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.

Technicians use a service system lift to access the target chamber interior for inspection and maintenance at the National Ignition Facility (NIF), a laser-based inertial confinement fusion research device, at Lawrence Livermore National Laboratory federal research facility in Livermore, California, United States in 2008. Philip Saltonstall/Lawrence Livermore National Laboratory/Handout via REUTERS

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.