Desktop fusion is back on the table
Physicist
claims to have definitive data, but can they be replicated?
Mark
Peplow
Imploding bubbles, caught on film emitting light.
Are they emitting energy too?© D.
Flannigan and K.S. Suslick, University of Illinois at
Urbana-Champaign
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 haystackTaleyarkhan'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liquid acetone in which hydrogen atoms had been replaced by their heavier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 sourcesThere 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 proofAnother 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
