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Tabletop Nuclear Fusion Claims Meet with Skepticism

bubbles
Image: COURTESY OF OAK RIDGE NATIONAL LABORATORY/RENSSELAER POLYTECHNIC INSTITUTE/RUSSIAN ACADEMY OF SCIENCES

A new report suggests that scientists have achieved nuclear fusion--the energetic process by which two light atoms join to form a third, heavier atom and energy as a by-product--in a beaker sitting atop a laboratory bench. The results, to be published Friday in the journal Science, have already attracted plenty of skepticism from within the scientific community.

The experiment in question exploits a phenomenon called acoustic cavitation, in which sound waves traveling through a liquid cause tiny bubbles to grow dramatically before collapsing. This disintegration releases the energy accumulated by the bubble during that growth. If the energy within the collapsing bubble is sufficiently high, light is also emitted in a process termed sonoluminescence.

Using an experimental setup approximately the size of three stacked coffee cups (see image), Rusi Taleyarkhan of the Oak Ridge National Laboratory, Richard T. Lahey, Jr., of Rensselaer Polytechnic Institute and colleagues used ultrasound to bombard a beaker of liquid acetone that had had its hydrogen atoms replaced by heavier deuterium atoms. The sound waves forced tiny bubbles (smaller than the size of a period) within the liquid to increase rapidly in size such that they measured almost two millimeters across before they collapsed in a flash of light. Calculations that support their observations, the team reports, suggest that temperatures within the imploding bubbles could approach 10 million kelvins--as hot as the center of the sun and energetic enough for nuclear fusion to occur.

When two deuterium atoms fuse, the reaction produces a third isotope of hydrogen known as tritium and a neutron with a characteristic energy of 2.5 million electron volts. In their Science paper, Taleyarkhan and colleagues report detecting both slightly elevated levels of tritium and neutrons with energies close to 2.5 million electron volts. Because the levels of both tritium and neutrons are small, such measurements are notoriously difficult to make. Indeed, when two other Oak Ridge National Laboratory scientists, Dan Shapira and Michael J. Saltmarsh, tried to replicate the neutron results using a different detector, they classified their results as insufficient to support the team's fusion claim. In a rebuttal, Taleyarkhan's group suggests the second experiment failed because of a faulty calibration of the detector, which differed from the one used for the published results. Neither supplemental report has been peer-reviewed or endorsed by Science.

Regardless of which measurements are more accurate, visions of a plentiful, cheap, clean and small energy source are premature, and scientists caution that scaling up the process is unlikely. "If the claim of nuclear fusion is indeed correct," says Lee Riedinger, deputy director for science and technology at Oak Ridge National Laboratory, "these experiments would still have produced only one tenth of a millionth of a watt of power--far too small to measure." But other uses may not be so far-fetched. "If the results are confirmed," Fred Becchetti of the University of Michigan writes in an accompanying commentary to be published in Science, "this new, compact apparatus will be a unique tool for studying nuclear fusion reactions in the laboratory." He adds, however, that scientists must remain skeptical until other groups reproduce the experiments. It seems that the one thing everyone can agree on for the time being is that more research is needed before small-scale fusion becomes a sure thing.

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