“Things are different now”

After decades of promise, the quest for viable fusion energy using stellarators is accelerating thanks to growing investment, advanced computer modelling, ambitious start-ups, calls for a public test facility in the United States, and an ITER initiative to share its expertise with private companies.

“I have zero doubt that we can make commercial fusion work with stellarators,” says Lucio Milanese, the co-founder and COO of Proxima Fusion, a European start-up that spun out from the Max Planck Institute for Plasma Physics and focuses on a type of stellarator known as quasi-isodynamic (QI). “There have always been engineering challenges, but things are different now because so many new solutions are possible.”

In the world of fusion devices, tokamaks such as ITER’s and the stellarators preferred by facilities like Wendelstein 7-X in Germany are considered cousins because they both use magnetic confinement to control the hot plasma. The key difference is that tokamaks confine the plasma through a combination of magnetic fields and induced current in the plasma, while stellarators rely on magnetic confinement alone using a twisting chamber and magnets to produce the required helical shape.

While tokamaks have held centre stage in recent decades due to landmark projects such as the Joint European Torus (JET) and ITER, the long-hailed renaissance in stellarator technology appears to have arrived. This is partly due to successful testing at Wendelstein 7-X, which showed that the energy losses that plagued earlier stellarators had been overcome.

There have also been breakthroughs concerning the complex shape of the stellarator chamber. First, computer modelling has allowed for greater magnet tolerances, which eases assembly requirements. Then, the design of the magnetic confinement region has been improved to achieve more precise symmetries. And, finally, a group of companies is working to simplify the stellarator so it has planar or cylindrical surfaces and is thus easier and cheaper to manufacture.

These various advances have inspired an investment boom. According to the sustainability consultancy Cleantech Group, in 2023 and early 2024 private-sector stellarator companies made the most venture capital deals. Meanwhile, in its 2024 annual report, the Fusion Industry Association noted that among the 45 private-sector fusion companies surveyed, stellarators are now the most common technology, with eight companies using stellarators followed by seven laser-driven inertial confinement projects and a combined six tokamak projects (traditional and spherical).

Stellarator momentum received a further boost in May 2024 when the ITER Organization convened a Private Sector Fusion Workshop to lay the groundwork for greater information sharing with fusion companies. A special panel was devoted to stellarators and featured six leading companies – Type One Energy, Renaissance Fusion, Thea Energy, Gauss Fusion, Helical Fusion, and Proxima Fusion – as well as the Max Planck Institute of Plasma Physics (IPP), which operates Wendelstein 7-X. ITER knowledge sharing makes particular sense to stellarator companies because progress can be mutually beneficial as there is so much overlapping technology. In a recent analysis of stellarator technology published for the World Economic Forum, a parallel was drawn with electric and traditional cars. Regardless of the specific magnetic confinement method, key elements such as breeder blankets or divertors are largely the same on tokamaks and stellarators, just as steering or braking technology can be applied to both electric and combustion engine cars. 

These commonalities are why a company like Gauss Fusion, which was founded by industrial companies that have provided components for ITER and which aims to build a fusion power plant having a stellarator at its heart, is relying on the information-sharing initiative. “ITER can provide expertise in fusion technologies that are cross-compatible with stellarators,” says Richard Kembleton, the Chief Scientific Officer at Gauss Fusion, who previously worked for EUROfusion on DEMO, the European fusion project. “The lessons learned at ITER in terms of development, procurement, and project management are also vital resources.”
 

The optimism around stellarators was further in evidence this summer when a group of two dozen fusion scientists published a paper calling for a public stellarator program in the United States—the Flexible Stellarator Physics Facility. Led by Felix Parra Diaz of the Princeton Plasma Physics Laboratory, the scientists argue that testing stellarator technology at scale will prove the technology can help the United States meet the clean energy goals outlined in its Decadal Vision for Commercial Fusion Energy. This proposal has been applauded by the American stellarator ecosystem, including Thea Energy, which is one of the companies seeking to simplify stellarators through planar magnet designs.

“The call for such a device by the broader fusion research community is no doubt due to the excitement generated by the important new confinement results from Wendelstein 7-X,” notes David Gates, co-founder and CTO of Thea Energy. “We hope the stellarator community is successful in their endeavour to create this facility and are eager to engage with them to make this important vision a reality.”

Of course, stellarators still face the same challenge as the broader fusion sector: money. Billions in investments are needed to fund the private-sector stellarator projects and the proposed public test facility. Indeed, Francesco Volpe, the founder and CEO of the French stellarator company Renaissance Fusion, says stellarator technology must be made more affordable for it to fulfil its promise. Advances in general fusion technology, like high-temperature superconductors (HTS) that reduce the cost of magnetic confinement and liquid metals that prolong the life of key components are a step forward. But Volpe believes approaches that are specific to stellarators, such as the simplified design that Renaissance is working on with a cylindrical chamber that has HTS coating that can be shaped through engraving, could also cut manufacturing costs.

“We need a paradigm shift,” says Volpe. “We understand physics well enough to start building the next stellarator, but the risk is that nobody will buy it as a power plant. Thus, we need ingenuity to reconcile this beautiful physics with pragmatic engineering and competitive economics.” 

This confluence of developments may indeed lead to the long-awaited breakthrough for stellarators. And if it does, the broader fusion movement could receive a lift. “A watershed moment will lead to a fusion boom for both tokamaks and stellarators,” predicts Lucio Milanese of Proxima Fusion. “We’re basically cousins, if not brothers.”