Beyond Inert Gases: The Promise of Plasma Cathodes

Plasma cathodes operate via the emission of electrons through an extraction sheath bounding a dense plasma. Because electrons are sourced from a plasma in contrast to thermionic emission, plasma cathodes can operate in principle on any feed gas. Such a capability means that in contrast to conventional electric propulsion hot cathodes, which operate exclusively on inert gases, plasma cathodes can operate on propellants ranging from green feedstock such as water vapor or reactive vapors such as air or even rocket fuel. The technology also enables in situ utilization of propellants gases derived from extraterrestrial sources as CO₂ derived from Mars or nitrogen and methane extracted from the atmosphere of Titan. The plasma cathode therefore extends the application space of electric propulsion systems and have the potential to simplify gas feed systems as well as reduce overall system mass. Here, we review the basic operation of plasma cathodes, its capabilities and limitations as well technical challenges for realizing a cathode that operates on arbitrary gases. Implementation of such cathodes on actual missions is reviewed as well as ongoing work aimed at operating such cathodes on arbitrary propellants.