Researchers at the Georgia Institute of Technology have won a $6.5 million grant to develop improved components that will boost the efficiency of electric propulsion systems used to control the positions of satellites and planetary probes. Sponsored by the U.S. Defense Advanced Research Projects Agency Defense Sciences Office (DARPA-DSO), the 18-month project seeks to demonstrate the use of propellant-less cathodes with Hall effect thrusters.

To maintain their positions in space or to reorient themselves, satellites must use small thrusters that are either chemically or electrically powered. Electrically-powered thrusters use electrons to ionize an inert gas such as xenon. The resulting ions are then ejected from the device to generate thrust.

In existing Hall effect thrusters, a single high-temperature cathode generates the electrons. A portion of the propellant – typically about 10 percent of the limited supply carried by the satellite – is used as a working fluid in the traditional hollow cathode. The DARPA-funded research would replace the hollow cathode with an array of field-effect cathodes fabricated from bundles of multi-walled carbon nanotubes. Powered by on-board batteries and photovoltaic systems on the satellite, the arrays would operate at low power to produce electrons without consuming propellant.

Walker and collaborators at the Georgia Tech Research Institute (GTRI) have already demonstrated field-effect cathodes based on carbon nanotubes. This work was presented at the 2009 AIAA Joint Propulsion Conference held in Denver, Colo. The additional funding will support improvements in the devices, known as carbon nanotube cold cathodes, and lead to space testing as early as 2015. This work depends on our ability to grow aligned carbon nanotubes precisely where we want them to be and to coat them to enhance their field emission performance.

In addition to reducing propellant consumption, use of carbon nanotube cathode arrays could improve reliability by replacing the single cathode now used in the thrusters. Existing cathodes are sensitive to contamination, damaged by the ionized exhaust of the thruster, and have limited life due to their high-temperature operation. The carbon nanotube cathode arrays would provide a distributed cathode around the Hall effect thruster so that if one of them is damaged, we will have redundancy.

The researchers are testing their cathodes with the same Busek Hall effect thruster that flew on the U.S. Air Force’s TacSat-2 satellite. In addition, the cathodes will be operated with Hall effect thrusters developed by Pratt & Whitney and donated to Georgia Tech. The researchers are also collaborating with L-3 ETI on the electrical power system and with American Pacific In-Space Propulsion on flight qualification of the hardware.

The use of carbon nanotubes to generate electrons through the field-effect process was reported in 1995 by a research team headed by Walt de Heer, a professor in Georgia Tech’s School of Physics. Field emission is the extraction of electrons from a conductive material through quantum tunneling that occurs when an external electric field is applied.

By Vasil Sidorov on November 11, 2009 from Georgia Tech

sidorovvasil@gmail.com