A few small companies and maverick university laboratories, including ... one at U.C.L.A. 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. Dr. 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. Dr. 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. 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. Impulse Devices, a small company in the small town of Grass Valley, Calif., is exploring the same sound-driven fusion as Dr. 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," Mr. 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."