Sputter Erosion Sensor for Anode Layer Type Hall Thrusters Using Cavity Ring-Down Spectroscopy

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We report the development of a sputter erosion monitoring system to study Hall thruster lifetime and contamination. The laser-based sensor uses the continuous-wave cavity ring-down spectroscopy (cw-CRDS) technique and allows for in situ measurements in near real-time. The cw-CRDS diagnostic allows direct probing of sputter products in their ground state, thereby providing a reliable quantitative measure of their overall number density. Combining the number density of sputtered particles with their velocity allows determination of the flux of sputtered articles and erosion rate. We perform proof of principle experiments in which sputtered manganese (Mn) atoms from the acceleration channel of an anode layer type Hall thruster are measured. The measurement strategy is to detect the Mn atoms via an absorption line from the ground state at a wavelength of 403.076 nm (air). The measured path-integrated number density of sputtered Mn atoms is 1.7 ± 0.3×1013 m-2 for an argon anode mass flow rate of 2.08 mg/s and discharge voltage of 250 V. A finite element sputter model is used to compare the CRDS results against validating mass loss measurements and shows good agreement.