Scientists at the California-based Lawrence Livermore National Laboratory repeated the breakthrough in an experiment in the National Ignition Facility (NIF) on 30 July that produced a higher energy yield than in December, a Lawrence Livermore spokesperson said.

The approach, which gives rise to the heat and light of the sun and other stars, has been hailed as having huge potential as a sustainable, low-carbon energy source.

In December, Lawrence Livermore first achieved a net energy gain in a fusion experiment using lasers.

The US Department of Energy called it “a major scientific breakthrough decades in the making that will pave the way for advancements in national defense and the future of clean power”.

Fusion energy raises the prospect of plentiful clean power: the reactions release no greenhouse gases or radioactive waste byproducts.