Impact of Energetic Electrons on Electric Propulsion Beam Expansion: Grid-Based Vlasov Simulations*

Plasma beam expansion is a fundamental problem in the field of electric propulsion (EP), traditionally approached by considering electrons as an equilibrium charge-neutralizing fluid governed by assumed thermodynamic relations, such as isothermal and polytropic behaviors. However, kinetic particle-in-cell (PIC) simulations have revealed significant discrepancies in plume properties resulting from this hybrid treatment. Therefore, a more accurate model of electrons is needed, necessitating a deep understanding of both microscopic electron kinetics and macroscopic thermodynamics. Recently, Cui and Wang investigated the effects of electron Velocity Distribution Function (VDF) skewness and electron trapping on collisionless electron heat flux using the grid-based Vlasov method, assuming initial ions and electrons following a Maxwellian distribution. However, in practical engineering applications, electrons may exhibit non-Maxwellian properties, characterized by an enhanced high-energy tail that can significantly impact the electron collisionless heat flux. Consequently, studying the effects of non-Maxwellian electron distributions becomes crucial. In this study, we employ the grid-based Vlasov method to examine the thermodynamics and kinetics of electrons in the beam expansion process. We consider various initial electron distribution functions with differing degrees of enhanced high-energy tails. Through this investigation, we aim to provide valuable insights that could enhance the future design of electron collisionless fluid models.