Carbon Nanotube Electron Sources for Space Propulsion Applications

Low-power, low-voltage, efficient field emission neutralizers for FEEPS, colloid thrusters, and other micro-propulsion engines are attractive for nanosatellites because they do not use mass flowrate to operate, unlike more conventional neutralizing solutions such as hollow cathodes. Electronics are field-emitted from the surfaces of metals and semiconductors by the application of a high elecrostatic field. Field emitters use high aspect ratio structures to generate very high fields even when low voltages are applied. The ideal field enhancing structure is a rounded whisker. Micro-engineered field emission neutralizers would have smaller starting voltages, better area usage, and more uniform I-V characteristics, compared to macro/meso fabricated field emitter versions.

Plasma-Enhanced Chemical Vapor Deposited (PECVD) Carbon Nanotubes (CNTs) are rounded whiskers with 100 nm or less of tip radius and 13 ?m or more tall. The adoption of CNTs as electron-emitting substrate has recently being shown to have advantages compared to Spindt emitters because of the higher aspect ratio of CNTs and their superior resistance to harsh environments. This research focuses on the development of a batch-fabricated MEMS neutralizer that uses PECVD CNTs as field enhancers. As a reference, a previously made Busek-MIT MEMS CNT device that uses a randomly oriented CNT matrix produced by Busek Co.(Natick MA) with a proprietary arc-based process yielded de- vices with Fowler-Nordheim emission, startup voltage as low as 100 V, and electron currents as large as 3.2 mA/cm2 with about 20% of gate current interception.