Proponents of generating clean energy from nuclear fusion, the reaction that powers the Sun, have had to live for decades with the taunt that a commercial fusion plant always seems to lie 30 years in the future. But the chances that the timescale might become considerably shorter rose with the official announcement on Tuesday that, for the first time, the energy output from an experimental reactor had exceeded the input.

The “net energy gain” at the National Ignition Facility (NIF), part of the Lawrence Livermore National Laboratory in California, was a relatively modest 50 per cent, far below what would be needed for a commercial reactor. But it still marks a scientific milestone that should encourage more investment in fusion research and development from both private and public sectors.

This achievement follows an encouraging flow of technical and financial news over the past year or so. In February the Joint European Torus in the UK recorded a world record energy output, though this was less than the power required to fire up the reaction. Funding for the growing band of fusion companies, while still far below the level for counterparts in other energy sectors, has doubled over the past year to reach a total of $5bn.