Igniting Fusion

It’s late April and workers are assembling the last parts of the National Ignition Facility (NIF), a sprawling building covering the area of three football fields at Lawrence Livermore National Laboratory in Livermore, CA. Dressed in hard hats, hair nets, lab coats, and latex gloves, they have gathered at the target chamber, a sphere 10 meters in diameter and bristling with 48 burnished-aluminum ducts that together house 192 separate laser beams. Each beam on its own is one of the world’s most powerful, says Bruno Van Wonterghem, operations manager at NIF. Together they deliver 50 to 60 times the energy of any other laser.

The workers are preparing to install a key piece of equipment–the target-alignment sensor–at the end of a tapered boom that can be extended into the center of the chamber. Scientists will use the sensor to position a gold canister the size of a pencil eraser at the center of the sphere and align it with the laser beams. In a series of experiments over the coming months, if all goes according to plan, those lasers will strike the gold canister with a pulse 3 to 20 nanoseconds long, generating a bath of high-energy x-rays. These in turn will cause a two-­millimeter pellet containing hydrogen isotopes to implode. “All of that kinetic energy gets transformed into heat,” says Van ­Wonterghem. The hydrogen pellet will reach a temperature of 100 million °C and a density 100 times that of lead–enough to start a fusion reaction.

Fusion, in which atomic nuclei combine to form atoms of a new element, is the key reaction fueling nuclear bombs and the sun. (In the NIF experiments, hydrogen isotopes combine to form helium nuclei while releasing neutrons and x-rays.) It has also long been held up as a potential source of abundant energy, if only the reactions could be harnessed in a controlled setting. That’s challenging, because a plasma hot enough for hydrogen nuclei in it to fuse is so hot that it would destroy any containment material. Scientists have conceived two general solutions. The first and most mainstream is to confine the plasma in a powerful electromagnetic field. That is what’s supposed to happen at the multinational, $14 billion ITER project in France, which is expected to be operational by 2018.

NIF takes a fundamentally different tack. By using lasers to compress the hydrogen fuel, it will mimic the extreme heat and density inside a star. The resulting fusion reaction is controlled not by confining it electromagnetically but by limiting the amount of fuel. NIF will produce a tiny thermonuclear explosion, so small that it can be studied in a 10-meter chamber. In fact, NIF’s primary mission is to shed light on high-­temperature and high-density physics, including the reactions in nuclear weapons, by re-creating conditions inside stars and bombs.

Researchers debate which approach will be the most useful for generating electricity; so far it’s too early to be sure. But it looks likely that NIF will be the first facility to reach a significant milestone in the quest for laser-based fusion power: the ignition of a self-sustaining reaction that produces more energy than was put in by the laser. Previous experiments and computer simulations suggest that the 192 lasers at NIF are powerful and precise enough to set off such a chain reaction–one that will continue to burn until the hydrogen fuel runs out.

There are still huge challenges to be met before fusion can be harnessed to generate electricity. But achieving controlled fusion burn “will be an incredible event,” says Edward Moses, a principal associate director at Livermore who’s in charge of NIF. “We think we’re coming to a new era.”

Firing Lasers
Igniting fusion won’t be easy. It requires a facility that can marshal vast amounts of power but control it so precisely that it can be aimed at targets measured in micrometers. That, says Ian Hutchinson, a professor of nuclear science and engineering at MIT, will be “an incredibly impressive technological achievement.”

On the same afternoon when technicians worked to install the target-­alignment sensor, others have started to gather in the facility’s control room, with its large screens and clusters of work­stations. They’re preparing for a test shot of the laser, minus the fusion pellet; as a safety precaution, it’s been scheduled for night, after the facility’s laser bays and target chamber have been cleared of workers.

Firing the laser requires setting 60,000 different control points. The sequence of events that delivers the laser pulse to the target is too complex for human control, Van Wonterghem says, so after the settings are selected, a network of 1,500 computers will take over and carry out the final countdown, with the researchers’ hands hovering near the many emergency-shutdown buttons arranged throughout the room.

If it all works, the lasers will deliver a pulse of power 500 times greater than the peak electricity-generating capacity of the United States. The pulse will ignite the thermonuclear explosion–essentially creating a tiny star.

Powering Up
Significant hurdles will remain before such a process can be used to generate electricity. The fusion reactions are expected to produce 10 to 20 times the amount of energy delivered by the lasers. But this does not take into account the energy needed to make the lasers in the first place: converting electricity into laser light is an inefficient process. Making up for the wasted energy, and producing enough extra to generate electricity, would require fusion reactions that generate about 100 times the energy delivered by the lasers.

Speaking in a cluttered office near NIF, Moses says there are at least two potential ways to get around this problem. One requires combining two laser pulses in a process called fast ignition. In theory, this could reduce the amount of laser energy needed to ignite a sustained reaction. NIF, however, isn’t currently set up for this; it’s an approach that will be taken by other laser fusion projects now under construction, and eventually by NIF as well.

The other approach, Moses says, is to combine fusion with fission, the reaction used in conventional nuclear power plants. This option doesn’t offer the same prospect of nearly limitless energy as fusion alone, but it could increase by orders of magnitude the amount of energy that can be extracted from uranium, greatly enhancing this already abundant source of fuel. At the same time, it could remove the chief objection to nuclear fission by eliminating almost all the long-lived radioactive waste it typically produces. “Right now we only get half a percent to 1 percent of the available energy,” Moses says. “We can get 99-plus out.”

The researchers at NIF have developed a detailed conceptual plan for pairing fusion and fission. The reason nuclear reactors use only a fraction of the energy in uranium is that as reaction products accumulate, they eventually interfere with the chain reactions needed to keep generating power. Fusion can supply a stream of neutrons that can keep these reactions going, using up almost all the energy in the fuel.

To be sure, not everyone agrees that laser-based fusion power will work. And some skeptics question whether NIF in particular can achieve self-sustained fusion, saying that the facility cannot produce sufficiently high-energy laser pulses without either damaging the laser optics or losing the tight focus on the target needed to compress the fuel evenly. Even if the facility achieves sustained fusion, producing electricity in a power plant would require lasers that could ignite a new fuel pellet 10 to 15 times a second. The NIF lasers, which have to be cooled down between shots, can be fired at most once every two to four hours. “Even if NIF is as successful as hoped, they’ll still be a very long way from being in a position to turn this into a practical energy source,” Hutchinson says.

NIF has already seen some signs of success. Earlier this year, all 192 lasers were fired at once and reached energy levels that will be enough to ignite fusion. Still, earlier laser projects at Livermore were supposed to achieve fusion ignition and didn’t. Although a lot has been learned since then, there’s no guarantee it will work this time. The good news is that it won’t be long until the researchers know: after a series of test shots, they hope for success within the next two years. “We’re looking forward to hearing some results,” Hutchinson says.