High-Specific Impulse Operation of the BPT-4000 Hall Thruster for NASA Science Missions

Abstract

To meet the propulsive requirements, cost constraints, and scientific objectives of NASA deep-space missions, high-performance, low-cost electric propulsion (EP) options are needed. Commercial EP technology has matured sufficiently that ion and Hall thrusters are now leading candidates for many of these missions. A commercial Hall thruster system is as much as 50% less expensive than an equivalent ion thruster system, but the maximum specific impulse of commercial, flight-qualified Hall thrusters is presently 2000 s. Extending the specific impulse to 2500-3000 s would increase the Hall thruster mission capture to include nearly all of the missions where EP was applicable. To assess the suitability of extending the specific impulse range of a candidate commercial Hall thruster, a test campaign designed to assess the performance and plasma properties of Aerojet’s BPT-4000 during high-specific impulse operation was executed. An inverted-pendulum thrust stand was used for performance measurements. An RPA, ExB probe, and emissive probe were used to measure the ion energy, ion species’ current fractions, and plasma potential, respectively, in the far-field plume. For operating conditions spanning 500-800 V discharge voltage and 2.5-5.5 kW discharge power, thrust varied from 134-288 mN, specific impulse from 1980-2720 s, and efficiency from 0.50-0.59. It was concluded that operating the BPT4000 at specific impulses of 2600 s and 4.5 kW discharge power was within the existing capabilities of the magnetic circuit, thermal margins, and voltage isolation. Based on these results, a delta-qualification for high specific impulse operation and higher propellant throughput via additional testing of the qualification thruster is recommended in order to ready the thruster for near-term infusion into NASA science missions.