Performance scaling of inductive pulsed plasma thrusters with coil angle and pulse rate

Abstract

A circuit model for an inductive pulsed plasma thruster was developed in order to investigate the performance of thrusters with conical coils; the model can accommodate cone-angles from 0° (a straight theta-pinch coil) to 90° (a planar coil). The plasma is treated as a deformable slug that moves both radially and axially in response to the force applied by the coil. The radial equation of motion includes a restoring force due to the plasma pressure, which is derived under the assumption that the electron population is isothermal, while the ions are isothermal, adiabatic, or shock-heated depending on the magnitude and sign of the radial velocity. The inductance of the coil and the plasma slug, and their mutual inductance, was determined using QuickField. A local maximum in efficiency and specific impulse was found for angles less than 90°; however the absolute maximum for both these quantities occurs at 90°. High pulse-rate operation was found to yield dynamic efficiencies (excluding ionization cost) as high as 60–70% for ISP in the range of 3000–5000 s, even for a device with modest jet-power (5 kW). This mode of operation also permits elimination of the pulsed gas valve, which would be a significant system-level simplification. An alternate mode of inductive recapture, in which the current is interrupted at the second zero-crossing, was found to result in a sacrifice of only 1–2% in efficiency, while offering other significant system-level benefits for this kind of thruster.