Determination of Number Density and Velocity of Sputtered Particles by Cavity Ring-Down Spectroscopy

Sputter erosion is a critically important process in many electric propulsion (EP) devices, from the point of view of both lifetime assessment and contamination assessment. We report the use of cavity ring-down spectroscopy (CRDS) as a diagnostic tool to quantify sputter erosion. We discuss measurements of sputtered particle number density and velocity obtained from a bench-top setup consisting of an ion beam incident on a planar target. The number density measurements are validated by comparison with a simple sputter model. The detection limits of our CRDS system are found to be very adequate for studies of EP devices. The CRDS velocity measurements are obtained by a Doppler shift method and are validated by comparison with sputter simulations. We also present CRDS studies of excited state population distributions and velocities of sputtered particles and discuss the measured distributions. In summary, we find that the CRDS technique provides a sensitive and versatile tool for sputter erosion studies for EP applications.