Researchers under the direction Chang Liu of Princeton Plasma Physics Laboratory (PPPL) have discovered a promising approach to mitigating damaging runaway electrons created by plasma disruptions in tokamak fusion reactors. This approach harnesses a unique type of plasma waves that bears the name of astrophysicists Hannes Alfvén, a 1970 Nobel laureate.
Alfvén waves have long been known to loosen the confinement of high-energy particles in tokamak reactors. This permits some particles to escape and reduces the efficiency of the donut-shaped fusion reactors. However, the new findings by Chang Liu and his team at General Atomics, Columbia University and PPPL have uncovered new techniques to deal with runaway electrons.
The researchers found that such loosening can diffuse or scatter high-energy electrons before they can turn into avalanches that damage tokamak components. This process was determined to be circular. The runaway electrons create plasma instabilities that give rise to Alfvén waves that keep avalanches from forming.
Chang Liu is a staff researcher at PPPL and the lead author of a paper that details the results of his work in the journal Physical Review Letters. He said, “The findings establish a distinct link between these modes and the generation of runaway electrons.”
Researchers have derived a theory for the circularity of these interactions. The results of their experiments align well with runaway electrons in experiments on the DIII-D National Fusion Facility which is a Department of Energy (DoD) tokamak that General Atomics operates for the U.S. DoE Office of Science.
Felix Parra Diaz is the head of the Theory Department at PPPL. He said, “Chang Liu's work shows that the runaway electron population size can be controlled by instabilities driven by the runaway electrons themselves. His research is very exciting because it might lead to tokamak designs that naturally mitigate runaway electron damage through inherent plasma instabilities.”
Plasma disruptions begin with sharp drops in the multi-million-degree temperatures required initiate and sustain fusion reactions. These drops in temperatures are called “thermal quenches”. They release avalanches of runaway electrons similar to earthquake-produced landslides. Liu said, “Controlling plasma disruptions stands as a paramount challenge to the success of tokamaks,”
Plasmas are the hot, charged states of matter composed of free electrons and atomic nuclei called ions. Fusion reactions combine light elements in the form of plasmas to release vast amounts of energy. Fusion processes power the Sun and stars. Mitigating the risk of plasma disruptions and runaway electrons would provide a significant benefit for tokamak facilities designed to reproduce the fusion process on Earth.
The new approach could have implications for the advancement of the International Thermonuclear Experimental Reactor (ITER). ITER is the international tokamak project under construction in France to demonstrate the practicality of fusion energy and could mark a key step in the development of commercial fusion power plants.
Liu said, “Our findings set the stage for creating fresh strategies to mitigate runaway electrons.” Experimental campaigns in which all three research centers aim to further develop the important findings with respect to runaway electrons.