Comment on "Adiabatic Expansion of Electron Gas in a Magnetic Nozzle" by Kazunori Takahashi, Christine Charles, Rod Roswell, and Akira Ando, Phys. Rev. Lett. Vol. 120, 045001 (2018)

TL;DR

This paper discusses the thermodynamic properties of partially ionized plasmas and explores the potential of Alfvén waves for plasma heating in magnetic nozzle propulsion systems.

Contribution

It provides a detailed analysis of heat capacities and polytropic indices in ionized plasmas and introduces the concept of using Alfvén waves for plasma heating in propulsion.

Findings

Heat capacities vary with ionization and temperature.

Polytropic index is not constant in partially ionized plasma.

Alfvén waves could be effective for plasma heating.

Abstract

Heat capacities at fixed volume and pressure as a function of the degree of ionization are graphically depicted as functions of reciprocal temperature and ionization degree. The polytropic index is calculated as a function of the same variables; as it is not constant a partially ionized plasma can be only approximately polytropic fluid. In parallel, it is launched the idea that Alfv\'en waves can be used to heat the plasma in a propulsion jet with magnetic nozzle.