Barium sensing in hollow cathode plasma using cavity ring-down spectroscopy (CRDS)

Abstract

Hollow cathodes (HCs) are ion propulsion devices commonly paired with Hall Effect Thrusters (HETs), which are devices of increasing importance in the ion propulsion community. Barium Oxide (BaO) cathodes are known to emit barium when operating under high-temperature conditions. Understanding barium densities in the cathode plasma provides experimental guidance for NASA barium modeling, including understanding of the physical characteristics and lifetime of the cathode. Based on modeling work, expected barium densities are ~1010 cm-3. A sensitive diagnostic is required such as CRDS. In this work, the detection of barium from the thermionic emitter of the Mark II 25 A BaO HC using the laser diagnostic technique of cavity ring-down spectroscopy (CRDS) is presented. CRDS detects ground state neutral barium via absorption of the probe laser beam in the vicinity of 553.548 nm (air wavelength). The cathode CRDS measurements are performed along the axis of the cathode since that is the control volume of interest. We report barium density as a function of heater current (plasma off) with results showing an approximately exponential density increase with current. Further parameters of study include keeper current, anode current (with the cathode operating), and propellant flow values. The measured signal-to-noise allows estimation of the barium density detection limit as ~106 cm-3 in the present configuration. An appendix to this work addresses the need for a diagnostic technique to measure krypton neutrals in HC plumes. In the krypton study, we enhance the krypton Two-Photon Absorption Laser Induced Fluorescence (TALIF) technique and apply it to a BaO HC plasma. We utilize a dye laser at 212.6 nm to excite TALIF fluorescence within the plume, with the fluorescence detected at 758.7 nm. We present spatial maps for krypton neutral densities at a cathode flow rate of 7.5 sccm and anode currents of 5A and 13A. These measurements provide insights into facility effects related to cathode coupling and cathode physics, such as the collisional damping of instabilities. Additionally, we discuss how plasma characteristics, including spot versus plume mode, and plasma luminosity, are influenced.