NEXT Ion Engine 2000 Hour Wear Test Results

Abstract

The results of the NEXT 2000 h wear test are presented. This test was conducted with a 40 cm engineering model ion engine, designated EM1, at a 3.52 A beam current and 1800 V beam power supply voltage. Performance tests, which were conducted over a throttling range of 1.1 to 6.9 kW throughout the wear test, demonstrated that EM1 satisfied all thruster performance requirements. The ion engine accumulated 2038 h of operation at a thruster input power of 6.9 kW, processing 43 kg of xenon. Overall ion engine performance, which includes thrust, thruster input power, specific impulse, and thrust efficiency, was steady with no indications of performance degradation. The ion engine was also inspected following the test. This paper presents these findings. I. IntroductionThe success of the NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR) program's ion propulsion system on the Deep-Space 1 spacecraft has secured the future for ion propulsion technology for other NASA missions.1 While the 2.3 kW NSTAR ion engine input power and service life capabilities are appropriate for low power missions, in-space propulsion technology analyses conducted by NASA identified the need for a higher power, higher throughput capability 25 kW ion propulsion system targeted for robotic exploration of the outer planets. As a result, NASA's Office of Space Science awarded a research project to a NASA Glenn Research Center (GRC)-led team to develop the next generation ion propulsion system. 2,3 The propulsion system, called NASA's Evolutionary Xenon Thruster (NEXT), consists of a 40 cm diameter ion engine, a lightweight, modular power processing unit with an efficiency and a specific power equal-to or better-than the NSTAR power processor, and a xenon feed system which uses proportional valves and thermal throttles to significantly reduce mass and volume relative to the NSTAR feed system.Ion engine performance requirements include a specific impulse and thruster efficiency of at least 4050 and 0.67, respectively, at full power. Based on NEXT Phase 1 design reference missions, the ion engine must provide a 270 kg propellant throughput capability at full power, which ultimately results in a 405 kg qualification throughput requirement.The service life capability of the NEXT ion engine is being assessed by engine wear tests and modeling of critical engine components, such as the ion optics and cathodes. The first wear test of a NEXT ion engine was planned for a 2000 hour duration at a 6.9 kW input power. The highest thruster input power level was chosen because modeling has predicted that this power level will cause the severest engine erosion. The objectives of this early wear test included: characterizing thruster operation and performance over the duration of the test; identifying thruster life-limiting phenomena; and measuring critical thruster component wear rates.This paper presents the results of the first NEXT wear test. A description of the test article is discussed in the next section. This is followed by a descripti...

Computational Ion Optics Design Evaluations

“…Problem During the NEXT 2000-hour wear test, accelerator apertures in the outer 47 mm of the grids exhibited an offcenter, hexagonal star-shaped erosion pattern. 12 The eroded points of the stars were chamfered through the electrode thickness and extent of the erosion pattern generally increased with increasing ion optics' radius (see Figure 2). It was determined that this erosion likely occurred early in the wear test and at lower beam current operation (1.20 A and 2.70 A).…”

Progress In NEXT Ion Optics Modeling

“…In the 2000 hour wear test of the NEXT ion thruster 6 , a large amount of erosion was observed in the apertures at the edge of the extraction grids compared to the central regions 7 . The erosion was in the form of irregular star patterns in general, and occurred past the 15.1 cm radius point on the grids.…”

Lifetime Assessment of the NEXT Ion Thruster

“…However, during the NEXT 2000 hour wear test, higher than anticipated keeper wear was found. 17 The cross section of the keeper after the test is shown in Figure 3. The wear profile can be seen and the maximum thickness eroded is roughly at a radius of 40 % of the total keeper radius.…”

“…This would correspond to 380 kg of xenon throughput and agrees with a pretest prediction of 375 kg. 17 This is less that than the 450 kg throughput qualification level, but does not constitute an end of life, rather the point at which the heater would be exposed to the plasma and experience increased wear. An effort is continuing to improve the keeper model and apply it to the NEXT graphite keeper.…”

“…The NEXT Engineering Model 1 (EM1) thruster was wear-tested for 2038 hours (43 kg of xenon throughput) to evaluate thruster wear. 17 Two wear mechanisms were revealed that could potentially lead to a premature failure. One was the excessive wear on the discharge cathode keeper orifice plate and the other was excessive wear on the accelerator grid holes beyond a radius of 15.3 cm.…”