Plasma Plume Mass Characterization of a Mini-Pulsed Plasma Thruster

Abstract

The ablation-fed pulsed plasma thruster (PPT) is noted for its easily stored Teflon propellant and ability to provide small impulse bits at high specific impulse. A principle concern of the PPT is contamination from the exhaust products. This contamination could affect sensitive instruments and components of either the spacecraft to which the PPT is mounted or any spacecraft in close proximity. The PPT exhaust is composed primarily of the dissociated and ionized products of the Teflon (C2F4) propellant bar, as well as products from the erosion of the thruster hardware. The reactivity of the ionized plume is of great concern. A direct method of characterizing the PPT exhaust plasma is presented which measures the energy distribution and drift speed of the particles in the multi-component plume. The diagnostic instrument couples a time of flight drift section with a gridded energy analyzer. The diagnostic will be described. The arrival time at a detector at the end of the drift section provides velocity information, and the energy analyzer provides the energy distribution information. The two data are combined to determine species concentrations of the ionic species in the plume based on mass and charge. Specifically, the plume is modeled with independent Maxwellian distribution functions for each likely species. The experimental data are fit using a global simulated annealing method. The data analysis method is described. The plasma plume of a mini-PPT with a parallel electrode design is analyzed. The PPT is rotated to determine the angular dependence of the exhaust plasma composition. The results reveal the relative concentrations in the PPT plasma plume with the dominant species being F + and C + . The plume concentrations are dependent on the plume angle. The peak of the plasma plume is canted towards the cathode.