Regenerable Field Emission Cathodes: Surface Morphology and Performance

Abstract

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

Influence of Magnetic Nanoparticles and Magnetic Stress on an Ionic Liquid Electrospray Source

“…8. A similar trend was also seen inFigure 7.9. between magnetic field strength and the ratio of the relative intensity of n = 0 to the relative intensity of the peak at 430 amu/e.…”

“…just the act of operating a source causes irreparable damage to the emission site. [8][9][10] A solution to the challenges associated with background structures is to eliminate them.…”

“…8 andFigure 4.11. Therefore, the physical mechanism for the increase in necessary extraction potential when using ILFF propellants remains uncertain, though it appears the NPs are at least partially responsible.The lower bound of flowrate for the CES stability island did not appear to correlate to the weight-percent of suspended nanoparticles.…”