Contamination flakes in the propulsion discharge chamber of a NASA Solar Electric Propulsion Technology Readiness (NSTAR) xenon-ion engine were evaluated at the NASA Glenn Research Center. NASA’s Deep Space 1 spacecraft, launched in October 1998, was a technology-validation mission that flew a single 30-cm-diameter NSTAR xenon-ion engine as its primary propulsion system, providing the first successful flight of an ion propulsion system. The NSTAR ion engine successfully completed the mission in December 2001 after 16,265 hr of operation in space. The success of the mission stimulated interest in future NASA science missions utilizing solar electric propulsion for required lifetimes in excess of 20,000 hr. As a result, assessing the ultimate service life capability of ion thrusters is vital, requiring extensive ground testing and data analysis.
To determine this capability, researchers conducted a long-duration test, called the Extended Life Test (ELT) of the Deep Space 1 spare flight ion thruster at the Jet Propulsion Laboratory (JPL) in collaboration with Glenn. The test was started in October 1998 and concluded on June 26, 2003, after 30,352 hr of operation. The ELT was terminated prior to its end of life so that the engine components could be analyzed to provide critical information to ion propulsion system designers. The primary purpose of the ELT was to determine the ultimate service life capability of the NASA 30-cm ion thruster technology. The left photograph shows the Deep Space 1 spare flight ion thruster taken shortly after completion of the ELT. Extensive post-test analyses were conducted at JPL and Glenn, with collaborative efforts from other groups.
Left: NSTAR ELT ion thruster after 30,352 hr of operation. Right: Optical micrograph of discharge chamber flakes.
Post-test inspection of the ELT engine revealed numerous small contaminant flakes uniformly distributed over the bottom of the discharge chamber. Some of these flakes are shown in the optical photograph on the right. Internally generated flakes are a major threat to thruster reliability and durability because they can short the ion optics or the cathode. However, if the flakes were generated externally (from the facility), they would not be a threat to in-space reliability and durability. Therefore, determining the origin of the flakes was critical to the understanding of the degradation mechanisms of long-life ion thruster operation.
Extensive analyses were conducted at Glenn on the NSTAR ELT discharge chamber contaminant flakes to determine the source of the particles (e.g., from within the thruster, from the screen or accelerator grids, or from outside the thruster). Analyses included optical microscopy and particle length histograms, field-emission scanning electron microscopy combined with energy-dispersive spectroscopy, and atomic-oxygen-plasma exposure tests. The results indicate that the majority of discharge chamber flakes consist of a layered structure, typically with either two or three layers. Flakes comprising two layers were typically found to have a molybdenum- (Mo-) rich layer on one side and a carbon- (C-) rich layer on the other side (see the next figure). Flakes comprising three layers were found to be sandwichlike structures with Mo-rich exterior layers and a C-rich interior layer. The presence of the C-rich layers indicates that these particles were produced by sputter deposition buildup on a surface external to the discharge chamber from ion sputter erosion of the graphite target in the test chamber. This contaminant layer became thick enough that particles spalled off and then were electrostatically attracted into the ion thruster interior, where they were coated with Mo from internal sputter erosion of the screen grid and cathode components.
Electron micrograph of the edge of a two-layered flake showing two distinct layers with corresponding chemistries (S1 is a C-rich layer, and S2 is a Mo-rich layer).
A particle-size histogram based on measurements of approximately 1500 flakes (shown in the bar chart) further indicated that the source of the particles was the spalling of carbon flakes from downstream surfaces. Analyses of flakes taken from the downstream surface of the accelerator grid supported this theory. The production of the downstream carbon flakes, and hence the potential problems associated with the flake particles in the ELT ion thruster engine, was determined by Glenn to be a facility-induced effect that would not occur in space.
Particle-size histogram based on the longest dimension of 1538 flakes.
Last updated: October 7, 2006
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