Performance characteristics of a LaB6 dry cathode coupled with a water-vapor Hall effect thruster

In this study, a lanthanum hexaboride (LaB${}_6$) dry cathode was tested in both diode mode with a graphite collector plate and coupling mode with a plume of a water-vapor Hall effect thruster (HET), in order to evaluate its compatibility. The work function of polycrystalline LaB${}_6$, initially 3.06 eV in diode mode, increased to approximately 3.6 ± 0.2 eV in coupling mode. This shift was consistent with post-operation oxygen deposition on the emitter surface, as revealed by energy-dispersive X-ray spectroscopy. The cathode emitted 200 mA of electron current with 100 V of negative bias voltage in the coupling configuration and it was roughly ten times higher than the space-charge-limited current observed in diode mode. Fitting equations were developed based on experimental data to predict the dry cathode performance under a broader range of operational conditions. Calculations indicated that electron production cost could be less than 150 W/A, surpassing that of a water radio frequency (RF) cathode. The estimated emitter erosion rate was below $8\times 10^{-3}\mathrm{mm/h}$, suggesting sufficient lifetime for HET operation. The thruster efficiency of a water-vapor HET employing the LaB${}_6$ dry cathode was estimated to reach 9.7% at a discharge voltage of 350 V and an anode mass flow rate of 0.68 mg/s, comparable to that of a xenon RF cathode. These results highlight the strong potential of LaB${}_6$ dry cathodes for use in water-vapor HETs.