Axisymmetric hybrid numerical modeling of pulsed plasma thruster plumes

Modeling of Pulsed Plasma Thruster (PPT) plumes is essential for the evaluation of potential plume/spacecraft interactions and for the successful integration of PPTs onboard spacecraft. Experimental PPT plume investigations are reviewed and the physical characteristics of the plume are established. The unsteady, partially ionized, collisional PPT plume plasma is modeled by a hybrid particle-fluid model implemented numerically in axisymmetric coordinates. Neutral and ion plume species are modeled with a combination of the Direct Simulation Monte Carlo (DSMC) method and a hybrid Particle-in-Cell (hybrid-PIC) method. Electrons are modeled as massless fluid particles with a momentum equation that includes terms due to electric fields, pressure gradients, as well as electron-ion and electron-neutral collisions. The model allows for a dual-grid structure to accommodate different ion and neutral length-scales, and incorporates sub-cycling for the efficient time-integration of neutral and ion equations of motion. The model includes an unsteady particle injection scheme at the thruster exit that incorporates PPT operational parameters. The Non-Time-Counter methodology is used for neutral-neutral, elastic ion-neutral, and charge exchange collisions. Ion-electron collisions are simulated using a collision force field. Electric fields are obtained from a charge conservation equation under the assumption of quasi-neutrality. Representative data of a NASA Glenn Research Center PPT operating at discharge energy levels of 5, 20 and 40 J are used as model input conditions. The simulation results demonstrate the expansion of the neutral and ion components of the plume during a pulse, the generation of low-energy ions and high-energy neutrals due to charge exchange reactions, and the generation of neutral and ion backflow. Model predictions of electron densities are compared with triple Langmuir probe data and show overall good qualitative agreement. Backflow is evaluated for the three discharge energy levels considered.