Nuclear fusion breakthrough leads to huge burst of laser energy

The experiment in December generated a storm of praise when it generated nearly three megajoules of energy – the equivalent of about 1.5 pounds of TNT, or about 1.5 times the energy of incoming lasers. This was the first time that a fusion reaction in a laboratory setting produced more energy than it took to initiate the reaction.

The July experiment was essentially the same as the December one. “We expected similar yields,” Dr. Towne said. “On the order of three megajoules.”

The actual output was 3.88 megajoules.

The better-than-expected results indicate that with some changes, laser fusion could become markedly more efficient. But even small variations can produce fusion pairs.

A fusion experiment just a month earlier in June was also predicted to produce about three megajoules, but it produced only between 1.6 and 1.7 megajoules, Dr. Towne said.

A recent effort this month, part of efforts to maintain nuclear weapons without underground nuclear tests, produced a little more than two megajoules, surpassing even laser energy.

“It was a little surprising that we couldn’t achieve ignition on all of them,” Dr. Towne said.

Analyzing the results, Livermore scientists now think they better understand what’s going on.

For one, 192 lasers are not perfect. “Every time you shoot a laser there are some changes that happen,” Dr. Towne said.

Instead of the laser energy being perfectly balanced to compress the hydrogen fuel capsule, a slight imbalance pushes the capsule in one direction. Some energy is dissipated, and the explosion inward does not heat the hydrogen as much.

The fuel capsule also has slight variations that affect fusion reactions. Computer simulations now indicate that the output energy may have a wide range.

“It could drop to 1.4 megajoules,” Dr. Towne said. “And if the stars align and everything works properly, it could reach seven megajoules.”

Siegfried Glanzer, a scientist at the SLAC National Accelerator Laboratory in Menlo Park, California, who led early fusion experiments at the Livermore facility years ago, said of the July advance, “The fact that the gains have increased in the last shot is encouraging news. and shows that current eruptions have not yet been fully adapted.”

A new series of experiments is about to begin at the National Ignition Facility as it aims to generate consistently higher fusion yields. The energy of the facility’s lasers is being upgraded from 2.05 to 2.2 megajoules. The latest progress comes after the last upgrade from 1.9 megajoules. Additional energy is expected to lead to further improvements.

“If you can effectively add more energy to the hot spot, you should get a higher yield,” Dr. Towne said. “You can do that with a big hammer.”

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