For decades, if you asked a fusion scientist to picture a fusion reactor, they would probably tell you about a tokamak. It is a chamber the size of a large room, shaped like a hollow doughnut. Physicists fill its interior with a not-so-tasty jam of superheated plasma. They then surround it with magnets in the hope of crushing the atoms together to create energy, just like the sun does.
But experts believe 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 upgrade for nuclear power. Thanks to recent research and a recently proposed reactor project, 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 plasmas better and therefore make better fusion reactors,” says Stephen Cowley, director of the Princeton Plasma Physics Laboratory.
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If you’re wondering how fusion power works, it’s the same process the sun uses to produce heat and light. If you can push certain types of hydrogen atoms past the electromagnetic forces that keep them apart and squish them together, you get helium and lot of energy—with almost no pollution or carbon emissions.
It sounds great. The problem is, to force the atoms together and for said reaction to take place, you need to achieve sky temperatures of millions of degrees for extended periods of time. That’s a difficult benchmark, and it’s one reason why the holy grail of fusion—a reaction that produces more energy than you put into it, also known as breakeven and gain—remains elusive.
The tokamak, in theory, is one way to get there. The idea is that by carefully sculpting the plasma with powerful electromagnets lining the donut shell, fusion scientists can keep this superheated reaction going. But tokamaks have been in use since the 1950s, and despite continued optimism, they have never been able to shape the plasma the way they should to fulfill their promise.
But there is another way to create fusion outside of a tokamak, called inertial confinement fusion (ICF). To do this, you take a grain-of-sand-sized hydrogen pellet, place it inside a special container, blast it with laser beams, and let the resulting shock waves perturb the inside of the pellet into a starting melt. Last year, an ICF reactor in California came closer than any other to this energy milestone. Unfortunately, since then, physicists have not been able to replicate the glow.
Stories like this show that if there is an alternative method, researchers will not hesitate to use it.
The idea of cutting the tokamak arose in the 1980s, when theoretical physicists—followed by computer simulations—suggested that a more compact shape could handle plasma more efficiently than a traditional tokamak.
Shortly thereafter, teams at the Culham Center for Fusion Energy in the United Kingdom and Princeton University in New Jersey began testing the design. “The results were almost instantaneous and very good,” says Cowley. That’s not something physicists can say with every new chamber design.
Despite the name, a spherical tokamak is not a true sphere: It looks more like a peanut without the shell. This shape, proponents believe, gives it some 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 throughout the reaction.
But there are also disadvantages. In a typical tokamak, the donut hole in the middle of the chamber contains some of these important electromagnets, along with the wiring and components needed to power the magnets and support them. Shrinking the size of the tokamak reduces this space to something like the core of an apple, which means accessories must be miniaturized to fit. “The technology of being able to fit everything down the narrow hole in the middle is quite a difficult job,” says Cowley. “We had some false starts in this one.”
In addition to placement issues, placing these components closer to the celestial hot plasma tends to wear them out faster. In the background, researchers are building new components to solve these problems. At Princeton, a team has shrunk these magnets and wrapped them with special wires that lack conventional insulation—which would have to be specially treated 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 development since the 1980s and will finally complete construction in southern France this decade. Hopefully it will pave the way for sustainable fusion power by the 2040s.
Meanwhile, fusion scientists are already planning something very similar in Britain with a Spherical Tokamak for Energy Production, or STEP. The chamber is nowhere near completion—the most optimistic plans won’t begin construction until the mid-2030s, and it won’t start generating power until around 2040—but it’s a sign that engineers are taking the spherical tokamak design very 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 thinks, are starting to enter that equation.