The Role of Magnetic Field Topography in Improving the Performance of High-Voltage Hall Thrusters

Investigations of high-voltage Hall thrusters have indicated a peak in the efficiency versus voltage characteristic that limited the maximum efficiency to specific impulses of less than 3000 s. This peak is believed to be primarily a trait of modern magnetic field design, which is optimized for discharge voltages of 300 V. The NASA-173M has been operated at 300-1000 V and 5 mg/s to investigate whether performance improvements could be realized through in situ variation of the magnetic field topography through the use of an auxiliary trim coil. Without the trim coil, a peak in the efficiency characteristic was observed at 600 V. The results with the trim coil energized indicate there is always some performance benefit to altering the magnetic field topography. Above 400 V, efficiencies were maintained at >50% and above 900 V, specific impulses >3000 s were demonstrated while using the trim coil. The largest gains in performance were observed at 1000 V, where the thrust, specific impulse, and efficiency improved by 10 mN, 200 s, and 5.5%, respectively, to 165 mN, 3360 s, and 51.5%. The results demonstrate that the peak in the efficiency characteristic observed without the trim coil can be mitigated when the magnetic field topography is tailored for high-voltage operation. Analysis of the magnetic field from numerical simulations has identified several important factors contributing to the performance benefits with trim coil operation. Introduction Mission studies have indicated that a variable specific impulse (VIPS) Hall effect thruster (HET) or ion thruster could provide substantial benefits to a variety of spacecraft missions. These have included LEO and GEO stationkeeping and orbit insertion, GEO reusable tug missions, and interplanetary probes. Ironically, to achieve VIPS a need was identified with both technologies to overcome lifetime issues at specific impulses (Isp) where the other technology has previously demonstrated high propellant throughput. In the case of the ion thruster, operation at Isp’s as low as 1800 s was required, where grid erosion is enhanced due to decreased beamlet focusing. For the HET, operation above 3000 s was needed, where the higher energy ions are expected to cause enhanced erosion rates of chamber walls. To address the needs identified in the mission studies, as well as on-going efforts to expand current HET technology, the NASA Glenn Research Center has in recent years initiated a multi-phased program for the development of high Isp HETs. In phase one, contracts were awarded to develop thrusters that demonstrate the feasibility of operating modern xenon HETs at Isp’s greater than 3000 s. The three engines that were delivered were the D-80 from TsNIIMASH under contract to Boeing, the SPT-1 from Fakel under contract to the Atlantic Research Corporation, and the BHT-1000 from Busek. The thrusters have been operated at maximum voltages and Isp’s of 1700 V / 4100 s, 1250 V / 3700 s, and 1000 V / 3300 s for the D80, SPT-1 and BHT-1000, respectively. Despite being different designs, each thruster exhibited a voltage range where the anode efficiency was maximized. This voltage was roughly between 500-800 V depending on the mass flow rate. The source of this peak efficiency was not identified, but there was speculation by some authors that the loss was being driven by increased electron current at high-voltages. Phase one successfully demonstrated that there were no fundamental restraints to building a HET capable of >3000 s Isp. Further, despite the observed peak, anode efficiencies were still maintained above 50% for a wide range of voltages and flow rates.