What Does It Mean When a Nuclear Fusion Lab Gets to 'Ignition'?

Fusion researchers at the U.S. National Ignition Facility started a reaction that made more energy than they put in.

Scientists at the world's largest nuclear fusion facility have created a nuclear reaction that gives off more energy than it uses. This is called "ignition." The results of the breakthrough at the US National Ignition Facility (NIF), which happened on December 5 and were announced today by the administration of US President Joe Biden, have gotten fusion researchers all over the world excited. The goal of that research is to use nuclear fusion, which is what makes the Sun work, to create a source of clean energy on Earth that is almost limitless.

Mark Herrmann, who is the deputy director for fundamental weapons physics at the Lawrence Livermore National Laboratory in California, where the fusion laboratory is, says, "It's an amazing achievement." Herrmann says that the experiment is a big deal because it is the result of years of work by many teams on things like lasers, optics, targets, and computer models. "Of course, that's what we're happy about."

The US Department of Energy's nuclear-weapons program's flagship experimental facility, NIF, was meant to study the reactions caused by nuclear weapons. Its original goal was to start up by 2012, but it has been criticized for delays and cost overruns. In August 2021, scientists from the NIF said that they had used their high-powered laser device to get a record reaction that crossed a critical threshold on the way to ignition. However, attempts to repeat that experiment, or shot, in the months that followed failed. In the end, scientists gave up on trying to recreate that shot and rethought the design of the experiment, which paid off last week.

“There were a lot of people who didn’t think it was possible, but I and others who kept the faith feel somewhat vindicated,” says Michael Campbell, former director of the fusion laboratory at the University of Rochester in New York and an early proponent of NIF while at Lawrence Livermore lab. “I’m having a cosmo to celebrate.”

Nature looks at NIF’s latest experiment and what it means for fusion science.

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What did NIF achieve?

With its 192 lasers, the facility sent 2.05 megajoules of energy to a gold cylinder the size of a pea that held a frozen pellet of deuterium and tritium isotopes of hydrogen. The energy pulse caused the capsule to break apart, creating temperatures that can only be found in stars and thermonuclear weapons. The hydrogen isotopes fused into helium, which released more energy and started a chain reaction of fusion reactions. The lab's analysis shows that about 3.15 megajoules of energy were released, which is about 54% more than the energy that went into the reaction and more than double the previous record of 1.3 megajoules.

Campbell says, "Research on fusion has been going on since the 1950s, and this is the first time in the lab that fusion has ever made more energy than it used."

The experiment is a safe example of ignition, which is a standard measure for fusion reactions that looks at how much energy went into the target and how much energy came out. But the fusion reactions may have made more than 3 megajoules of energy, which is more than what was sent to the target, but the NIF's 192 lasers used up 322 megajoules of energy in the process.

Dave Hammer, a nuclear engineer at Cornell University in Ithaca, New York, says, "It's a big step forward, but NIF is not a fusion energy machine."

Herrmann agrees with this, saying that getting to laser fusion energy is a long process with many steps. He says, "NIF was not made to be efficient." "It was made to be the biggest laser we could make so that we could get the information we need for the nuclear stockpile research program."

Before the most recent laser shot, scientists at the NIF made several changes, based in part on analysis and computer modeling of last year's experiments, in order to get the fuel to light. Along with increasing the power of the laser by about 8%, scientists made a new target with fewer flaws and changed how the laser energy was sent to the target to make the implosion more round. Scientists knew they were right on the edge of starting fusion, and Herrmann says that in that situation, "little changes can make a big difference."

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Why are these results significant?

On one level, the goal is to show what is possible, and many scientists have called the result a major step forward in the field of fusion science. But the results are especially important at NIF because the facility was built to help scientists who study nuclear weapons study the intense heat and pressures that happen inside thermonuclear explosions. This can only be done if the facility produces high-yield fusion reactions.

Stephen Bodner, a physicist who used to run the laser-fusion program at the US Naval Research Laboratory in Washington, DC, says, "They should be praised for reaching their goal." It took more than a decade, but they should be praised for getting there. Bodner says that the big question now is what the Department of Energy will do next: double down on weapons research at NIF or switch to a laser program that is specifically designed for fusion-energy research.

What does this mean for fusion energy?

The latest results have already stirred up interest in a future where clean fusion energy powers the world, but experts warn that there is still a long way to go.

Scientists at NIF are quick to admit that the facility wasn't built with commercial fusion energy in mind, and many researchers aren't sure that laser-driven fusion will be the way to get fusion energy in the end. Campbell, on the other hand, thinks that its most recent success could make people more optimistic about the promise of laser fusion power and, in the end, lead to a new program that focuses on energy applications. "This is a must if you want to be taken seriously and be able to sell an energy program," he says.

Kim Budil, who runs the Lawrence Livermore laboratory, called the success a "proof of concept." During a press conference in Washington, DC, she said, "I don't want you to think that we're going to plug the NIF into the grid. That's not how this works at all." "But this is the most important part of a plan to use fusion power with inertial confinement."

There are many other fusion experiments going on around the world that use different methods to try to achieve fusion for energy purposes. But there are still engineering problems to solve, like how to design and build plants that can take the heat made by fusion and use it to make a lot of energy that can be turned into electricity.

"This is good news, but this result is still a long way from the actual energy gain needed to make electricity," said Tony Roulstone, a nuclear energy researcher at the University of Cambridge in the UK, in a statement to the Science Media Centre.

Still, Anne White, a plasma physicist at the Massachusetts Institute of Technology in Cambridge, says that the NIF experiments that focus on fusion energy are definitely helpful on the way to commercial fusion power.

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What are the next major milestones in fusion?

To show that the type of fusion studied at NIF can be used to make energy, the efficiency of the yield—the energy released compared to the energy used to make the laser pulses—needs to increase by at least two orders of magnitude.

Time Luce, head of science and operation at the international nuclear fusion project ITER, which is being built in St. Paul-lez-Durance, France, says that researchers will also need to make a big change in how fast the lasers can produce pulses and how quickly they can clear the target chamber to get it ready for another burn.

"Enough fusion events that produce energy over and over again would be an important milestone," says White.

The US$22 billion ITER project is a collaboration between China, the European Union, India, Japan, Korea, Russia, and the United States. Its goal is to achieve self-sustaining fusion, which means that the energy from fusion creates more fusion, using a different method than NIF's "inertial confinement fusion" method. ITER will keep a plasma of deuterium and tritium inside a toroidal vacuum chamber, or tokamak, and heat it up until the nuclei fuse. Luce says that when it starts to do this in 2035, it will try to get to the "burning" stage, where the self-heating power is the main source of heat. Fusion that can keep going on its own is the key to making more energy than is put in.

What does it mean for other fusion experiments?

NIF and ITER are two ideas for fusion technology that governments all over the world are working on. Some of the methods are magnetic confinement, which is used by tokamaks and stellarators, inertial confinement, which is used by NIF, and a combination of the two.

White says that the technology needed to make electricity from fusion is mostly separate from the idea, and that the latest milestone won't necessarily cause researchers to give up on or combine ideas.

ITER and other facilities face different engineering challenges than NIF does. But the symbolic success could affect a lot of people. "A result like this will make people care more about the progress of all kinds of fusion, so it should be good for fusion research as a whole," says Luce.

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