A twisting ribbon of hydrogen gas, many times hotter than the surface of the sun, has given scientists a tentative glimpse of the future of controlled nuclear fusion—a so-far theoretical source of relatively “clean” and abundant energy that would be effectively fueled by seawater.
The ribbon was a plasma inside Germany’s Wendelstein 7-X, an advanced fusion reactor that set a record last May by magnetically “bottling up” the superheated plasma for a whopping 43 seconds. That’s many times longer than the device had achieved before.
It’s often joked that fusion is only 30 years away—and always will be. But the latest results indicate that scientists and engineers are finally gaining on that prediction. “I think it’s probably now about 15 to 20 years [away],” says University of Cambridge nuclear engineer Tony Roulstone, who wasn’t involved in the Wendelstein experiments. “The superconducting magnets [that the researchers are using to contain the plasma] are making the difference.”
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And the latest Wendelstein result, while promising, has now been countered by British researchers. They say the large Joint European Torus (JET) fusion reactor near Oxford, England, achieved even longer containment times of up to 60 seconds in final experiments before its retirement in December 2023. These results have been kept quiet until now but are due to be published in a scientific journal soon.
According to a press release from the Max Planck Institute for Plasma Physics in Germany, the as yet unpublished data make the Wendelstein and JET reactors “joint leaders” in the scientific quest to continually operate a fusion reactor at extremely high temperatures. Even so, the press release notes that JET’s plasma volume was three times larger than that of the Wendelstein reactor, which would have given JET an advantage—a not-so-subtle insinuation that, all other things being equal, the German project should be considered the true leader.
This friendly rivalry highlights a long-standing competition between devices called stellarators, such as the Wendelstein 7-X, and others called tokamaks, such as JET. Both use different approaches to achieve a promising form of nuclear fusion called magnetic confinement, which aims to ignite a fusion reaction in a plasma of the neutron-heavy hydrogen isotopes deuterium and tritium.
The latest results come after the successful fusion ignition in 2022 at the National Ignition Facility (NIF) near San Francisco, which used a very different method of fusion called inertial confinement. Researchers there applied giant lasers to a pea-sized pellet of deuterium and tritium, triggering a fusion reaction that gave off more energy than it consumed. (Replications of the experiment have since yielded even more energy.)
The U.S. Department of Energy began constructing the NIF in the late 1990s, with the goal to develop inertial confinement as an alternative to testing thermonuclear bombs, and research for the U.S.’s nuclear arsenal still makes up most of the facility’s work. But the ignition was an important milestone on the path toward controlled nuclear fusion—a “holy grail” of science and engineering.
“The 2022 achievement of fusion ignition marks the first time humans have been able to demonstrate a controlled self-sustained burning fusion reaction in the laboratory—akin to lighting a match and that turning into a bonfire,” says plasma physicist Tammy Ma of the Lawrence Livermore National Laboratory, which operates the NIF. “With every other fusion attempt prior, the lit match had fizzled.”
The inertial confinement method used by the NIF—the largest and most powerful laser system in the world—may not be best suited for generating electricity, however (although it seems unparalleled for simulating thermonuclear bombs). The ignition in the fuel pellet did give off more energy than put into it by the NIF’s 192 giant lasers. But the lasers themselves took more than 12 hours to charge before the experiment and consumed roughly 100 times the energy released by the fusing pellet.
In contrast, calculations suggest a fusion power plant would have to ignite about 10 fuel pellets every second, continuously, for 24 hours a day to deliver utility-scale service. That’s an immense engineering challenge but one accepted by several inertial fusion energy startups, such as Marvel Fusion in Germany; other start-ups, such as Xcimer Energy in the U.S., meanwhile, propose using a similar system to ignite just one fuel pellet every two seconds.
Ma admits that the NIF approach faces difficulties, but she points out it’s still the only fusion method on Earth to have demonstrated a net energy gain: “Fusion energy, and particularly the inertial confinement approach to fusion, has huge potential, and it is imperative that we pursue it,” she says.
Instead of igniting fuel pellets with lasers, most fusion power projects—like the Wendelstein 7-X and the JET reactor—have chosen a different path to nuclear fusion. Some of the most sophisticated, such as the giant ITER project being built in France, are tokamaks. These devices were first invented in the former Soviet Union and get their name from a Russian acronym for the doughnut-shaped rings of plasma they contain. They work by inducing a powerful electric current inside the superheated plasma doughnut to make it more magnetic and prevent it from striking and damaging the walls of the reactor chamber—the main challenge for the technology.
The Wendelstein 7-X reactor, however, is a stellarator—it uses a related, albeit more complicated, design that doesn’t induce an electric current in the plasma but instead tries to control it with powerful external magnets alone. The result is that the plasmas in stellarators are more stable within their magnetic bottles. Reactors like the Wendelstein 7-X aim to operate for a longer period of time than tokamaks can without damaging the reactor chamber.
The Wendelstein researchers plan to soon exceed a minute and eventually to run the reactor continuously for more than half an hour. “There’s really nothing in the way to make it longer,” explains physicist Thomas Klinger, who leads the project at the Max Planck Institute for Plasma Physics. “And then we are in an area where nobody has ever been before.”
The overlooked results from the JET reactor reinforce the magnetic confinement approach, although it’s still not certain if tokamaks or stellarators will be the ultimate winner in the race for controlled nuclear fusion. Plasma physicist Robert Wolf, who heads the optimization of the Wendelstein reactor, thinks future fusion reactors might somehow combine the stability of stellarators with the relative simplicity of tokamaks, but it’s not clear how: “From a scientific view, it is still a bit early to say.”
Several private companies have joined the fusion race. One of the most advanced projects is from the Canadian firm General Fusion, which is based near Vancouver in British Columbia. The company hopes its unorthodox fusion reactor, which uses a hybrid technology called magnetized target fusion, or MTF, will be the first to feed electric power to the grid by the “early to mid-2030s,” according to its chief strategy officer Megan Wilson. “MTF is the fusion equivalent of a diesel engine: practical, durable and cost-effective,” she says.
University of California, San Diego, nuclear engineer George Tynan says private money is flooding the field: “The private sector is now putting in much more money than governments, so that might change things,” he says. “In these ‘hard tech’ problems, like space travel and so on, the private sector seems to be more willing to take more risk.”
Tynan also cites Commonwealth Fusion Systems, a Massachusetts Institute of Technology spin-off that plans to build a fusion power plant called ARC in Virginia. The proposed ARC reactor is a type of compact tokamak that intends to start producing up to 400 megawatts of electricity—enough to power about 150,000 homes—in the “early 2030s,” according to a MIT News article.
Roulstone thinks the superconducting electromagnets increasingly used in magnetic confinement reactors will prove to be a key technology. Such magnets are cooled with liquid helium to a few degrees above absolute zero so that they have no electrical resistance. The magnetic fields they create in that state are many times more powerful than those created by regular electromagnets, so they give researchers greater control over superheated hydrogen plasmas. In contrast, Roulstone fears the NIF’s laser approach to fusion may be too complicated: “I am a skeptic about whether inertial confinement will work,” he says.
Tynan, too, is cautious about inertial confinement fusion, although he recognizes that NIF’s fusion ignition was a scientific breakthrough: “it demonstrates that one can produce net energy gain from a fusion reaction.”
He sees “viable physics” in both the magnet and laser approaches to nuclear fusion but warns that both ideas still face many years of experimentation and testing before they can be used to generate electricity. “Both approaches still have significant engineering challenges,” Tynan says. “I think it is plausible that both can work, but they both have a long way to go.”