Time-Resolved Electron Beam Fluorescence

ABSTRACT: An innovative diagnostic, Time-Resolved Electron Beam Fluorescence (TREBF), will be developed that possesses the measurement capabilities needed to understand test facility effects on electric propulsion (EP) systems. Even the most advanced test and evaluation vacuum chambers are incapable of replicating the pressures encountered in geostationary earth orbits. These elevated background pressures influence thruster stability, performance, lifetime, and plume behavior. The fundamental processes driving these effects are at present unknown, obfuscating predictive models and precluding standardized test conditions or physics-based performance corrections. A deeper understanding of neutral dynamics in non-equilibrium, partially-ionized EP plasma flows is required to address the facility effects problem; however, state-of-the-art neutral diagnostics do not possess the requisite spatial and temporal resolution. TREBF overcomes these limitations by exciting plasma neutral particles with an incident electron beam and measuring the resulting fluorescence. Using commercially available materials and sensors, TREBF is able to measure neutral particle densities down to 1017 particles/m3 at temporal and spatial resolutions on the order of 1 s and 1 mm, respectively. At the end of this program a fully functional TREBF system will have been developed and characterized, and its measurement capabilities will have been demonstrated on an operating Hall thruster at multiple US facilities.; BENEFIT: The Time-Resolved Electron Beam Fluorescence (TREBF) system proposed here is a portable, cost-effective diagnostic that is uniquely capable of probing neutral gas dynamics on length (~1 mm) and time (~1 s) scales relevant to electric propulsion (EP) plasmas, making it a crucial tool for the testing and evaluation of electric propulsion systems. Compared to laser-based diagnostics, TREBF possesses improved versatility because the electron-induced fluorescence manifests from the collisional excitation of plasma ions and neutrals. As a result TREBF is able to examine the ion, neutral, and molecular species of various propellants and erosion byproducts by simply observing the appropriate emission line. The ability to monitor these systems may lead to much longer lived thrusters, accurate prediction of on-orbit capabilities, higher efficiency thruster systems, and on-orbit real-time monitoring of thruster behavior. Beyond this, the ability to characterize thruster behavior at relatively high back pressures allows the testing of advanced propellants and higher power thruster systems that cannot be tested in conventional facilities. In addition to being a valuable tool for the EP industry, TREBF has potentially valuable commercial applications in the fields of plasma processing and waste treatment.