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23.07.2025 08:56

IPP Develops Superconducting Coil Models for Future Fusion Power Plants

Frank Fleschner Öffentlichkeitsarbeit
Max-Planck-Institut für Plasmaphysik

In the project “HTS4Fusion”, funded by the German Federal Ministry of Research, the Max Planck Institute for Plasma Physics (IPP) is working with partners to advance magnet technologies for stellarators – which could enable more compact and efficient fusion power plants.

Stellarators are among the most promising designs for future fusion energy systems. Their main strength lies in the ability to confine plasma at temperatures of many millions of degrees Celsius in a stable and steady state – made possible by a highly complex magnetic field. This magnetic field is typically generated by large, three-dimensional coils, such as those used in Wendelstein 7-X at the Max Planck Institute (IPP) in Greifswald, the world’s most advanced stellarator. These coils are made of superconducting material, meaning they carry current without resistance when cooled to around 4 Kelvin (minus 269 degrees Celsius).

For future power plants, high-temperature superconductors (HTS) are a promising option. These materials become superconducting at significantly higher temperatures – even up to 93 Kelvin (minus 180 degrees Celsius) – or enable stronger magnetic fields when operated at lower temperatures. The latter could lead to more compact and cost-effective fusion reactor designs.

Optimised Materials for Extreme Conditions

Building such coils is a major design and engineering challenge. HTS materials are mechanically brittle and are therefore deposited on stronger substrates in the form of “tapes” (rather than, e.g., traditional round wires) to make them usable; even so, these tapes must not be overly strained (either in their final configuration or during the winding process) to preserve their superconducting properties – a difficulty that becomes even greater with the non-planar coil shapes typically required in modern stellarator designs. There are also open questions about how the HTS tapes (and coils made from them) will be affected by neutrons produced by fusion reactors. The German Federal Ministry for Research (BMFTR) is therefore providing a total of €7 million to support the research and development of HTS tapes and coils with increased robustness and suitability for fusion applications.

The project is led by the company THEVA Dünnschichttechnik, based in Ismaning, which receives the largest share of the funding (€5.25 million). THEVA is developing novel composite HTS tapes tailored to the demanding requirements of fusion devices. The IPP site in Garching near Munich is helping to inform the development of these materials in terms of what properties are required or advantageous for their fabrication into non-planar coils, along with the experimental validation of small-scale test coils’ performance. The institute receives €948.000 in funding. A third partner in the project is the Research Neutron Source Heinz Maier-Leibnitz (FRM II) at the Technical University of Munich, which is investigating how the tapes’ performance is affected by neutron irradiation.

“In future fusion power plants, coils must withstand extreme demands, produce strong magnetic fields and operate reliably at cryogenic temperatures. They will also need to be large – on the scale of several meters,” explains Dr Eve Stenson, group leader and principal investigator for the IPP part of the project. “However, there are nevertheless fundamental questions about novel HTS tape and coil design that can be investigated through smaller-scale, lower-field, fast-prototyping methods and iteration between the public and private sectors. That is what our project is about.” For example, conventional superconducting tapes face physical limitations when wound into complex 3D shapes. Too much bending or unfavorable alignment with the magnetic fields they produce can drastically degrade or even destroy their superconducting properties. That’s why the “HTS4Fusion” project includes a particular emphasis on tests of the JANUS (“Joint ANgled Unconventional Superconductor”) concept.

Improving Performance through Layered Structure

Instead of using a single-layer HTS tape, the JANUS approach proposes a composite structure with multiple layers – for example, two superconducting layers joined by a conductive interlayer. This architecture should enable effective current sharing between the layers: when one layer reaches its strain or field limit – or suffers from a defect (e.g., due to neutron irradiation) – part of the current can transfer to the adjacent layer. As a result, the overall current-carrying capacity and robustness of the coil are improved – which could be a key innovation for reliable, high-performance fusion magnets. The design also allows better adaptation to the complex magnetic field distributions typical of stellarators.

“We aim to help THEVA produce HTS tapes that are better suited for making stellarator coils,” says Dr Stenson. “We will provide detailed specifications, based on simulation and design calculations that are validated with prototype testing.” The group draws on experience from previous projects that use a variety of HTS coils – planar and non-planar – for fundamental plasma science experiments.

One key issue in stellarator magnet design is the anisotropy of the critical current: the maximum current the HTS tape can carry with zero resistance depends not only on temperature and field strength, but also on the angle between the tape and the magnetic field. THEVA’s tapes show peak performance when the magnetic field is inclined by a 30-degree angle, relative to the tape face.

The JANUS concept proposes to exploit this angular dependence by combining two HTS layers with different orientations. This has the potential to be particularly beneficial for stellarator designs, but its further development requires additional calculations as well as experimental validation. The IPP team will draw on this background in HTS stellarator coil modeling and prototyping to shape JANUS tapes into non-planar coil geometries while both avoiding mechanical damage to the superconducting layers and improving coil performance robustness to potential sources of error and uncertainty. This involves optimising tape winding angles (this is like a roller coaster track that stragetically orients the tape, for a give stellarator design), selecting suitable support materials, and implementing controlled cryogenic cooling.

Stellarator Coil Modelling at IPP

Eve Stenson's group draws on experience gained in previous projects. For the EPOS (Electrons and Positrons in an Optimised Stellarator) device, for example, the group has been using and contributing to the open-source design software SIMSOPT – for example, working with collaborators at Columbia University and the start-up company Proxima Fusion to incorporate HTS mechanical strain constraints directly into stellarator design optimisation.

First test coils are being built on custom 3D-printed winding frames and tested at around 20 Kelvin (minus 253 degrees Celsius). “These tests are essential for understanding how composite tapes behave under realistic conditions,” says Dr Stenson. “Only by validating designs experimentally can we ensure that future magnet systems have a sound basis.”
In addition to their mechanical robustness, the IPP researchers will investigate the thermal behaviour of the coils – e.g., how efficiently heat is dissipated during current sharing. The demonstrator coils will be tens of centimetres, up to at most one metre in diameter, corresponding to less than one-quarter the size of reactor-scale systems. “Our work bridges cutting-edge superconductor technologies with the development of future fusion confinement concepts,” concludes Dr Stenson. “If we succeed at small scale in building higher-performance, more robust HTS stellarator coils, the results could then be scaled up to larger sizes and higher magnetic fields for fusion .” (Frank Fleschner)

Contact:
Max Planck Institute for Plasma Physics
Frank Fleschner
Press officer
Boltzmannstraße 2
85748 Garching
T: +49 89 3299-1317
M: press@ipp.mpg.de

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IPP scientist Paul Huslage assembles a model coil in the test stand. This allows magnetic coils to b ...

Copyright: MPI for Plasma Physics, Frank Fleschner

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