A 2021 experiment ignited nuclear fusion, which has now been confirmed by data analysis, but attempts to replicate this over the past year have failed.
It worked. An analysispublished in the physics journal Physical Review Letters, has confirmed that an experiment in 2021 caused a nuclear fusion reaction that produces enough energy to sustain itself, bringing the usefulness of nuclear fusion as an energy source one step closer.
The fusion ignition took place on August 8, 2021 in the National Ignition Facility (NIF) from Lawrence Livermore National Laboratory in California, but NIF researchers have since failed to reproduce this landmark. Over the past year, they’ve analyzed the experimental conditions that then led to inflammation to determine how they can repeat the trick.
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Powerful laser beams
In the experiment, 192 of the world’s most powerful laser beams hit a millimeter-sized capsule filled with hydrogen. This turned the capsule into a hot plasma that collapsed into a sphere about 18,000 times hotter than the sun’s surface. The pressure was more than 100 million times atmospheric pressure.
Under these extreme conditions, hydrogen nuclei fused into helium nuclei, releasing 1.3 megajoules of energy. That is the equivalent of powering 10 quadrillion watts for 100 trillionths of a second. This was the highest energy yield NIF has ever achieved.
The new analysis shows that the experiment also brought the facility closer to the kind of fusion reaction that could ultimately serve as an energy source, by meeting the so-called Lawson criterion for ignition. This states that the heating from the nuclear fusion must be powerful enough to overcome any physical processes that could cool the plasma.
Lawson Criterion
“We found that we met Lawson’s criterion, proving that not only is this possible, but that it is possible at the NIF,” said physicist and experiment designer Annie Kritcher. “This is the first time we’ve exceeded Lawson’s criterion in the lab.”
physicist Sam Wurzela consultant to the US Department of Energy, says exceeding this criterion provides critical evidence for nuclear fusion, and an outcome likely to accelerate research and development in fusion science for both national security and energy applications.
The Lawson criterion, formulated in 1955 by physicist John Lawson, takes into account variables such as the density of a plasma, and how long the plasma must be confined to produce a sustained reaction. “You get what is called a propagating combustion: fusion causes more fusion, which in turn causes more fusion, which in turn causes more fusion,” explains theoretical physicist Steven Cowley from Princeton University.
Energy yield
In the analysis, the experimental data was tested against nine different versions of Lawson’s criterion, each of which dictates how different sets of measurements should relate to each other during ignition. Inflammation was achieved according to all these versions, overcoming the natural tendency of a reaction to cool down and stop. If the reaction hadn’t heated itself up in this way, the energy yield would have been much smaller, Cowley said.
Since August 2021, NIF researchers have conducted four similar experiments, yielding energy yields up to two-thirds of the record value, but reaching no ignition, Kritcher says. According to her, the experiment is very sensitive to small changes, such as barely noticeable differences in the material structure of each hydrogen capsule, or small variations in the intensity of the lasers.
“If you start with a microscopically worse starting point, that will be reflected in a much larger difference in the final energy yield,” says plasma physicist Jeremy Chittenden from Imperial College London, who worked on the analysis. ‘The experiment of August 8 was the best scenario.’
Chia seed
Kritcher says she and colleagues have done “a lot of diagnostic and scientific digging” over the past year to uncover exactly what led to the inflammation. They found that even the size of the tiny tube that fills the capsule with hydrogen (which is itself smaller than a chia seed) makes a big difference. And so is the way hydrogen atoms arrange themselves inside the capsule. Kritcher says the NIF team wants to use this knowledge not only to replicate those conditions, but also to make the experiment more resistant to small deviations so that it can reliably ignite time and time again.
‘You don’t want to be in a position where you have to get everything just right to ignite,’ says Chittenden. Reliable and repeated ignition at NIF is important, but there is still a hurdle to overcome before fusion-based power plants become a realistic possibility. The amount of energy produced after ignition must be greater than the amount of energy that the lasers put into it. With the experiment of 8 August, this was achieved for about 72 percent.
That would make the use of fusion for clean power plants more feasible in the future, but that future could be many years away. In fact, for commercial energy production, the fusion reaction would have to produce more than a hundred times more energy than goes into achieving ignition, Cowley says.