Electron Field Emission and the Space Charge Limit: Techniques and Tradeoffs

Cold cathode electron emission technology is being developed in a variety of forms for spacecraft charge control applications, ranging from solar panel protection to deep space electric propulsion. Cold cathode emission can provide significant efficiency savings (power, weight, space, etc.) over classic approaches (such as thermionic emitters and plasma contactors) for many classes of missions. However, regardless of the technology approach (closely matched gate-tip arrays, gridded carbon nano-tube plates, or more exotic techniques) for applying an electric field to a novel emitting surface, all are constrained by the space charge limits which apply whenever charge moves (at a certain velocity and density) across a gap (with a certain size and potential). No matter how efficiently the electrons are extracted from the emitter material, at least enough energy must be added to avoid the space charge limit, and this can be a significant additional cost in some applications.