End-of-test Performance and Wear Characterization of NASA's Evolutionary Xenon Thruster (NEXT) Long-Duration Test

Abstract

The NASA's Evolutionary Xenon Thruster (NEXT) project is developing the nextgeneration solar electric ion propulsion system with significant advancements beyond the state-of-the-art NASA Solar Electric Propulsion Technology Application Readiness (NSTAR) ion propulsion system to provide future NASA science missions with enhanced capabilities. A Long-Duration Test (LDT) was initiated in June 2005 to validate the thruster service life modeling and to quantify the thruster propellant throughput capability. Testing was recently completed 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. As part of the test termination procedure, a comprehensive performance characterization was performed across the entire NEXT throttle table. This was performed prior to planned repairs of numerous diagnostics that had become inoperable over the course of the test. After completion of these diagnostic repairs in November 2013, a comprehensive end-of-test performance and wear characterization was performed on the test article prior to exposure to atmosphere. These data have confirmed steady thruster performance with minimal degradation as well as mitigation of numerous life limiting mechanisms encountered in the NSTAR design. Component erosion rates compare favorably to pretest predictions based on semi-empirical models used for the thruster service life assessment. Additional data relating to ion beam density profiles, facility backsputter rates, facility backpressure effects on thruster telemetry, and modulation of the neutralizer keeper current are presented as part of the end-of-test characterization. Presently the test article for the NEXT LDT has been vented to atmosphere with post-test disassembly and inspection underway. NomenclatureBOL = beginning-of-life CEX = charge exchange CRA = center radius aperture DCA = discharge cathode assembly ELT = extended life test EM = engineering model EM3 = engineering model 3 thruster EPC = end-of-test performance characterization GRC = NASA Glenn Research Center HiPEP = High-Power Electric Propulsion IPS = ion propulsion system J B = beam current, A J NK = neutralizer keeper current, A LDT = long-duration test ̇ = main plenum mass flow rate, sccm ̇ = discharge cathode mass flow rate, sccm 2 ̇ = neutralizer cathode mass flow rate, sccm NCA = neutralizer cathode assembly NEXT = NASA's Evolutionary Xenon Thruster NSTAR = NASA's Solar Electric Propulsion Technology Application Readiness P IN = input power, kW PM = prototype model PPC = post-test performance characterization PPU = power processing unit QCM = quartz-crystal microbalance TL = throttle level TT9 = Throttle Table 9 TT10 = Throttle Table 10 V A = accelerator grid voltage, V V B = beam power supply voltage, V VF = vacuum facility WT = wear test φ = aperture or orifice diameter

Overview of the Development of the Solar Electric Propulsion Technology Demonstration Mission 12.5-kW Hall Thruster

“…This assembly design is based on the discharge cathode design for NASA's Evolutionary Xenon Thruster (NEXT) that has recently completed over 50,000 hrs of operation; 38 and  Assembly 2: Uses a lanthanum hexaboride (LaB6) emitter that has been used on the flight SPT-100 and PPS-1350 thrusters and used in the H6MS thruster. Use of LaB6 emitter eases handling assembly requirements and reduces the propellant purity requirements.…”

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

“…The NEXT LDT was equipped with a number of imaging diagnostics that were used to map the temporal behavior of thruster erosion. 8 For the ion optics, these diagnostics included cameras that imaged apertures at the grid center, a radius of 16.3 cm, and the apertures at the edge of the perforated grid; a camera that measured the cold grid gap at the optics center; and, later in the test, the cameras at the center aperture were adapted to measure groove depth. 11 So, a goal of post-test inspection is to verify in situ measurements with post-test measurements.…”

“…The NEXT ion thruster service life capability is being assessed through a service life validation approach that utilizes a combination of testing and analyses. [4][5][6][7][8] The NEXT thruster, as a second-generation deep-space ion thruster, has made use of over 70,000 hours of ground and flight test experience (not including the accumulated hours from the NSTAR ion thrusters on the ongoing Dawn mission) in both the design of the NEXT thruster and the evaluation of thruster wear-out failure modes. A service life assessment was conducted at NASA GRC, employing several models to evaluate all known failure modes that were based upon the substantial amount of ion thruster testing dating back to the early 1960s.…”

“…Descriptions of the test setup and vacuum facility can be found in Ref. 8 and the references cited therein. The goals of the NEXT LDT were to demonstrate the initial project qualification propellant throughput requirement of 450 kg, validate thruster service life model's predictions, quantify thruster performance and erosion as a function of thruster wear and throttle level, and identify any unknown lifelimiting mechanisms.…”

“…Just prior to voluntary test termination, the thruster was retracted into its independentlypumped port and isolated from the main vacuum facility so that diagnostics that had failed during the test could be repaired. 8 A thruster performance test was conducted with the repaired diagnostics to fully characterize the end-oftest performance and condition of the thruster. In April 2014, the thruster was exposed to atmosphere for the first time in nearly nine years.…”