For decades, if you asked a fusion scientist to imagine a fusion reactor, he would probably tell you about a tokamak. It’s a chamber the size of a large room, shaped like a hollow doughnut. Physicists fill its insides with a not particularly tasty jam of superheated plasma. Then they surround it with magnets, hoping to smash the atoms together to create energy, just like the sun does.
But experts believe that you can make tokamaks in other shapes. Some believe that making tokamaks smaller and thinner could make them better at handling plasma. If the fusion scientists who propose it are right, then it could be a long-awaited improvement for nuclear power. Thanks to recent research and a recently proposed reactor design, the field is seriously considering generating electricity with a “spherical tokamak.”
“The indication from the experiments so far is this [spherical tokamaks] it can, pound for pound, confine the plasma better and therefore make better fusion reactors,” says Stephen Cowley, director of Princeton’s Plasma Physics Laboratory.
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If you’re wondering how fusion power works, it’s the same process the sun uses to generate heat and light. If you can push certain types of hydrogen atoms past the electromagnetic forces that keep them apart and smash them together, you get helium and a lot of energy – with virtually no pollution or carbon emissions.
Sounds wonderful. The problem is that in order to force the atoms to fuse and for said reaction to occur, you need to reach sky temperatures of millions of degrees for extended periods of time. That’s a tough metric, and it’s one reason the holy grail of fusion—a reaction that generates more energy than you put into it, aka profitability and profit—remains elusive.
The tokamak is in theory one way to get there. The idea is that by carefully sculpting the plasma with powerful electromagnets that line the donut’s shell, fusion scientists can sustain this super-hot reaction. But tokamaks have been in use since the 1950s, and despite continued optimism, they have never been able to shape plasma the way they need to to fulfill their promise.
But there is another way to create fusion outside of a tokamak called inertial fusion (ICF). To do this, you take a grain-of-sand-sized hydrogen pellet, place it in a special container, blast it with laser beams, and let the resulting shock waves rip apart the pellet’s interior in sudden fusion. Last year, the ICF reactor in California came closer than any other to this energy milestone. Unfortunately, in the year since, physicists have been unable to make lightning happen again.
Stories like this show that if there is an alternative method, researchers will not hesitate to use it.
The idea of shrinking the tokamak emerged in the 1980s, when theoretical physicists, followed by computer simulations, suggested a more compact form could handle plasma more efficiently than a traditional tokamak.
Before long, groups from the Culham Center for Fusion Energy in the United Kingdom and Princeton University in New Jersey began testing the design. “The results were almost instantly very good,” Cowley says. That’s not something physicists can say with every new camera design.
Despite the name, a spherical tokamak is not a true sphere: it is more like an unshelled peanut. Proponents believe this form gives it several key advantages. The smaller size allows the magnets to be placed closer to the plasma, reducing the energy (and cost) required to actually power them. The plasma also tends to act more stably in a spherical tokamak during the reaction.
But there are also disadvantages. In a standard tokamak, the donut hole in the middle of the chamber contains some of those important electromagnets, along with the wiring and components needed to power the magnets and maintain them. Reducing the size of the tokamak reduces this space to something like the core of an apple, meaning the accessories must be miniaturized to match. “The technology of being able to get everything through the narrow hole in the middle is a pretty tough job,” Cowley says. “We had some false starts on that.”
In addition to mounting issues, placing these components closer to the sky-hot plasma causes them to wear out more quickly. In the background, researchers are creating new components to solve these problems. At Princeton, a group has shrunk these magnets and wrapped them in special wires that lack conventional insulation—which would have to be specially machined in an expensive and error-prone process to fit the harsh conditions of fusion reactors. This development does not solve all problems, but it is a gradual step.
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Others dream of going even further. The world of experimental tokamaks is currently gearing up for ITER, a record-capacity test reactor that has been in the works since the 1980s and will finally finish construction in southern France this decade. It will hopefully pave the way for viable fusion power by 2040.
Meanwhile, fusion scientists are already designing something very similar in Britain with the Spherical Energy Production Tokamak, or STEP. The chamber isn’t close to completion—the most optimistic plans won’t start construction until the mid-2030s and start generating power until around 2040—but it’s an indication that engineers are taking the spherical tokamak design quite seriously.
“One of the things we always have to do is ask ourselves, ‘If I were to build a reactor today, what would I build?'” says Cowley. Spherical tokamaks, he believes, are starting to enter that equation.