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)

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.