Study of the ionization in air-breathing electric rockets with VLEO atmospheric propellants and typical laboratory mixtures

This work investigates the ionization of atmospheric propellant in air-breathing electric propulsion (ABEP) systems for satellites operating in Very-Low Earth Orbit (VLEO). Plasma formation inside the ionization chamber of an air-breathing thruster is studied to gain insights on physical phenomena involved in ABEP technology. A time-dependent zero-dimensional model of the plasma chamber is formulated, that accounts for a comprehensive set of reactions for nitrogen, oxygen and argon. This model is implemented within a numerical framework to simulate plasma ignition at different operating parameters defined to describe geometry, intake behaviour, input power, magnetic field interactions, acceleration mechanism, and VLEO environmental conditions. The simulation results include the evolution in time of plasma density, composition and reaction rates during the ignition transient until stationary conditions. The study identifies the plasma ignition envelope by analysing the dependencies of plasma properties on mass flow rate, electron temperature, and ion-to-neutral residence times' ratio. The composition and density of the plasma are characterized across different regimes, revealing a transition between molecular and atomic ions dominance. NO+ ion is observed as an important constituent for low-plasma-density regimes, where molecular ions dominate. Comparison between results on different mixtures are performed and approximation laws to facilitate experimental validation are derived, which can help improving the accuracy of plasma diagnostic measurements for experimental conditions relevant to ABEP testing. Furthermore, it is shown that the use of argon as substitute of atomic oxygen in low-density air-like mixture plasma does not provide representative results.