Ignition confirmed in a nuclear fusion experiment for the first time

We have ignition. An analysis has confirmed that an experiment conducted in 2021 created a fusion reaction energetic enough to be self-sustaining, which brings it one step closer to being useful as a source of energy.

The fusion ignition took place on 8 August 2021 at the Lawrence Livermore National Laboratory’s National Ignition Facility (NIF) in California, but NIF researchers haven’t been able to reproduce this landmark achievement since. They have spent the past year analysing the experimental conditions that got them to ignition to determine how to achieve it again.

In the experiment, 192 of the world’s most energetic lasers hit a millimetre-sized capsule filled with hydrogen. This turned the capsule into a hot plasma that quickly collapsed into a tiny sphere about 18,000 times hotter than the surface of the sun, and which experienced more than 100 million times the pressure of the Earth’s atmosphere.

Under these extreme conditions, hydrogen atoms underwent fusion and released 1.3 megajoules of energy – the equivalent of 10 quadrillion watts of power for 100 trillionths of a second. This was NIF’s highest ever energy yield.

The new analysis shows that the experiment also brought the facility closer to the kind of fusion reaction that could eventually be used as an energy source by satisfying the so-called Lawson criterion for ignition, which states that the fusion heating must be high enough to overcome all physical processes that might cool the plasma.

“We determined that we reached Lawson’s criterion, which proves that this is not only possible, but possible at NIF,” says Annie Kritcher at NIF. “This is the first time we have crossed Lawson’s criterion in the lab.”

Sam Wurzel at the US Department of Energy says that crossing this criterion is crucial proof of ignition and a result that is likely to accelerate research and development in fusion science for both national security and energy applications.

Lawson’s criterion, formulated by physicist John Lawson in 1955, takes into account variables such as a plasma’s density and how long it must be confined to create a sustained reaction. “You get what’s called a propagating burn: fusion causes more fusion which causes more fusion which causes more fusion,” says Steven Cowley at Princeton University.

The analysis tested experimental data against nine different versions of Lawson’s criterion, each dictating how different sets of measurements should relate to each other during ignition. Ignition was reached according to all of them, overcoming the natural tendency of a reaction to cool and stop. Had the reaction not self-heated in this way, the energy yield would have been much smaller, Cowley says.

Since August 2021, NIF researchers have conducted four similar experiments that produced energy yields up to two-thirds of the record value, but did not reach ignition, Kritcher says. She says that the experiment is highly sensitive to small changes like barely perceptible differences in the material structure of each hydrogen capsule or small variations in the intensity of the lasers.

“If you start from a microscopically worse starting point, it’s reflected in a much larger difference in the final energy yield,” says Jeremy Chittenden at Imperial College London, who worked on the analysis. “The 8 August experiment was the best-case scenario.”

Kritcher says that she and colleagues have done “lots of diagnostic and scientific digging” in the past year to uncover what exactly resulted in ignition. They determined that even the size of the tiny tube that fills the capsule with hydrogen, itself smaller than a chia seed, makes a big difference, as does the way hydrogen atoms arrange themselves inside of it. Kritcher says that the NIF team wants to use this knowledge to not only replicate those conditions, but also make the experiment more resilient to small errors so it can reliably achieve ignition again and again.

“You don’t want to be in a position where you’ve got to get absolutely everything just right in order to get ignition,” says Chittenden.

Reliably and repeatedly achieving ignition at NIF would be significant, but one more hurdle remains before fusion-based power plants become a realistic possibility. Namely, the amount of energy produced after ignition must be as large or larger than the amount of energy the lasers put in. The 8 August experiment made it about 72 per cent of the way there.

That would make harnessing fusion for clean power plants more feasible in the future, but that future may still be many years away – for commercial energy purposes, the fusion reaction would have to output over a hundred times more energy than it requires to get ignited, says Cowley.


H. Abu-Shawareb et al. Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment, Physical Review Letters, DOI: 10.1103/PhysRevLett.129.075001

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