Sophia Henneberg: Championing the Stellarator's Comeback in Fusion Energy

Fusion energy has long been dominated by tokamaks, but MIT's Sophia Henneberg is betting on a different approach. As the Norman Rasmussen Career Development Assistant Professor in the Department of Nuclear Science and Engineering, Henneberg is working to advance stellarator technology—a twist on the traditional fusion reactor design that could offer greater stability and efficiency.

Growing up in central Germany, Henneberg found herself drawn to both physics and mathematics, eventually choosing physics for its broader applications. Her path to fusion research began during her undergraduate studies at Goethe University in Frankfurt, where she discovered plasma physics. "Most of the visible material in the universe is in the form of hot, ionized gas called plasma, so studying that is really fundamental," she explains. "And there's this amazing application, fusion, which has the potential to become an unlimited energy source."

The Stellarator Advantage

Both tokamaks and stellarators use magnetic confinement to compress plasma and trigger fusion, but their designs differ significantly. While tokamaks create a simple donut shape, stellarators produce a twisted donut configuration. This twist, Henneberg discovered during her PhD work at the University of York, could make stellarators inherently more stable than tokamaks.

In tokamaks, plasma temperatures can exceed 100 million degrees Celsius, and the extreme conditions can lead to sudden energy surges that disrupt the fusion process. Stellarators, by contrast, can be designed to minimize these instabilities through careful optimization of their magnetic field configurations.

Optimization Breakthroughs

After joining the Max Planck Institute for Plasma Physics in Greifswald, Germany, in 2016, Henneberg focused on stellarator optimization. Her work there coincided with the operation of Wendelstein 7-X, the world's most advanced stellarator. One of her key innovations involves designing both the plasma boundary and the superconducting coils simultaneously, rather than as separate steps.

"We've now reached the point where stellarator performances can exceed those of tokamaks, because we're able to optimize them very well, but you have to put the effort in," Henneberg says. "You can't get good performance out of just any twisty donut."

The Hybrid Future

In a 2024 paper co-authored with Gabriel Plunk, Henneberg introduced the concept of a stellarator-tokamak hybrid reactor. The goal is straightforward: combine the strengths of both designs into a single device that outperforms either alone. One particularly promising approach involves converting existing tokamaks into stellarators by adding just a few specialized coils that can be turned on or off.

This hybrid approach could serve as a bridge for the tokamak community to explore stellarator benefits without completely abandoning existing infrastructure. At least one university has already secured funding to build such a hybrid reactor.

Growing Commercial Interest

Interest in stellarators has been steadily increasing, with Henneberg noting that most new fusion startups are now focusing on stellarator designs. Companies like Type One Energy and Thea Energy in the United States, and Proxima Fusion and Gauss Fusion in Germany, are pursuing commercial stellarator applications.

However, Henneberg cautions that technical challenges remain. "It can seem like the technical issues involved in fusion are already solved, but there are still many interesting open questions," she says. Her current work focuses on improving both the physics and economic feasibility of stellarator designs.

Training the Next Generation

At MIT, Henneberg is not only advancing research but also training future stellarator experts. She co-taught the renowned Fusion Design (22.63) course with Professor Dennis Whyte, where students compared stellarator designs with magnetic mirror machines. The course has historically led to published papers and even inspired company creation.

Henneberg praises her MIT students as "highly motivated and a lot of fun to work with," and she's confident her research group will soon make significant progress. She's also excited to be part of MIT's Plasma Science and Fusion Center, where she can collaborate with tokamak experts while pursuing her stellarator research.

As fusion energy moves closer to commercial viability, Henneberg's work on stellarators could prove crucial. By optimizing these complex machines and exploring hybrid designs, she's helping to ensure that fusion's promising underdog gets the chance to compete on equal footing with traditional tokamaks.