MIT Validates Science Behind New Nuclear Fusion Reactor Design

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MIT Validates Science Behind New Nuclear Fusion Reactor Design

Research shows Commonwealth Fusion Systems prototype should work, but huge engineering challenges remain.

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CFS’s Sparc machine could pave the way for the world’s first commercial fusion reactor. (Credit: CFS/MIT-PSFC; CAD rendering by T. Henderson)

A proposed fusion reactor design by Commonwealth Fusion Systems has received a boost after its underlying scientific principles were validated by peer review.

CFS, one of two fusion hopefuls that have attracted oil industry interest, published seven papers last month in the Journal of Plasma Physics that appeared to confirm that the company’s theoretical approach could yield real-life results, CEO Bob Mumgaard said in an interview.

The papers summarize about two years of work, including research into using new magnets to contain a fusion plasma — one of the critical challenges in trying to harness the power that fuels the sun and the stars.

“This set of papers pulls together in one spot, for the first time, the physics picture,” Mumgaard said. “There is a solution that pops out of all this: If you have these magnets, you could build much smaller fusion machines than we previously thought.”

CFS, which is a spinoff out of the Massachusetts Institute of Technology, is working with MIT on a prototype reactor called Sparc. It is planned to be the first compact fusion system capable of delivering a net energy gain, generating 50 megawatts from a 25 MW input.

Being able to get more energy out of the system than what is required to create and sustain the plasma will be a significant step in the path toward commercial fusion reactors. But the most pressing objective now is to prove out the new magnet, Mumgaard said.

“We’re in the thick of building a magnet that’s a hundred times beyond anything that’s been done before,” he said.


CFS hopes to start testing the magnets and begin construction on Sparc next year. If Sparc is successful, CFS aims to develop commercial reactors with an electric power output of around 200 megawatts, small enough to fit into a gymnasium.

The ability to contain a plasma in such a small area is potentially one of the keys to producing fusion energy in a cost-effective way.

Martin Greenwald, senior scientist and deputy director of MIT's Plasma Science and Fusion Center, told GTM that commercial fusion reactors would need to achieve plasma temperatures of up to 200 million degrees Celsius, which is hotter than the sun.

The only way to contain such plasmas is through magnetic fields that shield the plasma from ordinary matter. And the stronger the magnetic field, the better it works, he said.

“The quality of the thermal insulation increases as you increase the magnetic field strength,” he said. “You double the magnetic field [and] you can cut the linear size in half, which means the volume goes down by a factor of eight. And most of the costs scale with the weight.”

CFS is working on high-temperature superconducting magnets that could perform as well as those at the International Thermonuclear Experimental Reactor in France but are 10 times smaller and can be built on a significantly faster timeline, according to a press release.

The peer-reviewed papers tell researchers that it’s very likely to work, Greenwald said. But even if the science is sound, CFS still faces major engineering hurdles in creating a commercial fusion reactor. “In the end, you don’t make fusion with pencil and paper,” said Greenwald.


“It’s going to take steel and concrete,” he said. “We don’t want to appear complacent. Nature can still surprise us.”

Ironically, one of the challenges ahead will be to make sure CFS reactors do not produce too much energy. While Sparc is intended to produce twice as much power as it consumes, the theoretical basis for the machine predicts a potential output of up to 10 times the input.

Mumgaard said the CFS team still hasn’t figured out how they are going to extract the energy from a fusion reactor once it manages to create a surplus. This is likely to be a challenge for any proposed fusion machine since the excess heat that could drive a turbine could also threaten to melt the magnets holding the plasma.

As it stands, though, the promise of commercial fusion energy looks closer today than ever before. And even if the CFS approach ends up a dud, there are plenty more hopefuls lining up to have a go at the challenge. “We think the issue is too important for just one shot on goal,” said Greenwald.