How the world’s biggest laser smashed a nuclear-fusion record

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.

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