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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.

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Credit: Jason Madara
Video by Kevin Bullis

Tagged: Energy, lasers, fusion, fusion reactions

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