Impact of Neutral Backfill on Plasma Sheet Dynamics in Pulsed Inductive Thrusters

Abstract

The time-dependent effects of operating a pulsed inductive thruster (PIT) in a finite neutral backfill are investigated analytically and experimentally. In principle, a PIT could offer an efficient, in-space propulsion alternative without the lifetime-limiting use of electrodes. However, its traditional design has relied on MW-class power regimes to reach competitive performances. Recent advancements in electrical switching mechanisms have enabled higher pulse frequencies in the range of $1-10~$kHz at lower discharge energies. Operation at these rates has been hypothesized to allow for efficient, quasi-steady operation while also improving reliability. A theoretical model was developed to capture how non-equilibrium ionization processes influence plasma formation and acceleration while including neutral entrainment via charge-exchange (CX) collisions and finite skin depth. An inter-pulse evolution of neutral density was also derived to model PIT operation across multiple pulses and allow for comparisons between different ambient conditions. It was determined that testing in a backfill yielded similar time-evolved state properties to steady-state operation with constant gas injection, but with substantial differences in sheet velocity, energy partitioning, and CX collision frequency. Benchtop backfill testing of a prototype unit yielded visual confirmation of predicted dynamic trends in the plasma exhaust and demonstrated the formation of secondary current sheets using steady gas injection.