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

Integration of a pulsed plasma thruster (PPT) onboard spacecraft requires t he evaluation of potential plume/spacecraft interactions that can be determ ined through plume modeling and characterization. A PPT plume model, numeri cal results, and comparisons with experiments are presented. The physical c haracteristics of PPT plumes are reviewed first, and the outstanding modeli ng issues related to the unsteady, partially ionized, collisional PPT plume plasma are presented. The PPT plume model is axisymmetric and based on a h ybrid particle-fluid approach. Neutrals and ions are modeled with a combina tion of the direct simulation Monte Carlo and a hybrid-particle-in-cell met hod. Electrons are modeled as a massless fluid with a momentum equation tha t includes electric fields, pressure gradients, and collisional contributio ns from ions and neutrals. The nontime-counter methodology is used for neut ral-neutral, elastic ion-neutral, and charge-exchange collisions. Ion-elect ron collisions are modeled with the use of a collision force field. Electri c fields are obtained from a charge conservation equation under the assumpt ion of quasi neutrality. The code incorporates subcycling for the time inte gration and unsteady particle injection. Simulations are performed using PP T conditions representative of a NASA John H. Glenn Research Center at Lewi s Field laboratory-model PPT operating at discharge energies of 5, 20, and 40 J. The results demonstrate the expansion of the neutral and ion componen ts of the plasmoid during a pulse, the generation of low-energy ions and hi gh-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.