Microfabrication of Capillary Electrospray Emitters and ToF Characterization of the Emitted Beam

Microfabrication, assembly, and characterization of internally fed arrays of electrospray emitters for spacecraft propulsion are discussed. Several different emitter geometries were fabricated, and the sprayed beams from these emitters are characterized for their ion/droplet composition. It is shown that with smaller inner diameters, ionic mode of operation can be achieved more easily, due to the increase in the hydraulic impedance. Up to 2000 s Isp was measured from capillaries with 5 μm inner diameter and 100 μm with 750V extraction voltage using EMI-BF4. Due to the small dimensions of the microfabricated capillaries, the onset and ionic mode operation voltages can be significantly lower thanmacroscopic emitters. The major failure mode for the emitters was the liquid overflow from the capillary tip, which can be resolved through coatings that wet the ionic liquids differently. A detailed study of the wetting behavior of two ionic liquids is also presented for various materials compatible with silicon microfabrication technology.