Electron Properties in Collisionless Mesothermal Plasma Expansion: Fully Kinetic Simulations

This paper presents a fully kinetic particle-in-cell simulation of collisionless mesothermal plasma expansion with a focus on the macroscopic electron properties. The results show that the electron thermal properties are anisotropic and nonuniform. In the beam core region, the electrons are thermalized due to interactions between the trapped electrons and the potential well between the beam exit and the beam front. The electron expansion inside the beam is equilibrium and significant cooling occurs associated with the expansion. Immediately out of the beam boundary, the electrons undergo an isothermal expansion. In the outer expansion region, the electrons transition into a nonequilibrium expansion and again exhibit significant cooling. We find that the commonly used assumption of Boltzmann electron simplification is not valid for modeling electrons in mesothermal plasma expansion. An analysis of the electron temperature and density correlation along each individual ion streamlines suggests that simplified models for electron dynamics may be constructed using multiple polytropic laws in different plume regions.