The U.S. Department of Energy on Tuesday will announce that scientists at a national lab have made a breakthrough on fusion, the process that powers the sun and stars that one day could provide a cheap source of electricity, three sources with knowledge of the matter said.

The scientists at Lawrence Livermore National Laboratory in California have achieved a net energy gain for the first time, in a fusion experiment using lasers, one of the people said.

While the results are a milestone in a scientific quest that has been developing since at least the 1930s, the ratio of energy going into the reaction at Livermore to getting energy out of it needs to be about 100 times bigger to create a process producing commercial amounts of electricity, one of the sources said.

The FT first reported the experiment.

Fusion works when nuclei of two atoms are subjected to extreme heat of 100 million degrees Celsius (180 million Fahrenheit) or higher leading them to fuse into a new larger atom, giving off enormous amounts of energy.

But the process consumes vast amounts of energy and the trick has been to make the process self-sustaining and get more energy out than goes in and to do so continuously instead of for brief moments.

If fusion is commercialized, which backers say could happen in a decade or more, it would have additional benefits including the generation of virtually carbon-free electricity which could help in the fight against climate change without the amounts of radioactive nuclear waste produced by today's fission reactors.

Running an electric power plant off fusion presents tough hurdles however, such as how to contain the heat economically and to keep lasers firing consistently. Other methods of fusion use magnets instead of lasers.

Energy Secretary Jennifer Granholm is slated to hold a media briefing on Tuesday at 10:00 a.m. EST (1500 GMT) on a "major scientific breakthrough."

The department has no information ahead of the briefing, a spokesperson said.

Lawrence Livermore focuses mainly on national security issues related to nuclear weapons and the fusion experiment could lead to testing safer testing of the nation's arsenal of such bombs.

But advances at the labs could also help efforts at companies that hope to develop power plants fired by fusion including Commonwealth Fusion Systems, Focused Energy and General Fusion.

Investors including Bill Gates, Jeff Bezos and John Doerr have poured money into companies building fusion. Private industry secured more than $2.8 billion last year, according to the Fusion Industry Association for a total of about $5 billion in recent years.