Unfortunately for the world’s energy needs, fusion presents far greater technical challenges than fission, which physicists mastered in the 1940s. It takes relatively little energy to split a nucleus—fission can even happen spontaneously. But for fusion to occur—that is, to force two nuclei to join—physicists must replicate the hellish temperatures and pressures found inside stars.
NIF seeks to do that with 192 giant lasers, which occupy a space as large as three football fields. Fired simultaneously, the laser beams blast a peppercorn-size speck of frozen hydrogen suspended in a 30-foot-wide target chamber with about 500 trillion watts of power—about 1,000 times the amount of energy used by the entire United States during that same few trillionths of a second. (Because the lasers fire so briefly, NIF uses only about $20 of electricity for each burst.) Crushed to less than a thousandth of its original volume, the hydrogen becomes 100 times denser than lead and hotter than the center of a star; the nuclei fuse and release bursts of energy.
According to NIF’s computer simulations, the fused hydrogen should generate more energy than the lasers put in—a process called ignition. Nature, unfortunately, has stubbornly refused to cooperate. There has been no ignition at the National Ignition Facility.
When physicists first turned on all the lasers at NIF in February 2009, they set a goal of reaching ignition by October 1, 2012. NIF’s lasers routinely cause fusion, but the energy pumped in by the lasers still exceeds the energy created by the fusing hydrogen. The failure to meet that ignition deadline is the main reason the President, with the support of at least some in Congress, decided to cut NIF’s budget.
"From a back-of-the-envelope calculation, the lasers do deposit enough energy onto the hydrogen pellet to do the job," said Robert Rosner, a physicist at the University of Chicago and the former director of Argonne National Laboratory. "The $64,000 question—actually a lot more than $64,000—is, why is the actual energy captured by the pellet in its implosion so much lower than that, by close to a factor of ten?"
Like a Leaky Piston
Ed Moses, the photon science principal associate director at NIF, says the researchers there are focusing on solving two critical problems. For ignition to occur, the hydrogen pellet must remain perfectly spherical as the lasers compress it. Using X-ray cameras to track the imploding hydrogen, physicists have found that the pellet deforms just as fusion starts. It assumes a lumpy, clover shape, a sign that the hydrogen is losing heat and pressure during its compression. “It’s like a leaky piston, and the pressure doesn’t keep going up,” says Moses. The other problem concerns the thin plastic shell that encases the hydrogen fuel. Bits of it might be mixing with the hot imploding hydrogen, cooling it and squashing ignition.
"We have shown our ability to compress the diameter of the fuel to where it would ignite if it were round, which is something people would have found unbelievable a few years ago," says Moses. "What we haven’t shown yet is that we can get the shape we need as we go in, and that we can prevent mixing."
A recent report by the National Research Council recommended that NIF be given three more years to solve its problems and determine whether the facility is even capable of achieving ignition. Some critics argue that NIF needs to adopt a fundamentally different research strategy, a critique endorsed by the report. David Hammer, a physicist at Cornell University, says the NIF team treated their fusion experiments like an engineering project, and assumed that they could achieve ignition if they tweaked the lasers just right from one “shot” to the next.
"It was misplaced confidence," said Hammer. "They would not accept that the different stages of the experiments were not well understood, and they went on to the next step anyway." The NIF researchers should have been more systematic, he said, starting at lower energies to make sure the computer predictions matched reality. "If they didn’t get it right at some low level, then figure out what’s wrong, because it’s a lot easier to figure things out when you’re not driving an experiment to its limits. And once you’ve understood it at say, half-energy, then you gradually build up and see how the experiment moves away from predictions of the computer code. I think if they had started a more science-oriented program in 2009, when the lasers were finished, they’d be a lot closer to ignition now."