Re-generable field emission cathodes for electric propulsion

The thrusters that are used with electric space propulsion systems are equipped with an electron source for spacecraft neutralization and often for propellant ionization. The research presented in this dissertation was intended to explore a type of electron source that can be re-generated when its performance degrades -- restoring the electron source to its original like-new condition. The electron source of interest is a field emission electron source, which relies on electric field enhancement from nano- and micro-scale sharp emitter tips to create a beam of electrons. The re-generable emitter tips are formed by taking advantage of Taylor cone formation from an operating liquid metal ion source. Tip formation was accomplished by solidifying, or quenching, the ion-emitting cone to preserve the sharp ion-emitting nano-structures so that they can then be used for electron emission, and subsequently re-generated when they become damaged.To examine the feasibility of using re-generable field emitters with electric propulsion systems, a series of experiments were conducted to investigate the re-generable emitter tips. The experiments involved re-generating multiple emitter tips so that the electron performance of each re-generated tip could be evaluated and applied to the Fowler-Nordheim model to estimate the emitter tip radii. The re-generable emitter tips were also subjected to long duration electron emission tests under UHV conditions, as well as repeatedly being subjected to elevated pressures. The same duration and pressure elevation experiments were also performed with smooth and roughened pure tungsten field emitters for comparison purposes. Another experiment was conducted in the specimen chamber of a Field Emission Scanning Electron Microscope (FE-SEM) so that the nano-structures could be re-generated at ion emission current before quenching of 2 to 20 muA and then investigated while inside the microscope. After quenching, Fowler-Nordheim modeling was used to estimate the emitter tip radii and then the FE-SEM optics were used so that the surface morphology of the quenched emitters could be investigated visually to compare with the model.It was demonstrated that as the ion emission before quenching was increased, a decrease in emitter tip radius was observed. At ion emission currents greater than 10-15 muA a minimal plateau was reached for the tip radii that were formed. The re-generable emitter tips also demonstrated more stable electron emission than pure tungsten field emitters during 100's of hours of testing. Where the emission current from the pure tungsten emitters decreased over time, the emission current from the re-generable sources remained more stable. The FE-SEM investigation revealed that multiple nano-structures were formed upon quenching an operating liquid metal ion source and it was apparent that 30 seconds of ion emission operation is approximately the minimum time required before quenching to form all of the nano-structures that are generated. The multiple nano-structures could then be used to operate much like an array of electron field emitters since each of the nano-structures had similar electric field enhancement.The results of the experiments allowed for several conclusions to be made. The re-generable emitters proved to be more robust than pure tungsten emitters. Re-generable emitters continued to operate to higher vacuum pressure, on the order of 10-4 Torr, and for 10's of hours longer than the tungsten emitters. Also, repeated exposure to elevated pressure eventually caused catastrophic failure of the pure tungsten emitters whereas the re-generable emitters demonstrated the ability to re-generate sharp nano-structures after the emission deteriorated. The re-generated tips could then be used at lower extraction voltage than before re-generation. The FE-SEM investigation revealed that the Fowler-Nordheim estimation of emitter tip radii appeared to accurately predict the emitter tips to at least an order of magnitude. Higher resolution micrographs would be necessary but within the resolution of the acquired micrographs the model approximations appear realistic. The FE-SEM experiments also revealed multiple nano-structures that were never observed before. The structures created multiple locations of electric field enhancement which is beneficial for a field emitter. Combining all of information gathered from this research about emitter tip lifetime, tip robustness in elevated pressure, re-generability after performance degradation, and the fact that multiple nano-structures are created on the emitter tip, re-generable field emitters appear to have a great amount of potential to be further developed for electric propulsion systems.