Star in the Jar

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In the new work, Taleyarkhan and his collaborators used bursts of neutrons to fabricate clouds of short-lived, but extraordinarily large, sonoluminescence bubbles in acetone, the solvent in many nail-polish removers. In some tests, the researchers filled the flask with ordinary acetone, whose molecules each contain six hydrogen atoms. In other tests, they used deuterated acetone, in which deuterium atoms replace the hydrogen ones. When he was younger he’d trawl car boot sales and charity shops for uranium glass. “It’s just green glass, but if you put a UV light on it, it will glow bright alien green. And it’s sort of radioactive because it’s the uranium that gives it the colour,” explained Edwards. He saved up to buy a Geiger counter from the US. His headmaster was concerned about safety. “There was the chance that I could be electrocuted by the high-voltage power supply ... and then probably the chance of the vacuum chamber imploding because of all the forces on it,” says Edwards, “but quite a minimal aspect to it was the radiation.” Using his younger sister’s fish tank filled with a water and boron solution: “The radiation wasn’t really an issue.”

What causes the glow? Good question. According to [Justin], we just don’t know for sure what causes it, although the leading theory is that cavitation of the bubble causes the trapped gas to compress and heat violently, turning into a brief bit of plasma. But there are problems with that theory, which is one of the reasons he wanted to show just how easy the process can be – now that he’s shaken out the bugs with five years of effort. It wasn’t easy getting the transducers attached and the driver circuit properly tuned, but with little more than a signal generator, an audio amp, and a spool of magnet wire, you too can make your own “star in a jar.” Barber, B. P. and Putterman, S. J. (1991) Observation of synchronous picosecond sonoluminescence, Nature 352, 318–320; The measured pulse width is less than 50ps. Other critics say that the most damning indictment of the new work is an unpublished follow-up experiment by a pair of nuclear physicists, also of the Oak Ridge lab. He is already thinking about his next project. “I was thinking that maybe I could make a hand-held laser cutter,” says Edwards. “So I’ve been looking into some really high-powered lasers.” However, sonoluminescence flashes typically occur at temperatures of thousands of degrees, not millions. “Such high temperatures are unlikely to occur” in the bubbles of the Oak Ridge setup, notes Lawrence A. Crum of the University of Washington in Seattle.

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Moss, W. C. et al. (1997) Calculated pulse widths and spectra of a single sonoluminescing bubble, Science 276, 1398–1401. The new signs of fusion in bubbles were so extraordinary that Lee L. Reidinger, the Oak Ridge lab’s deputy director for science and technology, commissioned two of the lab’s nuclear physicists, Dan Shapira and Michael J. Saltmarsh, to monitor the sonoluminescence setup using different detectors. That last part is critical. Gates said he's encouraged by fact that the W7-X project, and nuclear fusion research in general, is the result of close collaboration among scientists from around the world.

In the current setup, creating sonoluminescence takes far more energy than the bubble collapse gives off, even if fusion is taking place, Taleyarkhan says. We applaud [Justin]’s determination to bring this project to a successful conclusion. It’s not unlike his dogged effort to make a cold plasma torch, or even his desktop radio telescope.

Under extreme pressure and at temperatures of millions of degrees, such as at the center of the sun, deuterium atoms fuse in a reaction whose products include tritium–hydrogen’s radioactive heavy isotope–and neutrons.