US scientists have announced an important milestone in the costly, decades-old quest to develop fusion energy, which, if harnessed successfully, promises a nearly inexhaustible energy source.
For the first time, experiments had produced more energy from fusion reactions than the amount of energy put into the fusion fuel, scientists at the federally funded Lawrence Livermore National Laboratory in California said.
The team of researchers, led by physicist Omar Hurricane, described the achievement as important but said much more work was needed before fusion could become a viable energy source. The scientists noted that they did not produce self-heating nuclear fusion, known as ignition, needed for any fusion power plant.
Researchers have faced daunting scientific and engineering challenges in trying to develop nuclear fusion - the process that powers stars including our sun - for use by humankind.
"Really for the first time anywhere, we've gotten more energy out of this fuel than was put into the fuel. And that's quite unique. And that's kind of a major turning point, in a lot of our minds," Hurricane said. "I think a lot of people are jazzed."
Unlike fossil fuels or the fission process in nuclear power plants, fusion offers the prospect of abundant energy without pollution, radioactive waste or greenhouse gases.
Unlike the nuclear fission energy that is derived from splitting atoms, fusion energy is produced by fusing atoms together.
Experts believe it still will be many years or decades before fusion can become a practical energy source.
Of the uncertain path ahead in fusion research, Hurricane compared it to "climbing half way up a mountain, but the top of the mountain is hidden in clouds. You can't see it. You don't have a map".
Scientists used 192 laser beams to zap a tiny target containing a capsule about 2mm in diameter filled with fusion fuel, consisting of a plasma of deuterium and tritium, which are two isotopes of hydrogen.
The fuel was coated on the inside of the capsule in a frozen layer less than the width of a human hair.
At very high temperatures, the nucleus of the deuterium and the nucleus of the tritium fuse, a neutron and something known as an "alpha particle" emerge, and energy is released.
The experiments, published in the journal Nature, created conditions up to three times the density of the sun.
In two experiments described by the researchers that took place in September and November, more energy came out of the fusion fuel than was deposited, but it was still less than the total deposited into the target.
The fusion-energy yield was increased by about tenfold from past experiments, in a series that started last May. One of the experiments produced more than half of the so-called Lawson criteria needed to reach ignition - but only about one-100th of the energy needed for ignition.
Lawrence Livermore National Laboratory is overseen by the US National Nuclear Security Administration.
Eager to exploit the potential this type of energy offers to reduce dependence on oil and other fossil fuels, the US and other nations have invested many millions of dollars into fusion research, often with uneven results.
Steve Cowley, director of the Culham Centre for Fusion Energy, called the new findings "truly excellent" but said different measures of success made it hard to compare with his type of research.