28 June 2011—Designers of satellites obsess about how little fuel their creations are able to carry into space. So the propulsion method they choose for maneuvers such as orbital transfers has to deliver a lot for a little.
Now a NASA engineer has come up with a new way to fling satellites through space on mere grams of fuel, tens of times as efficiently as today’s best space probe thrusters. The answer, he says, is fusion. You might be thinking, "Fusion? Really?" But it’s not as far-fetched as it sounds at first blush. The engineer delivered the details today at the IEEE Symposium on Fusion Engineering in Chicago.
Instead of using deuterium and tritium as the fuel stocks, the new motor extracts energy from boron fuel. Using boron, an "aneutronic" fuel, yields several advantages over conventional nuclear fusion. Aneutronic fusion, in which neutrons represent less than 1 percent of the energy-carrying particles that are the result of a reaction, is easier to manage. "Neutrons are problematic, because for one thing they’re difficult to harness," says John J. Chapman, the concept’s inventor and a physicist and electronics engineer at NASA’s Langley Research Center, in Virginia. To make use of neutrons, "you need an absorbing wall that converts the kinetic energy of the particles to thermal energy," he says. "In effect, all you’ve got is a fancy heat engine, with all its resultant losses and limitations."
In Chapman’s aneutronic fusion reactor scheme, a commercially available benchtop laser starts the reaction. A beam with energy on the order of 2 x 1018 watts per square centimeter, pulse frequencies up to 75 megahertz, and wavelengths between 1 and 10 micrometers is aimed at a two-layer, 20-centimeter-diameter target.
The first layer is a 5- to 10-µm-thick sheet of conductive metal foil. It responds to the teravolt-per-meter electric field created by the laser pulse by "acting as a de facto proton accelerator," says Chapman. The electric field releases a shower of highly energetic electrons from the foil, leaving behind a tremendous net positive charge. The result is a massive self-repulsive force between the protons that causes the metal material to explode. The explosion accelerates protons in the direction of the target’s second layer, a film of boron-11.