Walk into a convenience store and you may be met by a sharp blast of cold air the from air-conditioning units at the entrance. Like an atmospheric confinement, this rapid surge of air breaks up the turbulent flows swirling in and out of the shop and leads to improved insulation.
Similar turbulent flows are a critical phenomenon at the nuclear scale, posing a challenge for building commercialisable fusion reactors. In nuclear fusion, two elements combine into a heavier one, releasing heat energy during the collision and forming plasma gas in the process. If the reaction could be reliably controlled and heat leakages minimised, fusion energy could help power our cities in a clean and sustainable way.
“Turbulence is responsible for losses of both heat and particles out of the plasma, which decreases the temperature. If too much heat escapes the fusion core too quickly, the fusion reaction stops, like a candle in the wind,” said Valerian Hall-Chen, Head of Plasma Physics and Diagnostics at A*STAR’s Future Energy Acceleration and Translation Programme (FEAT).
Much like the fast-flowing air at the store entrance, fusion plasmas also have regions of zonal flows that can affect turbulence. Hoping to recreate this effect and improve the performance of fusion reactors, Hall-Chen and the team led by Juan Ruiz Ruiz of the University of Oxford, UK, zeroed in on Alfvén waves, which were theorised to influence these flows.
“Numerical simulations suggested that the Alfvén waves can produce stationary flows and hence suppress turbulence,” Hall-Chen said. “However, the idea remained highly speculative; there was no experimental evidence of it.”
Even then, there were signs that experimental observations may be possible. Some experiments in fusion devices like the Joint European Torus (JET), the world’s largest tokamak fusion reactor, had shown improvements in plasma insulation but had no explanation for the mechanism behind it. As such, Hall-Chen along with collaborators from France, Ukraine, the US, the UK and Belgium, headed to the JET tokamak on a quest to observe the behaviour of Alfvén waves.
The team used the Doppler Backscattering diagnostic at the JET, which shoots electromagnetic beams into the plasma. These beams are scattered differently depending on the types of turbulent flows present, much like how echolocation detects an object based on the returning signal.
Excitingly, the researchers detected, for the first time, Alfvén waves generating zonal flows in the plasma. Moreover, the improvement in plasma insulation could only be explained by these Alfvén-generated flows. “This discovery provides the missing link between the theoretical ideas and the recent experiments in JET,” said Hall-Chen.
Together with global collaborators, the researchers are next working to understand how plasma design can better suppress turbulence. “We believe that such engineering is key to developing future fusion reactors,” Hall-Chen remarked.
The A*STAR-affiliated researcher contributing to this research is from the A*STAR Institute of High Performance Computing (A*STAR IHPC).
