Theoretical modeling of ablation-fed, pulsed plasma thrusters (PPTs) with the MACH2 code has shown that after repeated pulsed operation, the total expelled mass is due to ablation during the discharge and solid decomposition that persists long after the pulse. The latter mass does not considerably contribute to the impulse-bit thus degrading thruster performance. For the rectangular PPT geometry, optimizing current waveforms in combination with channel widths are presented, that utilize all decomposed mass, electromagnetically. These waveforms are characterized by short rise times (<1 µsec) and prolonged decays (>25 µsec). Simplified modeling based on steady-state, one-dimensional flow reveals that the mass flow rate varies linearly with the square of the magnetic field and that the downstream flow speed is driven towards the Alfven wave speed when the magnetic pressure is much greater than the gasdynamic pressure. The model has been confirmed by MACH2. The mass flow requirement for such magnetosonic flow in turn, determines the surface temperature of the solid.
Numerical simulations of coaxial geometries show that, compared with the rectangular, annular and linear pinch configurations, only an arrangement which operates an inverse-pinch discharge offers the convenience of axisymmetry for better correlation between theory and experiment, and operation at relatively high magnetic fields with propellant temperatures below the decomposition limit. Design guidelines for an inverse pinch thruster are provided. The inverse-pinch discharge produced by a non-reversing, waveform that rises to 18 kAmps in 0.625 μsec and decays in 6 μsec, in a lcm-(propellant) radius thruster, is found to prevent solid decomposition while still providing ablated mass for accelaration. At these lower magnetic field levels (~0.4 T, maximum) it is found that thermal effects are driving the surface temperature of the solid, during the latter times of current decay.