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A-bomb blasts are hardly practical in power plants, and the sun's internal pressure is impossible to duplicate on earth. So fusion scientists put their nuclei in a bottle -- not a physical one, since any contact with the walls would instantly cool the gas and kill the reaction -- but a bottle made of magnetic fields. The researchers would make up for the comparatively low pressure inside by raising the temperature to unheard-of levels. (A competing idea that shows promise uses converging laser beams to compress and ignite a stream of tiny, gas-filled glass pellets.)
Confining a gas made of electrically charged atomic nuclei -- a plasma -- has proved to be far more complex than anyone had suspected, and so has heating it. While the first rudimentary fusion reactors were a few feet across and weighed a ton or two, the Tokamak weighs hundreds of tons and fills a gymnasium-size room. A commercial reactor would be much bigger still and with current technology would cost hundreds of billions of dollars.
The main attraction of fusion is the potentially limitless fuel supply. The ideal fuel is not plain hydrogen but the formula used last week: a mixture of deuterium and tritium, two isotopes of hydrogen that have extra neutrons in their nuclei. Even though they're rarer than ordinary hydrogen, scientists estimate that enough of these two isotopes could be extracted from the top 2 in. of water in Lake Erie to match the energy in all the world's oil reserves.
But no one knows for sure whether fusion on a large scale will be practical. The U.S. Department of Energy has canceled a bigger machine that was supposed to go beyond what Tokamak can achieve. Instead America will join the Europeans, Japanese and Russians in building the International Thermonuclear Experimental Reactor; when it goes into operation a decade or so from now, fusion scientists should finally have a device that generates more power than it consumes. Even then it will take decades of engineering before any households could possibly draw electricity from a commercial fusion plant.