University-industry partnership identifies new solutions for mirror fusion devices

UW–Madison researchers partnered with Realta Fusion to publish a performance assessment identifying trade-offs between power plant design variables and materials for mirror fusion device components.

Hitarth Shah, a graduate student in the Department of Nuclear Engineering and Engineering Physics, first-authored the publication in collaboration with his advisor, Associate Professor Ben Lindley; Tim Bohm, a scientist at UW-Madison; and researchers from Realta Fusion, a startup company that was co-founded by Lindley and other UW faculty members. 

“This collaboration leveraged UW-Madison’s world-leading expertise in fusion neutronics to support Realta Fusion in the development of a neutronic modelling capability for the tandem mirror, as well as advancing the science and engineering of tandem mirror fusion technology,” says Lindley. 

Magnetic mirror fusion devices confine plasma between two magnets, causing energetic particles to bounce back and forth in a mirror-like effect. Magnetic mirrors were popular in early fusion research but have only recently resurfaced as a viable path to fusion due to new physics and magnet engineering developments. 

Advancing to the next stage of magnetic mirror technologies requires thorough evaluation of their neutronics performance—or how well the device manages neutron behavior. Each fusion reaction produces a free, high-energy neutron. It’s important to understand how those neutrons interact with components surrounding the plasma, especially because those neutrons are a valuable commodity that the device can turn into tritium fuel. 

The most common fuel used for fusion reactors is a combination of deuterium and tritium. However, tritium does not occur naturally in sufficient quantities. To solve this problem, future fusion energy systems are being designed with mechanisms in place to produce their own tritium, making them self-sufficient. 

In many fusion devices, that mechanism is a lithium breeding blanket. The high-energy neutrons produced by fusion reactions collide with lithium atoms in the blanket to produce tritium. Breeding blankets also perform the essential function of shielding surrounding components from radiation.

The team investigated both tritium breeding performance and shielding performance through two studies. The first study evaluated the shielding effectiveness of four candidate magnet materials. The second study evaluated the tritium breeding performance of four breeding blanket materials, testing a range of blanket geometries and lithium enrichment levels for each material.

They used a parametric neutronics toolkit called ParaTAN for their analysis. Developed at UW–Madison in part through the collaboration with Realta, ParaTAN automates much of the neutronics workflow.

The studies revealed several key findings. A compound made of tungsten and boron emerged as the highest-performing shield material. The tool also highlighted the trade-offs associated with different breeding materials. For example, the helium-cooled pebble bed reactor design achieved the strongest tritium breeding performance but proved less desirable in other aspects. Other configurations, like lead lithium eutectic, performed well and offers benefits that the pebble bed design does not.

“Despite some major physics and engineering milestones—many of them from the WHAM machine at UW–Madison’s Physical Sciences Laboratory—mirror concepts have not been well explored in comparison to tokamak and stellarator concepts,” says Shah. “This tool will support rapid and extensive design exploration for magnetic mirror machines.”

Looking ahead, the team is working to extend ParaTAN’s capabilities for more detailed neutronics analysis and analysis of additional domains like tritium transport and thermal hydraulics. The collaboration with Realta Fusion will continue with the design of a new lead lithium MHD loop at UW–Madison, an effort led by Associate Professor Juliana Pacheco Duarte as part of the DOE-funded FIRE collaborative. 

Featured image caption: Hammir-DT, Realta Fusion’s preconceptual design for a deuterium-tritium tandem magnetic mirror fusion power plant.