A Carbon Nanotube Pillar Array Ionizer for Miniature Ion Thruster Applications

Satellites of the ‘micro’ class and smaller require scalable propulsion systems that minimize mass, volume and power. With a substantial reduction in ionizer size, ion thrusters may fulfill all of these requirements. This work explores field ionization with nanotube arrays for a highly-scalable ionizer. Fabrication and testing of carbon nanotube pillar array (CPA) ionizers is undertaken at the Nanotechnology Center, NASA Ames Research Center. The devices are built using conventional photolithography, ion beam sputtering and thermal chemical vapor deposition processes. Fabrication tribulations and solutions discovered are discussed. Testing is performed under both ultrahigh vacuum and low-pressure static gas atmospheres, with the devices configured as field electron emitters and as field gas ionizers. Argon is used as a propellant to demonstrate its suitability as an alternative to xenon. The four test samples turn on at applied fields as low as 5.3 V/μm and produce ion current densities greater than 10 mA/cm2. Tested as field emitters, the devices show very strong correlation with Fowler-Nordheim theory and exhibit field enhancement factors as high as 1500. A qualitative analysis is made of pre- and post-test CPA characterizations.