Two teams of scientists have announced a breakthrough in fusion energy research, demonstrating for the first time the ability to simultaneously achieve high plasma density and confinement in a tokamak reactor. Derived from a Russian acronym, a tokamak is a donut-shaped experimental device that uses magnetic fields to make use of the energy of nuclear fusion. The announcement from the Department of Energy’s Office of Science revealed that researchers achieved plasma conditions they had previously assumed were mutually exclusive — a density above the Greenwald limit and energy confinement quality roughly 50% better than standard high-confinement mode.

Understanding the Greenwald limit

The Greenwald limit, which physicist Martin Greenwald devised nearly 40 years ago, is a density threshold above which tokamak plasmas typically become unstable. Researchers have viewed the limit as a longstanding challenge in fusion research, as higher plasma densities are a prerequisite for improving the efficiency of fusion reactions. The equation for the limit is as follows:

In the equation, the notation refers to the following:

• \( n_{G} \): Density of particles.
• \( I_{p} \): Plasma current.
• \( a \): The radius of the circular area.
• \( a^{2} \): Square of the radius of the circular area.

One historical challenge lies in finding the balance for sustained fusion reactions. Increasing density often leads to instability and a loss of energy confinement, hampering the overall efficiency. The DOE noted in a press release that its scientists at the DIII-D National Fusion Facility were able to transcend the density limit “while simultaneously maintaining high confinement quality.” “Reaching this new and attractive operational space involved pushing the known limits of a specific approach for fusion device operation,” it noted.

DIII-D team’s approach

By experimenting with the advanced high poloidal beta scenario, the DIII-D team achieved an unexpected synergy: increased core density gradients led to greater plasma turbulence suppression and increased confinement quality, which subsequently allowed for even higher densities. They report this is the first time researchers have simultaneously achieved density above a historical empirical limit (>1.0 times the Greenwald limit) with very good confinement (~1.5 times better than standard). Nature has accepted a manuscript on the research.

Additionally, the increased density at the plasma edge suppressed edge instabilities typical of high-confinement plasmas and reduced plasma temperature near the surrounding walls, both effects extremely favorable for wall durability in a fusion power plant. This maintenance of a stable and cool plasma edge with high power in the core — referred to as “core-edge integration” —  addresses another central challenge for tokamaks.

University of Wisconsin–Madison researchers announce advance in tokamak plasma stability

In related news, researchers at the University of Wisconsin–Madison’s Wisconsin Plasma Physics Laboratory (WiPPL) have also announced an advance in tokamak plasma stability. Using the Madison Symmetric Torus (MST) device, researchers at the school produced a tokamak plasma that remained stable at 10 times the Greenwald limit.

“Our discovery of this unusual ability to operate far above the Greenwald limit is important for boosting fusion power production and preventing machine damage,” said Noah Hurst, a scientist with the Wisconsin Plasma Physics Laboratory (WiPPL) and lead author on the study, in a press release.

The WiPPL researchers published their findings in Physical Review Letters on July 29, 2024. While the research may have implications for tokamak fusion reactor design, the scientists stress that their plasma research is not equivalent to a fusion reactor.