Almost a year ago, scientists at the world’s largest laser fusion facility announced a historic achievement: it broke all records and produced, even for a split second, an energetic fusion reaction of the kind that fuels stars and thermonuclear weapons. However, efforts to replicate that experience were unsuccessful. temper nature He learned that earlier this year, researchers at the California facility changed direction, turning to rethink their experimental design.
The turn of events has renewed controversy over the future of the National Ignition Facility (NIF), a $3.5 billion device located at Lawrence Livermore National Laboratory and overseen by the National Nuclear Security Administration (NNSA), a branch of the United States. The Department of Energy, which manages nuclear weapons. The primary task of the NIF is to create high-throughput fusion reactions, and to inform the maintenance of the US weapons stockpile.
By some measures, a laser shot recorded on August 8, 2021 proved that the facility, which cost much more and produced far less than originally promised, had finally accomplished its main mission. With that said, repeated attempts yielded at best 50% of the energy produced late last year. The researchers did not expect smooth sailing while trying to replicate the experiment, because the bulky device is now operating at the cusp of fusion ‘flashes’, where unintended small differences from one experiment to another can have huge effects on output. However, for many, the failure to reproduce last August’s experiment underscores the inability of researchers to accurately understand, engineer, and predict experiments at these energies.
Omar Hurricane, chief scientist with Livermore’s inertial fusion program, called for pushing forward with the current experimental design to explore this energy system, rather than backing down to reassemble. “The fact that we do this is kind of a proof of existence that we can do it,” he says. “Our problem is to do it frequently and reliably.” However, he says, the program leadership has made the decision to stop iteration experiments and focus on next steps that could push NIF beyond the integration threshold and into an entirely new – and more predictable – system, where returns are much greater than in the August experiment. .
Some researchers in the community have long questioned the usefulness of NIF, and for them, the entire episode highlights the facility’s remarkable accomplishments—as well as its fundamental limitations. “I think they should call it a hit and a hold,” says Stephen Bodner, a physicist who previously headed the laser fusion program at the US Naval Research Laboratory in Washington, D.C. Bodner says NIF is a technological dead end, and that it is time to prepare for a new generation of lasers that could open the door to fusion energy.
NIF opened in 2009 with the promise of achieving fusion ignition, defined by the US National Academy of Sciences (NAS) as an experiment that generates more energy than it consumes. After the initial deadline for achieving ignition was passed in 2012, Livermore scientists began a ten-year effort to fine-tune the system (see: ‘The Path to Ignition’). Finally, last August, after a series of modifications to aspects of the facility, including the laser and the ignition target — a gold capsule containing frozen grains of the hydrogen isotopes deuterium and tritium — they had a breakthrough moment.
In less than 4 billionths of a second, 192 laser beams delivered 1.9 megajoules of energy to the target. When the capsule collapsed, isotopes of hydrogen in the pellet core began fusing into helium, releasing a torrent of energy and creating a chain reaction that eventually released more than 1.3 megajoules of energy — about 8 times the previous record and 1,000 times the improvement in First experiences.
Although it does not meet the NAS definition of ignition, the shot resulted in a high-throughput fusion reaction that was safely qualified as ignition according to the criteria used by scientists at NIF. Hurricane calls it “the Wright Brothers moment,” and even the NIF’s harshest critics, including Bodner, have lobbed their hats.
In September, leaders of the inertial fusion program in confinement laid out a plan for three experiments to determine if the August result could be replicated. Experiments began in October and produced only 400-700 kilojoules of energy. Although these results still represent an incremental change in the NIF process, they do not come close to the August breakthrough—nor did they exceed what the NIF scientists describe as the ignition threshold.
Hurricane says the team’s analysis of those experiments indicates that inconsistencies in target fabrication and inevitable shifts in laser performance due to its lifetime produce subtle, but significant, differences in the shape of the implosion. “We understand why repeated shots performed the way they did,” he says, “but we’re still trying to determine exactly what it is about these engineering aspects that we need to better control for.”
In light of these findings, Hurricane called for additional iterative experiments that could be used to better understand shot variance. However, program leaders have chosen to move forward, and Hurricane says the team is now looking at ways to boost laser power by more than 10%, as well as modifying targets that can make more efficient use of that energy.
Mark Hermann, deputy director of basic weapons physics at Livermore, says the lab got plenty of feedback from more than 100 scientists involved in the program. But he stresses that the long-term goal is to achieve returns twice as high as those managed even last August. He adds: “As long as we are doing a good, systematic scientific study, this is the most important in my view.”
To some extent, the lab’s failure to replicate Augustus’ experiment was to be expected, because the laser is now operating in a “ignition cliff,” says Ricardo Petti, who heads the Laser Fusion Center at the University of Rochester in New York. Provides independent assessments of trials in the NIF. “If you’re on one side of the cliff, you get too much fusion product, and if you’re on the other side of the cliff, you get too little,” he says. He says the lab does not yet have the empirical accuracy to predict which side a particular experiment will land on.
Questions about basic science and predictive power were at the center of a classified review of the NIF’s scientific contributions to the US nuclear weapons program that JASON, an independent scientific panel that advises the US government, submitted to the NNSA last year. In an unclassified executive summary of the report, obtained by temper nature Under the US Freedom of Information Act, the committee acknowledged the capabilities of the NIF, but stated that the facility was unlikely to achieve a “predictable and repeatable ignition” in the next several years.
The report was completed and released to the NNSA four months before the August shooting, and Hurricane and others argued it was untimely and overly pessimistic.
JASON team members called for a fundamental rethink of the software in their report, and this discussion has already begun in the broader laser fusion community. Scientists at NIF and elsewhere are studying ways to reconfigure existing lasers, while others are pushing toward entirely new designs that could provide more practical ways toward fusion energy.
For his part, the hurricane is not in a hurry. He asserts that the device is now operating in an important fusion system that will be useful for understanding and predicting the reliability of nuclear weapons.
“Once we get more energy and more predictability, you’re kind of ahead of the curve in interesting physics,” Hurricane says. If the understanding and the best scholars and rulers [of the nuclear stockpile] It’s your goal, that’s the system you’re working in.”
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