"The examination of carbon nanotubes for cathodes is a relatively new approach, but one of several that has been investigated over the last decade," says Michael Patterson, the principal investigator of the new ion-propulsion system that's part of NASA's Evolutionary Xenon Thruster (NEXT) program. Researchers at NASA's Glenn Research Center have investigated the use of microstructures made of diamond-like materials, but have had difficulty using them. "Generally they have a short lifetime when subjected to erosive environments or run at very low currents," says Patterson.

To create the carbon-nanotube cathodes, the Georgia Tech researchers grow the multiwalled carbon nanotubes using plasma instead of conventional chemical vapor deposition. "We need to be able to finely control the height of the carbon nanotubes, which for our design is 10 microns," says Ready.

Busek, a space propulsion company based in Natick, MA, is also developing carbon-nanotube cathodes that are already space-certified. Ready says the researchers have a good relationship with the company and would be interested in working with it to commercialize their own technology.

The Georgia Tech researchers have demonstrated the durability of their carbon nanotubes by showing they can survive the vibrations experienced during launch. The nanotubes have a lifespan of over 368 hours. The group has received a $6.5 million grant from DARPA, the research and development arm of the U.S. Department of Defense, and have begun a second phase of testing.

"Carbon nanotubes are a worthy area of research that could improve the overall system performance," says Patterson. He adds that carbon-nanotube cathodes may be most suitable for low-power spacecraft and small satellites because the standard cathode technology is most prohibiting on these systems. "A large fraction of the propellant is wasted on the cathode."