A Self-Consistent Open Boundary Condition for Fully Kinetic Plasma Thruster Plume Simulations

A new, charge-conserving, energy-based, open boundary condition (BC) is developed and used to perform steady-state plasma plume simulations. This BC prevents the numerical instability, which is typically observed for kinetic simulations that employ the traditional outflow boundary, and thus allows the fully kinetic plume simulations to reach a steady-state with finite computational domain sizes. In addition to modeling a plume with colocated electron-ion sources, the BC is also applied to separated electron-ion sources which model realistic thrusters with a separate external neutralizer configuration without assuming quasi-neutrality. Domain-independence simulations performed with the new open boundary construct showed that the electric field obtained from small and large domain sizes agree within 2%. For the separated electron-ion plume, the minimum domain size required to obtain convergence is such that it must accommodate the plume until the charge density decreases by an order of magnitude. The computational cost of the electrostatic particle-in-cell simulation was found to decrease by a factor of four when the domain size was decreased by one-half.