Energy output from nuclear fusion reaction exceeds input

For the first time ever, researchers have carried out a controlled nuclear fusion reaction, producing more energy than the amount pumped into the nuclear fuel to ignite it. The advance, which was announced Dec. 13 by US secretary of energy Jennifer M. Granholm, may lead to new ways to analyze nuclear weapons without needing to detonate such devices. The achievement also provides insights into the fusion process, which one day may be a source of abundant, inexpensive, and carbon-free energy.

The successful experiment was conducted Dec. 5 at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. It comes after more than 60 years of research in which scientists and engineers have worked on designing laboratory equipment that can simulate the extreme temperatures and pressures that drive fusion in the cores of the sun and other stars and inside exploding nuclear weapons.

Fusion can occur when two extremely energetic atoms slam together. The process causes the nuclei to coalesce into a single larger nucleus and emits a lot of energy. In the NIF experiment, a mixture of deuterium and tritium served as the nuclear fuel. The reaction fused the two heavy isotopes of hydrogen, forming helium.

To drive the reaction, 192 high-​energy lasers fired simultaneously on a centimeter-sized cylinder that held a spherical container the size of a ball bearing. The sphere was filled with the deuterium-tritium fuel and was cooled cryogenically—forming a layer of ice on the sphere’s inner surface. Directing the laser beams at the cylinder wall generated an intense flux of X-rays that bombarded the sphere. The radiation produced powerful compressive forces that caused the sphere to implode and heated it to millions of degrees for a few billionths of a second, igniting the fuel and triggering fusion. At the press conference, NIF researchers reported that the lasers pumped 2.05 MJ of energy into the target, and the fusion reaction generated 3.15 MJ of energy.

“This is a big deal, a very significant breakthrough,” says David N. Ruzic, a specialist in fusion at the University of Illinois Urbana-Champaign. Until now, no one has succeeded in getting more energy out than they put in to drive fusion, he says. He clarifies, however, that the lasers used 300 MJ of energy from the electrical grid to produce the 2 MJ pulse, which indicates that the full energy accounting does not yet show a net energy gain. NIF researchers addressed that point at the press conference, noting that newer laser systems could be far more energy efficient than the 20-year-old ones used in the experiment.

Years of development are needed before fusion power plants can become a reality, Ruzic says. Yet he is encouraged by the increase in private funding in recent years and the latest fusion results. “There is real cause for optimism,” he says.