Results of the 2000 hr Wear Test of the HiPEP Ion Thruster with Pyrlytic Graphite Ion Optics

Abstract

A 25 kW, long-life ion thruster was developed and wear tested at the NASA Glenn Research Center in support of Project Prometheus. The 2000 hr wear test was undertaken to quantify known erosion phenomena such as ion optics erosion due to charge exchange ion impingement and discharge cathode keeper erosion and to identify unknown wear mechanisms associated with such high-specific impulse, high-power thrusters. The discussion provides a comparison between predicted wear and deposition rates and an analysis of the impact of the various phenomena observed. Trends in observed erosion of the ion optics were consistent with expectations and the negligible wear of the discharge keeper and neutralizer keeper was less than expected. The HiPEP thruster was designed and developed at the NASA Glenn Research Center (GRC) during the ongoing development of the NASA Evolutionary Xenon Thruster (NEXT) 2 and following the successful demonstration of the NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR) ion thruster on the Deep Space 1 spacecraft. 3 Both of these programs incorporated relatively short duration wear tests (~2000 hr) at GRC early in their development efforts. 4,5 These tests were conducted to validate design approaches, identify unknown wear mechanisms, and quantify wear rates before the thrusters were developed to higher fidelity. It is in this context that the HiPEP thruster was incorporated into a wear test very early in its development. While this test was ongoing, a higher fidelity development model thruster (DMT) was being developed in collaboration between GRC and the Aerojet Corporation. An ion optics assembly electrostatically identical to the one being wear tested was successfully vibration tested. 6 A thermo -mechanical model of the DMT was also developed. The results of the 2000 hr wear test would then support and augment a detailed design process and significantly accelerate the delivery of high -fidelity hardware. However, before the assembly of the HiPEP DMT, NASA's Project Prometheus redirected the HiPEP design work to support the design of the Herakles ion thruster, which was in part based on advances made under the HiPEP program. The HiPEP thruster was designed to accommodate a large range of operational requirements and to facilitate the future development of higher -power ion thrusters. To this end, it has rectangular discharge chamber and incorporates pyrolytic graphite (PG) ion optics. HiPEP versions have been successfully operated with both dc (i.e., hollow cathode-based) and microwave discharges. 7,8 Operation at 25 kW over a specific impulse range of 6000 to 9000 s using a dc discharge was demonstrated. 8 Following the performance demonstration of the HiPEP thruster with PG ion optics and a dc discharge, 9 the thruster entered a 2000 hr wear test. The objectives of the test were to