Neutrons are a valuable tool for scientists in many fields, allowing them to probe the structure and dynamics of a range of materials. Today, the main drawback of neutron science is that intense beams of neutrons must be produced in either nuclear reactors or dedicated accelerator facilities – making a laser-based table-top source very attractive.
Laser-based sources involve creating very brief pulses of high-energy electromagnetic radiation, which ionize a small solid target and then propel the liberated electrons to the back of the target, so creating a very strong electric field that in turn accelerates the ions. The ions – typically deuterons, which comprise one proton and one neutron – then stimulate nuclear reactions in a second target, producing neutrons. Despite a decade of research, however, the resulting neutron fluxes have remained low. This is largely because charged molecules such as water vapour contaminate the target surface and are accelerated at the expense of the ions.
n 2006 Lin Yin and Brian Albright at Los Alamos National Laboratory in the US showed how this problem might be overcome. They used computer simulations to show that an intense laser beam can penetrate a thin solid target. Usually a solid object is opaque because the frequency with which its constituent electrons vibrate exceeds that of the incoming light. But Yin and Albright calculated that a very intense laser beam should be able to boost the speed of electrons in a plasma to such an extent that their relativistic mass significantly reduces the electrons' frequency to below that of an infrared laser.
Yin and Albright named this effect the "laser breakout afterburner" because in "breaking out" to the far side of the target the laser beam would re-energize electrons that have lost energy in accelerating ions, so allowing those ions to reach higher energies. The beam would also interact with the entire target, rather than just the atoms on the surface, meaning that many more deuterons would be accelerated, so increasing the neutron flux.
This scheme has now been put into practice by Markus Roth of the Technische Universität Darmstadt and colleagues at Los Alamos and Sandia National Laboratories. Roth's team directed extremely powerful and well defined pulses from the Los Alamos TRIDENT laser onto a 400-nm-thick plastic target doped with deuterium atoms. This was positioned just 5 mm in front of a secondary target made from beryllium.
Even though the pulses delivered less than a quarter of the energy employed in previous experiments, they produced neutrons that were nearly 10 times as energetic – up to 150 MeV – and also nearly 10 times as numerous. In addition, many of these neutrons were emitted in the forward direction, which the researchers attribute to one specific kind of nuclear reaction, the break-up of deuterons.
The next step, Wilks adds, will be to increase the laser's repetition rate, which, he predicts, "will be no small feat, but, given laser technology's rapid evolution, inevitable".
Note, high powered lasers with high repetition have been proposed for high performance space propulsion and for commercial nuclear fusion.
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 10^18 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.
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.
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