27 May 2025
Wendelstein 7-X makes fusion history: In the world's largest stellarator facility, high-energy helium-3 ions were generated for the first time using ion cyclotron resonance heating - a milestone for fusion research. This technology, developed as part of the TEC cluster by partners in Jülich and Brussels, not only helps in the development of sustainable energy, but also provides new insights into processes on the sun.
Wendelstein 7-X (W7-X) is the world's largest stellarator-type fusion reactor. The experimental facility is operated at the Max Planck Institute for Plasma Physics in Greifswald and plays a decisive role in the further development of fusion energy research. The heart of the facility is a torus-shaped magnetic field cage that encloses the multi million degree plasma.
One of the key challenges in such facilities is the efficient confinement of fast alpha particles (helium-4 nuclei) produced during fusion reactions. These high-energy particles are essential for maintaining the extreme temperatures required for sustainable fusion. If they escape too quickly, they cannot heat the plasma effectively.
Since W7-X is an experimental stellarator and not a fully-fledged fusion power plant, it is smaller in size. Therefore, the behaviour of the alpha particles must be simulated using lower-energy particles scaled to the dimensions of the facility. In practice, the lighter helium-3 ions are accelerated to a suitable energy for this purpose. The crucial question is: How can such particles be reliably produced in the W7-X?
Like a child on a swing
The answer lies in a process called resonant energy transfer, in which powerful high-frequency waves with a power output in the megawatt range are used to accelerate the particles. This is similar to pushing a child on a swing: to be effective, each push must be precisely in tune with the swing's natural frequency - in other words, it must be in resonance.
In W7-X, electromagnetic waves are fed in using a special antenna (see figure). In plasmas consisting of hydrogen and helium-4 in carefully selected ratios, the fed waves are tuned to the ion cyclotron frequency of the helium-3 ions - the frequency at which they orbit around the magnetic field lines. When resonance is achieved, the particles efficiently absorb energy from the waves and reach the necessary high energies. This is the first time that high-energy helium-3 ions have been produced in a stellarator using ion cyclotron resonance heating (ICRH): a world first in fusion research.
The ICRH system is being developed and operated at W7-X under the umbrella of the Trilateral Euregio Cluster (TEC) in close collaboration between the Plasma Physics Laboratory of the Royal Military Academy in Brussels and the Jülich institutes IFN-1 and ITE. It is the result of years of dedicated research and development work.
Insights into how the sun works
This research not only contributes to the development of a sustainable energy source, but also provides unexpected insights into how the sun works. The same resonance processes that excite helium-3 particles in W7-X may also explain the occasional occurrence of helium-3-rich clouds in its atmosphere.
Just like in W7-X, helium-3 particles there receive energy from electromagnetic waves generated in the sun - they are selectively accelerated and form large clouds. These can contain up to 10,000 times more helium-3 than usual and were recently rediscovered by the Solar Orbiter space probe on 24 October 2023. These findings show that fusion science is not only shaping the future, but also helping to unlock the mysteries of the cosmos around us.
