By SAM SCHECHNER

PITTSBURGH -- In the garage of his house, Frank Sanns spends nights tinkering with one of his prized possessions: a working nuclear-fusion reactor.

Mr. Sanns, 51 years old, is part of a small subculture of gearheads, amateur physicists and science-fiction fans who are trying to build fusion reactors in their basements, backyards and home laboratories. Mr. Sanns, who owns a banquet hall here, believes he's on track to make fusion a viable power source.

"I'm a dreamer," he says.

Many of these hobbyists call themselves "fusioneers," and have formed a loosely knit community that numbers more than 100 world-wide. Getting into their elite "Neutron Club" requires building a tabletop reactor that successfully fuses hydrogen isotopes and glows like a miniature star. Only 42 have qualified; some have T-shirts that read "Fusion -- been there...done that."

Called fusors and based on a 1960s design first developed by Philo T. Farnsworth, an inventor of television, the reactors are typically small steel spheres with wires and tubes sticking out and a glass window for looking inside. But they won't be powering homes anytime soon -- for now, fusors use far more energy than they produce.

Fusion, which releases energy by forcing two atoms close enough together that they join to become a heavier atom, is the process that powers the sun and stars. Replicating that on Earth requires enormous amounts of energy. For decades, scientists have been experimenting with various methods to fuse atoms, including using magnetic fields and lasers. Even a nearly $15 billion multinational project to build a fusion reactor in southern France is only intended to show that fusion power is technically feasible, not to actually tap it.

But the allure is strong. A fusion power plant would likely be fueled by deuterium and tritium, both isotopes of hydrogen that are in plentiful supply. Fusion advocates say reactors would be relatively clean, generating virtually no air pollution and little long-lived radioactive waste. Today's nuclear power plants, in contrast, are fission-based, meaning they split atoms and create a highly radioactive waste that can take millennia to decompose.

While some amateurs, like Mr. Sanns, think fusion power holds promise, others are less hopeful. "Basically, it's almost like, over the gates of hell, 'Abandon hope all ye who enter,'" says Richard Hull, who built his first. . .

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