A laser fusion breakthrough gets a bigger burst of energy

In July, scientists at the National Ignition Facility at Lawrence Livermore National Laboratory in California generated a burst of energy by bombarding a pellet of hydrogen with 192 lasers, reproducing for a brief moment the process of fusion that powers the sun. It was a repeat of an experiment last December, but this time the scientists generated even more energy, with nearly a factor of two in gain compared with the energy of the incoming lasers.

“We again repeated ignition,” Richard Town, the associate program director of the laser fusion program at Livermore, said in an interview. He gave a talk about the July experiment Monday at a conference in Denver.

The Livermore results raise hopes that fusion can one day be used to generate bountiful amounts of electricity without producing greenhouse gases or longlived radioactive waste.

The key number: 3.88 megajoules

The experiment in December

generated a whirlwind of accolades when it produced about 3 megajoules of energy — equivalent to about 1.5 pounds of TNT, or about 1.5 times the energy of the incoming lasers. It was the first time that a fusion reaction in a laboratory setting produced more energy than it took to start the reaction.

The July experiment was essentially identical to the December one. “We expected a similar yield,” Town said. “On the order of 3 megajoules.”

The actual output was 3.88 megajoules.

The better-than-predicted result indicates that with a few tweaks, laser fusion can become markedly more efficient. But minuscule variations could yield fusion duds as well.

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

A more recent attempt this month, as part of efforts to maintain nuclear weapons without underground nuclear tests, yielded slightly more than two megajoules, breaking even with the laser energy.

“It was a little bit surprising that we did not achieve ignition on all of them,” Town said.

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

For one, the 192 lasers are not perfect. “There are some variations every time you shoot the laser,” Town said.

Instead of laser energy arriving perfectly balanced to compress the hydrogen fuel capsule, a slight imbalance nudges the capsule off in one direction. Some of the energy is lost, and the inward implosion does not heat the hydrogen as much.

There are also slight variations in the fuel capsules that affect the fusion reactions. Computer simulations now indicate there can be a wide range in the output energy.

“It could fall as low as 1.4 megajoules,” Town said. “And if the stars align and everything works perfectly, it could get up to seven megajoules.”