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Brian Kappus, a physics graduate student at UCLA, tipped the clear cylinder to trap some air bubbles in the clear liquid inside. He clamped the cylinder, upright, on a small turntable and set it spinning. With the flip of another switch, powerful up-and-down vibrations, 50 a second, started shaking the cylinder.

A bubble floating in the liquid — phosphoric acid — started to shine, brightening into an intense ball of light like a miniature star.

The shining bubble did not produce any significant energy, but perhaps someday it might, just like a star. A few small companies and maverick university laboratories, including this one at UCLA run by Seth Putterman, a professor of physics, are pursuing quixotic solutions for future energy, trying to tap the power of the Sun — hot nuclear fusion — in devices that fit on a tabletop.

Putterman's approach is to use sound waves, called sonofusion or bubble fusion, to expand and collapse tiny bubbles, generating ultrahot temperatures. At temperatures hot enough, atoms can literally fuse and release even more energy than when they split in nuclear fission, now used in nuclear power plants and weapons. Furthermore, fusion is clean in that it does not produce long-lived nuclear waste.Putterman has not achieved fusion in his experiments. He and other scientists form a small but devoted cadre interested in turning small-scale desktop fusion into usable systems. Although success is far away, the principles seem sound.

Other researchers already have working desktop fusion devices, including ones that are descendants of the Farnsworth Fusor invented four decades ago by Philo T. Farnsworth, the television pioneer.

Achieving nuclear fusion, even in a desktop device, is not particularly difficult. But building a fusion reactor that generates more energy than it consumes is far more challenging.

So far, all fusion reactors, big and small, fall short of this goal. Many fusion scientists are skeptical that small-scale alternatives hold any promise of breaking the break-even barrier.

Impulse Devices, a small company in the small town of Grass Valley, Calif., is exploring the same sound-driven fusion as Putterman, pushing forward with venture capital financing. Its president, Ross Tessien, concedes that Impulse is a high-risk investment, but the potential payoffs would be many.

"You solve the world's pollution problems," Tessien said. "You eliminate the need for wars. You eliminate scarcity of fuel. And it happens to be a very valuable market. So from a commercial point of view, there's every incentive. From a moral point of view, there's every incentive. And it's fun and it's exciting work."

The Sun produces energy by continually pressing together four hydrogen atoms — a hydrogen atom has a single proton in its nucleus — into one helium atom, with a nucleus of two protons and two neutrons. A helium atom weighs less than the four original hydrogen atoms. So by Einstein's E=mc2 equation, the change in mass is transformed into a burst of energy.