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Nuclear fusion: A new solution for the instability problem

For fusion reactors like ITER, plasma instabilities are a major challenge. A research team around the nuclear fusion group at TU Wien has now found a promising solution.

Four people with a model of ITER

© David Rath, TU Wien

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Georg Harrer (TU Wien), Lidija Radovanovic (TU Wien), Elisabeth Wolfrum (IPP Garching), Friedrich Aumayr (TU Wien) holding a 3D printed 1:100 model of ITER

A large lab with metal constructions and pipes

© IPP Garching, Helmut Faugel

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The ASDEX Upgrade research facility at the Max-Planck-Institute for Plasma Physics in Garching near Munich, where the experiments have been carried out

two pictures of the reactor. one with people inside, the other with plasma

© IPP Garching

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Left: View inside the plasma vessel of the ASDEX Upgrade fusion reactor. The first wall is made of the element tungsten, a metal with an extremely high melting point, which withstands the high heat fluxes and the constant particle bombardment from the plasma. Right: View into the reactor interior of ASDEX Upgrade during a plasma discharge

Cross section of the plasma

© G. Harrer & L. Radovanovic, TU Wien

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Cross-section of the toroidal tokamak plasma in ASDEX Upgrade; The left picture shows the usual operation regime, where strong instabilities (called Type-I ELMs) occur; right, the new regime of operation, with its more triangular cross section. If at the same time the density of the plasma at the edge is increased, the dangerous Type-I ELMs can be prevented and a quasi-continuous exhaust (QCE) operational regime can be achieved.

Nuclear fusion power plants could one day provide a sustainable solution to our energy problems - but to date there is no commercial nuclear fusion reactor in operation. To realize fusion reactions, the plasma in the center must be very hot (about 100 million °C), and at the same time the wall of the reactor must not melt.  The edge of the plasma must therefore be well insulated from the reactor wall. In this region, however, plasma instabilities called ELMs occur frequently. During such events, energetic particles from the plasma may hit the wall of the reactor, potentially damaging it. These instabilities are one of the most important obstacles on the way to a commercial reactor.

Now the fusion research team of TU Wien, opens an external URL in a new window together with the Max Planck Institute for Plasma Physics (IPP) in Garching, opens an external URL in a new window (Germany) could demonstrate: There is an operational regime for fusion reactors that avoids this problem. Instead of large potentially destructive instabilities, one intentionally accepts many small instabilities that do not pose a problem for the reactor’s walls. The results have now been published in the journal Physical Review Letters as Editors' Suggestion.

The renaissance of a disregarded mode of operation

In a toroidal tokamak fusion reactor, ultra-hot plasma particles move at high speeds. Powerful magnetic coils ensure that the particles remain confined instead of hitting the reactor wall with destructive force. "However, you don't want to isolate the plasma perfectly from the reactor wall either; after all, new fuel has to be added and the helium produced during fusion has to be removed," explains Friedrich Aumayr, Professor of Ion & Plasma Physics at the Institute of Applied Physics of TU Wien in Vienna, Austria.

The details of the dynamics inside the reactor are complicated: The motion of the particles depends on plasma density, temperature and magnetic field. Depending on how one chooses these parameters, different regimes of operation are possible. In a long-standing collaboration between Friedrich Aumayr's group at TU Wien and the IPP Garching group coordinated by Elisabeth Wolfrum (group leader at IPP Garching and TU Vienna professor), a novel operating regime has now been developed and shown to prevent the particularly destructive plasma instabilities called “Type-I ELMs”.

Experiments already showed a few years ago: If one slightly deforms the plasma through the magnetic coils, so that the plasma cross-section no longer looks elliptical, but rather resembles a rounded triangle, and if one simultaneously increases the density of the plasma especially at the edge, then the dangerous Type-I ELMs can be prevented.

"At first, however, this was thought to be a scenario that only occurs in currently running smaller machines such as ASDEX Upgrade (IPP Garching) and is irrelevant for a large reactor," explains Lidija Radovanovic, who is currently working on her PhD thesis on this topic at TU Wien. "However, with new experiments and simulations, we have now been able to show: The regime can prevent the dangerous instabilities even in parameter ranges foreseen for reactors like ITER."

Like a pot with a lid

Due to the triangular shape of the plasma cross-section and the controlled injection of additional particles at the plasma edge, many small instabilities occur - several thousand times per second. "These small particle bursts hit the wall of the reactor faster than it can heat up and cool down again," says Georg Harrer, lead author of the paper, who received a two-year EUROfusion Researcher Grant from the EU to further study the new operation regime. "Therefore, these individual instabilities do not play a major role for the reactor wall." But as the team has been able to show through detailed simulation calculations, these mini-instabilities prevent the large instabilities that would otherwise cause damage.

"It's a bit like a cooking pot with a lid, where the water starts to boil," Georg Harrer explains. "If pressure keeps building up, the lid will lift and rattle heavily due to the escaping steam. But if you tilt the lid slightly, then steam can continuously escape, and the lid remains stable and doesn't rattle."

This fusion reactor operation regime can be implemented in a variety of reactors - not only at the ASDEX Upgrade reactor in Garching, Germany, but also at ITER, opens an external URL in a new window, currently under construction in France, or even in future DEMO, opens an external URL in a new window fusion plants. "Our work represents a breakthrough in understanding the occurrence and prevention of large Type I ELMs," says Elisabeth Wolfrum. "The operation regime we propose is probably the most promising scenario for future fusion power plant plasmas."

The research described is part of the Austrian fusion research program and was carried out within the EU project EUROfusion, opens an external URL in a new window

Origina lpublication

G. F. Harrer, et al. „A quasi-continuous exhaust scenario for a fusion reactor: the renaissance of small edge localized modes” Physical Review Letters., opens an external URL in a new window


Dr. Georg Harrer
Institute of Applied Physics
TU Wien
+43 1 58801 13434
+43 664 4001145

Prof. Friedrich Aumayr
Institute of Applied Physics
TU Wien
+43 1 58801 13430
+43 664 605883471