Imposing Chaos on Magnetic Fields Suppresses Runaway Electrons in a Fusion Plasma

The Science

Fusion energy research is based on trapping a high-temperature plasma, the fourth state of matter along with solids, liquids, and gases. In a plasma, the electrons and protons in atoms are separated from one another. One of the leading concepts for reaching fusion is the tokamak, a donut-shaped device in which the plasma is trapped with a strong magnetic field. In some tokamak plasmas, the device inadvertently accelerates electrons to extremely high energies. These “runaway” electrons can damage the interior wall of the vessel containing the plasma. This research demonstrates that a suitable perturbation to the magnetic field can suppress these runaway electrons.

The Impact

Researchers will take the next major step in harnessing fusion for energy generation at ITER, the largest tokamak in the world. But as a research device, ITER faces many challenges. One of the most important is the unwanted generation of runaway electrons. Researchers are using smaller tokamaks and computer models to test approaches for suppressing these runaway electrons. This research was achieved at a small university tokamak. It has attracted the interest of two Department of Energy (DOE) national labs for additional measurements and modeling to help optimize runaway electron suppression in ITER. The results could help improve the operation of ITER and other future fusion devices.

Summary

Researchers conducted experiments on suppressing runaway electrons at the Madison Symmetric Torus (MST) tokamak. The experiment intentionally produced runaway electrons by applying a voltage to the plasma. The electrons race the long way around the device, confined by the strong magnetic field. Researchers detect these electrons via their emission of X-rays. A set of 38 small electromagnetic coils perturbs the magnetic confinement field in various ways. The experiment found that only some perturbations affect the electrons. The researchers then used the NIMROD computer code to calculate the magnetic field patterns that result from different perturbations. Consistent with the MST experiment data, NIMROD predicts chaotic field lines in cases where the high-energy electrons are suppressed.

The present leading approach for runaway electron suppression in ITER is the injection of solid pellets but injecting these pellets fast enough to reach the hot plasma core is challenging. Magnetic perturbations may prove a crucial alternative, and researchers are developing new ideas for implementing this approach in ITER. Scientists from two DOE national labs are now helping to expand on the results of the experiments reported here. This DOE support includes fielding an advanced X-ray imaging camera on MST and helping a University of Wisconsin (UW)-Madison graduate student apply a new computer code to model the electron trajectories.

Funding

This work was supported by the DOE Office of Science, Office of Fusion Energy Sciences and the DOE Computational Sciences Graduate Fellowship program. The computational part of this research was performed using the computer resources and assistance of the UW-Madison Center for High Throughput Computing (CHTC) in the Department of Computer Sciences. The CHTC is supported by UW-Madison, the Advanced Computing Initiative, the Wisconsin Alumni Research Foundation, the Wisconsin Institutes for Discovery, and the National Science Foundation, and is an active member of the Open Science Grid, which is supported by the Office of Science and the National Science Foundation.

Publications

Munaretto, S. et al., Generation and suppression of runaway electrons in MST tokamak plasmas. Nuclear Fusion 60, 046024 (2020). [DOI: 10.1088/1741-4326/ab73c0]