essential in full-f fusion plasma simulations. Additionally, we discuss implications for the conserved quantities in drift-kinetic formulations when fluctuating magnetic and electric fields are present: we suggest how the accuracy of the algorithms could potentially be improved with minor modifications.