With historic explosion, a long sought fusion breakthrough Science

More energy out than in. For 7 decades, fusion scientists have chased this elusive goal, known as energy gain. At 1 am on December 5, researchers at the National Ignition Facility (NIF) in California finally did it, focusing 2.05 megajoules of laser light onto a tiny capsule of fusion fuel and sparking an explosion that produced 3.15 MJ of energy—the equivalent of about three sticks of dynamite.

“This is extremely exciting, it’s a major breakthrough,” says Anne White, a plasma physicist at the Massachusetts Institute of Technology, who was not involved in the work.

Mark Herrmann, who leads NIF as the program director for weapons physics and design at Lawrence Livermore National Laboratory, says it feels “wonderful,” adding: “I’m so proud of the team.”

The result, announced today by officials at the US Department of Energy (DOE), represents a shot in the arm for fusion researchers, who have long been criticized for overpromising and underdelivering. Fusion holds the tantalizing promise of plentiful, carbon-free energy, without many of the radioactive headaches of fission-driven nuclear power. But getting hydrogen ions to fuse into helium and release energy requires temperatures of millions of degrees Celsius—conditions that are hard to achieve and sustain. The NIF result shows it is possible, at least for a fraction of a second. “Three MJ is a hell of a lot of energy. It shows something is working,” says plasma physicist Steven Rose of Imperial College London.

Despite the fanfare, fusion power stations are still a distant dream. NIF was never designed to produce power commercially. Its primary function is to create miniature thermonuclear explosions and provide data to ensure the US arsenal of nuclear weapons is safe and reliable. Many researchers believe furnacelike tokamaks are a better design for commercial power because they can sustain longer fusion “burns.” In a tokamak, microwaves and particle beams heat the fuel and magnetic fields trap it. “The challenge is to make it robust and simple,” White says.

However, the leading tokamak device, the ITER reactor under construction in France, is anything but simple. It is vastly over budget, long overdue, and will not reach breakeven until the late 2030s at the earliest. With NIF’s new success, proponents of such laser-based “inertial fusion energy” will be pushing for funding to see whether they can compete with the tokamaks.

The $3.5 billion NIF began its “ignition” campaign in 2010. Its laser, housed in a building the size of three US football fields, delivers a powerful, nanosecondslong infrared pulse split into 192 beams that are converted to ultraviolet light. The beams are focused on the target—a gold can the size of a pencil eraser containing a peppercorn-sized fuel capsule. Heated to millions of degrees, the gold emits x-rays that vaporize the diamond shell of the capsule. The blasted diamond implodes the fuel, compressing and heating it.

If the compression of the fuel is symmetrical enough, fusion reactions begin in a central hot spot and propagate smoothly outward, with the heat from fusion sparking more burning. That self-sustaining burn is what defines ignition, and after more than a decade of effort NIF scientists declared they had achieved that milestone after a shot in August 2021 produced 70% of the input laser energy. But NIF’s funder, DOE’s National Nuclear Security Administration, set NIF’s goal as an energy gain greater than one—the threshold it passed last week.

Going that extra mile wasn’t easy. After the August 2021 shot, the NIF team found it couldn’t repeat it. Using a smooth diamond capsule turned out to be key: The one from August 2021 had been the most perfectly smooth and spherical they’d made. “We had to learn how to make the capsules better,” Herrmann says. They also made the capsule slightly thicker, which provided more momentum for the implosion but required a longer, more powerful laser pulse. So they tweaked the laser to squeeze out more juice, increasing the energy from 1.9 MJ to 2.05 MJ.

A shot in September produced 1.2 MJ, showing the NIF researchers they were on the right track, but the symmetry was poor: The fuel was squashed into a pancake rather than a tight ball. By adjusting the energy among the laser’s 192 beams, they were able to get a more spherical implosion, and last week they finally hit the jackpot. “The physics phenomenon has been demonstrated,” says Riccardo Betti of the Laboratory for Laser Energetics at the University of Rochester.

If gain meant producing more output energy than input electricity, however, NIF fell far short. Its lasers are inefficient, requiring hundreds of megajoules of electricity to produce the 2 MJ of laser light and 3 MJ of fusion energy. Moreover, a power plant based on NIF would need to raise the repetition rate from one shot per day to about 10 per second. One million capsules a day would need to be made, filled, positioned, blasted, and cleared away—a huge engineering challenge.

The NIF scheme has another inefficiency, Betti says. It relies on “indirect drive,” in which the laser blasts the gold to generate the x-rays that actually spark fusion. Only about 1% of the laser energy gets into the fuel, he says. He favors “direct drive,” an approach pursued by his lab, where laser beams fire directly onto a fuel capsule and deposit 5% of their energy. But DOE has never funded a program to develop inertial fusion for power generation. In 2020, the agency’s Fusion Energy Sciences Advisory Committee recommended it should, in a report co-authored by Betti and White. “We need a new paradigm,” Betti says, but “there is no clear path how to do it.”

Now that NIF has cracked the nut, researchers hope laser fusion will gain credibility and more funding may flow. After the long slog to get here, Betti jokes about passing the baton. “This is a very important first step,” he says. “We’ve done it now, so I can retire.”