This research investigates the discharge characteristics of a field emission cathode for use in electric propulsion that has the ability to be regenerated when the emitter tip becomes damaged. Emitter tip regeneration is achieved by taking advantage of Taylor cone formation from an operating liquid-metal ion source. Tip formation is accomplished by solidifying, or quenching, the ion-emitting cone to preserve the sharp protrusion so that it can then be used for electron emission. Electron emission I-V curves were taken after tips were formed by quenching the liquid-metal ion source at ion discharge currents ranging from 1 to 25 A. Fowler-Nordheim modeling was then used to estimate the emitter tip radii of each quenched liquid-metal ion source. Results of the Fowler-Nordheim modeling were promising, showing the ability to regenerate tips and to control the features of the resulting tips by varying the ion current during the quench process. The set of experiments that are reported demonstrated the regeneration process of emitter tip radii ranging from approximately 30-45 nm from a tip quenched at 2 A down to tip radii of 15-22 nm when quenched at 25 A.

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This paper reports on a field-emission cathode for use in electric propulsion that has the potential for very long lifetime due to its ability to be regenerated when the emitter tip(s) become damaged. The field-emitting tips were formed by applying an ion-extracting electric potential to a heated indium-coated tungsten needle, known as a liquidmetal ion source. The liquid-metal ion source was then cooled, freezing in a solid nanotip at the apex. When the modified emitter was then subjected to electron-extracting potentials, stable and long-lived electron emission was observed. The first goal of this investigation was to operate and quench a liquid-metal ion source at ion emission currents from 1 to 30 A to acquire micrographs of the surface morphology as a function of the ion emission current at quench. Micrographs of the quenched emitter tips revealed Taylor-cone-shaped structures. Some of the quenched emitters exhibited multiple nanoprotrusions on the tapered surface of the microscale Taylor cone, which were capable of electron field emission. The second goal of this investigation was to compare regenerable field emitters with single-needle tungsten field emitters. Each type of emitter was used to obtain electron emission at a vacuum chamber pressure of 10 8 Torr, and then the emitters were exposed to increased pressure, up to 10 5 Torr, to observe how long they could sustain emission. In all cases, emission from the regenerable emitters lasted 10s of hours longer at increased pressure, and it was demonstrated that the tungsten emitters would eventually permanently fail, whereas the regenerable emitters could be repaired when they became damaged. Nomenclature a = Fowler-Nordheim empirical relation b 0 = Empirical relation between tip radius and gap spacing k = Fowler-Nordheim field-voltage proportionality factor m = slope r t = emitter tip radius s m = standard error t e = elapsed time = Fowler-Nordheim image-correction factor ' = work function