Hollow cathodes are indispensable for the operation of Hall effect thrusters assisting with discharge ignition, maintenance, and plume neutralization. Over the past decades, cathodes were developed to produce currents ranging from hundreds of milliamperes to hundreds of amperes in operation with mainly inert gases such as xenon, krypton, and argon. Hydrogen was identified as an alternative propellant for a hollow cathode in a water electrolysis Hall effect thruster, with oxygen being ideal for the thruster’s anode feed. As such, hereinafter the development of a hydrogen-fed hollow cathode, referred
to as the Hydrocat, is presented. The approach to designing, building, and testing first a prototype model and then an engineering model is explained and results from cathode standalone testing with heavier propellants (Xe, Kr, Ar) as well as with hydrogen during coupling testing with the WET–HET and AQUAHET water electrolysis thruster anodes are discussed. During some coupling tests, hydrogen was directly produced by an electrolyzer before being fed to the Hydrocat. Lanthanum hexaboride was identified as the ideal emitter material due to its high tolerance to poisoning. The cathode showed stable operation at emission currents ranging from 0.6 to 5 A with coupling potentials below 50 V at hydrogen flow rates ranging from 0.054 to ∼ 0.3 mg s−1. Emission current levels of up to 10 A were achieved with Kr–fed Hydrocat together with O2–fed AQUAHET. The cathode power–to–system power ratio remained below 15% for system powers above 1 kW.