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Last Updated: Thursday, 28 April, 2005, 22:49 GMT 23:49 UK
Table-top fusion 'demonstrated'
Nuclear fusion, SPL
Previous claims for desktop fusion have been highly controversial
A US team has created a "pocket-sized" nuclear fusion reactor that generates neutrons, Nature magazine reports.

Scientists have tried to harness nuclear fusion - the same process that powers the Sun - for commercial uses but this goal has remained elusive.

The new device is expected only to have small niche applications, such as in fine-control thrusters on spacecraft.

Full-scale fusion is a key target because it would provide an abundant source of relatively clean energy.

It works on the principle that energy can be released by forcing together atomic nuclei - rather than by splitting them, as in the case of the fission reactions that drive current nuclear power stations.

But controlling fusion reactions is technically very challenging. And although a large fusion power station is thought to be feasible, its realisation could still be many years away.

Gentle heating

There are claims, too, for so-called "desktop" fusion devices. However, these have proven highly controversial.

The latest claim, though, seems to have convinced scientific peers.

In the Nature study, Brian Naranjo and colleagues, from the University of California-Los Angeles (UCLA), initiated fusion of heavy hydrogen, or deuterium, using the strong electric field generated in a pyroelectric crystal.

Materials like this crystal produce these electric fields when they are heated. The researchers concentrated the field at the tip of a tungsten needle connected to the crystal.

In an atmosphere of deuterium gas, the field generated positively charged deuteron ions and accelerated them to high energy in a beam.

When this beam struck a target of erbium deuteride, the team detected neutrons coming from the target with precisely the energy expected if they were generated by the nuclear fusion of two deuterium nuclei.

The neutron emission was about 400 times stronger than the usual background level.

"This is true 'hand-held fusion'," said co-author Seth Putterman.

"I believe that we could build an egg-sized device, where inside the egg is the crystal and the right proportion of deuterium gas; and by plunging it into ice and warming it with your hands, you can generate a reasonably large fusion signal," he told BBC News.

"What we have done is nowhere near the efficiency needed to use it as an energy source. What we have done is produce highly compact neutron generators which could conceivably be useful for handheld cameras or tiny X-ray sources that could be put into the body to deliver X-rays locally to destroy tumours."

Small devices that emit neutrons could also be used as microthrusters in miniature spacecraft. Such fine control would be employed in certain experimental set-ups in space where precise positioning of a craft was essential.

Collapsing bubbles

In 2002, Rusi Taleyarkhan at Oak Ridge National Laboratory, US, caused a sensation when he claimed to have made hydrogen nuclei fuse by blasting tiny bubbles in acetone with sound waves, forcing them to implode.

Taleyarkhan and colleagues argued that as the bubbles collapsed, the temperature inside would rise to millions of degrees, hot enough for two deuterium nuclei to fuse.

But measuring neutrons on a small, laboratory scale has proven notoriously difficult in the past because neutrons also occur naturally in the Earth's environment.

The claim met with deep scepticism from many of the key researchers in the field.

One attempt to re-run the experiment by Mike Saltmarsh and Dan Shapiro, then colleagues of Taleyarkhan's at the Oak Ridge National Laboratory, found no evidence of fusion.

In March 2004, Taleyarkhan published further evidence for his fusion claims. Despite being thoroughly reviewed and published in a respected scientific journal, the study did little to convince the sceptics.

On the subject of pyroelectric crystals, Saltmarsh told BBC News: "It's a great way of producing fusion reactions but you are guaranteed to use more energy than you ever produce through fusion because the incoming ions hit atoms in the target and they lose most of their energy in non-productive ways - as heat. Only a few of them produce fusion neutrons."


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Listen to a description of the experiments



SEE ALSO:
Nuclear fusion 'put to the test'
18 Feb 05 |  Science/Nature
Fusion experiment disappoints
25 Jul 02 |  Science/Nature
Fusion controversy rekindled
05 Mar 02 |  Science/Nature


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