Nearly one year ago, scientists at the world’s largest laser-fusion facility announced a landmark achievement: it had shattered all records and produced, if only for a fraction of a second, an energetic fusion reaction of the kind that powers stars and thermonuclear weapons. Yet efforts to replicate that experiment have fallen short. Nature has learnt that, earlier this year, researchers at the California facility changed direction, moving to rethink their experimental design.

US achieves laser-fusion record: what it means for nuclear-weapons research

The turn of events has renewed debate about the future of the National Ignition Facility (NIF), a US$3.5-billion device that is housed at Lawrence Livermore National Laboratory and overseen by the National Nuclear Security Administration (NNSA), a branch of the US Department of Energy that manages nuclear weapons. The NIF’s primary mission is to create high-yield fusion reactions, and to inform maintenance of the US weapons stockpile.

By some measures, the record-setting laser shot on 8 August 2021 proved that the facility, which has cost much more and yielded much less than originally promised, has at last accomplished its main mission. Repeat attempts, however, yielded at best 50% of the energy produced late last year. Researchers didn’t expect smooth sailing while trying to replicate the experiment, because the massive device is now operating at the cusp of fusion ‘ignition’, where tiny, inadvertent differences from one experiment to another can have huge impacts on output. Nonetheless, for many, the failure to reproduce last August’s experiment underscores researchers’ inability to understand, engineer and predict experiments at these energies with precision.