Surface Morphology and Lifetime Assessment of a Re- generable Field-Emission Cathode for Low Power Electric Propulsion

The research reported here focuses on the surface topography of a field-emission cathode for use in Electric Propulsion (EP) that has the potential for very long lifetime and the added ability to be re-generated when the emitter sites become damaged. The cathode was formed by the application of an ion-extracting electric potential applied to a heated indium-coated tungsten needle. For reasons not understood at this time, the electrostatic field caused modification of the indium-oxide surface layer such that a large number of nano-sharp crystal protrusions were created in solid In2O3. When the modified needle was then subjected to electron-extracting potentials very stable and long-lived electron emission was observed. Electron field emission from the needle was observed to be stable for over 1,700 hours – a lifetime attributed to the presence of multiple crystal protrusions that allowed the emission to naturally jump to the next-best site after a single tip became blunted. Nomenclature a = Fowler-Nordheim term (see Equation 2) A = total emitting area (m2) b ’ = Fowler-Nordheim term (see Equation 3) k = empirical relation for tip radius and gap spacing Vo = extraction voltage (V) rt = emitter tip radius (m) = Nordheim image-correction term = work function (eV) = Fowler-Nordheim term I