Ion-propulsion systems have propelled a handful of Earth-orbiting and interplanetary spacecraft over the past 50 years. Now researchers at Georgia Institute of Technology are developing more efficient ion thrusters that use carbon nanotubes for a vital component.
Ion propulsion works by accelerating electrically charged, or ionized, particles to propel a spacecraft. One of the most common ion engines, known as a “Hall Effect” thruster, ionizes gas using electrons trapped in a magnetic field. The resulting ions are then accelerated using the potential maintained between an anode and a cathode. But some of the emitted electrons must also be used to neutralize the ions in the plume emitted from the spacecraft, to prevent the spacecraft from becoming electrically charged. Existing Hall Effect thrusters must use about 10 percent of the spacecraft’s xenon gas propellant to create the electrons needed to both run the engine and neutralize the ion beam.
The Georgia Tech researchers created a field emission cathode for the thruster using carbon nanotubes. In this type of cathode, electrons are emitted after they tunnel through a potential barrier. The carbon nanotube design is especially efficient because nanotubes are incredibly strong and electrically conductive. “By using carbon nanotubes, we can get all the electrons we need without using any propellant,” says Mitchell Walker, principal investigator of the project and an assistant professor in the High-Power Electric Propulsion Laboratory at Georgia Tech. This means that 10 percent more of the ion thruster’s propellant is available for the actual mission, extending a spacecraft’s lifetime.
“We can pull the electrons from the tip of the material at less than .25 volts per micron,” which makes for a tremendously efficient system, says Jud Ready, coprincipal investigator of the project. In contrast, the hollow cathodes conventionally used in ion thrusters require heavy electronics and need to be heated to thousands of degrees to obtain the ample voltage.
Furthermore, since the nanotubes are thin and lightweight, they can be applied to the surface of the thruster body, potentially allowing the spacecraft to carry larger payloads and fit on smaller launch vehicles. Walker presented a paper on the new cathode earlier this year at the Joint Propulsions Conference and Exhibit in Denver and says the new system could be ready to launch in three to five years.
“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.”
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