Optimizing space-charge limits of electron emission into plasmas with application to in-space electric propulsion

The topic of this thesis is the improvement of space-charge limits for electron emission into a plasma, primarily as applied to in-space electric propulsion applications. The space-charge limit is the point at which the electrostatic forces in a beam of emitted charged particles becomes sufficient to slow down and reflect new particles as they are emitted. Any emitter operating above this limit will suffer severe efficiency losses, as an average of 50% or more of the beam is reflected back to the emitter. This limit is especially salient to small spacecraft electric propulsion and other applications where power spent on electron emission is at a premium, such that it is not acceptable to improve the space-charge limit by simply adding energy to the emitted beam. The primary motivation of this work is field effect electron emitters because they are capable of high-current, low-power emission, but the results are applicable to thermionic and other types of emitters as well.; Several techniques were studied including variation of emitter size and spacing, the addition of defocus rings, and time and spatial modulation of the emitted beam. Separating large emitters into multiple small emitters provided the greatest improvement, and trade studies are given showing the balance between integration cost and emission power. Defocus rings were found to be capable of improving emission from small emitters by 40% even in their most simple form of a grounded ring. Modulation of the beam was less effective at improving the space-charge limit, with a modulated beam having at best the same time-average space-charge limit as an un-modulated beam. It was discovered, however, that beam modulation significantly increases the efficiency of emission when the emitter must operate at current levels higher than the space-charge limit. Finally, it was confirmed that these results are consistent both for emission across a vacuum gap to a conducting anode, and across a plasma sheath into a plasma.; This research was performed primarily via particle-in-cell (PIC) simulations, using the code XOOPIC developed at Berkley, with support by analytical calculations. Some preliminary experimental work is presented in the appendices.