Precision assessment of micro-thruster performance: A comparative study of indium field emission electric propulsion thrust measurement methods with a force-feedback pendulum

Accurate thrust assessment is crucial for characterizing the performance of micro-thrusters. This paper presents a comprehensive evaluation of the thrust generated by a needle-type indium field emission electric propulsion (In-FEEP) micro-thruster using three methods based on a pendulum: direct thrust measurement, indirect plume momentum transfer and beam current diagnostics. The experimental setup utilized capacitive displacement sensors for force detection and a voice coil motor as a feedback actuator, achieving a resolution better than 0.1 μN. Key performance factors such as ionization and plume divergence of ejected charged particles were also examined. The study reveals that the high applied voltage induces significant electrostatic interference, becoming the dominant source of error in direct thrust measurements. Beam current diagnostics and indirect plume momentum measurements were conducted simultaneously, showing strong agreement within a deviation of less than 0.2 μN across the operational thrust range. The results from all three methods are consistent within the error margins, verifying the reliability of the indirect measurement approach and the theoretical thrust model based on the electrical parameters of In-FEEP.