Fusion’s Breakthrough: Liquid Metal Revolutionizes Heat Management

liquid metal fusion heat management

Researchers at the Princeton Plasma Physics Laboratory (PPPL) are at the forefront of utilizing liquid lithium within spherical tokamaks to bolster fusion energy output.

Recent computational models highlight the strategic placement of lithium vapor as a protective barrier for the tokamak’s interior, shielding it from the extreme thermal conditions produced by the plasma.

Novel structural concepts, such as the lithium “cave” and porous, plasma-facing surfaces, promise to streamline design complexities and enhance thermal dissipation, paving the way for the future of fusion energy.

In the cutting-edge fusion reactors known as spherical tokamaks, scientists at the U.S. Department of Energy’s PPPL are exploring an innovative idea: a hot zone with liquid metal, akin to a subterranean cave, designed to shield the interior from the intense plasma heat. This concept, which harkens back several decades, leverages one of the Lab’s key strengths—expertise in liquid metals.

Progress in Tokamak Design through Liquid Metal Innovation

“PPPL’s deep knowledge of liquid metals, especially liquid lithium, is refining our approach to its application within a tokamak,” remarked Rajesh Maingi, head of tokamak experimental science at PPPL and co-author of a recent Nuclear Fusion paper detailing the optimal lithium placement.

Scientists have been conducting simulations to determine the ideal positioning of a lithium vapor “cave” within the fusion device.

Achieving commercial fusion requires the precise arrangement of each component in the toroidal-shaped tokamak. The lithium vapor cave concept is designed to keep lithium within the boundary layer, away from the hot, core plasma, while still near the areas where excess heat accumulates.

A heated surface, functioning as an evaporator, releases lithium atoms, directing them to regions where heat is most concentrated.

Researchers evaluated three potential locations for the lithium vapor cave. It could be positioned near the center stack at the bottom of the tokamak, in the area known as the private flux region; it could be placed at the outer edge, or common flux region; or it could stem from both regions.

Recent simulations have pinpointed the private flux region, near the tokamak’s center stack, as the most effective location for the lithium vapor cave.

These simulations are the first to incorporate neutral particle collisions, offering a more comprehensive understanding of the lithium’s behavior.

Advantages of Lithium Placement in the Private Flux Region

“The evaporator is only effective when positioned in the private flux region,” explained Eric Emdee, a research physicist at PPPL and lead author of the Nuclear Fusion paper.

When lithium is evaporated in this region, it becomes positively charged ions, interacting with the excess heat and protecting the adjacent walls.

Once ionized, lithium particles follow the magnetic field lines of the plasma, spreading and dissipating heat across a broader area, thus reducing the risk of damage to the tokamak’s components.

The private flux region serves as the optimal target for evaporated lithium since it remains separate from the core plasma, which must stay hot. “It’s crucial to keep the core plasma uncontaminated by lithium, yet still allow the lithium to mitigate heat before exiting the cave,” Emdee added.

Reconceptualizing Lithium Containment:

From Box to Cave Initially, researchers believed that a “metal box” with an opening at the top would be the best containment for lithium, allowing plasma to flow into the gap so the lithium could absorb heat before it reached the metal walls.

However, they now propose that a cave—a simpler geometric form of the box—would be more effective. This shift in thinking is more than a semantic difference; it fundamentally alters how lithium disperses heat.

“For years, the assumption was that a full, four-sided box was necessary, but now we’ve found that a simpler design is just as effective,” Emdee said. Data from the new simulations led the research team to realize that they could achieve the same containment and heat dissipation by halving the box. “We now refer to it as the cave,” Emdee noted.

In this cave configuration, the structure would include walls on the top, bottom, and the side closest to the tokamak’s center. This setup optimizes the path for evaporating lithium, guiding it to capture the most heat from the private flux region while reducing the overall complexity of the device.

Innovative Heat Management Strategies

Another approach proposed by PPPL scientists suggests achieving similar heat dissipation without significantly altering the tokamak’s wall structure. In this method, liquid lithium would flow beneath a porous, plasma-facing wall located at the divertor, where excess heat is most intense.

This wall allows lithium to penetrate its surface, delivering liquid metal precisely where it’s most needed. This capillary porous system, detailed in an earlier Physics of Plasmas paper, is designed to place liquid lithium directly into areas of highest thermal stress.

Andrei Khodak, PPPL Principal Engineering Analyst and lead author of that earlier study, favors using a porous plasma-facing wall independently, as tiles embedded in the tokamak. “

The benefit of the porous plasma-facing wall is that it doesn’t require changing the confinement vessel’s shape; you simply swap out the tiles,” Khodak explained. He also contributed to the recent paper alongside former Lab Director Robert Goldston.

Utilizing lithium evaporation on the divertor surface creates a strong coupling between the plasma edge and the plasma-facing component in terms of heat and mass transfer.

The heat from plasma causes lithium to evaporate, which then modifies the plasma heat flux to the liquid lithium component. A new model, described by the same authors in IEEE Transactions on Plasma Science, accounts for this dynamic interaction.

PPPL scientists and engineers will continue to test and refine these concepts as part of their ongoing mission to make fusion a viable component of the global energy grid.

Reference

“Optimization of lithium vapor box divertor evaporator location on NSTX-U using SOLPS-ITER” by E.D. Emdee, R.J. Goldston, A. Khodak, and R. Maingi, July 2, 2024, Nuclear Fusion.

The work presented in the new Nuclear Fusion paper was supported by the Department of Energy under contract number DE-AC02-09CH11466. The Physics of Plasmas study was also backed by the DOE, Office of Science, Office of Fusion Energy Sciences, and conducted by Princeton University under the same contract number.