Nuclear fusion reactor instabilities solved by new operational regime

A group of scientists have devised a new operational regime that potentially solves the instability dangers of nuclear fusion reactors, a development that may see the potential of nuclear power fully harnessed.

In the future, nuclear fusion power plants could provide a sustainable energy solution to decarbonise the planet; however, due to instability issues, there are currently no commercial nuclear fusion reactors in operation.

Experts from the fusion research team at TU Wien and the Max Planck Institute for Plasma Physics (IPP) have designed a novel operational regime to overcome these challenges. Instead of large and potentially destructive instabilities, their new method concedes many small instabilities that do not threaten the reactor’s walls.

Nuclear fusion challenges

To perform nuclear fusion, the plasma in the centre of the reactor must be extremely hot – about 100 million °C. Additionally, the wall of the reactor must not melt. This means the edge of the plasma must be well insulated from the reactor wall; however, plasma instabilities in this region called ELMs occur frequently.

During these events, the plasma’s energetic particles can smash against the wall of the reactor, which can cause damage and is a significant barrier to developing a commercial reactor. In a toroidal tokamak fusion reactor, ultra-hot plasma particles move at high speeds, but specialised magnetic coils stop the particles from hitting the reactor wall with destructive force.

Friedrich Aumayr, Professor of Ion & Plasma Physics at the Institute of Applied Physics of TU Wien, explained: “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.”

The reactor’s dynamics are extremely complicated. The particle’s motion depends on plasma density, temperature, and magnetic field. Different regimes are possible depending on how these parameters are set up. The researchers have now pioneered a new operating regime that can prevent destructive plasma instabilities called Type-I ELMs.

Eliminating instabilities

Previous experiments demonstrated that if the plasma is slightly deformed by the magnetic coils so that the plasma cross-section no longer looks elliptical, instead looking like a rounded triangle, and if the plasma density, especially at the edge, is simultaneously increased, Type-I ELMs can be prevented.

Lidija Radovanovic, who is currently working on her PhD thesis on this topic at TU Wien, commented: “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. However, with new experiments and simulations, we have now been able to show that the regime can prevent the dangerous instabilities even in parameter ranges foreseen for reactors like ITER.”

The triangular shape of the plasma cross-section coupled with the controlled injection of additional particles at the plasma edge causes many small instabilities to occur – thousands of times per second.

Georg Harrer, the lead author of the paper, said: “These small particle bursts hit the wall of the reactor faster than it can heat up and cool down again. Therefore, these individual instabilities do not play a major role in the reactor wall.”

A range of detailed simulation calculations illuminated that the mini instabilities prevent the large instabilities from occurring.

Harrer explained: “It’s a bit like a cooking pot with a lid, where the water starts to boil. If the 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.”

The team’s groundbreaking fusion reactor operation regime can be employed in a variety of reactors or even in future nuclear fusion plants.

Elisabeth Wolfrum, the group leader at IPP Garching and TU Vienna professor, concluded: “Our work represents a breakthrough in understanding the occurrence and prevention of large Type I ELM. The operation regime we propose is probably the most promising scenario for future fusion power plant plasmas.”

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