Frontiers Physics World  May 2018

Nanowire arrays boost nuclear fusion

Fire me up The target chamber used to achieve laser fusion in deuterated polyethylene nanowires. (Advanced Beam Laboratory / Colorado State University)

Smaller, cheaper neutron sources and new opportunities for simulating the extreme conditions at the centre of stars are two possible benefits of new work carried out by physicists in the US and Germany. Led by Jorge Rocca of Colorado State University, the researchers directed rapid-fire pulses of intense blue light from a compact laser at arrays of nanowires. The process generated a dense plasma yielding lots of neutrons from nuclear fusion.

Most attempts to show nuclear fusion’s feasibility as an energy source involve huge, energetic lasers. The National Ignition Facility (NIF) in California, for example, is three football pitches big and generates pulses with an energy of 1.8 MJ, which compress tiny pellets of deuterium and tritium until the nuclei fuse and emit neutrons. NIF’s aim is ignition, with the alpha particle released by the fusing nuclei providing heat for a self-sustaining reaction – and the energy of the neutrons being tapped to produce electricity.

However, NIF fires only a few times a day and some researchers are instead working on less energetic but more rapid-fire lasers. These will never near ignition, but can still achieve exceptionally high intensities – thanks to the extreme brevity, and hence power, of their pulses. In the latest work, Rocca and colleagues used a titanium-sapphire laser to generate pulses lasting just 60 fs with up to 1.65 J of energy.

Capable of being fired three times a second at arrays of deuterated polyethylene nanowires, each about 5 μm long, the pulses rip electrons from the wires’ surfaces. The electrons then get accelerated to very high energies within the void between the wires, which heat up rapidly and explode. The resulting plasma accelerates deuterons to energies up to several megaelectronvolts, causing the deuterons to fuse and generate rapid bursts of neutrons.

Rocca and colleagues were able to generate up to two million fusion neutrons per joule. While these efficiencies were higher than those from similar-sized lasers, they were lower than those at NIF, which recently yielded some 8 × 1015 neutrons per pulse, or about four billion neutrons per joule. The plasma, which has a high energy density, could, however, be ideal for studying extreme astrophysical environments (Nature Comms 9 1077).

Edwin Cartlidge