A Quasi-Neutral Full PIC Model for the Study of Electron Thermodynamics in a Plasma Thruster Plume*

Classical electron fluid formulations for the simulation of electric propulsion plumes employ the simplification of a diagonal and isotropic temperature tensor. However, experimental and kinetic numerical data point to the presence of relevant temperature anisotropies. To overcome this limitation, a 2D 3V full PIC model has been developed, in which ion and electron particles are moved in a potential map obtained by assuming quasi-neutrality and solving the inertialess electron momentum and continuity equations. To account for an anisotropic temperature tensor, a revisited definition of the thermalized potential is used, permitting a reduced noise in the solution for the potential. The quasi-neutrality assumption allows to overcome the constraint of the discretization of the Debye length and plasma frequency typical of classical full PIC codes, largely reducing the otherwise prohibitive computational costs of full-scale plume simulations. The model is applied to the case of a plasma thruster plume expanding into vacuum; and is used to evaluate the accuracy and limitations of an anisotropic polytropic closure. The simulation results show that an anisotropic formulation of the polytropic closure is fundamental to capture the evolution of the electron temperature tensor in a plume; and optimal values of the global polytropic coefficients are obtained through the comparison with the kinetic electron population.