How the world’s biggest laser smashed a nuclear-fusion record
The US National Ignition Facility is the only laboratory where a nuclear-fusion reaction has generated more energy than it consumed. Here’s how it achieved this historic milestone and sparked fresh interest in fusion energy.
The Sun shone brightly on Livermore, California, on 8 June 2011, when researchers charged up the world’s largest laser for its first major fusion experiment. It might have seemed like a good omen for the stadium-sized facility, which is a flagship project of the US nuclear weapons programme.
That day, the laser at the US National Ignition Facility (NIF) blasted a pea-sized target with a huge jolt of energy. It was an important first step, but the test ended with a brief flash and a fizzle. This result would become frustratingly familiar.
The US$3.5-billion facility was designed with a singular goal: to compress hydrogen isotopes into a white-hot core, where their nuclei would meld to create helium and enough surplus energy to drive a cascade of fusion reactions. Nobody had expected success straight away, but by June 2011 the researchers were already eight months into a two-year effort that was expected to achieve ‘ignition’: when an experiment generates more energy than the laser supplies. Those 2 years would drag into 12.
In December 2022, the laboratory finally reached its ignition goal as laid out by the US National Academy of Sciences 25 years earlier, and the researchers have since upped their game. NIF shattered records this February by producing double the amount of fusion energy that the laser provided (see ‘Steady progress’), and the facility confirmed its sixth successful ignition experiment this month.
The success at NIF has opened up fresh avenues of research into nuclear weapons and buoyed the budding field of fusion energy: both governments and businesses are now pouring money into the idea that humans might one day generate a limitless source of clean energy to help solve the climate crisis. NIF is not designed to provide that energy. The facility instead serves as a proving ground for fusion research, as one of the only places where scientists can explore the fundamental physics that would underpin any fusion-energy future.
“There is really no better place to be,” says Annie Kritcher, a physicist at NIF who led the experimental campaign that finally achieved ignition.
Annie Kritcher, a physicist at NIF, led the experimental campaign that finally achieved ignition.
Annie Kritcher, a physicist at NIF, led the experimental campaign that finally achieved ignition.
NIF researchers had to solve a mountain of technical issues to achieve their goal. Over the years, they went back to the drawing board time and again, even as pressure mounted from critics over the facility’s mammoth price tag and limited results. Here is the inside story of how the laser team achieved success and is now pushing towards even greater fusion milestones.
The fizzle
The decision to build NIF was inspired by the end of the Cold War. Just months before its dissolution in 1991, the Soviet Union announced it would stop testing nuclear weapons. The United States followed suit the next year, and then the question arose: how would the country test the reliability and safety of nuclear bombs and missiles without occasionally blowing one up? The answer was science, and a big part of that was NIF.
On 15 January 1993, the US Department of Energy (DoE) approved construction of the laser facility, which would enable researchers to replicate and study the reactions of thermonuclear weapons in a laboratory setting for the first time. NIF was billed as a central player in the country’s ‘science-based stockpile stewardship’ strategy, which was designed to provide government scientists with the information they needed to predict the performance of tens of thousands of weapons in an ageing nuclear arsenal. Other research in the stewardship programme would focus on issues such as the longevity of weapons materials and components.
NIF encountered controversy from the outset. Wary lawmakers and government watchdogs raised flags about construction delays, cost overruns and alleged mismanagement by the DoE and the National Nuclear Security Administration (NNSA), an independent agency in the DoE that manages nuclear weapons. By the time it was completed at the Lawrence Livermore National Laboratory (LLNL) in 2009, the massive facility — ten storeys high, with the footprint of three American football fields — was seven years behind its original schedule and $2.3 billion over budget. But the science mission was just beginning.
“The pressure was on,” says Richard Town, associate director of the lab’s inertial-confinement fusion programme.
The attempt that fizzled on 8 June 2011 was the first real shot at ignition, after a long series of tests and experiments that were designed to tune equipment. Over the next year the facility’s team ramped up the laser and tweaked the experiment designs. That work came to a head on 20 September 2012, when the researchers conducted their final experiment of the initial drive to achieve ignition.
Taking a shot
At the beginning of an experiment, NIF creates a weak infrared laser pulse, with an energy around one billionth of a joule, that is split into 192 beams. These beams are amplified more than one quadrillion (1015) times and converted into ultraviolet energy.
They then converge in a 130,000-kilogram spherical ‘target’ chamber made of aluminium and concrete.
The chamber is outfitted in a steampunk style with dozens of ports for diagnostic instruments and other equipment. On that day in 2012, once the beams entered the chamber, they delivered 447 trillion watts of peak power — around 900 times more power than the United States uses at any given moment — to a target.
The 192 ultraviolet beams entered the target through plastic windows at the top and bottom of a pencil-eraser-sized gold cylinder, known as a hohlraum.
They smashed into the walls of the hohlraum, generating an X-ray ‘oven’ that heated and compressed a plastic capsule smaller than a peppercorn containing a frozen pellet of the hydrogen isotopes deuterium and tritium.
The goal was for the isotope nuclei to fuse, creating a torrent of fusion energy.
But the experiment again ended with a fizzle that generated less than 1.6 kilojoules of fusion energy — roughly 1,000 times below the ignition threshold. The first phase of work at NIF came to a close less than ten days later, and it was clear that the team had a lot more work to do.
“Just turning the knobs all the way up and pushing the button wasn’t enough,” says Robert Goldston, a physicist who studies fusion energy at Princeton University in New Jersey. “It required taking a step back and really understanding, scientifically, what was getting in the way of getting ignition.”
One of the fundamental problems, the researchers realized, was one of symmetry.
The X-ray oven was supposed to evenly compress and heat the pellet like a spherical vice, forcing the tritium and deuterium nuclei to fuse. NIF scientists often compare the task to trying to squeeze a spherical object the size of a basketball to the size of a pea without distorting its shape.
Diagnostics and modelling would reveal that instead of a spherical implosion, the scientists were creating spiky implosions, with uneven compression that allowed cooler material from the capsule shell into the centre (see ‘Spherical vice’).
“It was sort of a mess,” says Mordecai Rosen, a physicist at the LLNL who was involved in the effort from the beginning.
The NNSA’s final report on the campaign, issued on 27 December 2012, concluded that reaching ignition remained “a considerable technical challenge with an uncertain outcome”.
Punching up the power
NIF started fresh in 2013 after bringing in physicist Omar Hurricane and other scientists who had been working on the core nuclear-weapons design programme at the LLNL. Hurricane’s team looked at the evidence from previous experiments and suggested that one way to create a more stable and less spiky implosion would be to adjust NIF’s laser pulses, to make them shorter and stronger when they initially hit the target.
Imagine a photon ‘train’ plowing into the target at the speed of light: in the original configuration, the train was around six metres long and most of its energy was packed into the carriage at the end. Although the new train carried a similar amount of energy, it was shortened by roughly one-quarter and given a more powerful locomotive on the front end. The result was a faster and more powerful punch.
Physicists Omar Hurricane (left) and Richard Town (right) sit in the control room at NIF.
Physicists Omar Hurricane (left) and Richard Town (right) sit in the control room at NIF.
Scientists had theorized that part of the problem was the way in which the plastic coating on the isotope-filled capsule was burning up. Hurricane’s idea was to deliver a more powerful burst of energy up front to blow off the plastic coating more quickly and uniformly. It seemed to work.
“We got about a factor of ten increase in fusion performance out of these experiments,” says Hurricane, who was named chief scientist of the inertial-confinement fusion programme in 2014. The team also saw1 the first indication of sustained heat from the fusion reactions after making this change, he says. “But that was just one tactic that solved one problem.”
As progress plateaued, however, doubts increased about the campaign, which was burning through a budget of roughly $500 million annually. In May 2016, the NNSA issued a report acknowledging that it was unclear whether NIF would ever achieve ignition.
Around the same time, Hurricane and his colleagues decided a new approach was needed. The physicist organized teams to come up with various solutions, including a rethink of the capsule coating and a fresh design for the hohlraum, which was increasingly being made from gold-lined uranium. The work of one physicist who was new to the programme — Kritcher — stood out, and she was appointed as a lead designer.
NIF’s laser beams converge on a target called a hohlraum, which is held in place by a pair of silicon arms. Suspended inside the hohlraum is a diamond-coated capsule holding a frozen pellet of hydrogen isotopes.
NIF’s laser beams converge on a target called a hohlraum, which is held in place by a pair of silicon arms. Suspended inside the hohlraum is a diamond-coated capsule holding a frozen pellet of hydrogen isotopes.
One of the most promising avenues, dubbed the high-yield big-radius implosion design (HYBRID), increased the size of both the capsule and the hohlraum. Kritcher’s team also swapped out the plastic coating on the capsule for a diamond one. The dense diamond shell propagated energy to the frozen pellet of isotopes much faster.
In parallel, Kritcher’s team experimented with removing most of the helium gas inside the hohlraum. The helium, it was originally posited, would shield the fusion reaction from plasma generated when the laser beams smashed into the gold walls of the hohlraum. But it turned out that the gas was also scattering the laser beams before they reached the capsule.
Although still short of ignition, the energy yield from experiments more than doubled from 2016 to 2019. “This is when things started to work,” Kritcher says.
While Kritcher and her team continued to work on the target design, others focused on improving capsule fabrication in an effort to eliminate troublesome voids and imperfections in the diamond shell. The researchers also began deploying another technique, one that would soon push them over the ignition finish line: tweaking the energy distribution of the 192 laser beams to control the shape of the implosions, which were no longer spiky but often looked a bit irregular, like thick pancakes and sausages.
This technique came into play after an experiment in November 2020.
Starting to believe
Although the fusion yield of that experiment set yet another record, the implosion looked more like an upright sausage than a sphere.
This indicated an excess of energy in the laser beams — too much squeeze from the vice — hitting the equator inside the hohlraum.
Kritcher’s team ran more calculations and adjusted the laser beams for the next experiment, redistributing energy back towards the top and bottom of the gold chamber.
This squeezed the sausage back into shape.
Source: sausage and sphere implosions adapted from Ralph, J. E. et al. Nature Comms. 15, 2975 (2024).
It worked: in February 2021, the scientists bested the previous record by 70% and achieved nearly one-tenth of the output needed for ignition. More importantly, both experiments breached a threshold known as burning plasma2, in which the fusion reaction generates more heat than the laser. “It really wasn’t until we hit this threshold that people started to believe,” Hurricane says.
Scepticism continued on the outside — an independent scientific panel known as JASON that advises the US government raised questions about NIF’s chances of success in April 2021 — but inside NIF confidence was mounting. Yet another experiment in August achieved a runaway fusion reaction that met the technical criteria for ignition used by scientists inside NIF3. After a few attempts to repeat that experiment failed, they decided it was time to go big.
In September 2022, NIF scientists ran an experiment with a new, more powerful laser configuration. This time, diagnostics revealed a pancake-shaped implosion. Once again, Kritcher’s team redistributed the energy of the beams, this time pumping more energy towards the equator of the hohlraum.
A little more than two months later, on 5 December 2022, researchers fired 2.05 megajoules of ultraviolet energy from NIF’s laser into their target, and the resulting implosion yielded 3.2 megajoules of fusion energy, a gain of more than 50%. After more than 12 years of effort, they had ignition4.
Fail, refine, improve
NIF is now regularly producing fusion yields that are 1,000 or more times as large as those of its first ignition campaign more than a decade ago. This has mollified many critics.
Ignition doesn’t mean a future of clean energy coming from NIF: the most successful experiment so far generated a little over 5 megajoules of energy, but more than 300 megajoules are required to fire its colossal laser.
The facility is instead becoming the experimental tool that was promised to physicists in the nuclear-weapons programme after the end of the Cold War, says David Hammer, a nuclear engineer at Cornell University in Ithaca, New York. In particular, the LLNL scientists are already exposing nuclear-weapons components to the blast of radiation that is generated during fusion experiments to better understand the vulnerability of these components in a nuclear war.
This fabrication facility at NIF is a ‘clean room’ where researchers prepare the target, including the hohlraum and the frozen pellet of hydrogen isotopes.
This fabrication facility at NIF is a ‘clean room’ where researchers prepare the target, including the hohlraum and the frozen pellet of hydrogen isotopes.
Yet NIF is still a work in progress, Hammer says. After the record-shattering experiment in February, three subsequent attempts at ignition came up short. For all their success, NIF’s scientists have yet to fully demonstrate predictability and reproducibility in their experiments, he says. “It’s still a science programme, not an engineering programme.”
Town acknowledges as much. There is a pattern: fail, refine, improve. “It’s part of the process,” he says. NIF’s latest successful attempt, on 18 November, involved a repeat of the conditions used in the record-setting experiment in February and was designed in part to set a baseline for further experiments.
In the long term, the team is hoping to boost NIF’s laser energy by another 18%, a move that could push fusion yields into the 30-megajoule range in a decade’s time. This would require reinforcing the massive target chamber with more concrete for safety.
In the meantime, Hammer says he already sees the effect that NIF’s success is having on the next generation of scientists, who are suddenly eager to pursue fusion energy as a climate solution. That is an appealing avenue for Kritcher, too.
“This facility will never be used for energy generation, but it can help answer questions that are relevant to a variety of fusion-energy approaches,” she says. “The more we learn from NIF, the better.”
- Author: Jeff Tollefson
- Taking a shot infographic: Tomáš Müller
- Photography: Rocco Ceselin for Nature
- Photo editor: Amelia Hennighausen
- Art editor: Jasiek Krzysztofiak
- Subeditor: Joanna Beckett
- Editor: Lauren Wolf
References
- Hurricane, O. A. et al. Nature 506, 343–348 (2014).
- Zylstra, A. B. et al. Nature 601, 542–548 (2022).
- Abu-Shawareb, H. et al. Phys. Rev. Lett. 129, 075001 (2022).
- Abu-Shawareb, H. et al. Phys. Rev. Lett. 132, 065102 (2024).
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