Development of a Cavity Ring-Down Spectroscopy Sensor for Boron Nitride Erosion in Hall Thrusters

Sputter erosion of boron nitride (BN) is a critically important process in many Hall thrusters from the point of view of both lifetime assessment and contamination effects. This contribution describes the development of a laser based sensor for in situ monitoring of sputtered BN from Hall thrusters. We present a laser electro-optic system and its demonstrative measurement results from BN sputtering experiments. A frequencyquadrupled diode laser at 250 nm is used as the light source for continuous-wave cavity ringdown spectroscopy. The optical cavity is mounted on a rail system and fiber optics are used to deliver light in/out of the vacuum chamber. Details of fiber optic delivery and future prism-based cavities are also discussed.