US physicists have confirmed that they achieved a stage in nuclear fusion called "burning plasma" last year.
There's a longstanding effort to crack fusion power because it promises an unlimited source of clean energy.
Burning plasma occurs when fusion reactions become the dominant source of heating in the process, rather than energy introduced from outside.
The stage was seen in experiments carried out at the National Ignition Facility (NIF) in California. The achievement is described in two papers published in the academic journal Nature.
Existing nuclear energy relies on a process called fission, where a heavy chemical element is split to produce lighter ones. Fusion works by combining two light elements to make a heavier one.
Researchers have been working on the nuclear fusion problem since the 1950s. It's the process that powers the Sun, and the effort has sometimes been likened to building a star on Earth.
However, turning nuclear fusion into a commercially viable energy source has proven elusive.
Getting the reactions going is not the problem; the trick is getting more energy out of the fusion process than you put in.
To this end, NIF uses a powerful laser to heat and compress hydrogen fuel inside a capsule.
The 192 beams from this laser - the highest-energy example in the world - are directed towards a peppercorn-sized capsule containing deuterium and tritium - different forms of the element hydrogen.