Magnetohydrodynamic operating regimes of pulsed plasma accelerators for efficient propellant utilization

Abstract

The presence of magnetohydrodynamic (MHD) acceleration modes in gas-fed pulsed plasma thrusters has been verified using the magnetic extension of Rankine–Hugoniot theory. However, the impact of initial conditions within the accelerator volume on the formation and structure of these modes remains poorly understood. This work develops a regime map to clarify how key initial conditions—such as propellant gasdynamics, pulse energy, and the timing between propellant injection and discharge initiation—govern transitions between two distinct MHD operating modes, a magneto-detonation and magneto-deflagration, along with an unstable transition regime that connects them. To characterize these regimes, a combination of time-of-flight and thrust stand diagnostics was used to assess their properties, scalability, and the structure while operating with air. Time-of-flight measurements reveal that reducing the initial downstream propellant mass (mdwn) of air shifts the thruster from the magneto-detonation to the magneto-deflagration regime, increasing exhaust velocity (vex). In this regime, the thruster exhibits improved propellant utilization as less mass is injected. At a constant 8 kA of peak current, the utilization efficiency (ηutil) increased from 5% to 35% as mdwn decreases from 70 to 10 μg. Thrust-to-power ratios, measured using a thrust stand, also improve with peak current in the magneto-deflagration regime. This work provides critical insights into how the initial conditions in gas-fed pulsed plasma thrusters dictate the formation of ionization waves, the structure of plumes, and the performance of thrusters. These insights show how pulse shaping methods can be used to optimize the performance of electromagnetic plasma thrusters in environments where high specific impulse and ηutil are crucial.