For decades, if you asked a fusion scientist to imagine a fusion reactor, they would probably tell you about a tokamak. It’s a chamber, about the size of a large room, shaped like a hollow donut. Physicists fill its insides with a not-so-tasty jam of superheated plasma. Then they surround it with magnets in hopes of squeezing atoms together to create energy, just like the sun.
But experts believe that tokamaks can be made in other shapes. Some believe that smaller and slimmer tokamaks could make them better at handling plasma. If the fusion scientists proposing it are right, it could be a long-awaited upgrade to nuclear power. Thanks to recent research and a newly proposed reactor project, the field is seriously considering generating electricity with a “spherical tokamak.”
“The previous experiments show that [spherical tokamaks] pound for pound can confine plasmas better and thereby make better fusion reactors,” says Steven Cowley, director of the Princeton Plasma Physics Laboratory.
[Related: Physicists want to create energy like stars do. These two ways are their best shot.]
If you’re wondering how fusion power works, it’s the same process the sun uses to create heat and light. If you can push certain types of hydrogen atoms past the electromagnetic forces that keep them apart and push them together, you get helium and a a lot of of energy with virtually no pollution or carbon emissions.
It sounds wonderful. The problem is that in order to force atoms together and cause this reaction, you have to reach heavenly temperatures of millions of degrees for long periods of time. It’s a difficult yardstick, and it’s one of the reasons why the holy grail of fusion — a reaction that produces more energy than you put into it, aka breakeven and gain — remains elusive.
The tokamak is theoretically one way to reach it. The idea is that by carefully shaping the plasma with powerful electromagnets lining the donut’s shell, fusion scientists can keep this super-hot reaction going. But tokamaks have been used since the 1950s, and despite continued optimism, they’ve never been able to shape the plasma in the way they needed to deliver on their promise.
But there is another way to generate fusion outside of a tokamak, called inertial confinement fusion (ICF). To do this, you take a hydrogen pellet the size of a grain of sand, place it in a special container, irradiate it with laser beams and let the resulting shock waves ruffle the inside of the pellet to start fusion. Last year, an ICF reactor in California came closer to that energy milestone than anyone before. Unfortunately, the year after, the physicists did not succeed in generating the flash again.
Stories like this show that researchers don’t hesitate to jump in when there’s an alternative method.
The idea of making the tokamak smaller arose in the 1980s, when theoretical physicists – followed by computer simulations – suggested that a more compact shape could handle the plasma more effectively than a traditional tokamak.
Not long after, groups at the Culham Center for Fusion Energy in the UK and Princeton University in New Jersey began testing the design. “The results were very good almost immediately,” says Cowley. Physicists can’t say that about every new chamber design.
Despite the name, a spherical tokamak is not a true sphere: it more closely resembles an unshelled peanut. This form, proponents think, gives it some key advantages. The smaller size allows the magnets to be placed closer to the plasma, reducing the energy (and cost) needed to actually power them. Plasma also tends to appear more stable throughout the reaction in a spherical tokamak.
But there are also disadvantages. In a standard tokamak, the donut hole in the center of the chamber contains some of these vital electromagnets, along with the wiring and components needed to power and support the magnets. By shrinking the size of the tokamak, that space is reduced to something like an apple seed, which means the accessories have to be miniaturized accordingly. “The technology of getting everything through the small hole in the middle is pretty hard work,” says Cowley. “We had a few false starts there.”
In addition to the adjustment issues, these components tend to wear out faster when placed closer to the heavenly hot plasma. In the background, researchers are making new components to solve these problems. At Princeton, a group shrunk these magnets and wrapped them in special wires that lack traditional insulation — which would need special treatment to withstand the harsh conditions of fusion reactors, in an expensive and error-prone process. This development does not solve all problems, but it is an incremental step.
[Related: At NYC’s biggest power plant, a switch to clean energy will help a neighborhood breathe easier]
Others dream of going even further. The world of experimental tokamaks is preparing for ITER, a record-breaking capacity test reactor that has been underway since the 1980s and will finally be built in southern France this decade. It will hopefully pave the way for viable fusion power by the 2040s.
Meanwhile, fusion scientists in the UK are already designing something very similar using a spherical power generation tokamak, or STEP. The chamber is far from complete — the most optimistic plans call for it to begin construction no earlier than the mid-2030s and to generate power until about 2040 — but it’s an indication that engineers are taking the spherical tokamak design very seriously take seriously.
“We have to keep asking ourselves, ‘If I were to build a reactor today, what would I build?'” says Cowley. Spherical tokamaks, he thinks, are beginning to enter this equation.