Analysis of composition and dynamics of the plasma plume emitted by a 1 J pulsed plasma thruster fed with polytetrafluoroethylene and determination of thruster efficiency components

Abstract

The plasma plume of a 1 J pulsed plasma thruster fed with polytetrafluoroethylene (PTFE) was studied with electric probes to obtain the shape and composition of the beam of ejected ions. Two ion diagnostic tools—Faraday cup (FC) and retarding potential analyzer (RPA), were employed together with a time-of-flight approach. The FC was used to obtain spatially and time-resolved data of the mean ion charge expelled from the thruster in each pulse. With the RPA the beam was examined for the presence of specific ion species. The results of this investigation indicated the presence of both elements of PTFE in the beam—fluorine and carbon as well as copper from the discharge electrodes. Fluorine ions (identified in charge-states from F+ up to F6+) constituted the majority of ions in the plume with only trace amounts of C+ detected, which raises the question on the whereabouts of the remaining carbon. Energy distribution and relative abundance of fluorine ion species on axis were retrieved and it was found that F2+ constitutes over 40% of the plasma—in both quantity and energy fraction. Angular profiles of ion charge density, apart from the expected azimuthal asymmetry, showed heightened flux of ions in the area shaded by the discharge electrodes. The results obtained from both diagnostics allowed us to calculate propellant utilization, beam divergence, and energy utilization. By combining this information, the total thruster efficiency was retrieved, which turned out to be comparable to the value obtained from impulse bit measurements.