Part of our weekly "In Focus" series—stepping back, looking closer.
A large banner hangs from the front of the stadium-size building that houses the world's most powerful array of lasers: "Bringing Star Power To Earth."
For the past four years, physicists at the National Ignition Facility, or NIF, in Livermore, California, have been trying to harness nuclear fusion, the same reaction that powers the sun and the stars. Supporters of the $3.5 billion facility believe that a successful outcome to the experiments could help usher in an era of nearly limitless energy. But the ambitious fusion research program at NIF now faces an uncertain future, both politically and scientifically.
On the political side, President Obama's proposed budget for fiscal year 2014 would reduce funding for fusion experiments at NIF by more than $60 million, putting it nearly 14 percent below the 2013 level. Key committees in both the House and Senate favor restoring part of NIF's funding, and a compromise will eventually emerge, but budget constraints aren't the only challenge facing NIF. Physicists working on the project expected to have succeeded in their quest for fusion energy by now. They're currently struggling to figure out what went wrong.
Tiny Stars, Big Lasers
There's an old joke about fusion: It's the energy source of the future, and always will be. Physicists have been pursuing the dream of controlling fusion energy for some 60 years now. Unlike nuclear fission, which releases energy when the nucleus of a heavy atom like uranium splits into two lighter nuclei, fusion generates energy when two separate light nuclei smash together to form a single, heavier nucleus. In fission, the energy comes from breaking the bonds of force that held the original heavy atom together; with fusion, the energy source is more esoteric—some of the mass from each of the two light nuclei is converted directly into energy when they fuse, in accordance with Einstein's iconic law, E=mc2.
Both fission and fusion release tremendous amounts of energy. One pound of enriched uranium used in a conventional nuclear power plant contains about as much energy as a million gallons of gasoline. Fusion yields even more energy—about three to four times as much as fission reactions. And while fission reactions generate waste that remains radioactive for millennia, fusion's byproducts become harmless within decades. Moreover, the world possesses a nearly infinite source of fusion fuel—the hydrogen atoms found in water.
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."
NIF isn't the only fusion project competing for federal dollars. The United States is also investing in an international collaboration that plans to harness fusion using a completely different strategy from NIF's. Now under construction in France, ITER, short for International Thermonuclear Experimental Reactor, will use powerful magnetic fields to compress a plasma—essentially hydrogen gas heated to such high temperatures that the electrons and protons in the hydrogen fly apart—until the protons fuse. The $20 billion project, which is scheduled to begin its first experiments in November 2020, aims to produce ten times the amount of energy needed to run it. But that 2020 deadline is likely to recede, given that President Obama's budget would cap future United States contributions to ITER at $225 million. The budget would also cut funds for a fusion laboratory at MIT, one of the three American projects conducting experiments related to ITER.
A final decision on NIF's funding is months away, as budget wrangling continues on Capitol Hill.
"Right now we're in the era of incremental government," said Representative Eric Swalwell, a Democrat, whose district includes the facility at Livermore. "We govern by crisis these days, which is really unfortunate, because while science is very unpredictable, when it comes to funding, scientists need certainty."
Nuclear Weapons and Getting to the World Series
Even with the proposed budget cuts, NIF will continue to operate for decades. Achieving ignition is only one aspect of the lab's mission. Its primary purpose—one that will most likely overshadow fusion research in the years ahead—is to enable the United States to maintain its stockpile of nuclear weapons. The country has observed a ban on explosive testing since 1992, and classified work at NIF tests components of nuclear weapons without the need to blow anything up. That aspect of NIF's research has broad bipartisan support, and the President's budget for 2014 would increase funding for the lab's weapons-testing program.
But ignition is the game-changing research that inspires most of the physicists who work at NIF.
"I've dedicated my life to this," said Moses. "I'm committed to understanding it. I think it's likely we'll work through all these issues. We have this three-year time line we've agreed to. If we're funded and can do our experiments, we think we can explore this phenomenon pretty completely in that time period. In sports, over a long season, some things go well, sometimes you boot the ball. The question is, how do you get to the World Series? And that's what we're trying to do."