The pivotal discovery signals a huge leap forward in the progress of fusion power reactors

Researchers have used Alfvén waves to attenuate runaway electrons in tokamak fusion devices, providing major implications for future fusion energy projects, including ITER in France.

Scientists led by Zhang Liu of Princeton’s Plasma Physics Laboratory (PPPL) revealed a promising approach to mitigate the damage of runaway electrons caused by turbulence in tokamak fusion devices. The key to this approach was harnessing a unique genre plasma A wave named after astrophysicist Hans Alvvén, who won the Nobel Prize in 1970.

Alfvén waves have long been known to loosen the confinement of high-energy particles in tokamak reactors, allowing some to escape and reducing the efficiency of the donut-shaped devices. However, new findings by Zhang Liu and researchers at General Atomics, Columbia University and PPPL have revealed useful results in the case of runaway electrons.

Great circular process

Scientists found that such loosening could scatter or scatter high-energy electrons before they turn into avalanches that damage the tokamak’s components. This process is determined to be remarkably circular: the escapees create instabilities that give rise to Alfvén waves that prevent the avalanche from forming.

“These findings provide a comprehensive explanation for the direct observation of Alfvén waves in inactivation experiments,” said Liu, a researcher at PPPL and lead author of a paper detailing the findings. Physical review letters. “The results demonstrate a clear link between these patterns and the generation of runaway electrons.”

Chang Liu. Credit: Elle Starkman

The researchers deduced a theory for the observed circuit of these interactions. The results fit well with the escapees in experiments conducted at the National Fusion Facility DIII-D, a Department of Energy tokamak operated by General Atomics for the Office of Science. Tests of the theory have also proven positive on the Summit supercomputer located at Oak Ridge National Laboratory.

“Zhang Liu’s work shows that the size of the pool of escaping electrons can be controlled by instabilities driven by the escaping electrons themselves,” said Felix Parra Diaz, Head of Theory at PPPL. “His research is very exciting because it may lead to tokamak designs that naturally mitigate runaway electron damage through inherent instability.”

Thermal quenching

The turbulence begins with a sharp drop in the million-degree temperatures required for fusion reactions. These drops, called “thermal quenching,” release landslide avalanches similar to landslides triggered by earthquakes. “Controlling turbulence is a major challenge for tokamak success,” Liu said.

Fusion reactions combine light elements in the form of plasma — the hot, charged state of matter made up of free electrons and atomic nuclei called ions — to release the massive energy that powers the sun and stars. Mitigating the risk of turbulence and runaway electrons would thus provide a unique benefit to tokamak facilities designed to reproduce the process.

Mitigating the risk of turbulence and runaway electrons would thus provide a unique benefit to tokamak facilities designed to reproduce the process.

Nuclear fusion energy can be a pivotal source of sustainable energy to complement renewable energy sources. The world’s largest fusion experiment, ITER, is now being built in France. Credit: ITER

The new approach could have implications for the progress of the ITER project, an international tokamak under construction in France to demonstrate the practical application of fusion energy and could represent a major step in the development of fusion power plants.

“Our findings pave the way for creating new strategies to mitigate runaway electrons,” Liu said. Now in the planning stage there are experimental campaigns in which the three research centers aim to further develop the amazing results.