Magnetic Field Design, Optimization, Physical Assembly, and Characterization for a High-Power ECR Thruster

Abstract

This thesis presents the design, manufacturing, and magnetic characterization of a hybrid solenoid–permanent magnet field source for a high-power electron cyclotron resonance (ECR) thruster. The magnetic assembly was developed to support a Space Force high-power ECR thruster concept requiring an approximately 875 gauss resonance region near the base of a boron nitride discharge chamber while maintaining a field topology that guides plasma through the chamber. The final design combines a large-radius solenoid with a fourteen-element permanent magnet array. The permanent magnets provide a static bias field that reduces the solenoid current requirement, while the solenoid provides an approximately axisymmetric contribution that tunes the ECR surface location and improves axial field-line guidance. The design evolved from an initially permanent-magnet-dominated concept into a manufacturable hybrid assembly. Early radial permanent magnet concepts were rejected because they were cost prohibitive, mechanically complicated, and incompatible with the need for a split assembly. The final permanent magnet architecture instead uses fourteen off-the-shelf rectangular N42 magnets arranged around a tetradecagonal inner boundary. A key design improvement was increasing the magnet dimensions from 1 × 1 × 1 in. to 1 × 1 × 2 in.; this increased the useful field contribution while preserving nearly the same chamber-facing magnetic boundary. The solenoid was moved outward to a larger radius to improve field-line straightness, and the final coil was manufactured with 648 turns in a 27-turn by 24-layer winding. The measured coil resistance was approximately 1 ohm, consistent with pre-manufacturing estimates. Magnetic characterization was performed using a gaussmeter probe at a fixed solenoid current of 8.5 A. The measured centerline axial field reached 877.4 gauss at the upstream measurement location, satisfying the 875 gauss ECR target. A two-component B(r,z) field map confirmed that the measured field structure was consistent with the intended magnetic nozzle-like topology, with a strong upstream field, downstream decay, and controlled outward field-line divergence. A 37-point backplate honeycomb measurement showed that the axial field varied smoothly in azimuth and followed the same general trend as the simulated field. The resulting characterization verified that the manufactured hybrid magnetic assembly achieved the required ECR-strength field, preserved the intended field shape, and maintained acceptable backplate axial-field uniformity despite the use of discrete permanent magnets.