Modeling of ablation-fed pulsed plasma thrusters

Design of the proper power system for an improved ablation-fed pulsed plasma microthruster (PPT) requires the ability to specify amplitudes, risetimes, waveforms and pulsetimes in relation to propellant properties and thrust chamber dimensions. Numerical modeling techniques, such as the MACH2 code, along with analytical modeling, are needed to provide guidance. Stationary operation in the limit of high magnetic Reynolds number may be characterized by a discharge thickness adjacent to the surface that scales inversely with the exhaust speed, resulting in a resistance that is approximately constant for fixed surface proportions and material. Heat transfer to the surface occurs in a thinner layer within this discharge, and provides a mass flow rate proportional to the electrical power. The mass ablated per shot then scales as the stored electrical energy, in agreement with experiments over a wide range of operations. Startup and shut-down transients, (e.g., flux -vs thermal skin-depths), can significantly alter this simple behavior. A new ablation-physics routine has been added to the MACH2 code to examine the time-dependent operation of the PPT discharge. Comparisons of numerical results with data from the LES 6 PPT are encouraging, even with present, idealized models for Teflon plasma properties.