Recent fusion simulations conducted within tokamaks have pinpointed the optimal location for a ‘cave’ containing liquid lithium. This ideal area is located towards the bottom, near the center stack, where evaporating metal particles can effectively dissipate excess plasma heat.
Scientists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have envisioned a heated area resembling an underground cave, filled with flowing liquid metal, in the next generation of fusion devices known as spherical tokamaks. They propose that liquid metal evaporation can shield the tokamak’s interior from the extreme heat generated by plasma. This concept has been in discussion for many years and highlights one of the laboratory’s key areas of expertise: liquid metal utilization.
“Our proficiency at PPPL in leveraging liquid metals, particularly liquid lithium, to enhance fusion efficiency is refining our understanding of its best usage within a tokamak,” explained Rajesh Maingi, head of tokamak experimental science at PPPL and co-author of a recent paper published in Nuclear Fusion regarding the lithium vapor cave.
Researchers have recently employed computer simulations to identify the optimal placement for the lithium vapor “cave” within the fusion reactor. For practical fusion energy production, precise positioning across the doughnut-shaped tokamak is critical. The lithium vapor cave is intended to keep the lithium in a boundary layer, shielding it from the core plasma while being close enough to manage excess heat. An evaporator, which heats surfaces to vaporize lithium, directs the vapor particles toward areas where excess heat accumulates. The team evaluated three potential cave positions: near the bottom of the tokamak in a region called the private flux area; on the outer edge of the tokamak, referred to as the common flux region; or from both locations.
The outcomes from numerous computer simulations suggest that the optimal site for the lithium vapor cave is at the bottom of the tokamak by the center stack. These new simulations incorporate additional data, marking the first instance where collisions between neutral particles—those lacking an overall charge—are taken into account.
“The lithium evaporator functions effectively only when it is situated in the private flux area,” noted Eric Emdee, an associate research physicist at PPPL and lead author of the new paper. In the private flux region, evaporated lithium forms positively charged ions in an area that contains a significant amount of excess heat, thus safeguarding the nearby structures. Once ionized, the lithium particles follow the same magnetic fields as the plasma, distributing and dissipating heat more broadly across the tokamak to minimize melting risks for its components.
The private flux area effectively separates evaporated lithium from the core plasma, which must remain at high temperatures. “We need to ensure that the core plasma doesn’t get contaminated with lithium and cool down, while still allowing lithium to perform some heat management before exiting the cave,” he elaborated.
Comparing containment methods: box versus cave
Initially, researchers believed that enclosing the lithium in a “metal box” with an opening at the top would be the most effective method. Plasma would enter through this opening, enabling the lithium to absorb excess heat before it reached the metallic walls of the vessel. However, the team now proposes that a cave structure—a geometric simplification of a box—filled with lithium vapor would be more efficient. This change is not merely a matter of terminology; it significantly influences the path of the lithium and its heat dissipation capabilities.
“For years, we thought a complete four-sided box was necessary, but we now realize we can create a simpler design,” remarked Emdee. Insights from recent simulations led the research team to conclude that they could effectively contain the lithium by simply halving the box, thus designating it a ‘cave.’
In this cave configuration, walls would be present on the top, bottom, and the side closest to the center of the tokamak. This setup optimizes the trajectory for the evaporating lithium, maximizing its absorption of heat from the private flux area while reducing device complexity.
Exploring a capillary porous system for lithium management
Another innovative idea from PPPL scientists in the new paper involves liquid lithium flowing swiftly beneath a porous, plasma-facing wall. This wall would be positioned at locations where the excess heat impacts the tokamak most intensely—specifically at the divertor. The porous material allows lithium to penetrate directly into the area most affected by plasma heating, ensuring liquid lithium reaches precisely where it’s needed most: the spot with the highest thermal intensity. This capillary porous system has been previously detailed in a paper published in the journal, Physics of Plasmas.
The lead author of that paper, PPPL Principal Engineering Analyst Andrei Khodak, expressed a preference for using a porous plasma-facing wall as standalone tiles embedded within the tokamak, stating, “The benefit of a porous plasma-facing wall is that it does not require changing the shape of the confinement vessel; only the tiles need adjustment.” Khodak also co-authored the new paper alongside former Lab Director Robert Goldston.
Implementing lithium evaporation at the divertor surface fosters strong interactions between the plasma edge and the plasma-facing component, enhancing heat and mass transfer. Heating from the plasma instigates lithium evaporation, which subsequently affects the plasma’s heat flux to the liquid lithium interface. A recent model, elaborated in a paper by the same authors in IEEE Transactions on Plasma Science, considers this robust two-way interaction. The scientists and engineers at PPPL will continue to explore and advance these concepts as part of their ongoing mission to make fusion a viable energy source for the power grid.
This research was supported by the DOE’s work under contract number DE-AC02-09CH11466. The manuscript on the Physics of Plasmas was produced with the support of the DOE, Office of Science, Office of Fusion Energy Sciences, in collaboration with Princeton University under contract number DE-AC02-09CH11466.
