Last year, scientists confirmed that, for the first time in the lab, they achieved a fusion reaction that is self-perpetuating (rather than fading), bringing us closer to replicating the chemical reaction that powers the Sun.
However, they are not exactly sure how to recreate the experiment.
Nuclear fusion occurs when two atoms combine to create a heavier atom, releasing a large burst of energy in the process.
It’s a process that’s often found in nature, but it’s very difficult to replicate in the lab because it needs a high-energy environment to sustain the reaction.
The Sun generates energy through nuclear fusion, smashing hydrogen atoms to create helium.
Explosive suns, supernovae, also take advantage of nuclear fusion for their cosmic fireworks shows. The power of these reactions is what creates heavier molecules like iron.
In artificial environments here on Earth, however, heat and energy tend to escape through cooling mechanisms such as X-ray radiation and heat conduction.
To make nuclear fusion a viable energy source for humans, scientists must first achieve something called “ignition,” where self-heating mechanisms overcome all energy loss.
Once ignition is achieved, the fusion reaction is ignited.
In 1955, physicist John Lawson created the set of criteria, now known as the “Lawson-like ignition criteria,” to help recognize when such ignition occurred.
The ignition of nuclear reactions usually occurs within extremely intense environments, such as supernovae or nuclear weapons.
Researchers at the National Ignition Facility at Lawrence Livermore National Laboratory in California have spent more than a decade perfecting their technique and have now confirmed that the historic experiment conducted on August 8, 2021, did indeed produce the first successful ignition of a nuclear fusion reaction.
In a recent analysis, the 2021 experiment was judged against nine different versions of Lawson’s criterion.
“This is the first time we’ve crossed the Lawson criterion in the lab,” nuclear physicist Annie Kritcher of the National Ignition Facility told New Scientist.
To achieve this effect, the team placed a tritium-deuterium fuel capsule in the center of a gold-lined depleted uranium chamber and fired 192 high-energy lasers into it to create a bath of intense x-rays.
The intense atmosphere generated by the inward directed shock waves created a self-sustaining fusion reaction.
Under these conditions, the hydrogen atoms fused, releasing 1.3 megajoules of energy for 100 trillionth of a second, which is 10 quadrillion watts of power.
Over the past year, the researchers tried to replicate the result in four similar experiments, but only managed to produce half the energy yield produced in the initial record-breaking experiment.
The ignition is very sensitive to small changes that are barely perceptible, such as differences in the structure of each capsule and the intensity of the lasers, explains Kritcher.
“If you start from a microscopically worse starting point, it’s reflected in a much larger difference in the final energy yield,” says plasma physicist Jeremy Chittenden of Imperial College London. “The August 8 experiment was the best case scenario.”
The team now wants to determine what exactly is required to achieve ignition and how to make the experiment more resilient to small errors. Without this knowledge, the process cannot be scaled up to create fusion reactors that can power cities, which is the ultimate goal of this type of research.
“You don’t want to be in a position where you have to get everything right in order to turn it on,” says Chittenden.
This article was published in Physical Review Letters.