1. List of investigators Near Real-Time Hall Thruster Erosion Diagnostics Prof. Alec Gallimore and Dr. Timothy Smith Plasmadynamics and Electric Propulsion Laboratory The University of Michigan In Collaboration with Prof. Azer Yalin, Colorado State University

2. Background and Motivation HET Erosion Diagnostics HET lifetime generally limited by sputter erosion of the BN channel. Sputtered particles can coat spacecraft surfaces, posing risk to mission. Effect of varying thruster operating parameters on lifetime not well understood. Lack of understanding makes qualifying new HETs challenging and expensive….Life Tests, which: are expensive, and labor and facility intensive; allow only post-facto bulk analysis; do not allow real-time erosion sensitivity measurements; are inflexible and dependent on prescribed mission profiles; and typically provide little information about spacecraft deposition/erosion rates

3. Objective of Research HET Erosion Diagnostics Need method for measuring thruster erosion rates non- intrusively in real-time. Such a technique would allow one to evaluate HET design and operating condition changes for performance and life simultaneously. Cavity Ring-Down Spectroscopy (CRDS) measurement of thruster erosion rates non-intrusively and in real-time. CRDS combined with wear testing would enable monitoring of thruster performance and erosion rates simultaneously, in real time.

4. Technical Approach CRDS method: –UV diode laser beam absorbed by BN erosion product (neutral, ground-state B) –number density directly proportional to change in cavity ring-down time  –narrow linewidth permits high SNR (~100) CSU (initial development by Prof. Azer Yalin) –ion gun rig for BN sputtering –initial pulsed laser (lower SNR) tests complete –proof-of-concept CW testing underway PEPL (deployment in HET plume) –UV diode laser installed –optical cavity will sweep across HET exit plane –line-integrated B number density from  –Abel inversion yields radial distribution Cavity Ring-down Spectroscopy (CRDS) Payoff: Improves thruster wear testing for faster and cheaper Hall thruster development and qualification cycles HETs with enhanced lifetimes - Higher Total Impulse Planned CRDS setup (PEPL) BN sputtering data from CRDS (CSU)

5. Anticipated Results HET Erosion Diagnostics Ultra-Sensitive CRDS Detection CRDS using frequency-quadrupled UV external cavity diode laser— $300K investment (Air Force DURIP); identical lasers installed at Michigan and CSU. Integration with Large Vacuum Test Facility CRDS system undergoing testing at CSU - excellent results CRDS system integration with LVTF at Michigan (6 x 9 m) Proof of Concept Testing at Michigan CRDS measurements in HET plume in LVTF Implementation Plan to Continue Technique Development Incorporation of M&S Deposition measurements & LIF