Laser fusion facility returns to the drawing board

Nearly a year ago, scientists at the world’s largest laser fusion facility announced a historic achievement: It had broken all records and produced, if only for a fraction of a second, an energetic fusion reaction of the kind that powers stars and thermonuclear weapons. However, efforts to replicate that experiment have fallen short. Nature has learned that earlier this year, researchers at the California facility changed direction and reconsidered their experimental design.

The turn of events has renewed debate over the future of the National Ignition Facility (NIF), a $3.5 billion device housed at the Lawrence Livermore National Laboratory and overseen by the National Fire Administration. Nuclear Security (NNSA), a branch of the US Department of Energy that manages nuclear weapons. The NIF’s primary mission is to create high-yield fusion reactions and inform the maintenance of US weapons stockpiles.

By some measures, the record-setting laser shot on August 8, 2021 proved that the facility, which has cost far more and produced far less than originally promised, has finally fulfilled its primary mission. However, repeated attempts generated, at best, 50% of the power produced at the end of last year. The researchers weren’t expecting smooth sailing as they tried to replicate the experiment, because the massive device is now operating at the cusp of fusion “turn-on,” where small unintended differences from experiment to experiment can have a big impact on output. For many, however, the inability to replicate last August’s experiment underscores researchers’ inability to accurately understand, design, and predict experiments at these energies.

Omar Hurricane, chief scientist for the Livermore inertial confinement fusion program, has advocated moving forward with the existing experimental design to investigate this energetic regime, rather than stepping back to regroup. “The fact that we’ve done it is kind of proof of existence that we can do it,” he says. “Our problem is doing it repeatedly and reliably.” However, he says, the program’s leadership made the decision to stop the replication experiments and focus on the next steps that could take the NIF well beyond the fusion threshold and into an entirely new and more predictable regime, where the yields are significantly higher than in the August experiment.

Some researchers in the community had long questioned the usefulness of the NIF, and for them, the entire episode highlights the facility’s remarkable achievements as well as its fundamental limitations. “I think they should call it a success and stop,” says Stephen Bodner, a physicist who previously led the laser fusion program at the US Naval Research Laboratory in Washington DC. Bodner says the NIF is a technological dead end and it’s time to prepare for a next-generation laser that could open the door to fusion power.

chasing the ignition

The NIF was launched in 2009 with the promise of achieving fusion ignition, which the US National Academy of Sciences (NAS) has defined as an experiment that generates more energy than it consumes. After missing the initial 2012 ignition deadline, Livermore scientists began a decade-long effort to fine-tune the system (see ‘The Road to Ignition’). Finally, last August, after a series of adjustments to aspects of the facility, including the laser and ignition target, a gold capsule containing a frozen pellet of the isotopes of hydrogen, deuterium, and tritium, had its defining moment. .

In less than 4 billionths of a second, 192 laser beams delivered 1.9 megajoules of energy to the target. As the capsule collapsed, the hydrogen isotopes in the core of the pellet began to fuse into helium, releasing a rush of energy and creating a cascade of reactions that ultimately released more than 1.3 megajoules of energy, about 8 times the record above and 1,000-fold improvement in early experiments.

Although it did not meet the NAS definition of ignition, the firing resulted in a high-yield fusion reaction that safely qualified as an ignition according to the criteria used by the NIF scientists. Hurricane calls it a “Wright Brothers moment,” and even the NIF’s harshest critics, including Bodner, doffed their hats.

In September, leaders of the inertial confinement fusion program drew up a plan for three experiments to determine if the August result could be replicated. The experiments began in October and produced only 400 to 700 kilojoules of energy. Although those results still represent a sea change in the operation of the NIF, they did not come close to the August breakthrough, nor did they exceed what NIF scientists describe as the ignition threshold.

Hurricane says the team’s analysis of those experiments indicates that inconsistencies in target fabrication and inevitable changes in laser performance due to its age produced minute but important differences in the shape of the implosion. “We understand why the repeat shots worked the way they did,” she says, “but we’re still trying to pin down what exactly we need to better control over these engineering aspects.”

In light of those results, Hurricane argued for additional repeated experiments that could be used to better understand shot-to-shot variability. However, program leaders chose to go ahead, and Hurricane says the team is now looking at ways to increase laser power by more than 10%, as well as modify targets that could make more efficient use of that power.

Mark Herrmann, deputy director of fundamental weapons physics at Livermore, says the lab received a lot of feedback from the more than 100 scientists involved in the program. But he stresses that the long-term goal is to achieve returns two orders of magnitude higher than those achieved even last August. “As long as we do a good, careful, systematic scientific study, that’s the most important thing from my perspective,” he adds.

a critical report

To some extent, it was to be expected that the lab would not replicate the August experiment, because the laser is now operating on the “cliff of ignition,” says Riccardo Betti, who directs the center for laser fusion at the University of Rochester in New York. and provides independent evaluations of experiments at the NIF. “If you’re on one side of the cliff, you can get a lot of fusion results, and if you’re on the other side of the cliff, you get very little,” he says. The lab does not yet have the experimental precision to predict which side a given experiment will land on, he says.

Questions about fundamental science and predictive ability were at the heart of a classified review of the NIF’s scientific contributions to the US nuclear weapons program provided to the NNSA last year by JASON, an independent scientific advisory panel. to the US government in an unclassified executive summary of the report, obtained by Nature under the US Freedom of Information Act, the panel acknowledged the capabilities of the NIF, but stated that the facility is unlikely to achieve a “predictable and reproducible turn-on” for years to come.

The report was completed and delivered to the NNSA four months before the August shooting, and Hurricane and others have argued that it was ill-timed and overly pessimistic.

The JASON panelists advocated a fundamental rethink of the program in their report, and that discussion has already begun in the broader laser fusion community. Scientists at the NIF and elsewhere are examining ways to reconfigure the current laser, while others are pushing entirely new designs that could provide more practical pathways to fusion power.

For its part, Hurricane is not in a hurry. He maintains that the device is now operating in a crucial fusion regime that will be useful in understanding and predicting the reliability of nuclear weapons.

“Once we get more power and more predictability, you’ve skipped the interesting physics,” says Hurricane. “If understanding and being better scientists and stewards [of the nuclear stockpile] is your goal, this is the regimen to work with.”