On the compatibility of C12A7 electride and iodine in planar and hollow cathodes

Iodine is gaining attention as an alternative propellant for electric propulsion systems due to its high storage density, low system complexity and reduced cost. While gridded ion thrusters and low-current neutralizers have demonstrated promising performance with iodine, the long-term compatibility of high-current thermionic neutralizers remains unresolved. This work examines the performance and degradation behavior of planar and hollow cathodes with C12A7 electride emitters operated with krypton and iodine at Airbus Friedrichshafen. A dual-gas feed system enabled heaterless ignition and stable krypton operation prior to transition to iodine. Both planar configurations achieve stable krypton discharges, whereas the hollow cathode exhibited quasi-periodic power oscillations attributed to surface melting of the electride material. Upon introduction of iodine, none of the configurations sustained a stable discharge, with rapid onset of instabilities and irreversible emitter damage. Post-test analysis revealed surface melting, grain coarsening, iodine accumulation in near-surface cavities and pronounced calcium depletion within 250\, m of the surface. Thermodynamic analysis suggests that reactions with positively charged iodine species can drive the formation of volatile calcium iodides, enabling chemically driven destruction of the electride structure. In the hollow cathode tests, oxygen incorporation converted the electride to insulating ceramic, likely due to trace water contamination of the iodine propellant. The combined thermal, chemical, and oxidative degradation mechanisms indicate that C12A7 electride is not compatible with iodine under the tested conditions. These findings suggest that C12A7-based emitters require significant material improvements before use in iodine-fed hollow cathodes and that alternative neutralizer concepts may be more suitable for long-duration iodine operation.