How a Compact Fusion Reactor Tames Star‑Hot Plasma

Nuclear fusion reactors offer the hope of vast, clean energy from the same process that powers stars. But despite decades of research, a fusion reactor that can supply practical amounts of power has proven elusive. Now startup Commonwealth Fusion Systems has revealed in depth what it says is the most complex aspect of the reactor it is constructing—the way the reactor controls the plasma responsible for generating power.

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High-Temperature Superconductor Magnets

Managing Plasma Disruptions in Tokamaks

The company says its findings support its vision—a reactor that can generate 1.1 gigawatts of fusion power and deliver 400 megawatts of net electricity to the grid. “That can power about 280,000 average American homes for a year, all using an amount of fuel you could deliver in a pickup truck,” says Brandon Sorbom, cofounder and chief science officer of Commonwealth Fusion Systems (CFS) in Devens, Mass.

The ARC (affordable, robust, compact) fusion reactor that CFS is developing is a tokamak. This is essentially a doughnut-shaped bottle that magnetically traps plasma at pressures and temperatures high enough to force atomic nuclei to fuse. A fraction of the mass of these atoms gets converted into energy. “We’re basically creating a miniature star,” Sorbom says.

High-Temperature Superconductor Magnets

The key innovation of the ARC reactor is the use of high-temperature superconductor (HTSC) magnets instead of typical superconducting magnets, which require frigid temperatures near absolute zero to work. Although HTSCs still require temperatures in the range of about 20 to 77 kelvins (-200 to -250 °C), the relative warmth in which they operate means they require dramatically less cooling equipment. This makes ARC significantly more compact and simple than previous fusion reactor designs, such as the International Thermonuclear Experimental Reactor (ITER).

The fusion reactions generate neutrons, whose energy heats a continuously flowing loop of molten salt around the reactor’s magnetic bottle. This blanket of molten salt then heats a fluid to drive a turbine that generates electricity.

CFS researchers collaborated with scientists at MIT, Columbia, the Max Planck Institute for Plasma Physics and other institutions around the world to describe the scientific underpinnings of the ARC reactor. They detailed their research in five peer-reviewed studies published today in the Journal of Plasma Physics.

“We demonstrate that the ARC power plant has a solid foundation in physics,” Sorbom says. “The papers confirm that when we build the ARC fusion power plant, it will work.”

Roughly two-thirds of the 58 authors of the studies come from outside CFS. “These papers are not just the stamp of our validation, but that of the global fusion-science community,” Sorbom says. “And then they underwent peer review from more institutions for independent checks to make sure all our calculations were correct.”