  Sputter erosion of boron nitride (BN) is a critically important process in
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 continuous-wave cavity ring-down spectroscopy (cw-CRDS) system and
its demonstrative measurement results from BN sputtering experiments.
