On the compatibility of LaB6 and Iodine in Planar and Hollow Cathodes

Iodine is increasingly considered a viable alternative to noble gases for electric propulsion systems, particularly due to its high storage density and non-pressurized form. While various thruster types have been successfully tested with iodine as a propellant, the compatibility of iodine with high-current neutralizers remains insufficiently understood. This work investigates the performance and degradation of LaB6-based hollow and planar cathodes operated with iodine at Airbus Friedrichshafen. A dual-gas feed system enabled heaterless ignition and stable operation with krypton, followed by the transition to iodine. While the planar cathode failed to sustain a stable discharge, the hollow cathode operated for up to 8.5 hours at 2 to 3 A. Post-test SEM/EDX analysis of the emitter revealed significant surface degradation including oxide and iodide formation, and an emitter erosion of up to 15%. The oxygen poisoning of the emitter is attributed to water residues in the iodine tank, which remained despite cleaning procedures. The loss of the emitter material and the high reactivity of lanthanum and boron with iodine suggest an erosion mechanism through volatile reaction products, which is confirmed by thermodynamic considerations. This degradation mechanism resembles the halogen cycle used for tungsten filaments, however without the redeposition of the evaporated material. The short-term effect is a fluctuating discharge voltage due to surface reactions. In the long-term, the cathode performance is expected to degrade due to emitter depletion. Considering the experimental results and the analysis of possible surface reactions, these findings suggest that LaB6 is not suitable as emitter material for long-term operation in hollow cathodes with iodine. A cathode lifetime below 100 hours is projected under the tested configuration.