A critical review of published thrust models for applied-field magnetoplasmadynamic thrusters is presented, along with a new model addressing shortcomings related to electrode and magnet geometry. While numerous theoretical thrust models have been presented in the literature, there has not been a comprehensive comparison to determine which best predicts thruster behavior across a large parameter space. In order to make this determination, all thrust data available in the literature were collected into a single database and tested against each model. Unlike previous comparisons between prediction and measurement, only the regime in which the applied-field thrust component dominates is examined, allowing for a direct comparison without invoking models of self-field and gasdynamic components of thrust. The degree to which each model deviates from measurement is determined for each controllable parameter. It is found that the largest deviations are due to incorrect representation of the effects of electrode and solenoid geometries. In light of this comparative study, an improved empirical model is derived as a function of nondimensional parameters representing these geometric variables. The improved agreement is attributed to the effects of magnetic field topology near the anode, which sets the effective anode radius that controls the magnitude of the Lorentz force for certain electrode geometries.

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. Nomenclature a 0 Ion sound speed, m/s A Area, m 2 B A Applied magnetic field, T J Current, A k Thrust coefficienṫ m Mass flow rate, kg/s p Pressure, N/m 2 r Electrode radius, m T Thrust, N Subscript ae Anode exit plane AF Applied-field c Cathode GD Gasdynamic SF Self-field

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