The energy source has been long on promise and short on reality. Now private companies think they can succeed where the government has failed. For more than half a century governments around the world have been trying to solve the challenge of nuclear fusion. In theory it could provide a cheap, clean, and almost boundless source of energy. Consider this: One tablespoon of liquid hydrogen fuel—a mix of deuterium and tritium—would produce the same energy as 28 tons of coal. But smashing two hydrogen atoms together at 100 million degrees centigrade to create a fusion reaction has proved to be a costly and elusive endeavor. The ITER international project in France has been plagued by cost overruns—the original 5-billion-euro project is now budgeted at 13 billion euros (about $15 billion)—and its 23,000-ton Tokamak experimental reactor, three times heavier than the Eiffel Tower, is still many years from completion. In […]