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Laser fusion facility returns to drafting board


Within the target chamber of the US National Ignition Facility, scientists focus 192 laser beams on gold capsules containing deuterium and tritium to achieve fusion.Credits: Lawrence Livermore National Laboratory / Scientific Photo Library

Almost a year ago, scientists at the world’s largest laser fusion facility announced groundbreaking results. It broke all records and, in the blink of an eye, created the kind of energetic fusion reaction that powers stars and thermonuclear weapons. However, efforts to reproduce the experiment were inadequate. Nature Earlier this year, researchers at the California facility learned that they had turned around and moved to rethink their design of experiments.

The order of the events is a discussion of the future of the National Ignition Facility (NIF), a US $ 3.5 billion device housed at the Lawrence Livermore National Laboratory and supervised by the US branch of the National Nuclear Security Administration (NNSA). Has been renewed. The Department of Energy, which manages nuclear weapons. NIF’s main mission is to produce high-yielding fusion reactions and to inform the maintenance of US weapons stockpiles.

The record laser shot, taken by some means on August 8, 2021, was finally its main mission, with the facility having a much higher cost and much lower yield than originally promised. Proved that it was achieved. However, repeated attempts yielded at most 50% of the energy produced at the end of last year. While trying to reproduce the experiment, the researchers did not expect a smooth voyage. This is because large devices are currently operating at the tip of the “ignition” of fusion, and small inadvertent differences between experiments can have a significant impact on power output. Nevertheless, for many, the failure to reproduce last August’s experiments emphasizes that researchers cannot accurately understand, design, or predict experiments with these energies.

Road to Ignition: A bar graph showing the fusion reactions achieved by the National Ignition Facility since 2012.

Source: Lawrence Livermore National Laboratory

Omar Hurricane, Chief Scientist of Livermore’s Inertial Confinement Fusion Program, advocated advancing existing design of experiments to scrutinize this energy regime rather than returning to reorganization. “The fact that we did it is proof of a kind of existence that we can do it,” he says. “Our problem is to make sure it’s done over and over again.” But he said the program’s leadership stopped replication experiments and far exceeded the NIF fusion threshold, yielding August. Experiment.

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

Chasing ignition

NIF opened in 2009 with the promise of achieving fusion ignition. This is defined by the National Academy of Sciences (NAS) as an experiment that produces more energy than it consumes. After missing the first deadline to achieve ignition in 2012, Livermore scientists began a decade-long effort to fine-tune the system (see “The Road to Ignition”). Finally, in August of last year, they celebrated a milestone after making a series of adjustments to the sides of the facility, including lasers and ignition targets (gold capsules containing frozen pellets of hydrogen isotope hydrogen and tritium). I did.

In less than a quarter of a second, 192 laser beams delivered 1.9 megajoules of energy to the target. When the capsule collapsed, the hydrogen isotopes in the core of the pellet began to fuse with helium, releasing a torrent of energy, creating a cascade of reactions that eventually released more than 1.3 megajoules. This is about eight times the previous record and 1,000-a multiple improvement in early experiments.

Although it did not meet the NAS ignition definition, this shot resulted in a high yield fusion reaction that was safely certified as ignition according to the standards used by NIF scientists. The hurricane called it the “Wright Brothers Moment” and even the toughest critics of NIF, including Bodner, turned their hats over.

In September, the leader of the Inertial Confinement Fusion Program created a plan for three experiments to determine if the August results could be repeated. The experiment began in October and produced only 400-700 kilojoules of energy. These results still represent a gradual change in NIF operations, but never approach the breakthrough in August and do not exceed what NIF scientists describe as ignition thresholds. did.

According to the hurricane, analysis by a team of these experiments shows that inconsistencies in the manufacture of targets and the inevitable changes in laser performance over time made a slight but significant difference in the shape of the implosion. I am. “I understand why repeat shots worked that way, but I’m trying to pinpoint exactly what these engineering aspects need to be better controlled,” he says.

In light of these results, Hurricane proposed additional iterative experiments that could be used to better understand variations between shots. However, the program leader chose to move on. According to the hurricane, the team is currently looking at ways to boost laser energy by more than 10% and change targets to use that energy more efficiently.

Mark Harman, Deputy Director of Basic Weapons Physics in Livermore, says the lab 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 double orders of magnitude higher than those managed even in August last year. “As long as we do good, careful, systematic scientific research, that’s the most important thing from my point of view,” he adds.

Important report

Riccardo Betti, who heads the laser fusion center at the University of Rochester in New York, said it was expected that the laser could not reproduce the August experiment to some extent because it is currently operating on an “ignition cliff.” Provides an independent assessment of experiments at NIF. “If you’re on one side of the cliff, you get a lot of fusion power, and if you’re on the other side of the cliff, you get very little,” he says. He says the lab doesn’t yet have the experimental accuracy to predict which side a particular experiment will land on.

Questions about basic science and predictive power were central to a categorized review of NIF’s scientific contributions to the US nuclear weapons program provided to the NNSA last year by JASON, an independent scientific committee that advises the US government. In the uncategorized executive summary of the report, Nature Under US Disclosure Law, the Panel acknowledged NIF’s capabilities, but said the facility was unlikely to achieve “predictable and reproducible ignition” in the coming years.

The report was completed and released to the NNSA four months before the August shot. Hurricanes and others claim that the timing is bad and that they are too pessimistic.

JASON panelists advocated a radical rethinking of the program in their report, a discussion that has already begun in the broader laser fusion community. Scientists at NIF and elsewhere are looking at ways to reconstruct current lasers, while other scientists are pushing for a whole new design that can provide a more practical means for fusion energy. doing.

On his side, the hurricane is not in a hurry. He claims that the device is currently operating in an important fusion regime that helps to understand and predict the reliability of nuclear weapons.

“When we get more energy and more predictability, it’s like you skipped interesting physics,” says Hurricane. “Understand better scientists and stewards, and if so [of the nuclear stockpile] Your purpose is the system in which this works. “