A drift kinetic model for the expander region of a magnetic mirror

TL;DR

This paper develops a drift kinetic model for plasma expansion in magnetic mirror devices, capturing trapped and passing particles, and validates it with kinetic simulations, relevant for space propulsion and fusion environments.

Contribution

Introduces a comprehensive drift kinetic model for plasma expansion in magnetic mirrors, including trapped/passing particles and fast ions, validated against kinetic simulations.

Findings

Model accurately predicts ambipolar potential drop.

Good agreement with kinetic simulation data.

Extends to include neutral beam heated ions.

Abstract

We present a drift kinetic model for the free expansion of a thermal plasma out of a magnetic nozzle. This problem relates to plasma space propulsion systems, natural environments such as the solar wind, and end losses from the expander region of mirror magnetically confined fusion concepts such as the Gas Dynamic Trap. The model incorporates trapped and passing orbit types encountered in the mirror expander geometry and maps to an upstream thermal distribution. This boundary condition and quasineutrality require the generation of an ambipolar potential drop of $\sim5 T_e/e$, forming a thermal barrier for the electrons. The model for the electron and ion velocity distributions and fluid moments is confirmed with data from a fully kinetic simulation. Finally, the model is extended to account for a population of fast sloshing ions arising from neutral beam heating within a magnetic mirror, again resulting in good agreement with a corresponding kinetic simulation.