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Direct Measurement of the Applied-Field Component of the Thrust of a Lithium Lorentz Force Accelerator

Abstract

The operation of an inverted-pendulum thrust stand designed to directly measure the applied-field thrust component of a steady-state applied-field magnetoplasmadynamic thruster is presented. This measurement, which is necessary for improving and validating thrust models, is achieved by mechanically isolating the solenoid from the thruster so that the solenoid can move independently. The deflection of the solenoid is compared to deflection by known forces to determine the applied-field thrust component. The thrust stand is found to be accurate to ±9 mN over a total range of 1200 mN. Two measurement methods are implemented to account for tare forces resulting from azimuthal currents to the thruster electrodes and are shown to agree with one another. As a proof-of-concept, the first direct measurements of the applied-field component of the thrust from a 30 kW lithium Lorentz force accelerator operating at 400 A, 8 mg/s lithium mass flow rate, and 0.056 T applied-field strength give a measured force of 108 ±14 mN. This measurement agrees with the value predicted by measuring the total thrust and subtracting out all other thrust components.