An astonishingly simple demonstration of nuclear fusion in a tabletop device has been performed, involving heating an ordinary crystal soaked in deuterium gas.
While the technique is unlikely to lead to power generation, such a device could act as a portable source of neutrons for analysing materials and medical imaging, and perhaps even spacecraft propulsion.
The key to the system is a crystal made of lithium tantalate. The crystal is asymmetric and, as a result, heating the material causes positive and negative charges to migrate to opposite ends of the crystal, setting up an electric field. The phenomenon is known as the pyroelectric effect.
In 1992, James Brownridge at the State University of New York in Binghamton, US, used crystals of lithium tantalate to generate X-rays by heating the crystals to about 100ºC in a dilute gas. The resultant electric field strips electrons from the gas molecules and accelerates them to huge energies. The electrons then collide with stationary nuclei in the crystal and generate X-rays.
When Seth Putterman at the University of California, Los Angeles, US, heard of the phenomenon a few years ago, he immediately realised that the electric fields were powerful enough for nuclear fusion to occur, specifically to fuse nuclei of an isotope of hydrogen called deuterium.
To test whether these fields could indeed cause nuclear fusion, Putterman and UCLA colleagues Brian Naranjo and James Gimzewski first bathed a crystal of lithium tantalate in deuterium gas. The setup was then cooled to -33ºC and then heated to about 7 ºC over three and a half minutes.
The resultant electric field accelerated deuterium nuclei over a distance of 1 centimetre to energies in excess of 100 kiloelectronvolts. The accelerated nuclei then collided and fused with deuterium nuclei that had permeated the surface of the crystal lattice. The fusion produced 400 times more neutrons than found in background measurements.
Fusion science is littered with hype and over-optimistic claims, but Putterman has convinced his peers that something interesting is going on. “They make a very strong case for having seen fusion,” says Nigel Hawkes, a nuclear physicist at the National Physical Laboratory in Teddington, UK.
But he is cautious about the potential for desktop neutron machines: “It’s too early to say where this might lead.”
One problem is the small number of neutrons the experiment produces – a few hundred per second. A commercial neutron generator would need to produce at least tens of millions of neutrons per second.
Today, neutrons are created in nuclear reactors or particle accelerators which can cost millions of dollars to build and maintain. The prospect of a desktop alternative is a powerful incentive to continue the research and Putterman’s team hopes to increase the yield by operating at lower temperatures and by using an array of crystals.
Putterman also suggests the crystals could be used as microthrusters for tiny spacecraft. By accelerating deuterium in one direction, the spacecraft would be propelled in the opposite direction.
Journal reference: Nature (vol 434, p 1115)