A technique that some scientists claim generates thermonuclear fusion in a benchtop apparatus works even without its controversial neutron trigger. So say the researchers who, since 2002, have reported that nuclear-fusion reactions can occur in a vat of chilled solvent agitated by ultrasound (SN: 3/6/04, p. 149: Available to subscribers at Bubble Fusion: Once-maligned claim rebounds). If this method of sparking fusion proves to be valid—a big if, critics insist—it could lead to a remarkably simple, cheap, inexhaustible power source.
Fusion reactions take place in the vat because clusters of bubbles form and then violently collapse, explains nuclear engineer and team leader Rusi P. Taleyarkhan of Purdue University in West Lafayette, Ind. A neutron or another energetic particle triggers a bubble to form in a low-pressure trough of the ultrasound waves, he says. Then, high pressure from the wave crushes the orb to an enormous density and temperature that fuse some atomic nuclei of the bubble’s gas.
Taleyarkhan and his colleagues have measured neutron emissions as a sign of fusion reactions. Because the group had used neutron pulses to trigger the process, other researchers have been skeptical of its neutron readings.
In an upcoming Physical Review Letters, Taleyarkhan’s team presents evidence of fusion in bubbles initiated by a uranium-based trigger that emits alpha particles instead of neutrons. “We got away from the idea of using neutrons to produce neutrons,” Taleyarkhan notes.
Nonetheless, the findings still face intense skepticism. Criticisms range from doubts about experimental procedures to quarrels with interpretations of the data. “I simply do not find the results significant and/or believable,” comments physicist Dan Shapira of Oak Ridge (Tenn.) National Laboratory.
Critics note that Taleyarkhan’s team admits in its report that its experimental outcomes vary greatly, many of them producing no evidence of fusion. Yet to D. Felipe Gaitan of Impulse Devices in Grass Valley, Calif., the uneven outcomes are encouraging. They “could explain our inability, and that of other researchers so far, to replicate [Taleyarkhan’s] results consistently,” says Gaitan. Impulse Devices plans to commercialize bubble fusion.
Lawrence A. Crum of the University of Washington in Seattle says that the new work “increases the credibility” of bubble fusion. But “unless it’s reproduced in someone else’s lab, I’m not going to believe it,” he adds.
Taleyarkhan claims that his team’s findings were independently verified last year by other Purdue researchers, whom he guided. Other physicists are unconvinced.
A welcome consequence of the latest results, Crum adds, is that other researchers should find the uranium-based triggering method easier to reproduce than the neutron one. So, he says, the new work “is an important step toward determining if the results of Rusi’s experiments are true.”