Why unwanted plasma eruptions can be avoided with additional coils

Researchers at the Max Planck Institute for Plasma Physics (IPP) evidence a physical phenomenon that suppresses these energy eruptions in tokamaks. Their results have now been published in the journal ‘Nature Physics.’

These eruptions, known as ELMs, are one of the major challenges in the development of fusion power plants. They can lead to energy losses that can destroy vessel walls. The IPP team's work proves an old hypothesis.

There are numerous obstacles to overcome on the way to a fusion power plant. In addition to material issues, ELMs (“edge localized modes”), the periodically occurring, violent eruptions of particles and energy at the edge of a tokamak plasma, are among the most stubborn. The eruptions occur mainly in the so-called H-mode, the desired operating mode for most reactor designs due to the high energy confinement time.

A single ELM is capable of ejecting up to 20% of the energy from the plasma interior to the wall of the vacuum vessel - a severe energy loss with potentially serious consequences for the vessel wall. Suppressing or even completely preventing the bursts is therefore an important research question in tokamak physics.

"ELMs are harmless for the IPP tokamak ASDEX Upgrade, the facility is too small for this to be critical. However, they can lead to problems in future large-scale experiments such as ITER - and even more so in a potential demonstration power plant,” says Dr. Matthias Willensdorfer. The physicist has been working at the Max Planck Institute for Plasma Physics since 2013. He adds: “In the worst case, the damage can be so extensive that parts of the vessel wall have to be replaced.”

Since their discovery in 1982, researchers have been working to prevent the eruptions. They have now been successful: special resonant magnetic perturbation coils (RMPs) are able to limit ELMs - and in the best case even suppress them completely. The coils are used in tokamaks worldwide and are also planned for the next generation of tokamaks. "With RMPs, we only disturb the symmetry of the confined plasma in the per mille range. But these perturbations are strong enough to prevent ELMs".

Why RMPs prevent plasma eruptions has been the subject of controversial debate. “There have been various theories,” says Willensdorfer, “but the most common one is certainly that magnetic islands prevent the formation of ELMs - we have now been able to confirm this hypothesis experimentally for the first time.”

The magnetic islands investigated by Willensdorfer are oval structures that can form in the field lines disturbed by RMP magnetic coils. Normally, magnetic islands are undesirable side effects, as they are capable of locally worsening the confinement of the plasma. However, when used correctly, the islands can ensure that no ELMs occur. “Small short circuits take place within the plasma, so to speak - this leads to a lower pressure gradient locally,” explains Willensdorfer, “which in turn reduces the drive of the ELM instability and thus ultimately prevents the ELM.”

The use of RMPs to suppress the unwanted plasma outbursts is like walking a tightrope: if the field lines are disturbed too little, magnetic islands are missing and small ELMs occur. If, on the other hand, they are disturbed too much, the pressure gradient required for the H-mode is too low. “And then less fusion processes take place,” says Matthias Willensdorfer.

With the help of his colleagues, Willensdorfer was able to prove the existence of magnetic islands during the application of RMPs. The special feature: By cleverly interconnecting the coils, Willensdorfer was able to rotate the interference field - without affecting the ELM-suppressing effect of the RMP magnets. “Together with the high-resolution measuring devices available to us at IPP, we were able to provide evidence of the magnetic islands,” summarizes Willensdorfer. “The combination of precise measurements of the electron temperature and the ability to rotate the interference field during operation is unique in the world.”  

The experimental research results were supported by computer simulations carried out by IPP doctoral student Verena Mitterauer. With the help of the simulation code JOREK, which is specially designed for non-linear plasma simulations, Mitterauer was able to confirm Willensdorfer's observations. The theory of magnetic islands has thus also been confirmed by computer simulations.

It remains to be seen to what extent the research results will also have an impact on the large-scale ITER experiment being built in Cadarache, France. However, RMP coils will be used there in any case. “Of course, we can't know 100 percent whether what we're planning will work exactly the same way in ITER,” says Willensdorfer with a laugh. “After all, if we knew everything in advance, we wouldn't have to build ITER.”