Proxima fusion: a start-up on the home stretch to fusion power

Proxima Fusion leverages the agility of a start-up to advance its stellarator concept toward power plant maturity

A fusion power plant promises virtually inexhaustible and clean energy. Numerous research institutions and startups around the world are working to make this a reality. One of them is Proxima Fusion. It was spun out from the Max Planck Institute for Plasma Physics and is based on a concept that was substantially developed there. Its goal is to put a commercial fusion power plant on the grid within the 2030s.

Text: Andreas Merian

To the point

• Proxima Fusion is constructing a fusion reactor based on the stellarator concept. It aims to be relatively simple and cost effective to build thanks to its design, which is more compact than previous concepts.
• Powerful magnetic coils: To contain the fusion plasma in a more compact reactor, highly powerful magnetic coils made from novel high-temperature superconductors are required. Proxima Fusion is developing these in collaboration with the Paul Scherrer Institute.
• The Munich-based startup plans to begin construction of the Alpha demonstration facility in 2027, which is intended to show in 2031 that it can generate more energy than is put into it.

Moonshot: Since the successful landing of a US astronaut on the moon, the term has come to represent a challenging and innovative endeavor with ambitious goals. It is only fitting that the founding team of the startup Proxima Fusion refers to its project as a moonshot—because its goal is to develop a fusion power plant and thereby provide virtually inexhaustible clean energy. Jorrit Lion, chief scientist and one of the founders of Proxima Fusion, completed his doctoral thesis at the Max Planck Institute for Plasma Physics in Greifswald, where he researched how the reactor concept pursued there could be developed into a power plant. It was during his time at the Institute that he met most of his future co-founders. Together, they conceived the idea of advancing the goal of a fusion power plant more quickly via a startup. “Germany’s basic research on nuclear fusion has achieved remarkable results for decades. Now, for the development of a commercial, electricity-generating power plant, a startup is the right environment,” Lion explains. A company can focus on the technical and economic aspects crucial for building a practical fusion reactor. With this in mind, Lion and four other scientists and engineers decided to found Proxima Fusion in 2023

Lion has since moved out of the Institute in Greifswald into a modern, open office space in Munich that exudes startup spirit. The decision to base the company in Munich was driven by several factors. The city has a thriving startup scene and offers young companies the necessary support through innovation centers, and proximity to both public and private investors. Additionally, Munich is also home to a branch of the Max Planck Institute for Plasma Physics, specifically its Garching location. The city’s two major universities serve as both potential research partners and sources of highly qualified talent. When it comes to the job market, Munich is also attractive to international talent—an important factor for Proxima Fusion, as it aims to recruit the brightest minds for its mission. Today, the startup employs 75 people across three locations: Munich, Villigen in Switzerland, and Oxford in England. 

To bring their project to life, the team has already raised over EUR 60 million—half from public funding and the other half from private investors. With this initial capital, the founders aim to prove that their approach to constructing a fusion reactor can be successful. “If we achieve this, the next step will be to secure the necessary funding for Alpha, our demonstrator,” says Lion. Proxima Fusion has set an intentionally ambitious timeline: construction of Alpha is set to begin in 2027, and by 2031, the team aims to show that the demonstrator produces more energy than it consumes. Many other companies are working toward the goal of building a commercial fusion reactor. The most promising project with the shortest timeline currently comes from the United States: Commonwealth Fusion Systems has already secured around USD 2 billion in funding and began constructing its demonstrator, Sparc, at the end of 2021. The company aims to achieve net energy with Sparc as early as 2027, though whether it can overcome all technical challenges by then remains to be seen.

History shows that skepticism is warranted when it comes to fusion research. Since the end of World War II, scientists have been working on harnessing nuclear fusion for electricity generation. Yet, despite all efforts, no research team has been able to build a fusion reactor with a net energy gain. Fusion energy is generated when light atomic nuclei merge. However, this only occurs under extreme pressure and at extremely high temperatures, as seen in the Sun. There, hydrogen nuclei fuse into helium under a pressure of around 200 billion bar and temperatures of approximately 15 million degrees Celsius. Under such conditions, matter exists as plasma, meaning that electrons and positively charged atomic nuclei are no longer bound together

On Earth, the most technically feasible fusion reaction involves heavy and super-heavy hydrogen—better known as deuterium and tritium. However, no research team has yet succeeded in extracting more energy from hydrogen plasma than is put into generating it (see MaxPlanckResearch 4/2022). One way to create the conditions necessary for nuclear fusion is to confine plasma within a toroidal magnetic field. This prevents plasma from contacting the reactor walls, as such contact would cool the plasma and cause the fusion reaction to collapse. Tokamak and stellarator reactors both rely on magnetic confinement to minimize plasma–wall interactions and maintain stable plasma. A tokamak is a donut-shaped vessel that is relatively easy to construct but cannot sustain fusion indefinitely. While a tokamak can be used in pulsed operations, this places strain on the system due to constant cycling between shutdown and restart. The large international fusion reactor ITER and Commonwealth Fusion Systems’ demonstrator Sparc are both based on the tokamak principle.

In a stellarator, the plasma vessel and magnetic field resemble a twisted roll of pastry rather than a simple donut shape. This complex geometry makes a stellarator more difficult to construct than a tokamak. For a long time, optimizing a stellarator’s magnetic field was hindered by a lack of crucial physical insights and insufficient computing power. However, since the 1980s, sufficiently precise calculations have become possible, enabling the Max Planck Institute for Plasma Physics to develop the modern stellarator concept with its Wendelstein experiments. Stellarators offer significant advantages for power plant operation: fusion is easier to control and can be maintained continuously. For these reasons, Proxima Fusion has chosen the stellarator concept for its approach

A first fusion power plant 2030s

In both reactor types, the necessary temperature for fusion is primarily achieved using microwave radiation. When conditions are right, the nuclei of deuterium and tritium fuse, producing a helium nucleus and a neutron, both carrying significant kinetic energy. The uncharged neutron is not confined by the magnetic field, allowing it to collide with the reactor wall at full force. The resulting heat is then used for electricity generation, similar to a conventional power plant. Fusion-based electricity produces no greenhouse gases or harmful byproducts. The only radioactive waste is the reactor wall material, which must be replaced periodically and stored as low-level radioactive material for several decades. Unlike nuclear fission reactors, fusion power plants do not carry the risk of catastrophic meltdown or explosion. Moreover, since the fuel sources are virtually inexhaustible, fusion advocates describe it as a source of nearly unlimited and clean energy.

Yet despite all the promises, plans, and efforts, fusion research remains far from delivering a power-generating plant. This has led some to sarcastically refer to the “fusion constant”: the idea that commercial fusion energy is always 30 or even 50 years away. Nevertheless, Lion remains optimistic: “The first fusion power plant will be operational in the 2030s. And in our view, the stellarator concept carries the lowest technological risk. The results from Wendelstein 7-X prove that the stellarator fundamentally works.” The decisive factor in founding Proxima Fusion was significant scientific progress made in 2021 and 2022. Based on these advancements, Lion and his team became convinced that a stellarator like Wendelstein 7-X is viable for power generation. The startup’s approach is to modify the Wendelstein 7-X design only where absolutely necessary. By re-optimizing its complex geometry, Proxima Fusion aims to develop a fully functional power plant within just a few years.

Proxima Fusion is focusing on a reactor design that is more compact than previous concepts, making it cheaper and faster to build. However, smaller reactors require significantly stronger magnetic fields to confine plasma and generate energy efficiently. These intense fields can only be achieved with novel high-temperature superconductors, which are not yet sufficiently developed for use in stellarator magnet coils. To tackle this challenge, Proxima Fusion is collaborating with experts at the Paul Scherrer Institute in Villigen to develop such high-field coils. Research is also underway in a large workshop adjacent to the startup’s Munich offices. The problem: the high-temperature superconductor material is a brittle ceramic that cannot simply be wound into magnetic coils. To overcome this, the ceramic is applied to steel tapes, which are then stacked and wrapped around copper windings.

Yet, several hurdles remain before a fully functional highfield coil can be achieved. The Proxima Fusion team is working behind closed doors to refine the details, aiming to complete its first demonstration magnet by 2027. “If the coils work and generate sufficiently strong magnetic fields, then we’ve done it!” says co-founder and lab director Jonathan Schilling. The team considers this the biggest technical challenge on the path to success. 

The demonstrator reactor, Alpha, is expected to produce more energy than it consumes—what fusion researchers refer to as a Q greater than 1. Q represents the ratio of power generated by fusion to the power directly input into the reaction. To date, only one fusion experiment worldwide has achieved a Q value greater than 1: the National Ignition Facility (NIF) in the U.S., which in 2022 demonstrated laser-based inertial confinement fusion. However, despite the media excitement, this milestone did not translate into practical power generation. Producing the laser energy required around 150 times more power than what was ultimately delivered to the reactor chamber. As a result, the fusion process released only about 1 percent of the total input energy as heat—of which, at best, 50 percent could be converted into electricity.

In the magnetic confinement method, the discrepancy between the energy balance of the fusion reaction and the net energy yield of the reactor is not as large. The energy required for heating the plasma and cooling the magnetic field coils is lower compared to the energy needed to generate laser pulses. Proxima Fusion would need to achieve a Q value of approximately 10 in order to generate electricity. In a recent study, the startup and the Max Planck Institute for Plasma Physics in Greifswald scientifically and technically demonstrated that a stellarator with high-field coils is a suitable reactor type for a power plant. The reactor described in the study, named Stellaris, would have a diameter of about 25 meters and could generate around one gigawatt of electricity, roughly equivalent to the yield of a modern nuclear power plant.

Stellaris shows a power plant is possible

However, Proxima Fusion does not intend to build entire power plants. This step, along with the operation, is expected to be taken on by energy companies. Proxima Fusion itself aims to offer the heat-generating stellarator as a product. Francesco Sciortino, co-founder and CEO of the startup, says, “We are already in discussions with energy companies and large energy consumers, such as data center operators in Europe and the US.”

However, before commercial electricity generation can begin, there are still several challenges to overcome beyond achieving a positive net energy yield. The availability of tritium is one issue that still needs to be addressed. While deuterium is naturally abundant, tritium is currently only available as a byproduct of nuclear fission in nuclear power plants. Since this supply is limited, companies like Proxima Fusion plan to eventually produce tritium themselves. Once operational, a fusion reactor would “breed” its own tritium. Breeding refers to the process in which some of the high-energy neutrons from the fusion reaction interact with lithium in the reactor walls, generating helium and tritium. The exact details of how this process works and how it can be controlled are still untested, as there are no sources of high-energy neutrons available to experimentally investigate reactions in the wall material. Proxima Fusion hopes that other companies, such as Kyoto Fusioneering, will develop the necessary technology.

Proxima Fusion expects that state institutions and numerous companies will work together to create the conditions necessary for fusion. Examples of these challenges include the development of the reactor wall material, which will be subjected to extreme conditions, and regulatory issues such as waste disposal and reactor safety. The startup is collaborating with other fusion startups and is also well connected internationally. Ultimately, the success of fusion power plants, should they prove technically feasible, will depend on their economic viability and compatibility with the existing power grid. Despite the uncertainties surrounding fusion power, the Proxima Fusion team remains motivated. Lion states: “The prospect of unlimited and clean energy is simply too good to not give it a try.”

Glossary

Q VALUE is the ratio between the energy produced and the energy directly input into the plasma to heat it.

STELLARATOR is a fusion reactor whose design resembles a twisted roll of pastry. Its construction is more complex than a tokamak, but it allows for continuous operation of the reactor without the issue of pulsing.

TOKAMAK is a fusion reactor that has the shape of a doughnut. It can only operate in pulses, meaning at intervals.