Self-Neutralizing Air Breathing Plasma Thruster for Very Low Earth Orbits
Open Access DepositedThe Air-Breathing Plasma Thruster is a novel propulsion system developed to enable electric propulsion for satellites in very low earth orbits. These thrusters use air in very low earth orbit as propellants by ionizing them to create air plasma and accelerating it using Lorentz force. Experimental and computational approaches are used to design, study, and test this air-breathing plasma thruster. Firstly, air ionization is investigated at altitudes of 80-110 km by performing plasma chemistry simulations to observe the behavior of various species under different mean electron energies. The key findings show that higher mean electron energies initially have minimal impact on results, with notable increases in electron density resulting from plasma chemistry reactions such as ionization, electron detachments, and ion-neutral collisions. Simulations at 80 km reveal significant densities of electrons and ions (N+, O2-, N2+, O-, O+, O2+), particularly at 1-3 eV where O2+ and O2- densities dominate. At 5 eV, electron density rises as O2- decreases due to detachment reactions, while higher energies (6-8 eV) yield substantial increases in electron and N2+ densities. At 90 km, similar trends are observed, with distinct peaks in O2+ and O2- densities at 5 eV and elevated electron densities at 15-20 eV. At altitudes of 100 and 110 km, ionization efficiency drops due to lower air density and longer mean free paths, resulting in reduced peaks in O2- ions. The air-breathing plasma thruster’s performance model estimates thrust, power, and thrust-to-power ratios, demonstrating its capability for efficient thrust generation and drag compensation for various geometries of the thruster at high altitudes. At 80 km, the ABPT achieves an average thrust of 59 N against a drag force of 58 N with a power supply of 1.37 MW at 300V, while at 90 km, it generates 12.63 N against a drag force of 5.5 N. The findings suggest that efficient thrust and drag compensation at these altitudes are feasible with appropriate ion densities and extraction voltages.Secondly, the research explores a neutralizer-less self-neutralizing air-breathing plasma thruster concept for very low earth orbit applications, highlighting the importance of electron sources in generating high-density air plasma and examining various ionization mechanisms. This thruster would achieve beam neutralization using the extraction of positive and negative ions such that their charge densities would cancel out. Experiments with different electron and ion sources, including DC glow discharge, electron cyclotron resonance ion source, and coaxial and circular arc electron source, reveal that air plasma with >1018 m-3 density could be generated. Various invasive and noninvasive diagnostics (such as Langmuir probe, Faraday cup, negative ion current measurements, scanning electron microscopy, high-speed imaging, conductance measurements, time of flight, etc.) are performed for the sources. For coaxial arc electron source, the maximum positive and negative ion currents were 20.2 mA and 40 mA, respectively, with an electron density of about 3 × 10¹⁵ m⁻³ and an electron temperature of 1.1-1.2 eV. In comparison, circular arc electron source produced higher electron densities (1.62 × 10¹⁸ m⁻³) and maximum ion current densities of 8.06 × 10² A/m² (positive) and 1.8 × 10² A/m² (negative) at 0.05 Torr pressure, while exhibiting lower sensitivity to pressure variations. Improved electron cyclotron resonance designs and magnetic field enhancements in coaxial and circular arc electron sources show promise, with circular arc electron sources demonstrating higher efficiency and self-healing capabilities. Lastly, the SABPT version 1 and 2 configurations aim to revolutionize electric propulsion for very-low earth orbit scenarios, focusing on ion extraction and charge neutralization. Version 1 features a scramjet-like design with high-speed airflow, while version 2 eliminates internal drag and introduces CAES and cusp acceleration for higher efficiency. Diagnostics using Time of Flight and Langmuir probe techniques provide insights into ion velocity profiles and current densities, guiding further optimization. Thruster’s misalignment with the nitrogen gas inlet initially led to lower ion velocities (at a high gas flow rate ~3.7 km/s for positive ions, ~3.25 km/s for negative ions), which increased significantly with proper alignment (up to ~11 km/s for positive ions, ~7 km/s for negative ions). Gas flow rate increase resulted in velocity losses due to collisions. Air use lowered velocities (high gas flow rate, 1-2 km/s for positive ions) due to oxygen presence and increased collision losses. Negative ions accelerated with higher voltage and flow rates, while positive ions decelerated due to collisions. Higher voltage increased current densities, peaking at ~24 A/m², with increased flow rates boosting charge densities. Additionally, the study proposes a novel self-neutralization mechanism involving the neutralization of positive ions by negative ions and additional electron neutralization, supported by simulation and experimental evidence. This research advances the understanding of air ionization and propulsion technology, offering significant implications for future very low-earth orbit satellite missions.
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