Post-test Inspection of NASA’s Evolutionary Xenon Thruster Long-Duration Test Hardware: Discharge and Neutralizer Cathodes

The NEXT Long-Duration Test is part of a comprehensive thruster service life assessment intended to
demonstrate overall throughput capability, validate service life models, quantify wear rates as a function
of time and operating condition, and identify any unknown life-limiting mechanisms. The test was
voluntarily terminated in February 2014 after demonstrating 51,184 h of high-voltage operation, 918 kg
of propellant throughput, and 35.5 MN-s of total impulse. The post-test inspection of the thruster
hardware began shortly afterwards with a combination of non-destructive and destructive analysis
techniques, and is presently nearing completion. This paper presents relevant results of the post-test
inspection for both discharge and neutralizer cathodes. Discharge keeper erosion was found to be
significantly reduced from what was observed in the NEXT 2 kh wear test and NSTAR Extended Life
Test, providing adequate protection of vital cathode components throughout the test with ample lifetime
remaining. The area of the discharge cathode orifice plate that was exposed by the keeper orifice
exhibited net erosion, leading to cathode plate material building up in the cathode-keeper gap and causing
a thermally-induced electrical short observed during the test. Significant erosion of the neutralizer cathode
orifice was also found and is believed to be the root cause of an observed loss in flow margin. Deposition
within the neutralizer keeper orifice as well as on the downstream surface was thicker than expected,
potentially resulting in a facility-induced impact on the measured flow margin from plume mode.
Neutralizer keeper wall erosion on the beam side was found to be significantly lower compared to the
NEXT 2 kh wear test, likely due to the reduction in beam extraction diameter of the ion optics that
resulted in decreased ion impingement. Results from the post-test inspection have led to some minor
thruster design improvements.