Experimental study on hybrid multi-emitter gallium FEEP thruster*

Field emission electric propulsion (FEEP) thrusters are CubeSat-compatible electric thrusters characterized by high specific impulse and micronewton scale thrust. Since the thrust of a single-emitter FEEP thruster ranges from 1 to 10 μN, a multi-emitter configuration is required to satisfy various thrust demands for CubeSats. In this study, a multi-emitter with a 2x2 array of needle-capillary hybrid emitters with various extractor shapes was tested to derive an optimal design for uniform emission and high emitter packing density. For an extractor with a single hole, non-uniform emission with a tilted Taylor cone caused by a radially biased electric field was observed. Conversely, for an extractor with a 2x2 array of holes, uniform and axis-aligned ion emission from all four emitters was confirmed. Although the grid array of extractor holes demonstrated stable and uniform emission, further improvement in the emitter packing density is still required. To enhance packing density, a linear array of emitters with slit-shaped extractor holes is planned for future investigation.