April 28, 2005
BBC
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 is 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.
"Although the reported fusion is not useful in the power-producing sense, we anticipate that the system will find application in a simple palm-sized neutron generator," the researchers write in Nature.
Small devices that emit neutrons could 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.
http://news.bbc.co.uk/2/hi/science/nature/4489821.stm