With a powerful laser zap, scientists have blasted toward a milestone for nuclear fusion.
A fusion experiment at the world’s biggest laser facility released 1.3 million joules of energy, coming close to a break-even point known as ignition, where fusion begins to release more energy than required to detonate it. Reaching ignition would strengthen hopes that fusion could one day serve as a clean, plentiful energy source, a goal that scientists have struggled to make progress toward (SN: 2/8/18).
By pummeling a tiny capsule with lasers at the National Ignition Facility, or NIF, at Lawrence Livermore National Laboratory in California, scientists triggered fusion reactions that churned out more than 10 quadrillion watts of power over 100 trillionths of a second. In all, the experiment, performed August 8, released about 70 percent of the energy of the laser light used to set off the fusion reactions, putting the facility much closer to ignition than ever before.
Notably, because the capsule absorbs only a portion of the total laser energy focused on it, the reactions actually produced more energy than directly went into igniting them. “That, just fundamentally, is a truly amazing feat,” says plasma physicist Carolyn Kuranz of the University of Michigan in Ann Arbor, who was not involved with the research. By that metric, the fusion reactions produced about five times as much energy as was absorbed.
“It’s a really exciting result, and it wasn’t clear that NIF would be able to get to this result,” Kuranz says. For years, NIF scientists have strived to reach ignition, but they have been plagued with setbacks (SN: 4/4/13). While the new results have yet to be published in a scientific journal, NIF scientists went public with their discovery after word got out to the scientific community and excitement mounted.
“It makes me very hopeful … for fusion in the future,” Kuranz says.
Nuclear fusion, the same process that powers the sun, would be an appealing source of energy on Earth because it checks several boxes for environmental friendliness: It wouldn’t generate climate-warming greenhouse gases or dangerous, long-lived radioactive waste. In nuclear fusion, hydrogen nuclei meld together to form helium, releasing energy in the process. But fusion requires extreme temperatures and pressures, making it difficult to control.
NIF is not alone in the fusion quest. Other projects, such as ITER, an enormous facility under construction in southern France, are using different techniques to tackle the problem (SN: 1/27/16). But those efforts have also met with difficulties. Perhaps unsurprisingly, controlling reactions akin to those in the sun is challenging no matter how you go about it.
In NIF’s fusion experiments, 192 laser beams converge on a small cylinder containing a peppercorn-sized fuel capsule. When that powerful laser burst hits the cylinder, X-rays stream out, vaporizing the capsule’s exterior and imploding the fuel within. That fuel is a mixture of deuterium and tritium, varieties of hydrogen that respectively contain one or two neutrons in their atomic nuclei. As the fuel implodes, it reaches the extreme densities, temperatures and pressures needed to fuse the hydrogen into helium. That helium can further heat the rest of the fuel, what’s known as alpha heating, setting off a fusion chain reaction.
That last step is crucial to boosting the energy yield. “What’s new about this experiment is that we’ve created a system in which the alpha heating rate is far larger than we’ve ever achieved before,” says NIF physicist Arthur Pak.
Scientists navigated a variety of quagmires to get to this stage. “There’s a whole host of physics issues … that we’ve faced off and mitigated,” Pak says. For example, researchers took pains to make the capsule absorb more energy, to eliminate tiny defects in the capsule and to carefully tune the laser pulses to maximize fusion.
In 2018, researchers began seeing the payoff of those efforts. NIF achieved a then-record fusion energy of 55,000 joules. Then, in spring 2021, NIF reached 170,000 joules. Further tweaking the design of the experiment, scientists suspected, could increase the output even more. But the new experiment went beyond expectations, producing nearly eight times the energy of the previous effort.
Further studies will help NIF scientists determine exactly how their changes created such bountiful energy and how to enhance the output further. Still, even if NIF achieves full-fledged ignition, using fusion to generate power for practical purposes is still a long way off. “There will be a huge amount of work needed to turn the technology into a viable source of energy,” says laser plasma physicist Stuart Mangles of Imperial College London, who was not involved with the research. “Nevertheless, this is a really important milestone on the way.”
sciencenews.org, 18 August 2021