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.
The NASA engineer acknowledges that this collection of ideas is still a long way from being a practical device. For example, losses from the alpha particles striking the walls of the exhaust nozzle or each other lower the net power output. Figuring out how to control the particles’ path is an important consideration.
Asked how long it will be before his fusion reactor is pushing spacecraft toward Mars, Chapman acknowledges that a decade of work might be required before that happens. "It takes teamwork to get something to the point where you put it in space," he says. His aim so far is "to get the idea out so other minds can begin thinking about it."
This article was updated on 26 July 2011.
