Interview: How materials can shape the magnetic fields inside a stellarator

Developing a stellarator fusion power plant requires engineers to consider how its different systems interact, often in ways that are not immediately obvious.

One such challenge is the potential effect of the materials used in a fusion reactor’s breeding blanket on the magnetic fields that confine the plasma. Understanding this interaction is relevant to the development of Gauss Fusion’s GIGA™ stellarator power plant concept, where the different systems must work together as part of an integrated design.

We spoke to Luisa Hübner, a PhD researcher at the Technical University of Denmark (DTU), working in collaboration with Gauss Fusion, about her research into this challenge, her early findings and what they could mean for future stellarator power plants.

Q. How did you become interested in fusion, and how did your PhD collaboration with Gauss Fusion come about?

I studied physics at Friedrich-Alexander University (FAU) in Erlangen, Germany. I became interested in fusion energy during my studies and spent an Erasmus semester in Prague, where I was able to take courses in plasma physics.

I subsequently joined Gauss Fusion as a working student, focused on stellarator coil optimisation. I eventually began working closely with Head of Magnets, Dr. Neil Mitchell and Director for Stellarator & Device, Dr. Sam Lazerson. That gave me an opportunity to gain practical experience working with computational tools used to model and analyse stellarator magnetic fields.

In parallel, I was working on my MA degree at DTU in Copenhagen. As I approached graduation, the possibility of continuing with a PhD in collaboration with Gauss Fusion emerged.

Gauss Fusion and DTU already had a cooperation agreement in place, so we worked with my professors to identify a suitable research topic.

It was important to find something that would be scientifically interesting and suitable for a PhD, while also addressing a practical engineering challenge for Gauss Fusion. The research also needed to be sufficiently independent that I could publish my findings academically.

The effect of magnetic materials in the breeding blanket turned out to be a good fit. It is an interesting problem that has received relatively little attention in stellarator research, but one that could become increasingly important as we move towards power plant designs.

Q. What is the scientific problem you're investigating, and why does it matter for a stellarator design?

Stellarators use carefully designed magnetic fields to confine extremely hot plasma without allowing it to touch the surrounding reactor walls.

Designing those fields is a complex optimisation problem. Typically, researchers first determine the magnetic configuration they want and then design the coils needed to produce it.

However, there is another factor that is often left out of this process: the materials surrounding the plasma can also affect the magnetic field.

In a fusion power plant, the breeding blanket sits around the plasma and performs several essential functions, including absorbing energy from fusion neutrons and helping produce the tritium needed to sustain the fuel cycle.

The blanket has to withstand demanding conditions, including exposure to high-energy neutrons. This makes the choice of structural materials particularly important.

One option is low-activation ferritic steel, which is being investigated for its suitability in this environment. The complication is that ferritic steel is ferromagnetic. When exposed to the strong magnetic field produced by the stellarator coils, it becomes magnetised and generates an additional magnetic field of its own.

That additional field can change the magnetic configuration we have so carefully optimized.

For a stellarator, where the precise shape of the magnetic field is particularly important, the effect of this additional field is something we need to understand.

My research aims to quantify that effect and determine how significant it could be for future stellarator power plants.

Q. How are you investigating this, and what have you found so far?

I'm combining two computational tools to model how the coils and the magnetic materials interact. The first calculates the magnetic field generated by the stellarator coils, while the second calculates how the ferritic steel becomes magnetised when exposed to that field.

We can then calculate the additional field produced by the steel, combine it with the original coil field and trace the resulting magnetic field lines around the stellarator.

This allows us to examine whether the presence of the material changes the magnetic configuration, including the shape of the magnetic surfaces that help confine the plasma.

An important part of my work so far has been getting these tools to work together reliably, resolving software issues and checking the calculations against a known analytical example.

I presented some preliminary results at the Culham Plasma Physics Summer School, in July 2026 and at the FuseNet PhD event, in Viterbo, in October 2026. The coils are designed to produce a field of 6 tesla on the magnetic axis. Compared to this, the field produced by the steel is very small. However, my simulations also show that even a small change in the field can have a substantial effect on important characteristics, such as the angle of the magnetic field lines.

With all of that said, it’s important to remember that these are early modelling results. We still need to understand how exactly the additional field affects the overall magnetic configuration and which consequences those changes would have for plasma confinement.

Q. What are the next steps in your research?

The first major challenge has been establishing a reliable modelling approach. I'm now reaching the stage where I can spend more time investigating the physics itself, which is what I'm particularly looking forward to.

One priority is to study how the results change when we vary the amount of ferritic steel in the blanket.

I also want to explore more realistic blanket geometries, including openings or ports for other reactor systems. These details matter because an actual power plant won't have a perfectly uniform blanket surrounding the plasma.

Another important objective is to validate the modelling against experimental measurements, if suitable opportunities become available.

Once we have a better understanding of the effects, we can start investigating how they might be addressed through the design.

For example, would the stellarator coils need to be reoptimised to account for the materials? Could additional correction coils help compensate for unwanted changes?

There is also an interesting possibility that magnetic materials might be used deliberately to improve certain aspects of the field, rather than simply treating their influence as a problem to overcome.

These are questions for the next stages of the research, rather than conclusions we have reached. The immediate priority is to establish the scale and characteristics of the effect.

Q. How does this research connect to Gauss Fusion's ambition to develop industrial stellarator power plants?

One of the challenges in moving from plasma physics experiments to fusion power plants is that we have to design much more than the plasma and the magnets.

We also need a breeding blanket, systems for handling heat and fuel, and equipment that can operate and be maintained in a demanding environment. All these systems have to fit together.

The research I'm doing is a good example of why that matters.

A material might be attractive for the breeding blanket because of how it performs under neutron exposure, but its magnetic properties could create another consideration for the stellarator design.

If we understand those interactions early enough, we have more opportunities to account for them when developing the plant.

For Gauss Fusion's GIGA™ concept, the wider objective is to develop an integrated stellarator power plant rather than optimise individual technologies in isolation.

My research contributes to that approach by investigating one of the interactions between the reactor's materials and its magnetic configuration.

We are still at an early stage, but understanding these effects could help inform future decisions about blanket materials, coil design and how the different parts of a stellarator power plant are engineered together.