Hybrid (particle-fluid) modeling of pulsed plasma thruster plumes

Integration of a pulsed plasma thruster (PPT) onboard spacecraft requires the evaluation of potential plume/spacecraft interactions that can be determined through plume modeling and characterization. A PPT plume model, numerical results, and comparisons with experiments are presented. The physical characteristics of PPT plumes are reviewed first, and the outstanding modeling issues related to the unsteady, partially ionized, collisional PPT plume plasma are presented. The PPT plume model is axisymmetric and based on a hybrid particle-fluid approach. Neutrals and ions are modeled with a combination of the direct simulation Monte Carlo and a hybrid-particle-in-cell method. Electrons are modeled as a massless fluid with a momentum equation that includes electric fields, pressure gradients, and collisional contributions from ions and neutrals. The nontime-counter methodology is used for neutral-neutral, elastic ion-neutral, and charge-exchange collisions. Ion-electron collisions are modeled with the use of a collision force field. Electric fields are obtained from a charge conservation equation under the assumption of quasi neutrality. The code incorporates subcycling for the time integration and unsteady particle injection. Simulations are performed using PPT conditions representative of a NASA John H. Glenn Research Center at Lewis Field laboratory-model PPT operating at discharge energies of 5, 20, and 40 J. The results demonstrate the expansion of the neutral and ion components of the plasmoid 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. Numerical predictions are compared with unsteady plume electron density data and show good quantitative agreement. Backflow predictions are presented for the three discharge energy levels considered.