Connecting Ablative and Gas-Fed Propellant Utilization in Electromagnetic Thrusters

Electromagnetic propulsion systems are limited fundamentally by poor propellant utilization and life-limiting electrode erosion. While conventional architectures are subdivided into gas-fed or solid-ablative propellant modes, this study demonstrates that electromagnetic thrusters operate continuously between these limits based on the density profile of the injected propellant and the integrated current of the discharge. We characterize this transition using a coaxial pulsed plasma accelerator by varying the ratio of number of electrons to number of injected gas particles (Ne/Ng) to sweep from conditions where propellant is extracted from injected air to ablated mass from electrodes. The mechanism of ablation is identified as the formation of explosive cathode spots that are characterized through scanning electron microscopy, optical emission spectroscopy, and profilometry. These measurements reveal that the ablation rate increases in the gas-starved regime (Ne/Ng > 2) up to ~ 30 ug/shot, with cathode spots generating a consistent population of 0.85 um diameter copper particles. The number density of the ablated macro-particles increases exponentially with peak current (from 500 to 3000 #/mm^2). Measurements of impulse bit indicated there was a transition from gaseous to ablative propellant at Ne/Ng = 2, with T/P increasing from 2.5 mN/kW to 3 mN/kW when Ne/Ng = 3. The use of a refractory metal (tungsten) cathode delays this transition to Ne/Ng = 2.4. Additionally, the tungsten cathode reduced the total ejected particle count by a factor of 3, but shifted the erosive stress to the outer electrodes. Maintaining operation below the propellant starvation threshold was also found to reduce the magnitude of ablative erosion.