Since achieving 1.3 megajoules of fusion energy in August, the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) has been trying to repeat that breakthrough result. (See “Lawrence Livermore claims a milestone in laser fusion,” Physics Today online, 17 August 2021.) Three inertial confinement fusion experiments conducted since the August shot have fallen well short. Each one, however, topped the performance of prior experiments. Researchers will take their next shot at producing more fusion energy than the laser energy needed to start the reaction—the definition of ignition used by the lab—in January.
A 7 November experiment produced 700 kilojoules of fusion energy, and a second shot last month yielded 430 kJ, says NIF director Mark Herrmann. Although he declined to disclose the yield of a 21 November test pending data analysis, Herrmann acknowledges that it didn’t top the August record.
In each case, as with the August shot, the NIF laser’s 192 beams fired 1.9 MJ of UV light onto a cylindrical gold target containing a BB-size capsule filled with deuterium and tritium (D–T) fuel. Inside the cylinder—known as a hohlraum—the UV is converted to x rays that implode the capsule and set off fusion in the compressed fuel.
Because NIF is used to conduct other types of experiments relevant to the nuclear weapons stockpile, it is limited to 20–30 ignition shots each year. The August record occurred on the 177th try since ignition attempts began in 2009.
“The intention was to replicate as closely as we could the parameters of the August shot,” says Herrmann. The fact that the three subsequent experiments performed well above the previous best mark of 170 kJ indicates that NIF has entered a regime in which even tiny variances in capsules and laser performance have an outsize impact on the outcome. Those variables aren’t well understood yet, he adds.
“All [three shots] were letting out more energy than the capsule absorbs, all were showing strong signs of alpha particles raising temperatures, and all were getting to higher temperatures than we’ve ever been able to reach in implosions,” Herrmann says. Ignition is a runaway reaction that requires the alpha particles generated by the fusion to deposit their energy inside the surrounding plasma.
In the coming months, LLNL scientists will use some of their limited supply of the precisely manufactured capsules—which can require eight months to fabricate—for experiments in which the fusion burn is “turned off” by altering the D–T ratio. Those experiments will allow researchers to more closely observe the hydrodynamics of the implosion, Herrmann says. In a shot having a high fusion yield, it’s difficult to take time-gated x-ray snapshots that show how the hot spot at the fuel’s center propagates.
Bedros Afeyan, an expert on laser–plasma instabilities who has worked in laser-fusion programs at LLNL and at Los Alamos and Sandia National Laboratories, says the results are “very encouraging,” but exactly where the fusion burn is occurring in the fuel isn’t known. NIF’s diagnostics indicate that the fusion reactions are “neither thermal nor isotropic.” More important, he says, even the record shot burned only 1.6% of the D–T in the capsule. “That’s fizzling, in my opinion,” he says. “You want to know what limited that.” Burning half of the fuel would be “absolutely a success.”
Herrmann says factors contributing to the lower yields of the recent shots may include variation in the diameter of the fill tube through which the cryogenic D–T mix is injected into the capsule and differences in the layering of that mix inside the shell, which is performed manually and isn’t easily controlled. Both could result in implosion asymmetries, as could slight variations in the energy of the individual laser beams. Capsules with imperfections over an area exceeding 100 μm2, out of a total capsule surface of 1 million μm2, are rejected.
“If we can get to a regime that’s robust, it won’t be as sensitive to small variations. But that’s not where we started on August 8 with the rapid advance,” Herrmann says.
Although 12 years old, NIF remains the world’s most energetic laser, sustaining its peak power of about half a petawatt in nanosecond-long pulses. Newer lasers employing chirped pulse amplification can produce petawatt-scale power in femtosecond- to picosecond-long pulses. (See “The commercial drive for laser fusion power,” Physics Today online, 20 October 2021.)
NIF operated at its peak output during each experiment, but Afeyan says an even more powerful laser may not overcome the remaining hurdles. “It’s not clear that pushing on central hot-spot ignition with a better-polished target is the only thing we can do or even the right thing to do,” he says. Researchers must figure out what is happening to limit the fuel burn. Possibilities include laser–plasma instabilities created when the beams intersect the gold plasma generated by the hohlraum—another factor not considered in the LLNL researchers’ modeling, he says. “Sometimes they have to think outside the box.”