Lawrence Livermore National Laboratory

Scientists at Lawrence Livermore National Laboratory have reported an important step on the way to fusion energy: a reaction in which fusing hydrogen gave off more energy than the lasers put in to initiate the reaction.

Fusion, the reaction that powers the sun and the more powerful part of thermonuclear explosions, combines lightweight atoms like hydrogen and releases a lot of energy in the process. In contrast, heavy elements such as uranium are split to release energy in the fission reactions that powered the first atomic weapons and today's nuclear power reactors.

Scientists long have hoped to harness fusion's power to produce energy free from the radioactive byproducts that are so troublesome with fission reactors. But controlled fusion has been extremely hard to create: it requires an extraordinarily high concentration of energy to get the reaction started and to produce enough extra energy to achieve a self-sustaining reaction.

The researchers at LLNL's National Ignition Facility (NIF) achieved "fuel gains," meaning they got more energy out of fusion from a tiny capsule about a millimeter across that contains deuterium and tritium, isotopes of hydrogen with one and two neutrons, respectively. It's machined with extremely high precision and mounted at the center of 192 ultraviolet lasers that pack a walloping 1.85 megajoules of energy.

The results, published Wednesday in the journal Nature, yielded results 10 times better than previous deuterium-tritium experiments, the researchers said.

However, it was still well short of "ignition," in which the energy produced exceeds what the entire experiment used, not just the smaller amount that actually reached the fuel. Controlled fusion has proved a famously elusive idea, and NIF has worked for years to get this far.

The researchers did see progress on another front called boot-strapping, a phenomenon that's part of achieving a self-sustaining fusion reaction. The boot-strapping process takes place when helium nuclei -- each one a pair of protons and a pair of neutrons produced by the fusion reaction -- impart energy to further fusion rather than escaping.

"We also see...evidence for the 'bootstrapping' required to accelerate the deuterium-tritium fusion burn to eventually 'run away' and ignite," the researchers said.

NIF's funding comes from the US government's Stockpile Stewardship program, designed to ensure nuclear weapons' reliability and storage safety even without underground nuclear tests. Improving the country's energy security, though, also is a goal.