Almost a year ago, scientists at the world’s largest laser fusion facility announced a groundbreaking feat: it had smashed all records and created, if only for a split second, an energetic fusion reaction of the kind that powers stars and thermonuclear weapons. However, efforts to repeat this experiment were unsuccessful. Nature has learned that researchers at the California facility changed direction earlier this year and revised their experimental design.
The turn of events has renewed debate about the future of the National Ignition Facility (NIF), a $3.5 billion device housed at Lawrence Livermore National Laboratory and managed by the National Nuclear Security Administration (NNSA). , a subsidiary of the United States, oversees the Department of Energy, which administers nuclear weapons. The primary mission of the NIF is to generate high-efficiency fusion reactions and inform US weapons inventory maintenance.
The record-breaking laser shot of August 8, 2021 proved through a number of measures that the facility, which cost much more and performed much less than originally promised, has finally fulfilled its main task. However, repeated attempts yielded at best 50% of the energy produced late last year. Researchers didn’t expect to go smoothly when attempting to replicate the experiment, as the massive device is now operating on the cusp of fusion “ignition,” where tiny, unintended differences from one experiment to another can have huge impacts on performance. Nonetheless, for many, the failure to reproduce last August’s experiment underscores the inability of researchers to accurately understand, design, and predict experiments at these energies.
Omar Hurricane, chief scientist for Livermore’s inertial fusion program, has advocated advancing the existing experimental design to study this energy regime, rather than stepping back and regrouping. “The fact that we made it is kind of an existential proof that we can make it,” he says. “Our problem is doing it repeatedly and reliably.” However, he says the program leadership made the decision to halt replication experiments and focus on the next steps that will take the NIF well beyond the fusion threshold and into an entirely new – and more predictable – regime where yields are significantly larger than in The August Experiment.
Some researchers in the community had long questioned the usefulness of the NIF, and for them the entire episode underscores 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 used to lead the laser fusion program at the US Naval Research Laboratory in Washington DC. Bodner says NIF is a technological dead end and that it’s time to prepare for a next-generation laser that could open the door to fusion energy.
The NIF opened in 2009 with the promise of achieving fusion ignition, which the US National Academy of Sciences (NAS) has defined as an experiment that produces more energy than it uses. After missing the initial deadline to achieve ignition in 2012, Livermore scientists began a decade-long effort to fine-tune the system (see “The Road to Ignition”). Finally, they broke through last August after making a number of adjustments to aspects of the facility, including the laser and the firing target — a gold capsule containing a frozen pellet of the hydrogen isotopes deuterium and tritium.
In less than 4 billionths of a second, 192 laser beams delivered 1.9 megajoules of energy to the target. As the capsule collapsed, isotopes of hydrogen in the core of the pellet began to fuse into helium, releasing a stream of energy and creating a cascade of reactions that eventually released more than 1.3 megajoules of energy – about eight times the previous record and a 1,000- fold improvement over the earliest experiments.
Although it did not fit the NAS definition of ignition, the shot resulted in a high-yield fusion reaction that could safely be qualified as ignition according to the criteria used by NIF scientists. Hurricane calls it a “Wright Brothers moment,” and even the NIF’s harshest critics, including Bodner, tipped their hats.
In September, leaders of the inertial fusion program drew up a plan for three experiments to see if August’s result could be replicated. The experiments began in October and delivered only 400-700 kilojoules of energy. While these results still represent a game-changing change in NIF operations, they didn’t come close to August’s breakthrough — nor did they surpass what NIF scientists call the ignition threshold.
According to Hurricane, the team’s analysis of these experiments suggests that inconsistencies in the manufacture of the target and inevitable shifts in the power of the laser due to its age created tiny but important differences in the shape of the implosion. “We understand why the retakes worked the way they did,” he says, “but we’re still trying to figure out what exactly we need to control more about those technical aspects.”
Given these results, Hurricane advocated additional replicate experiments that could be used to better understand shot-to-shot variability. However, program leaders chose to go ahead, and according to Hurricane, the team is now looking at ways to increase laser energy by more than 10% and modify targets that could use that energy more efficiently.
Mark Herrmann, associate director of fundamental weapons physics at Livermore, says the lab has received a lot of feedback from the more than 100 scientists involved in the program. However, he stresses that the long-term goal is to generate returns two orders of magnitude higher than those achieved as recently as August. “As long as we do good, careful and systematic scientific studies, I think that’s the most important thing,” he adds.
A critical report
To some extent, the lab’s failure to replicate the August experiment was to be expected given that the laser is now working on the “spark cliff,” says Riccardo Betti, who directs the laser fusion center at the University of Rochester in New York, a provides independent Reviews of experiments at the NIF. “If you’re on one side of the cliff, you can get a lot of fusion power, and if you’re on the other side of the cliff, you get very little,” he says. The lab doesn’t yet have the experimental accuracy to predict which side a given experiment will end up on, he says.
Questions about basic research and predictive ability were the focus 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 body that advises the US government. In an unclassified summary of the report obtained from Nature Under US Freedom of Information Act, the panel recognized the NIF’s capabilities, but noted that the facility was unlikely to achieve “predictable, reproducible firing” in the next few years.
The report was completed and submitted 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 exploring ways to reconfigure the current laser, while others are pushing for entirely new designs that could open up more practical routes toward fusion power.
For his part, Hurricane is in no hurry. He claims 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 have more energy and more predictability, you’ve skipped the interesting physics,” says Hurricane. “If you understand scientists and administrators and get better [of the nuclear stockpile] is your goal, this is the regime in which you must work.