Carbon nanotube Field Emission Cathode for Electric Space Propulsion

Low power electrostatic thrusters are greatly penalized by the lack of small efficient cathodes. Conventional hollow cathodes are relatively large and consume power and propellant at a rate approaching that required by the small thruster, thus greatly reducing its overall efficiency and Isp. An attractive alternative to hollow cathodes are fieId emission array (FEA) cathodes. These do not require propellant or a heater and operate at low power. Many FEA cathodes can be considered including the original metallic Spindt cathode, diamond like carbon (DLC) and diamond based cathodes. Most recently, carbon nanotube (CN) emitters have demonstrated excellent characteristics. Therefore the development of a CN based cathode will be the focus of our effort. In Phase I, we will grow carbon nanotubes using our proprietary process which yields CN bundles in a form directly usable for the FEA cathode. Busek has already demonstrated such cathode, achieving emission on-set at voltages an order of magnitude lower than the best known alternatives. The CN growth process will be optimized and emission from the CN bundles will be characterized. SRI International, our subcontractor, will perform SEM and TEM analysis of individual nanotubes. In Phase II, FEA cathodes suitable for operation with electrostatic thrusters will be constructed and tested on an existing low power Hall thruster. Small, power-limited satellites require low power, high Isp thrusters. Electrostatic thrusters (ion and Hall) are ideally suited for such applications. These thrusters require a reliable FEA cathode for efficient operation. A carbon nanotube based FEA cathode promises to satisfy this requirement. Terrestrial applications include all types of vacuum electronics, as in flat panel displays, microwave devices, radars, SEM, TEM devices, etc.