A piece of plastic (2.5cm across) loaded with deuterium atoms glows blue where an incoming beam of deuterium triggers fusion.© Seth Putterman, UCLAAt first, it sounds like the biggest science story of the century: scientists have invented a desktop fusion machine.
If nuclear fusion can be made to happen at room temperatures and pressures in an average lab, then one might think the world's energy crisis is over. But the inventors of the device stress that their gadget cannot generate power at all, because it does not support a self-sustaining thermonuclear reaction. Instead, they say, it has a whole host of other applications, from treating cancer to powering spacecraft.
The inventors are led by Seth Putterman, a physicist from the University of California, Los Angeles. Putterman is known for debunking claims of 'bubble fusion' and 'cold fusion' that promised revolutionary advances in energy production.
His toaster-sized device, detailed in this week's Nature1, relies on a pyroelectric crystal of lithium tantalate, which produces a strong electric field when heated to room temperature from freezing. This field is focused until it is powerful enough to accelerate a beam of deuterium ions (proton-neutron pairs) to about 1% of the speed of light.
When these ions hit a target containing deuterium nuclei, they fuse to form helium-3, a combination of two protons and a neutron. The process emits about 1,000 neutrons a second, and by allowing the crystal to heat up slowly, fusion can be sustained for as long as eight hours.
Low-power wonder
This type of fusion is already used in commercially available instruments that determine the chemical composition of materials at a distance. Such devices blast neutrons down to the bottom of oil wells, for example, to determine the quality of oil. They are also used at airports to study in detail the contents of suspicious luggage.
However, such applications currently require bulky, expensive particle accelerators with large electricity supplies. Replacing those with a small crystal is a big step. "The amazing thing is that the energy fields of a crystal can be used without plugging it in to a power station," says Putterman.
"They've built a really neat little accelerator," agrees Mike Saltmarsh, a nuclear physicist formerly at Oak Ridge National Laboratory, Tennessee.
It will probably make its first big splash in labs looking for an easy neutron source. But, predicts Putterman, "there will be a lot of spin-offs from this technology".
Radiation on tap
"Everyone will be talking about the fusion, but this crystal can also give off X-rays as it accelerates electrons," says Putterman. This effectively creates a tiny radioactive source that can be turned on and off at will. Such a device could one day be used to target radiation at cancerous cells: a smaller version could be injected into the body and directed towards a tumour before being switched on. In contrast, today's radiation therapies tend to blast healthy cells along with cancerous ones.
Putterman also thinks that rocket propulsion could benefit. Space probes such as the European Space Agency's SMART-1, which recently arrived at the Moon, already use ion engines that eject a stream of charged xenon gas to produce a gentle forward thrust. The pyroelectric accelerator could produce a similar beam of ions moving at much greater speed, which would increase the thrust considerably, says Putterman.
The team is now trying to boost the number of neutrons generated by the machine, as well as miniaturizing the device even further.
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