Renaissance Fusion Targets Cost-Competitive Fusion

Renaissance Fusion, the French stellarator startup based in Grenoble, quietly replaced its founder-physicist CEO last October. Francesco Volpe, who spent two decades working on stellarators at Princeton and the Max Planck Institute before founding the company in 2020, stepped down. His replacement: Sam Guilaumé, a semiconductor industry executive with no apparent background in plasma physics. That transition tells you more about where Renaissance Fusion actually is than any press release.

The company has been billing itself as the only fusion startup pursuing "economic fusion by design" — a machine architected from the ground up to produce electricity cheaper than fossil fuels, rather than one that might eventually be optimized into competitiveness. It is an audacious claim. It is also, so far, unproven. No peer-reviewed paper from Renaissance Fusion has demonstrated that its specific design can achieve the fusion gains required, let alone the economic thresholds. Proxima Fusion, a German-Munich spinout from the Max Planck Institute, and Type One Energy, a U.S. company working from MIT technology, have both published stellarator blueprints in peer-reviewed journals. Renaissance Fusion has not.

What Renaissance Fusion does have is a real technical approach, and it is genuinely different from the mainstream tokamak track that companies like Commonwealth Fusion Systems and ITER are building. The company proposes two interlocking innovations: a liquid lithium wall to handle the heat flux from the plasma, and 3D-printed high-temperature superconducting (HTS) magnets produced via a laser deposition process on rotating tubes. According to Lowercarbon Capital, which participated in the company's €32 million Series A1 round in April 2025, the magnet printing technique makes HTS coils seven times faster to produce and at a fraction of the cost of conventional winding. If that claim holds at scale, it is significant — magnet manufacturing cost and consistency have been a persistent bottleneck for compact fusion.

The liquid lithium wall is the more speculative element. The concept is sound in principle: a flowing liquid metal layer protects the structural wall from the plasma's radiative heat, eliminating the neutron damage and thermal stress that require solid first-wall components to be replaced on a tokamak. The problem is temperature. Renaissance Fusion's design requires liquid lithium circulating at 850°C. Most competing fusion approaches either run colder or don't require a liquid metal boundary at all. In November 2025, the company demonstrated levitation of liquid tin in an HTS magnetic field at 20 kelvin — a useful engineering milestone, but tin is not lithium, and 20 K is not 850°C. The leap to a hot liquid lithium wall that can be integrated into a stellarator geometry and survive sustained neutron bombardment has not been demonstrated by anyone, including Renaissance Fusion.

The company is aware of the gap. Volpe, who remains as president, has described the 850°C lithium as a target rather than an achievement. The October CEO change is consistent with a company moving from physics R&D toward engineering integration — a phase that tends to reward different skills than building a research program. Guilaumé's background is not public in detail, but the pattern matches a well-worn Silicon Valley script: founder-scientist steps back, operational executive enters to build toward a demo.

The demonstrator is the only test that matters. Renaissance Fusion expects to have an integrated machine — combining tubular HTS magnets with the liquid lithium wall — ready by the end of 2026. That machine will not produce fusion energy. It will test whether the magnet system and the liquid metal boundary can operate together in a stellarator geometry under fusion-relevant conditions. If it works, the path to a net-energy stellarator in the early 2030s becomes slightly more credible. If it doesn't, the company will have to explain why the physics that looked clean on paper didn't survive contact with reality.

Fusion startups have drawn more than $10 billion in investment across the sector, and the field has never been better funded or more crowded. Several approaches — tokamaks, inertial confinement, field-reversed configurations — are all advancing in parallel. Stellarators have a structural advantage in steady-state operation but a structural disadvantage in complexity: the plasma geometry that makes them stable also makes them extraordinarily difficult to build and tune. Renaissance Fusion's specific bet on liquid lithium at high temperature adds another layer of engineering risk on top of an already demanding design.

The honest summary: Renaissance Fusion has a coherent physics case, a credible team (Volpe's credentials include the 2003 Otto Hahn Medal for his PhD on the Wendelstein 7-AS stellarator and a 2011 DOE Early Career Award for first stabilization of a locked plasma mode), and real funding. It does not yet have a demonstrated path to 850°C liquid lithium, a validated HTS magnet geometry, or an integrated stellarator plasma. The October CEO change is a signal that the company itself may be making that same assessment. We'll find out what it's worth by the end of next year.