Progress in Development of Modeling Capabilities for a micro-Pulsed Plasma Thruster

In this work we report on progress in the development of models for a Micro-Pulsed Plasma Thruster (μPPT) and its exhaust plume. As a working example we consider a μPPT developed at the Air Force Research Laboratory. This is a miniaturized design of the axisymmetric PPT with a thrust in the 10 μN range that utilizes Teflon as a propellant. The plasma plume is simulated using a hybrid fluid-PIC-DSMC approach. The plasma plume model is combined with Teflon ablation and plasma generation models that provide boundary conditions for the plume. This approach provides a consistent description of the plasma flow from the surface into the near plume. The magnetic field diffusion into the plume region is also considered and plasma acceleration by the electromagnetic mechanism is studied. Teflon ablation and plasma generation analyses show that the Teflon surface temperature and ablation rate are strongly non-uniform in the radial direction and have a maximum near the central electrode. As a result the propellant surface has the form of a cone with an apex at the central electrode. Analysis predictions for the ablation depth as well as the ablation profile shows close correspondence to experimentally observed dependencies. Longterm operation is associated with propellant and central electrode recession. To this end a model of the plasma flow inside of the anode tube is developed. Some preliminary results are reported. Electron and neutral densities predicted by the plume model are compared with near field plume measurements using a two-color interferometer and good agreement is obtained.