Post-test Examination of NASA’s Evolutionary Xenon Thruster Long-Duration Test Hardware: Ion Optics

Abstract

A Long Duration Test (LDT) was initiated in June 2005 as a part of NASA's Evolutionary Xenon Thruster (NEXT) service life validation approach. Testing was voluntarily terminated in February 2014, with the thruster accumulating 51,184 hours of operation, processing 918 kg of xenon propellant, and delivering 35.5 MN-s of total impulse. The post-test inspection objectives for the ion optics were derived from the original NEXT LDT test objectives, such as service life model validation, and expanded to encompass other goals that included verification of in situ measurements, test issue root causes, and past design changes. The ion optics cold grid gap had decreased only by an average of 7% of pretest center grid gap, so efforts to stabilize NEXT grid gap were largely successful. The upstream screen grid surface exhibited a chamfered erosion pattern. Screen grid thicknesses were ≥ 86% of the estimated pretest thickness, indicating that the screen grid has substantial service life remaining. Deposition was found on the screen aperture walls and downstream surfaces that was primarily composed of grid material and back-sputtered carbon, and this deposition likely caused the minor decreases in screen grid ion transparency during the test. Groove depths had eroded through up to 35% of the accelerator grid thickness. Minimum accelerator aperture diameters increased only by about 5-7% of the pretest values and downstream surface diameters increased by about 24-33% of the pretest diameters. These results suggest that increasing the accelerator aperture diameters, improving manufacturing tolerances, and masking down the perforated diameter to 36 cm were successful in reducing the degree of accelerator aperture erosion at larger radii.

Grid Deformation Real-Time Measurement System of Ion Thruster Based on Videometrics

“…The reduction of the grid gap will affect the divergence angle of the plasma beam and the magnitude of thrust, increase the threshold value of electron backstreaming, and aggravate grid erosion [5][6][7]. More seriously, it will increase the probability of the short circuit of grids, then causing the thruster to shut down [8,9]. Therefore, it is necessary to accurately measure the amount of grid thermal deformation and the change of grid gap, and provide support for the control of the thermal deformation of the ion optics and the optimization of the thruster performance.…”

Rapid fabrication of ion optics by selective laser melting

“…Recently, a set of grids for the T6 ion thruster that will be used in the BepiColombo mission to Mercury in 2018 was built with molybdenum (screen grid) and graphite (accelerator grid) (Hutchins et al , 2015; Rudwan et al , 2011). Since the 2000, NASA has been developing a large ion thruster for long space mission, NEXT, for which many grid sets have been fabricated with titanium and molybdenum, and tested (Soulas et al , 2002; Soulas and Shastry, 2016).…”

3D Simulations of Ion Thruster Accelerator Grid Erosion Accounting for Charge Exchange Ion Space Charge

“…Interpretion of the results of this wear test has been complicated by the significant quantity of carbon facility material back-sputtered onto the grids during thruster operation [16]; thus there was an opportunity for simulations of both the LDT itself as well as NEXT operation under space vacuum to make valuable contributions to the flight service life assessment.…”