Desktop fusion is back on the table
Physicist claims to have definitive data, but can they be replicated?
Mark Peplow
Can the popping of tiny bubbles trigger nuclear fusion, a potential source
of almost unlimited energy? This controversial idea is back on the table,
because its main proponent has new results that, he claims, will silence
critics. But others say that the latest experiment simply comes with its own
set of problems.
The idea is simple enough. Blast a liquid with waves of ultrasound and tiny
bubbles of gas are created, which release a burst of heat and light when
they implode. The core of the bubble reaches 15,000 °C, hot enough to wrench
molecules apart. Physicists have even suggested that the intense conditions
of this sonoluminescence could fuse atomic nuclei together, in the same
process that keeps our Sun running.
Physicist Rusi Taleyarkhan of Purdue University in West Lafayette, Indiana,
published the first evidence1 of this 'sonofusion' in 2002; he has been
dogged by sceptics ever since.
The underlying physics behind the idea is valid, says Ken Suslick. An expert
in sonoluminescence at the University of Illinois in Urbana-Champaign,
Suslick tried and failed to replicate Taleyarkhan's first results. If the
bubbles' collapse is sufficiently intense, it should indeed be able to crush
atoms together. Taleyarkhan just hasn't done enough to prove it, says
Suslick.
Needle in a haystack
Taleyarkhan's first experiments were conducted while he was based at Oak
Ridge National Laboratory in Tennessee. His idea was to use liquid acetone
in which hydrogen atoms had been replaced by their heavier brethren,
deuterium. When deuterium nuclei fuse together, they emit a characteristic
burst of neutrons. But critics pointed out that Taleyarkhan was using an
external source of neutrons to 'seed' the bubbles, and that these were
swamping his measurements of neutrons produced by the fusion reaction
itself.
"This time round there are no external neutrons," he explains. Instead, his
team loaded a mixture of deuterated acetone and benzene with a uranium salt.
As the uranium undergoes radioactive decay it releases alpha particles,
which can also seed bubble formation, says Taleyarkhan.
"In this experiment we use three independent neutron detectors and a
gamma-ray detector," he adds. The results from the four instruments prove
that fusion is happening inside his experiment, asserts Taleyarkhan.
Although uranium can release neutrons during fission reactions, Taleyarkhan
rules them out because the neutrons he finds bear the energetic hallmark of
having come from the fusion of two deuterium nuclei2.
Taleyarkhan's test reactor still puts out a lot less energy than it takes
in, making it impractical for generating power. "We have a way to go before
we break even," he admits. But in the meantime, he adds, it could be a cheap
source of neutrons for analysing the structure of materials. The results are
to be published in Physical Review Letters in a few weeks' time.
Unreliable sources
There is one big problem, however: the experiment doesn't always work, and
the group is not sure why. Seth Putterman, a physicist at the University of
California, Los Angeles, who has also tried to verify some of Taleyarkhan's
experiments, notes that the paper does not reveal how many failed runs were
required before the team saw a trace of fusion neutrons. "As a paper it
doesn't convince me," says Putterman.
Putterman notes that the team did not continuously monitor background
neutron levels. Although the neutron count doubles at some points in the
experiments, Putterman says that neutrons produced in random showers of
cosmic rays, rather than fusion events, could be responsible. But
Taleyarkhan points out that the neutron count was smaller in detectors
further from the reaction chamber.
To prove that the neutrons are coming from fusion as bubbles burst,
Putterman and Suslick suggest that the team closely monitor exactly when the
neutrons appear. The current experiment simply counts up the number of
neutrons detected over minutes, so correlations with bubble bursts cannot be
seen. "The key to improving the signal is timing," says Putterman.
Finding proof
Another obvious way to confirm that fusion is happening would be to look for
tritium, a heavier isotope of hydrogen produced by fusion reactions. Tritium
leaves a telltale signature of high-energy electrons when it decays and
Taleyarkhan claimed to see this in similar previous experiments1,3. But in
the current tests, tritium's signature is overwhelmed by ?-decay from the
uranium, making it impossible to spot.
Given that Suslick and Putterman have both investigated Taleyarkhan's past
claims, they think it odd that they were not consulted by the editors of
Physical Review Letters about the paper. "There are other people who are
very knowledgeable about this," comments Martin Blume, editor-in-chief of
the American Physical Society.
Taleyarkhan says that Suslick and Putterman are welcome to visit his lab to
see the results for themselves. Both are eager to go as soon as possible.
"We look forward to seeing the experiment run," says Putterman.
http://www.nature.com/news/2006/060109/full/060109-5.html
