Published online 8 April 1999 | Nature | doi:10.1038/news990408-2


Table-top nuclear fusion

Researchers in the United States have achieved thermonuclear fusion using table-top laboratory equipment: and before you ask, this isn't a replay of the largely discredited 'cold' fusion reported a few years ago. Writing in Nature [8 April], Todd Ditmire and colleagues from the Lawrence Livermore National Laboratory in Livermore, California describe how they used compact but powerful lasers to excite thermonuclear fusion in a jet of deuterium ('heavy' hydrogen) gas. The deuterium is cooled to minus 170 °C, so in that sense only, the fusion is 'cold'.

Although the researchers achieved a respectable yield of fusion neutrons - comparable with those found in large-scale laser fusion experiments - their compact experimental set-up means that many researchers will have access to sources of neutrons for all kinds of research applications in their own research institutes, without having to travel to use the giant-sized facilities only found in the very largest national-level laboratories.

Livermore is just such a laboratory, and prides itself on research engineering on a grand scale. The Nova laser at Livermore routinely produces fusion neutrons by subjecting materials to intense blasts of coherent light - but the Nova facility is as big as an aircraft hangar, and the barrel of the laser is like the trunk of a giant redwood tree laid on its side. But since the end of the Cold War, Livermore scientists have been encouraged to see the beauty in smallness, so Ditmire and his colleagues in the Laser Program at Livermore have been experimenting with fusion using a variety of laser-based techniques. Their latest report is about what happens when intense but extremely brief laser pulses (of the order of femtoseconds - that is, billion-billionths of a second) are fired at 'clusters' of deuterium atoms. 'Clusters' represent an intriguing no-man's-land, halfway between atoms and molecules on the one hand, and bulk materials on the other. They have all kinds of interesting properties, and their response to laser excitation is one of them.

Laser light passing through large clusters - of 1,000 atoms or more - dislodges electrons from the constituent atoms. As a result, these atoms acquire an electric charge - that is, they become 'ions'. Heated and energized by the laser light, the dislodged electrons form a strong electric field that radiates outwards from the cluster. This electric field agitates the ions, leading to a small explosion as the cluster can no longer contain the energy it has absorbed from the laser.

The researchers put these small explosions to work: by energizing a gas jet of deuterium atoms, they produced explosions in which deuterium ions accelerating out of clusters had a strong chance of colliding with accelerated deuterium ions from adjacent clusters with sufficient woomph to create nuclear fusion - the deuterium ions would fuse to create helium nuclei, releasing neutrons of a characteristic energy.