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The dream of fusion power inched closer to reality in December 2022, when researchers at Lawrence Livermore National Laboratory (LLNL) revealed that a fusion reaction had produced more energy than what was required to kick-start it. According to new research, the momentary fusion feat required exquisite choreography and extensive preparations, whose high degree of difficulty reveals a long road ahead before anyone dares hope a practicable power source could be at hand.
The groundbreaking result was achieved at the California lab’s National Ignition Facility (NIF), which uses an array of 192 high-power lasers to blast tiny pellets of deuterium and tritium fuel in a process known as inertial confinement fusion. This causes the fuel to implode, smashing its atoms together and generating higher temperatures and pressures than are found at the center of the sun. The atoms then fuse together, releasing huge amounts of energy.
“It showed there’s nothing fundamentally limiting us from being able to harness fusion in the laboratory.”—ANNIE KRITCHER, LAWRENCE LIVERMORE NATIONAL LABORATORY
The facility has been running since 2011, and for a long time the amount of energy produced by these reactions was significantly less than the amount of laser energy pumped into the fuel. But on 5 December 2022, researchers at NIF announced that they had finally achieved breakeven by generating 1.5 times more energy than was required to start the fusion reaction.
A new paper published yesterday in Physical Review Letters confirms the team’s claims and details the complex engineering required to make it possible. While the results underscore the considerable work ahead, Annie Kritcher, a physicist at LLNL who led design of the experiment, says it still signals a major milestone in fusion science. “It showed there’s nothing fundamentally limiting us from being able to harness fusion in the laboratory,” she says.
While the experiment was characterized as a breakthrough, Kritcher says it was actually the result of painstaking incremental improvements to the facility’s equipment and processes. In particular, the team has spent years perfecting the design of the fuel pellet and the cylindrical gold container that houses it, known as a “hohlraum”.
Why is fusion so hard?
When lasers hit the outside of this capsule, their energy is converted into X-rays that then blast the fuel pellet, which consists of a diamond outer shell coated on the inside with deuterium and tritium fuel. It’s crucial that the hohlraum is as symmetrical as possible, says Kritcher, so it distributes X-rays evenly across the pellet. This ensures the fuel is compressed equally from all sides, allowing it to reach the temperatures and pressures required for fusion. “If you don’t do that, you can basically imagine your plasmas squirting out in one direction, and you can’t squeeze it and heat it enough,” she says.
The team has since carried out six more experiments—two that have generated roughly the same amount of energy as was put in and four that significantly exceeded it.
Carefully tailoring the laser beams is also important, Kritcher says, because laser light can scatter off the hohlraum, reducing efficiency and potentially damaging laser optics. In addition, as soon as the laser starts to hit the capsule, it starts giving off a plume of plasma that interferes with the beam. “It’s a race against time,” says Kritcher. “We’re trying to get the laser pulse in there before this happens, because then you can’t get the laser energy to go where you want it to go.”
The design process is slowgoing, because the facility is capable of carrying out only a few shots a year, limiting the team’s ability to iterate. And predicting how those changes will pan out ahead of time is challenging because of our poor understanding of the extreme physics at play. “We’re blasting a tiny target with the biggest laser in the world, and a whole lot of crap is flying all over the place,” says Kritcher. “And we’re trying to control that to very, very precise levels.”
Nonetheless, by analyzing the results of previous experiments and using computer modeling, the team was able to crack the problem. They worked out that using a slightly higher power laser coupled with a thicker diamond shell around the fuel pellet could overcome the destabilizing effects of imperfections on the pellet’s surface. Moreover, they found these modifications could also help confine the fusion reaction for long enough for it to become self-sustaining. The resulting experiment ended up producing 3.15 megajoules, considerably more than the 2.05 MJ produced by the lasers.
Since then, the team has carried out six more experiments—two that have generated roughly the same amount of energy as was put in and four that significantly exceeded it. Consistently achieving breakeven is a significant feat, says Kritcher. However, she adds that the significant variability in the amount of energy produced remains something the researchers need to address.
This kind of inconsistency is unsurprising, though, says Saskia Mordijck, an associate professor of physics at the College of William & Mary in Virginia. The amount of energy generated is strongly linked to how self-sustaining the reactions are, which can be impacted by very small changes in the setup, she says. She compares the challenge to landing on the moon—we know how to do it, but it’s such an enormous technical challenge that there’s no guarantee you’ll stick the landing.
Relatedly, researchers from the University of Rochester’s Laboratory for Laser Energetics today reported in the journal Nature Physics that they have developed an inertial confinement fusion system that’s one-hundredth the size of NIF’s. Their 28 kilojoule laser system, the team noted, can at least yield more fusion energy than what is contained in the central plasma—an accomplishment that’s on the road toward NIF’s success, but still a distance away. They’re calling what they’ve developed a “spark plug“ toward more energetic reactions.
Both NIF’s and LLE’s newly reported results represent steps along a development path—where in both cases that path remains long and challenging if inertial confinement fusion is to ever become more than a research curiosity, though.
Plenty of other obstacles remain than those noted above, too. Current calculations compare energy generated against the NIF laser’s output, but that brushes over the fact that the lasers draw more than 100 times the power from the grid than any fusion reaction yields. That means either energy gains or laser efficiency would need to improve by two orders of magnitude to break even in any practical sense. The NIF’s fuel pellets are also extremely expensive, says Kritcher, each one pricing in at an estimated $100,000. Then, producing a reasonable amount of power would mean dramatically increasing the frequency of NIF’s shots—a feat barely on the horizon for a reactor that requires months to load up the next nanosecond-long burst.
“Those are the biggest challenges,” Mordijck says. “But I think if we overcome those, it’s really not that hard at that point.”
Source: Spectrum.ieee.org
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