Physicists starry-eyed after nuclear fusion success

Scientists at last produced nuclear fusion reactions that create more energy than was in the fuel.

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LOS ANGELES — It took 192 lasers and a building big enough to contain three football fields, but physicists have finally produced a pair of nuclear fusion reactions that created more energy than was in the fuel to start with.

The reactions lasted less than a billionth of a second and released only a few thousand joules — enough to power a 100-watt light bulb for less than three minutes. But it marks the first time scientists have been able to harness the power of stars on Earth.

“This is really an important milestone,” said Warren Mori, a plasma physicist at the University of California, Los Angeles who was not involved.

The experiment, conducted at Lawrence Livermore National Laboratory in the Bay Area, is still a very long way from “ignition,” the point at which the reaction generates more energy than was required to kick it off with lasers. Scientists agree that significant hurdles remain before that goal can be reached.

But the tests, described Wednesday in the journal Nature, give researchers a promising sign that they’re finally on the right path to reaching this goal — one that could ultimately lead to cleaner nuclear energy, safer weapons arsenals and a more profound understanding of astrophysics.

“We are closer than anyone has gotten before,” said study leader Omar Hurricane, a plasma physicist at Lawrence Livermore National Laboratory.

Nuclear fusion is the process of combining the nuclei of two atoms to create a heavier atom, releasing an incredible amount of energy in the process. It’s what powers the stars and generates their light.

In some ways, it’s the reverse of nuclear fission, which releases energy — and a significant amount of dangerous radiation — by breaking large atoms into smaller ones. Fission is used in nuclear power plants today.

Hurricane and his colleagues used a comparatively simple fusion recipe. For their fuel, they used two “heavy” hydrogen isotopes: deuterium (which has one proton and one neutron in its nucleus) and tritium (which has one proton and two neutrons). When they fuse together, they create a single atom of helium (two protons and two neutrons), along with a spare neutron — and a massive amount of energy.

In theory, that should prompt more atoms in the deuterium-tritium fuel to merge, triggering a chain reaction. It’s similar to dropping a match on a pile of kindling — if you have the right match and the right wood under the right conditions, it’s easy to make it burn.

But outside of a star, it’s very difficult to create the high pressure and high temperature needed to get the process going.

The scientists at the Livermore lab’s National Ignition Facility tried to simulate conditions in a star by using an array of lasers to squeeze down and heat up a tiny jot of fuel. The lasers shot high-energy beams through two tiny holes on either end of a pill-sized gold can.

When the lasers hit the can’s interior walls, the surface gave off X-rays that bathed an even tinier plastic sphere about the size of a small bead. The sphere contained a layer of deuterium-tritium fuel that was just 70 microns thick, barely a human hair width.

With a precise combination of shocks from the laser beams, the pressure inside the sphere was several times greater than at the center of the sun. It lasted for about a seventh of a billionth of a second, long enough for the fuel inside to start fusing together, Hurricane said.

After two runs, the scientists measured the energy output by tracking the energy level of the spare neutrons flying out from the tiny sphere. The first released 14,400 joules, slightly more than the 12,000 joules in the deuterium-tritium fuel to start with. The second was better: 17,300 joules out for just 9,400 joules in.

“I think it’s a promising advance,” said Mark Herrmann, physicist at Sandia National Laboratories in Albuquerque who was not involved in the study. “I was excited.”

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