Scientists have reached a major milestone in the pursuit of nuclear fusion, the process that powers the stars and could one day provide a globally accessible, long-term supply of carbon-free energy.

U.S. government officials said the breakthrough was achieved last week at the National Ignition Facility at Lawrence Livermore National Laboratory, the site of a long-running effort to achieve fusion by blasting specially designed targets with powerful laser beams.

During a test on Dec. 5, the nuclear reaction triggered in a tiny target released about 50 per cent more energy than it took to trigger the reaction with a powerful blast from the laser.

The result marks the first time any such experiment has achieved a net energy gain – a critical threshold that is the primary goal of fusion research.

“This demonstrates that it can be done,” Energy Secretary Jennifer Granholm said at a news briefing in Washington, D.C., Tuesday.

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While the announcement does not immediately change the timeline for a practical application, it adds to the growing sense of momentum in a field long regarded as offering a scientifically feasible but technically challenging solution to the world’s energy needs.

Thanks to public excitement and the urgent need to cut the fossil fuel emissions that drive climate change, the development could indirectly boost the prospects of several private companies working on quicker pathways to commercial fusion power.

But experts caution that Tuesday’s exciting news should be tempered with the reality of how far scientists have yet to go to turn fusion into a useful energy source.

For example, last week’s experiment generated 3.15 megajoules of energy from 2.05 megajoules of input from the laser. Yet the laser draws about 300 megajoules from the grid just to operate.

Having now achieved a net energy gain from their target, researchers said the next step is making the process more efficient and easily reproduced. A longer-term goal is a system that can fire repeatedly and power the entire laser system with energy to spare.

“A few decades of research on the underlying technologies could put us in a position to build a power plant,” said Kim Budil, the director of the laboratory.

Dr. Budil said the laboratory brought in an independent team to peer-review the experiment’s results ahead of Tuesday’s announcement.

The National Ignition Facility was constructed in the late 1990s, and the first tests were conducted in 2009. After years of incremental progress, scientists reported last year that they were 70 per cent of the way to net energy gain and were optimistic about reaching that goal.

Their approach uses the world’s most powerful laser to blast BB-size targets consisting of isotopes of hydrogen packed inside a diamond shell. During a test, the laser beam is divided into several separate beams, which converge on the target from multiple directions and rapidly compress it in time scales of about one billionth of a second.

At that point, the hydrogen isotopes – deuterium and tritium – are so squeezed that they are converted into helium, momentarily releasing energy in the process.

The process, known as inertial confinement, is one of two routes to fusion being pursued by large government-funded megaprojects.