Cavity Ring-Down Spectroscopy Measurements of Barium Density in a Hollow Cathode

Hollow cathodes are ion propulsion devices that provide electrons and neutralize charges. Barium oxide (BaO) cathodes will experience evaporation of the barium emitter and sputtering because of plasma-heating and ion charge-exchange reactions. Measurement of barium densities in the plasma of the cathode will inform modeling efforts of the physical characteristics and lifetime of the cathode. Here, the detection of barium from the thermionic emitter of the Mark II 25 A barium oxide hollow cathode using the laser diagnostic technique of cavity ring-down spectroscopy (CRDS) is presented. The CRDS diagnostic detects ground-state neutral barium via absorption at 553.702 (vacuum wavelength). The cathode mount is designed to allow optical access along the axis of the cathode and CRDS measurements are performed along this direction. Varying the parameters of anode current and propellant flow rate shows a positive correlation in barium density to increasing current values, with corresponding positively correlated temperature trends from a thermocouple mounted on the cathode. A potential inverse relationship between propellant flow and barium density is found. An overall detection limit of ~3x106 cm-3 is found. The robustness of the CRDS diagnostic under plasma conditions is considered.