Can Aneutronic Fusion Space Thrusters Open the Solar System?
If humans are ever to travel beyond the inner solar system, they will need to devise space propulsion methods beyond conventional chemical rockets. Nuclear reactions are orders of magnitude more powerful than chemical reactions, and seem the natural approach for space propulsion to the asteroid belt and the outer solar system. But working out the best form of nuclear space propulsion is apt to take time and a lot of work. NASA physicist and engineer John Chapman thinks that fusion -- aneutronic fusion -- is the way to go.
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.The more powerful and efficient your propulsion, the more payload you can carry, the less fuel, and the greater your choices for timely flight path and orbital selection.
"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.
...There, a complicated nuclear dance begins. The protons (which carry energy on the order of roughly 163 kiloelectron volts) strike boron nuclei to form excited carbon nuclei. The carbons immediately decay, each into a helium-4 nucleus (an alpha particle) and a beryllium nucleus. Almost instantaneously, the beryllium nuclei decay, with each one breaking into two more alpha particles. So for each proton-boron pair that reacts, you get three alpha particles, each with a kinetic energy of 2.9 megaelectron volts.
...Electromagnetic forces push the target and the alpha particles in the opposite directions, and the particles exit the spacecraft through a nozzle, providing the vehicle’s thrust. Each pulse of the laser should generate roughly 100 000 particles, making the method tremendously efficient, says Chapman. And according to his calculations, improvements in short-pulse laser systems could make this form of thruster more than 40 times as efficient as even the best of today’s ionic propulsion systems that push spacecraft around. Even at 50 percent efficiency, burning off 40 milligrams of the boron fuel would deliver a gigajoule of energy. The amount of power depends on the laser pulse rate. The motor could generate 1 megawatt per second if the pulses are frequent enough to start reactions that consume that amount of boron in 1000 seconds. (According to Chapman, using this aneutronic fusion technique with helium-3 isotopes would yield roughly 60 percent more energy per unit mass. But boron is a more attractive fuel source because it is abundant on Earth and helium-3 is scarce.)
Another big advantage of fusion space propulsion, Chapman claims, is that some of the energy can be converted into electricity to power a spacecraft’s onboard control systems. "A traveling wave tube—basically an inverse klystron—captures most of the particles’ flux kinetic energy and efficiently converts it into electrical energy," says Chapman. The process, he says, is 60 to 70 percent efficient. _IEEESpectrum