The key number: 3.88 megajoules.
The December experiment generated a flurry of praise when it produced about three 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 produced more energy than was necessary to start the reaction.
The July experiment was essentially identical to the December one. “We expected similar performance,” Dr. Town said. “On the order of three megajoules.”
The actual production was 3.88 megajoules.
The better-than-expected result indicates that with some adjustments, laser melting can become noticeably more efficient. But tiny variations could also lead to fusion failures.
A fusion experiment conducted in June, just a month earlier, was also predicted to produce about three megajoules, but generated only between 1.6 and 1.7 megajoules, Dr. Town said.
A more recent attempt this month, as part of efforts to keep nuclear weapons free from underground nuclear testing, produced just over two megajoules, matching the energy of the laser.
“It was a little surprising that we didn’t achieve ignition in all of them,” Dr. Town said.
Why it’s important: Much more than learning.
By analyzing the results, the Livermore scientists now believe they have a better understanding of what is happening.
For one thing, the 192 lasers are not perfect. “There are some variations each time the laser is fired,” Dr. Town said.
Instead of the laser energy arriving perfectly balanced to compress the hydrogen fuel capsule, a slight imbalance pushes the capsule in one direction. Some of the energy is lost and the internal implosion does not heat the hydrogen as much.
There are also slight variations in the fuel capsules that affect fusion reactions. Computer simulations now indicate that there may be a wide range in output energy.
“It could drop as low as 1.4 megajoules,” Dr. Town said. “And if the stars align and everything works perfectly, up to seven megajoules could be generated.”
What happens next: Experiment update and optimization.
Siegfried Glenzer, a scientist at the SLAC National Accelerator Laboratory in Menlo Park, California, who led the initial fusion experiments at the Livermore facility years ago, said of the July breakthrough: “The fact that the gain has increased in the last “This is encouraging news and shows that current implosions are not yet fully optimized.”
A new series of experiments is about to begin at the National Ignition Facility, aimed at generating higher fusion yields more consistently. The power of the facility’s lasers is being increased from 2.05 to 2.2 megajoules. The latest developments came after the latest 1.9 megajoule update. The additional energy is expected to lead to further improvements.
“If you can effectively couple more energy to the hot spot, you should get better performance,” Dr. Town said. “You can do it if you have a bigger hammer.”
