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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.