Almost a year ago, scientists at the world’s largest laser fusion center announced an outstanding achievement: it broke all records and produced, if only for a fraction of a second, an energetic thermonuclear reaction similar to that that powers stars and thermonuclear weapons. However, attempts to reproduce this experiment have not been successful. Nature it became known that earlier this year, researchers at the California-based institution changed direction, moving to rethink their experimental design.
The turn of events has rekindled debate about the future of the National Ignition Facility (NIF), a $3.5 billion device that is housed at Lawrence Livermore National Laboratory and overseen by the National Nuclear Security Administration (NNSA), a US affiliate. Department of Energy, which manages nuclear weapons. The main mission of NIF is to create high-throughput fusion reactions and inform the maintenance of the US weapons stockpile.
By some measures, the record-breaking laser shot on August 8, 2021 proved that the installation, which cost much more and delivered much less than originally promised, had finally fulfilled its main task. However, repeated attempts gave at best 50% of the energy produced at the end of last year. The researchers didn’t expect things to go smoothly when trying to replicate the experiment because the massive device is now operating on the cusp of fusion fire, where tiny, unintended differences from one experiment to the next can have a huge impact on the outcome. However, for many, the failure to replicate last August’s experiment highlights the failure of researchers to accurately understand, design, and predict experiments at these energies.
Omar Hurricane, chief scientist at the Livermore Inertial Confinement Fusion Program, advocates moving forward with the existing experimental plan to investigate this energy regime rather than backtracking to regroup. “The fact that we did it is kind of proof that we can do it,” he says. “Our challenge is to do it repeatedly and reliably.” However, according to him, program management has made the decision to stop the replication experiments and focus on the next steps, which could push NIF far beyond the synthesis threshold and enter a completely new – and more predictable – mode, where the yields are significantly higher than in the previous one. . August experiment.
Some researchers in the community have long questioned the usefulness of NIF, and for them, the entire episode highlights the object’s remarkable accomplishments as well as its fundamental limitations. “I think they should recognize this as 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 NIF is a technological dead end and it’s time to prepare for the next generation laser that could open the door to fusion power.
Chasing the ignition
NIF opened in 2009 with the promise of igniting fusion, which the US National Academy of Sciences (NAS) has defined as an experiment that generates more energy than it consumes. Having missed the initial deadline of achieving ignition in 2012, Livermore scientists began a decade-long work to fine-tune the system (see Path to Ignition). Finally, last August, after a series of adjustments to aspects of the setup, including the laser and the ignition target—a golden capsule containing a frozen pellet of deuterium and tritium hydrogen isotopes—the breakthrough moment came.
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 balloon’s core began to fuse into helium, releasing a torrent of energy and creating a cascade of reactions that released more than 1.3 megajoules of energy—about 8 times the previous record and a 1,000-fold improvement over most early experiments.
Although it did not meet the NAS definition of ignition, the shot resulted in a highly efficient fusion reaction that safely qualified as ignition according to the criteria used by NIF scientists. Hurricane calls it “the Wright Brothers moment” and even the harshest NIF critics, Bodner included, raised their hats.
In September, leaders of the inertial confinement fusion program developed a plan for three experiments to determine if the August result could be repeated. The experiments began in October and produced only 400–700 kilojoules of energy. While these results still represent a step change in NIF’s work, they did not come close to the August breakthrough or surpass what NIF scientists call the ignition threshold.
Uragan says the team’s analysis of these experiments shows that inconsistencies in target fabrication and inevitable changes in laser performance due to its age resulted in minor but important differences in implosion shape. “We understand why the re-shots were performed the way they were,” he says, “but we are still trying to determine what exactly in these engineering aspects we need to have more control over.”
In light of these results, Hurricane advocated for additional repeated experiments that could be used to better understand shot-to-shot variability. However, program leaders decided to move on, and Hurricane says the team is now looking at ways to increase the power of the laser by more than 10%, as well as modify targets that could use this energy more efficiently.
Mark Herrmann, Livermore’s deputy director of fundamental weapons physics, says the lab has received a lot of feedback from more than 100 scientists involved in the program. But he emphasizes that the long-term goal is to achieve yields that are two orders of magnitude higher than even last August. “As long as we do good, rigorous, systematic research, this is the most important thing from my point of view,” he adds.
To some extent, the lab’s inability to replicate the August experiment was to be expected because the laser is now running on “ignition cutoff,” says Riccardo Betti, who heads the Laser Fusion Center at the University of Rochester in New York. provides independent evaluations of experiments at the NIF. “If you are on one side of a cliff, you can get a lot of fusion production, and if you are on the other side of a cliff, you can get very little,” he says. According to him, the laboratory does not yet have the experimental accuracy to predict which side a given experiment will land on.
Questions about basic science and predictive capabilities were at the center of a classified review of NIF’s scientific contributions to the US nuclear weapons program, provided by NNSA last year to JASON, an independent scientific group that advises the US government. In an unclassified summary of the report obtained by Nature under the U.S. Freedom of Information Act, the commission acknowledged the capabilities of the NIF, but stated that the facility was unlikely to achieve a “predictable reproducible fire” in the next few years.
The report was completed and submitted to the NNSA four months before the August shot, and Hurricane and others argued that it was out of time and too pessimistic.
The JASON panellists advocated a fundamental rethinking of the program in their report, and this discussion has already begun in the broader laser fusion community. Scientists at NIF and elsewhere are exploring ways to reconfigure the existing laser, while others are pushing for entirely new designs that could provide more practical paths to fusion power.
For its part, Hurricane is in no hurry. He claims that the device is now operating in a critical thermonuclear mode, which will be useful for understanding and predicting the reliability of nuclear weapons.
“Once we get more power and more predictability, you kind of miss out on some interesting physics,” Hurricane says. “If you understand and be the best scientists and stewards [of the nuclear stockpile] your goal is the mode of operation.”
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