Last September, under x-ray assault, the rapid implosion of a plastic shell into icy isotopes of hydrogen produced fusion at Lawrence Livermore National Laboratory's National Ignition Facility (NIF). This wasn't just a run-of-the-mill fusion reaction: it was the first one NIF has ever produced wherein the fuel released more energy than it absorbed.

The laboratory's 192 lasers have been pumping energy into a succession of tiny fuel pellets since 2010. In this instance, the scientists got the timing right. Instead of ramping up the lasers over the course of the blast, which lasts 20 trillionths of a second, Livermore physicist Omar Hurricane and his team started the blast at maximum intensity and then let it taper off. That change made the fuel in the two-millimeter pellet hotter sooner—reaching temperatures of about 50 million degrees Celsius and pressures of 150 billion Earth atmospheres. Such conditions enable fusion, and, in this case, the fusing fuel yielded nearly twice as much energy as the roughly 10,000 joules that triggered it. The results were published in February in Nature. (Scientific American is part of Nature Publishing Group.)

“This is closer than anyone's gotten before” to self-sustaining energy, Hurricane says. Yet scientists still have a lot of work to do. Although the fuel pellet yielded 17,000 joules of energy, the entire fusion experiment fell far short of breaking even. The NIF experiment required more energy to run than it generated; feeding the lasers alone required a burst of at least 190 million joules.* Doing better than breaking even—or “ignition,” as the NIF folks put it—will require even more extreme pressures and other conditions. A source of nearly unlimited, clean energy is still decades away.

*Correction (7/31/14): This sentence was changed after posting from the original print version, which erroneously stated the amount of energy necessary to feed the lasers.