1 American Institute of Aeronautics and Astronautics ELECTRON FIELD EMISSION AND THE SPACE CHARGE LIMIT: TECHNIQUES AND TRADEOFFS Dave Morris * and Brian Gilchrist † University of Michigan, Ann Arbor, MI, 48109 * Graduate Student, Electrical Engineering and Space Systems † Professor, Electrical Engineering and Space Systems, AIAA Associate Fellow ABSTRACT 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. Analytic solutions for the space charge limit in simple geometries have been developed up to three dimensions, but there are a variety of factors that can be manipulated in the design of a physical system that cannot be practically and generically included in such analysis. Via particle-in-cell modeling, a number of techniques for mitigating the space charge limit are being studied, ranging from atypical geometries to spatial and chronological distribution of emission. Some results of explorations of these techniques will be presented here. NOMENCLATURE FE=field emitter SCL=space charge limit PIC=particle-in-cell Vgt = gate tip voltage ε 0 : permittivity of free space e: electron charge [C] m e : electron mass [kg] T o : electron emission energy [eV] D: gap spacing [m] V: gap voltage [V]- equal to spacecraft bias with respect to the plasma W: emitter width r b : emitter radius A: emitter area [m 2 ] s= sheath size [m] J CL (N) = N dimensional Child-Langmuir current limit [A/m 2 ] INTRODUCTION There are a variety of space applications that require electron charge emission. Any electric propulsion thruster (Hall Thrusters, Ion thrusters such as DS-1, etc.) requires the simultaneous emission of electrons to balance the emitted ion charge. Space electrodynamic tethers generate thrust via 39th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit 20-23 July 2003, Huntsville, Alabama AIAA 2003-4792 Copyright © 2003 by David Morris. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.