Space Based Applications for FEA Cathodes (FEAC)
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Space Based Applications For FEA Cathodes (FEAC) B. E. Gilchrist, U. Michigan, Ann Arbor, MI 48109-2143 K. L. Jensen, Naval Research Lab, Washington, DC 20375-5000 A. D. Gallimore, U. Michigan, Ann Arbor, MI 48109-2140 J. G. Severns, Consultant ABSTRACT Cold cathodes such as field emitter arrays offer the potential to benefit or enable space-based applications of critical commercial, government, or military importance by providing an electron source that is low power, low cost, requires no consumables, potentially robust as well as highly reliable. Applications that would especially benefit from such cold cathodes include low power electric propulsion (EP) thruster technology, electrodyanamic tethers (ED) for propellantless propulsion in low-Earth orbit, and spacecraft negative potential charge control. In controlled environments, field emitter arrays have shown substantial capability, but have failed in harsher environments more typical of space applications. We argue that a combination of localized arc suppression coupled with a low work function, but nevertheless robust, coating such as zirconium carbide would provide the needed ruggedness to withstand energetic ions, oxygen fluxes, and adsorbates typical of a spacecraft environment. We have found that arc-protected and coated FEACs that can operate in a 1-10 microTorr pressure environment with current densities of less than 0.1 Amps/cm^2 and gate voltages between 50-100 Volts, would enable reliable, lowcost devices capable of operating in the required space environment. 1. Introduction FEACs appear to be a technology ready for rapid development as a new class of electron charge emission space-based applications which should lower the cost and/or be an enabling technology. The use of FEACs in these applications will significantly lower power consumption and eliminate or reduce consumable requirements over competing approaches. Specifically, it will enable miniaturized fuel-efficient electric propulsion (EP) systems such as the Closed-Drift Hall thruster (CDT) that enhance performance of small satellites by reducing spacecraft propellant mass. FEACs also enable use of “propellantless” ED tether propulsion for efficient spacecraft deorbit, atmospheric drag make-up, and orbit raising (250 to 2,000 km altitude). Additionally, FEACs provide a simple method to control charging of large spacecraft solar arrays. These space applications will require FEAC devices that: (1) emit on the order of 0.1 A/cm2; (2) operate in the 10-5 to 10-6 Torr pressure range (survive after operational exposure to 10-3 Torr); (3) require bias potentials of 50-100 V or less; and, (4) have sufficient life for multi-year space operations. Recent investigations suggest that each of these requirements can be achieved. What remains is to bring together the correct set of FEAC fabrication technologies to validate and qualify them for operation in the space environment. Our investigation suggests that to demonstrate FEACs for space applications, four critical FEAC development issues must be addressed:
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