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Benedikt Geiger and PhD student Michael Gerard with HSX
June 5, 2024

How modifying magnetic fields can tame turbulence in fusion devices

Written By: Adam Malecek

In magnetic confinement fusion devices, an unruly plasma is a big obstacle to harnessing fusion as a clean energy source.

This turbulence in the plasma, which isn’t fully understood, causes heat and particles to flow out of the plasma and prevents the ionized gas from reaching the extremely hot temperatures necessary for fusion reactions to occur.

Now, using numerical simulations, University of Wisconsin-Madison engineers have uncovered ways to reduce turbulence in the Helically Symmetric eXperiment, or HSX, at UW-Madison by modifying the three-dimensional magnetic field geometry.

The researchers detailed their findings in a paper published May 1, 2024, in the journal Physics of Plasmas.

“These exciting results are relevant not only for the HSX experiment but also for other fusion devices as we’ve obtained new insight on turbulence and the underlying instabilities through this research,” says Benedikt Geiger, an assistant professor of nuclear engineering and engineering physics who led the research.

HSX is a stellarator-type fusion device housed in the UW-Madison Department of Nuclear Engineering and Engineering Physics. It uses electromagnetic coils to create three-dimensional magnetic fields that confine the high-temperature plasma inside a vacuum chamber. The researchers discovered that altering the electrical currents in individual coils allowed them to change the geometry of the magnetic field, and this new geometry reduced a particular instability that is a main driver of turbulence.

“We were able to develop a solid understanding of how this stabilization was occurring in the plasma and how it relates back to the geometry of the magnetic field,” says Michael Gerard, a PhD student in nuclear engineering and engineering physics and first author on the paper. “These new insights will be useful when considering the design of future fusion devices.”

Gerard says a key insight from this research is that the way individual particles drift in the plasma is a major driver of the instability. The researchers plan to conduct experiments to validate their simulations.

With HSX, the researchers had to work with the experiment’s existing coil set, which limited how much they were able to modify the magnetic field geometry. “It will be interesting to see how our findings could further enhance the plasma stability in future fusion devices where there’s a lot more design freedom to shape the magnetic field geometry,” Gerard says.

This research was supported by the U.S. Department of Energy (grant no. DE-SC0020990).

Additional UW-Madison co-authors on the journal paper include Chris Hegna, Harvey D. Spangler Professor of nuclear engineering and engineering physics, Physics Professor Paul Terry, research assistant Joey Duff and scientist Benjamin Faber.

Featured image caption: NEEP Assistant Professor Benedikt Geiger (left) and PhD student Michael Gerard with HSX. Credit: Joel Hallberg.