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Sputter monitoring system using continuous-wave cavity ring-down spectroscopy (cw-CRDS) was built for both lifetime assessment and contamination effects in Hall thrusters. First, we have performed proof of principle measurements of sputtered boron atoms from BN targets by argon ions using pulsed CRDS. The measurement strategy is based upon detection of boron atoms via an absorption line from ground state at a wavelength of 249.773 nm. The path-integrated number density is 1.8 × 1013 m-2 at ion beam current of 45 mA and argon ion energy of 1,000 eV. The number density is proportional to the ion beam current, as expected. These results show the validity of the boron sensor for detecting sputtered boron atoms. Next, in order to achieve the required detection sensitivity and time response, we implement CRDS with a continuous-wave (cw) laser for enhanced sensitivity. The target was changed to manganese (λ=403.07 nm) instead of boron. The results show that a detection limit of per-pass absorbance of 0.6 ppm for a 1-s measurement time. Scaling the experimental results and accounting for changes in laser system and mirror reflectivity indicate that the BN detection system should have sufficient signal-to noise for expected Hall thruster conditions.

Development of Real-time Boron Nitride Erosion Monitoring System for Hall Thrusters by Cavity Ring-Down Spectroscopy