Kinetic Electron Cooling in Magnetic Nozzles: Experiments and Modeling

TL;DR

This paper reviews the physics of electron cooling in magnetic nozzles used in plasma thrusters, combining experimental and theoretical insights to understand energy balance and ion acceleration in these nearly collisionless plasmas.

Contribution

It provides a comprehensive review of kinetic electron cooling mechanisms in magnetic nozzles, highlighting recent kinetic modeling approaches and future research challenges.

Findings

Kinetic electron effects significantly influence plasma energy balance.

Electron cooling impacts ion acceleration and particle detachment.

Recent models improve understanding of plasma behavior in magnetic nozzles.

Abstract

As long-distance space travel requires propulsion systems with greater operational flexibility and lifetimes, there is a growing interest in electrodeless plasma thrusters that offer the opportunity of improved scalability, larger throttleability, running on different propellants, and limit device erosion. The majority of electrodeless designs rely on a magnetic nozzle (MN) for the acceleration of the plasma, which has the advantage of utilizing the expanding electrons to neutralize the ion beam without the additional installation of a cathode. The plasma expansion in the MN is nearly collisionless, and a fluid description of electrons requires a non-trivial closure relation. Kinetic electron effects, and in particular electron cooling, play a crucial role in various physical phenomena such as energy balance, ion acceleration, and particle detachment. Based on the experimental and theoretical studies conducted in recognition of this importance, the fundamental physics of the electron cooling mechanism revealed in MNs and magnetically expanding plasma are reviewed. Especially, recent approaches from the kinetic point of view are discussed, and our perspective on the future challenges of electron cooling and the relevant physical subject of MN is presented.