Fluid Models from Kinetic Models using a Geometric Averaging Procedure

TL;DR

This paper demonstrates that under certain conditions, kinetic models of charged particles can be effectively approximated by fluid models using a geometric averaging approach, especially in ultra-relativistic regimes.

Contribution

It introduces a geometric averaging method to derive fluid models from kinetic equations without additional assumptions on higher moments.

Findings

Charged cold fluid models are valid for narrow, ultra-relativistic particle distributions.

The method does not require assumptions on higher moments of the distribution.

The approach can be extended to warm plasmas and other plasma models.

Abstract

We interpret the Lorentz force equation as a geodesic equation associated with a non-linear connection. Using a geometric averaging procedure, we prove that for narrow and smooth one-particle distribution functions whose supports are invariant under the flow of the Lorentz equation, a bunch of charged point particles can be described by a charged cold fluid model in the ultra-relativistic regime. The method used to prove this result does not require additional hypotheses on the higher moments of the distribution. This is accomplished by estimating the expressions that include the differential operators appearing in the charged cold fluid model equation. Under the specified conditions of narrowness and ultra-relativistic dynamics, it turns out that these differential expressions are close to zero, justifying the use of the charged cold fluid model. The method presented in the work can also be applied to justify the use of warm plasmas and other models.