Plasma instabilities pose a significant problem for fusion reactors like ITER. A research team centered on the nuclear fusion department at TU Wien has recently discovered a viable solution.
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. Image Credit: David Rath, TU Wien
Nuclear fusion power plants might one day offer a long-term answer to global energy issues; however, there is currently no commercial nuclear fusion reactor in use. The plasma in the middle must be extremely hot (approximately 100 million °C), yet the reactor wall must not melt to achieve fusion reactions.
Therefore, there must be adequate insulation between the plasma’s edge and the reactor wall. However, plasma instabilities known as ELMs regularly develop in this region. In such situations, powerful plasma particles may strike the reactor’s wall and cause possible damage. One of the biggest challenges in the path of a commercial reactor is these instabilities.
There is an operational regime for fusion reactors that avoids this issue, as the TU Wien fusion research team and the Max Planck Institute for Plasma Physics (IPP) in Garching, Germany, have now been able to show.
One purposefully accepts numerous tiny instabilities that do not affect the reactor’s walls rather than a few massive potentially damaging instabilities. As an Editors’ Suggestion, the findings have now been published in the journal Physical Review Letters.
The Renaissance of a Disregarded Mode of Operation
The high-speed motion of ultra-hot plasma particles occurs in a toroidal tokamak fusion reactor. The particles are kept contained rather than slamming violently into the reactor wall due to strong magnetic coils.
However, you do not 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.
Friedrich Aumayr, Professor, Ion and Plasma Physics, Institute of Applied Physics, TU Wien
The specifics of the reactor’s internal dynamics are challenging: particle velocity is influenced by the density, temperature, and magnetic field of the plasma. Different operating regimes are conceivable depending on how these parameters are selected.
Elisabeth Wolfrum, Group Leader at IPP Garching and Professor at TU Vienna, has long collaborated with Friedrich Aumayr’s group at TU Wien. As a result of this collaboration, a novel operating regime has now been created and demonstrated to prevent the hugely detrimental plasma instabilities known as “Type-I ELMs.”
Recent experiments have demonstrated that the harmful Type-I ELMs can be avoided by gradually deforming the plasma via the magnetic coils so that the plasma cross-section resembles a rounded triangle rather than an ellipse and by increasing plasma density throughout, especially at the edge.
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: The regime can prevent the dangerous instabilities even in parameter ranges foreseen for reactors like ITER.
Lidija Radovanovic, Institute of Applied Physics, TU Wien
Like a Pot with a Lid
Several thousand minor instabilities take place per second as a result of the plasma’s triangle cross-section and the controlled injection of extra particles at the plasma edge.
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 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.
Georg Harrer, Study Lead Author, Institute of Applied Physics, TU Wien
To further investigate the new operation framework, Harrer was awarded a two-year EUROfusion Researcher Grant by the EU.
“It is a bit like a cooking pot with a lid, where the water starts to boil. 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 does not rattle,” added Harrer.
This fusion reactor operation regime can be used in a variety of reactors, including the ITER reactor now being built in France, the ASDEX Upgrade reactor in Garching, Germany, and even future DEMO fusion plants.
Our work represents a breakthrough in understanding the occurrence and prevention of large Type I ELMs. The operation regime we propose is probably the most promising scenario for future fusion power plant plasmas.
Elisabeth Wolfrum, Research Group Leader, Max Planck Institute for Plasma Physics
The study was conducted as part of the EU project EUROfusion and is a part of Austria’s fusion research program.
Harrer, G. F., et al. (2022) Quasicontinuous Exhaust Scenario for a Fusion Reactor: The Renaissance of Small Edge Localized Modes. Physical Review Letters. doi:10.1103/PhysRevLett.129.165001