美国国家公共电台 NPR--U.S. reaches a fusion power milestone. Will it be enough to save the planet?（在线收听）

U.S. reaches a fusion power milestone. Will it be enough to save the planet?

Transcript

Scientists with the U.S. Department of Energy have reached a breakthrough in nuclear fusion.

For the first time ever in a laboratory, researchers were able to generate more energy from fusion reactions than they used to start the process. The total power out was around 150% of the power that was put in by 192 laser beams.

"America has achieved a tremendous scientific breakthrough," Energy Secretary Jennifer Granholm said at a press conference.

The achievement came at the National Ignition Facility (NIF), a $3.5 billion laser complex at Lawrence Livermore National Laboratory in California. For more than a decade, NIF has struggled to meet its stated goal of producing a fusion reaction that generates more energy than it consumes.

But that changed in the dead of night on Dec. 5. At 1 AM local time, researchers used the lasers to zap a tiny pellet of hydrogen fuel. The lasers put out 2.05 megajoules of energy, and the pellet released roughly 3.15 megajoules.

It's a major milestone, one that the field of fusion science has struggled to reach for more than half a century.

"In our laboratory we've been working on this for almost 60 years," says Mark Herrmann, who oversees the NIF program at Livermore. "This is an incredible team accomplishment."

Researchers say that fusion energy could one day provide clean, safe electricity without greenhouse gas emissions. But even with this announcement, independent scientists believe that dream remains many decades away.

Unless there's an even larger breakthrough, fusion is unlikely to play a major role in power production before the 2060s or 2070s, says Tony Roulstone, a nuclear engineer at Cambridge University in the U.K., who's done an economic analysis of fusion power.

"I think the science is great," Roulstone says of the breakthrough. But many engineering obstacles remain. "We don't really know what the power plant would look like."

At that rate, fusion power won't come soon enough for the Biden administration, which is seeking to bring America's net greenhouse gas emissions to zero by 2050 — a goal that experts say must be met to avoid the worst effects of climate change.

Laser power

Fusion power has long fired the imaginations of nuclear scientists and engineers. The technology would work by "fusing" light elements of hydrogen into helium, generating an enormous amount of energy. It's the same process that powers the sun, and it's far more efficient than current nuclear "fission" technology. What's more, fusion power plants would generate relatively little nuclear waste, and they could run off of hydrogen readily found in seawater.

The ten-story-tall NIF facility is the world's most powerful laser system. It is designed to aim its 192 beams onto a tiny cylinder of gold and depleted uranium. Inside the cylinder is a diamond capsule smaller than a peppercorn. That capsule is where the magic happens — it's filled with two isotopes of hydrogen that can fuse together to release astonishing amounts of energy.

When the lasers are fired at the target, they generate x-rays that vaporize the diamond in a tiny fraction of a second. The shockwave from the diamond's destruction crushes the hydrogen atoms, causing them to fuse and release energy.

NIF first opened in 2009, but its initial laser shots fell well short of expectations. The hydrogen in the target was failing to "ignite", and the Department of Energy had little to show for the billions it had invested.

Then in August 2021, after years of slow but steady progress, physicists were able to ignite the hydrogen inside the capsule, creating a self-sustaining burn. The process is analogous to lighting gasoline, says Riccardo Betti, the chief scientist of the laboratory for laser energetics at the University of Rochester. "You start with a little spark, and then the spark gets bigger and bigger and bigger, and then the burn propagates through."

Bang in a box

This self-burning ignition actually resembles a process similar to that of a modern thermonuclear warhead, albeit on a much smaller scale.

The United States has not tested a nuclear weapon since 1992, and the primary purpose of the NIF facility is to conduct very small-scale bangs that closely mimic nukes. The data from these tiny explosions are fed into complex computer simulations that help physicists understand whether the nation's nuclear weapons remain reliable, despite decades on the shelf.

"We use these experiments to get experimental data to compare to our simulations," says Herrmann, who also oversees nuclear weapons research at the lab. In addition, he says the radiation from the explosions can be used to test components. Such tests will make sure that new and refurbished parts of nuclear weapons behave as expected.

More out than in

Even after last-year's achievement, there was still one more goal to reach – producing more power from the tiny capsule than the lasers put in.

Herrmann says that the August 2021 shot gave the team a starting point. "That put us on the threshold," he says. "We actually made a lot of progress in the last year." Steady improvements in the lasers, targets, and other components gradually put the facility in a position where it could finally generate energy from the capsule.

"It is a big scientific step," says Ryan McBride, a nuclear engineer at the University of Michigan. But, McBride adds, that does not mean that NIF itself is producing power. For one thing, he says, the lasers require more than 300 megajoules worth of electricity to produce around 2 megajoules of ultraviolet laser light. In other words, even if the energy from the fusion reactions exceeds the energy from the lasers, it's still only around one percent of the total energy used.

Moreover, it would take many capsules exploding over and over to produce enough energy to feed the power grid. "You'd have to do this many, many times a second," McBride says. NIF can currently do around one laser "shot" a week.

Still, the long-term potential is staggering, says Arati Dasgupta, a nuclear scientist with the U.S. Naval Research Laboratory. Whereas a giant pile of carbon-spewing coal might generate electricity for a matter of minutes, the same quantity of fusion fuel could run a power plant for years–with no carbon dioxide emissions. "This is a great demonstration of the possibility," Dasgupta says. But, she adds, many technical issues remain. "It's a huge undertaking."

And getting economical power out of a fusion reactor is even tougher, says Roulstone. He and his team looked at a rival technology known as a tokamak and concluded that there were still an enormous number of challenges to making fusion work economically. His analysis estimated that fusion won't be ready for the grid before the second half of this century. He believes the same timeline holds for NIF's technology. "It's not very easy to see how you scale this into a power reactor quickly," he says.

By then most climate experts believe the world will have to have already made drastic cuts to carbon emissions to avoid the worst effects of climate change. To limit warming to 2.7 degrees Fahrenheit by the end of the century, the world must nearly halve its carbon output by 2030 — a far shorter timescale than what's needed to develop fusion.

Betti agrees that the timeline to building a fusion plant is "definitely decades". But, he adds, that could change. "There's always a possibility of breakthrough," he says. And the new NIF results could help spur that breakthrough forward. "You're going to get more people to look into this form of fusion, to see whether we can turn it into an energy-making system."

NPR's Rebecca Hersher contributed to this report.