On Ohm's law in reduced plasma fluid models

TL;DR

This paper examines the formulation of Ohm's law in drift-reduced MHD models, highlighting the importance of including electromagnetic and electron mass effects for accurate plasma dynamics simulations.

Contribution

It clarifies the conditions under which Ohm's law yields physically consistent dispersion relations in reduced plasma models.

Findings

Including electron pressure requires electromagnetic and electron mass effects for correct dispersion.

A modified plasma vorticity improves simulation of low density plasma regions.

The study impacts numerical modeling of magnetised plasma phenomena.

Abstract

Drift-reduced MHD models are widely used to study magnetised plasma phenomena, in particular for magnetically confined fusion applications, as well as in solar and astrophysical research. This letter discusses the choice of Ohm's law in these models, the resulting dispersion relations for the dynamics parallel to the magnetic field, and the implications for numerical simulations. We find that if electron pressure is included in Ohm's law, then both electromagnetic and finite electron mass effects must also be included in order to obtain physical dispersion relations. A simple modification to the plasma vorticity is also found which improves handling of low density regions, of particular relevance to the simulation of the boundary region of magnetised plasmas.