Scientists have been working for more than 50 years to duplicate the process by which the sun generates energy, namely fusing together hydrogen.

It's not just heat that does the trick, but extremely high pressure. One way to experiment on Earth has been to zap tiny fuel-filled capsules with lasers to make them compress, which packs the material inside tighter and tighter together.

At some point, the resulting nuclear fusion takes off like wildfire, creating a cascade of reactions and bringing the dream of power plants that produce more energy than they consume one giant leap closer to reality.

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Scientists report a small but significant step this week when, for the first time, the amount of energy produced by fusing deuterium-tritium ions exceeded what was naturally stored inside the fuel.

Previously, the capsules, which start out at just 2 mm in diameter and end up less than the width of a human hair, blew apart before that state of fusion could be achieved.

"The pressures in this tiny little spot are 150 billion atmospheres. The density of the fuel that we finally achieve in these experiments is 2.5 to three times the density of the center of the sun," said physicist Omar Hurricane, with the Lawrence Livermore National Laboratory.

"It's hot. It's pushing back ... the conditions are quite ferocious. Because of that it doesn't last very long," he said.

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Much more work lies ahead. Scientists will continue to tweak how they use the lab's 192 lasers to heat the fuel. They also are thinking about different shapes and materials to better control the reactions.

In the current experiments, for example, the capsule was compressed by about 35 times.

"If you started with a basketball, that would be like compressing it down to the size of a pea," said Deborah Callahan, also with Lawrence Livermore.

Conditions will have to be even more extreme to reach the next milestone -- getting as much energy out of the fuel as what goes in with the lasers.

"We have waited 60 years to get close to controlled fusion -- we are now close," Steven Cowley, with the Culham Centre for Fusion Energy in the United Kingdom, wrote in an email to Discovery News.

Cowley works on another branch of fusion energy research which uses magnetic fields to confine the hot plasma fuel.

“The engineering milestone is when the whole plant produces more energy than it consumes," he said.

The research appears in this week's Nature.