Daniel A. Herman

Performance Evaluation of the Prototype-Model NEXT Ion Thruster

Abstract The performance testing results of the first prototype model NEXT ion engine, PM1, are presented. The NEXT program has developed the next generation ion propulsion system to enhance and enable Discovery, New Frontiers, and Flagship-type NASA missions. The PM1 thruster exhibits operational behavior consistent with its predecessors, the engineering model thrusters, with substantial mass savings, enhanced thermal margins, and design improvements for environmental testing compliance. The dry mass of PM1 is 12.7 kg. Modifications made in the thruster design have resulted in improved performance and operating margins, as anticipated. PM1 beginning-of-life performance satisfies all of the electric propulsion thruster mission-derived technical requirements. It demonstrates a wide range of throttle-ability by processing input power levels from 0.5 to 6.9 kW. At 6.9 kW, the PM1 thruster demonstrates specific impulse of 4190 s, 237 mN of thrust, and a thrust efficiency of 0.71. The flat beam profile, flatness parameters vary from 0.66 at low-power to 0.88 at full-power, and advanced ion optics reduce localized accelerator grid erosion and increases margins for electron backstreaming, impingement-limited voltage, and screen grid ion transparency. The thruster throughput capability is predicted to exceed 750 kg of xenon, an equivalent of 36,500 hr of continuous operation at the full-power operating condition.

Application of the NEXT Ion Thruster Lifetime Assessment to Thruster Throttling

Abstract Ion thrusters are low thrust, high specific impulse devices with typical operational lifetimes of 10,000 to 30,000 hr over a range of throttling conditions. The NEXT ion thruster is the latest generation of ion thrusters under development. The NEXT ion thruster currently has a qualification level propellant throughput requirement of 450 kg of xenon, which corresponds to roughly 22,000 hr of operation at the highest input power throttling point. This paper will provide a brief review the previous life assessment predictions for various throttling conditions. A further assessment will be presented examining the anticipated accelerator grid hole wall erosion and related electron backstreaming limit. The continued assessment of the NEXT ion thruster indicates that the first failure mode across the throttling range is expected to be in excess of 36,000 hr of operation from charge exchange induced groove erosion. It is at this duration that the groove is predicted to penetrate the accelerator grid possibly resulting in structural failure. Based on these lifetime and mission assessments, a throttling approach is presented for the Long Duration Test to demonstrate NEXT thruster lifetime and validate modeling .

NEXT multi-thruster array test -Engineering demonstration

A multi-thruster array test was executed at NASA Glenn Research Center, focusing on the characterization of individual thruster, and array, performance and behavior – as affected by the simultaneous operation of multiple ion thrusters; a key step in development of the NEXT ion propulsion system. The subject of this characterization effort was a four engineering model NEXT thruster array in a 3+1 flight-representative configuration where one thruster was dormant (a spare). This test was executed concurrent with detailed plasma environments and plume measurements documented elsewhere. The array was operated over a broad range of conditions including the simultaneous firing of 3 thrusters at 20.6 kW total input power, yielding a total thrust of about 710 mN, at 4190 seconds specific impulse and approximately 71 percent efficiency. Major findings from a series of tests include: the performance observed for a thruster during operation in an array configuration appears to be consistent with that measured during singular thruster operation with no apparent deleterious interactions; and, operation of 1 neutralizer to neutralize 2-or-more thruster beams appears to be a potentially viable fault-recovery mode, and viable system architecture with significant system performance advantages. Overall, the results indicating single thruster operations are generally independent of array configuration have potentially significant implications with respect to testing requirements and architectural flexibility for multi-thruster systems.

Discharge Chamber Plasma Structure of a 30-cm NSTAR-Type Ion Engine

†‡ Langmuir probe plasma parameter data are presented over a two-dimensional array of spatial locations in the discharge chamber of a 40-cm diameter ring-cusp NEXT ion thruster. The discharge plasma structure is determined by the magnetic circuit, which reduces the diffusion of electrons across magnetic field lines. Number density profiles exhibit a highly collimated plume structure confined by the axial magnetic field near the discharge cathode assembly (DCA). A maximum value of 2x10 13 cm -3 is measured along centerline at the keeper exit plane. A number density of ~5x10 11 cm -3 is observed outside of the discharge cathode plume. Electron temperatures in the cathode plume range from 2 – 4 eV increasing off-axis in the radial direction to 4 – 8 eV in the bulk discharge plasma. This increase is explained by electron acceleration across the free-standing potential gradient, termed a double layer, forming the transition between the discharge cathode plume and bulk discharge plasma. Between the magnetic cusps, near the anode, the electron temperature increases significantly. Only the high-energy electrons can cross the magnetic field lines to occupy these regions resulting in an increased electron temperature. Shorting of the discharge keeper to discharge cathode common, a condition of interest from ground-based NSTAR testing, does not have an effect on the near plasma structure outside of the DCA keeper sheath.

Performance Test Results of the NASA-457M v2 Hall Thruster

Abstract Performance testing of a second generation, 50 kW-class Hall thruster labeled NASA-457M v2 was conducted at the NASA Glenn Research Center. This NASA-designed thruster is an excellent candidate for a solar electric propulsion system that supports human exploration missions. Thruster discharge power was varied from 5 to 50 kW over discharge voltage and current ranges of 200 to 500 V and 15 to 100 A, respectively. Anode efficiencies varied from 0.56 to 0.71. The peak efficiency was similar to that of other state-of-the-art high power Hall thrusters, but outperformed these thrusters at lower discharge voltages. The 0.05 to 0.18 higher anode efficiencies of this thruster compared to its predecessor were primarily due to which of two stable discharge modes the thruster was operated. One stable mode was at low magnetic field strengths, which produced high anode efficiencies, and the other at high magnetic fields where its predecessor was operated. Cathode keeper voltages were always within 2.1 to 6.2 V and cathode voltages were within 13 V of tank ground during high anode efficiency operation. However, during operation at high magnetic fields, cathode-to-ground voltage magnitudes increased dramatically, exceeding 30 V, due to the high axial magnetic field strengths in the immediate vicinity of the centrally-mounted cathode. The peak thrust was 2.3 N and this occurred at a total thruster input power of 50.0 kW at a 500 V discharge voltage. The thruster demonstrated a thrust-to-power range of 76.4 mN/kW at low power to 46.1 mN/kW at full power, and a specific impulse range of 1420 to 2740 s. For a discharge voltage of 300 V, where specific impulses would be about 2000 s, thrust efficiencies varied from 0.57 to 0.63.

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

NASA is developing mission concepts for a solar electric propulsion technology demonstration mission. A number of mission concepts are being evaluated including ambitious missions to near Earth objects. The demonstration of a high-power solar electric propulsion capability is one of the objectives of the candidate missions under consideration. In support of NASAs exploration goals, a number of projects are developing extensible technologies to support NASAs near and long term mission needs. Specifically, the Space Technology Mission Directorate Solar Electric Propulsion Technology Demonstration Mission project is funding the development of a 12.5-kW magnetically shielded Hall thruster system to support future NASA missions. This paper presents the design attributes of the thruster that was collaboratively developed by the NASA Glenn Research Center and the Jet Propulsion Laboratory. The paper provides an overview of the magnetic, plasma, thermal, and structural modeling activities that were carried out in support of the thruster design. The paper also summarizes the results of the functional tests that have been carried out to date. The planned thruster performance, plasma diagnostics (internal and in the plume), thermal, wear, and mechanical tests are outlined.

NEXT Long-Duration Test Plume and Wear Characteristics after 16,550 h of Operation and 337 kg of Xenon Processed

The NASA’s Evolutionary Xenon Thruster (NEXT) program is developing the nextgeneration ion propulsion system with significant enhancements beyond the state-of-the-art. The NEXT ion propulsion system provides improved mission capabilities for future NASA science missions to enhance and enable Discovery, New Frontiers, and Flagship-type NASA missions. As part of a comprehensive thruster service life assessment utilizing both testing and analyses, a Long-Duration Test (LDT) was initiated to validate and qualify the NEXT propellant throughput capability to a qualification-level of 450 kg, 1.5 times the missionderived throughput requirement of 300 kg. This wear test is being conducted with a modified, flight-representative NEXT engineering model ion thruster, designated EM3. As of June 25, 2008, the thruster has accumulated 16,550 hours of operation: the first 13,042 hours at the thruster full-input-power of 6.9 kW with 3.52 A beam current and 1800 V beam power supply voltage. Operation since 13,042 hours, i.e., the most recent 3,508 hours, has been at an input power of 4.7 kW with 3.52 A beam current and 1180 V beam power supply voltage. The thruster has processed 337 kg of xenon surpassing the NSTAR propellant throughput demonstrated during the extended life testing of the Deep Space 1 flight spare. The NEXT LDT has demonstrated a total impulse of 13.3x10 6 N·s; the highest total impulse ever demonstrated by an ion thruster. Thruster plume diagnostics and erosion measurements are obtained periodically over the entire NEXT throttle table with input power ranging 0.5 – 6.9 kW. Observed thruster component erosion rates are consistent with predictions and the thruster service life assessment. There have not been any observed anomalous erosion and all erosion estimates indicate a thruster throughput capability that exceeds ~750 kg of xenon, an equivalent of 36,500 hrs of continuous operation at the fullpower operating condition. This paper presents the erosion measurements and plume diagnostic results for the NEXT LDT to date with emphasis on the change in thruster operating condition and resulting impact on wear characteristics. Ion optics’ grid-gap data, both cold and operating, are presented. Performance and wear predictions for the LDT throttle profile are presented.

Status of the NEXT Ion Thruster Long-Duration Test After 10,100 hr and 207 kg Demonstrated

The NASA s Evolutionary Xenon Thruster (NEXT) program is developing the next-generation ion propulsion system with significant enhancements beyond the state-of-the-art in ion propulsion to provide future NASA science missions with enhanced mission capabilities at a low total development cost. As part of a comprehensive thruster service life assessment utilizing both testing and analyses, a Long-Duration Test (LDT) was initiated to validate and qualify the NEXT propellant throughput capability to a qualification-level of 450 kg, 1.5 times the mission-derived throughput requirement of 300 kg. This wear test is being conducted with a modified, flight-representative NEXT engineering model ion thruster, designated EM3. As of June 21, 2007, the thruster has accumulated 10,100 hr of operation at the thruster full-input-power of 6.9 kW with 3.52 A beam current and 1800 V beam power supply voltage. The thruster has processed 207 kg of xenon and demonstrated a total impulse of 8.5 106 N-s; the highest total impulse ever demonstrated by an ion thruster in the history of space propulsion. Thruster performance tests are conducted periodically over the entire NEXT throttle table with input power ranging 0.5 to 6.9 kW. Overall ion thruster performance parameters including thrust, input power, specific impulse, and thruster efficiency have been nominal with little variation to date. Lifetime-limiting component erosion rates have been consistent with the NEXT service life assessment, which predicts the earliest failure sometime after 750 kg of xenon propellant throughput; well beyond the mission-derived lifetime requirement. The NEXT wear test data confirm that the erosion of the discharge keeper orifice, enlarging of nominal-current-density accelerator grid aperture cusps, and the decrease in cold grid-gap observed during the NSTAR Extended Life Test have been mitigated. This paper presents the status of the NEXT LDT to date.

NEXT Single String Integration Test Results

As a critical part of NASAs Evolutionary Xenon Thruster (NEXT) test validation process, a single string integration test was performed on the NEXT ion propulsion system. The objectives of this test were to verify that an integrated system of major NEXT ion propulsion system elements meets project requirements, to demonstrate that the integrated system is functional across the entire power processor and xenon propellant management system input ranges, and to demonstrate to potential users that the NEXT propulsion system is ready for transition to flight. Propulsion system elements included in this system integration test were an engineering model ion thruster, an engineering model propellant management system, an engineering model power processor unit, and a digital control interface unit simulator that acted as a test console. Project requirements that were verified during this system integration test included individual element requirements ; integrated system requirements, and fault handling. This paper will present the results of these tests, which include: integrated ion propulsion system demonstrations of performance, functionality and fault handling; a thruster re-performance acceptance test to establish baseline performance: a risk-reduction PMS-thruster integration test: and propellant management system calibration checks.

NASA HERMeS Hall Thruster Electrical Configuration Characterization

The NASA Hall Effect Rocket with Magnetic Shielding (HERMeS) 12.5 kW Technology Demonstration Unit-1 (TDU-1) Hall thruster has been the subject of extensive technology maturation in preparation for development into a flight ready propulsion system. Part of the technology maturation was to test the TDU-1 thruster in several ground based electrical configurations to assess the thruster robustness and suitability to successful in-space operation. The ground based electrical configuration testing has recently been demonstrated as an important step in understanding and assessing how a Hall thruster may operate differently in-space compared to ground based testing, and to determine the best configuration to conduct development and qualification testing. This paper describes the electrical configuration testing of the HERMeS TDU-1 Hall thruster in NASA Glenn Research Centers Vacuum Facility 5. The three electrical configurations examined were 1) thruster body tied to facility ground, 2) thruster floating, and 3) thruster body electrically tied to cathode common. The HERMeS TDU-1 Hall thruster was also configured with two different exit plane boundary conditions, dielectric and conducting, to examine the influence on the electrical configuration characterization.