Michael J. Patterson

NEXT: NASA's Evolutionary Xenon Thruster

NASA’s Glenn Research Center has been selected to lead development of NASA’s Evolutionary Xenon Thruster (NEXT) system. The central feature of the NEXT system is an electric propulsion thruster (EPT) that inherits the knowledge gained through the NSTAR thruster that successfully propelled the Deep Space 1 to asteroid Braille and comet Borrelly, while significantly increasing the thruster power level and making improvements in performance parameters associated with NSTAR. The EPT concept under development has a 40 cm beam diameter, twice the effective area of the Deep-Space 1 thruster, while maintaining a relatively-small volume. It incorporates mechanical features and operating conditions to maximize the design heritage established by the flight NSTAR 30 cm engine, while incorporating new technology where warranted to extend the power and throughput capability.

Validation of the NSTAR Ion Propulsion System on the Deep Space One Mission: Overview and Initial Results

Deep Space 1 is the first interplanetary spacecraf t to use an ion propulsion system for the p r imary delta-v maneuvers. The purpose of the mission is to validate a number of technologies, including ion propulsion and a high degree of spacecraft autonomy, on a fiyby of an asteroid and two comets. The ion propulsion system has operated now for a total of 1791 hours at engine power levels ranging from 0.48 to 1.94 kW and has completed the deterministic thrusting requi red for an encoun te r w i th the asteroid 1992KD in late July, 1999. The system has worked extremely well after an initial grid short was cleared after launch. O p eration during this primary mission phase has demonstrated all ion propuls ion system and au tonomous navigation functions. All propulsion system operating parameters are ve ry close to the expected values wi th the excep tion of the thrust at higher power levels, which is about 2 percent lower than calculated values. This paper provides an overview of the system and presents the first flight validation data on an ion propulsion system i n interplanetary space.

Performance Evaluation of the NEXT Ion Engine

The performance test results of three NEXT ion engines are presented. These ion engines exhibited peak specific impulse and thrust efficiency ranges of 4060 4090 s and 0.68 0.69, respectively, at the full power point of the NEXT throttle table. The performance of the ion engines satisfied all project requirements. Beam flatness parameters were significantly improved over the NSTAR ion engine, which is expected to improve accelerator grid service life. The results of engine inlet pressure and temperature measurements are also presented. Maximum main plenum, cathode, and neutralizer pressures were 12,000 Pa, 3110 Pa, and 8540 Pa, respectively, at the full power point of the NEXT throttle table. Main plenum and cathode inlet pressures required about 6 hours to increase to steady-state, while the neutralizer required only about 0.5 hour. Steady-state engine operating temperature ranges throughout the power throttling range examined were 179 303 C for the discharge chamber magnet rings and 132 213 C for the ion optics mounting ring.

Low-Isp derated ion thruster operation

The performance and lifetime expectations of 30 cm xenon ion thruster technology at low values of specific impulse were evaluated, with emphasis on 1000-2500 s operation. Power levels of up to 2.0 kW, appropriate for auxiliary and orbit maneuvering propulsion, were processed at thrust-to-power ratios up to 57 mN/kW. These tests were conducted using a derated 30 cm ion thruster with high-perveance design two-grid ion optics with xenon propellant. Lifetime projections were made based on a simple analysis of critical component erosion rates, and it was found that a strong correlation exists with the ratio of the specific impulse-to-input power. Under all operating conditions for which the projected thruster lifetime is less than 10,000 hrs, the life-limiting component of this technology is erosion of the accelerator grid due to charge-exchange ions. The use of alternative grid materials such as carbon is estimated to increase useful thruster lifetimes by as much as an order of magnitude and may enable long-life high thrust-density, sub-2500 s Isp operation. The performance and life of the derated thruster appears similar to that of the Russian SPT-100 thruster in the 1.0-2.0 kW, 1600-2000 s operational envelope.

Performance of 10-kW class xenon ion thrusters

Presented are performance data for laboratory and engineering model 30 cm-diameter ion thrusters operated with xenon propellant over a range of input power levels from approximately 2 to 20 kW. Also presented are preliminary performance results obtained from laboratory model 50 cm-diameter cusp- and divergent-field ion thrusters operating with both 30 cm- amd 50 cm-diameter ion optics up to a 20 kW input power. These data include values of discharge chamber propellant and power efficiencies, as well as values of specific impulse, thruster efficiency, thrust and power. The operation of the 30 cm- and 50 cm-diameter ion optics are also discussed.

NEXT Study of Thruster Extended-Performance (NEXT STEP)

As ion propulsion technology under NASA’s Evolutionary Xenon Thruster (NEXT) project progresses through its development, the existing capability of the NEXT xenon ion thruster was reviewed. Specifically the possibilities of extending the NEXT thruster performance and life time capabilities beyond those demonstrated under the existing program were considered. Both high thrust-to-power operation and high thrust density high power operation were evaluated. Preliminary test data were obtained on an engineering model thruster at these conditions. Study and test data support the extension of NEXT thruster operation up to 50 mN/kW at sub-3000 seconds specific impulse, and increasing the dynamic power throttling range from the existing 13:1 ratio to 25:1 (> 13 kW operation).

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

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, NASAs 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

Current Density Measurements of an Annular-Geometry Ion Engine

The concept of the annular-geometry ion engine, or AGI-Engine, has been shown to have many potential benefits when scaling electric propulsion technologies to higher power. However, the necessary asymmetric location of the discharge cathode away from thruster centerline could potentially lead to non-uniformities in the discharge not present in conventional geometry ion thrusters. In an effort to characterize the degree of this potential non-uniformity, a number of current density measurements were taken on a breadboard AGI-Engine. Fourteen button probes were used to measure the ion current density of the discharge along a perforated electrode that replaced the ion optics during conditions of simulated beam extraction. Three Faraday probes spaced apart in the vertical direction were also used in a separate test to interrogate the plume of the AGI-Engine during true beam extraction. It was determined that both the discharge and the plume of the AGI-Engine are highly uniform, with variations under most conditions limited to +/-10% of the average current density in the discharge and +/-5% of the average current density in the plume. Beam flatness parameter measured 30 mm from the ion optics ranged from 0.85 - 0.95, and overall uniformity was shown to generally increase with increasing discharge and beam currents. These measurements indicate that the plasma is highly uniform despite the asymmetric location of the discharge cathode.

Annular engine development status

This publication documents the development status of the Annular-Geometry Ion Engine (AGI-Engine). The AGI-Engine is the design core for a new class of electric propulsion thrusters, referred to as the Next-Generation Electric Propulsion Thruster (NGEPT). The NGEPT is a single thruster concept, implementing an annular ring architecture, which potentially can be scaled and adapted for any power level at any specific impulse of interest. The AGI-Engine itself provides a pathway to substantial increases in thrust density (≥ 9 N/m 2 ), power density (≥ 260 kW/m 2 ) and input power (10’s-100’s of kW) relative to conventional ion thrusters, at < 5000 seconds specific impulse. Progress was demonstrated in the maturation of the concept with the design, fabrication, and testing of high-perveance flat annular geometry ion optics electrodes manufactured from pyrolytic graphite. The annular electrodes exhibit high beam collimation and high beam flatness while achieving a span-to-gap ratio of less than 1⁄2 that of the NEXT ion optics with a beam area ~ 97% of that of NEXT. Sufficient data were obtained to support the development of a full-scale highpower Annular Engine of 65 cm diameter with a potential input power capability of 21-62 kW over 2730-4670 seconds Isp.

Next-Generation Electric Propulsion Thrusters

*This publication documents the design, anticipated operational, and performance characteristics of a new class of (‘Next-Generation’) Electric Propulsion Thrusters. Two of several possible physical manifestations of the class are presented: one a new ion thruster design; and the second an electric-propulsion hybrid concept. In the former, the AnnularGeometry Ion Engine (AGI-Engine), substantial increases in input power (>10X, up to 100’s of kWe), and power density (>3X), might be realized relative to conventional ion thrusters. Other salient features of the scalable design may afford additional significant performance and life advantages. In the latter, a hybrid referred to as the Dual-Mode Hybrid-Engine (DMH-Engine), may provide a single device with the throttling range and specific impulse capability of both Hall-Effect thrusters and ion thrusters while exceeding both the efficiency (at fixed input power) and input power capability (at fixed specific impulse) of conventional ion thrusters of equivalent beam area.