By Ashwini Sakharkar 21 Aug, 2024
Collected at: https://www.techexplorist.com/liquid-metal-evaporator-protect-inside-tokamak-intense-heat/87515/
In the next generation of fusion vessels, the spherical tokamaks, scientists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have conceived an innovative approach. They imagine a hot region with flowing liquid metal, reminiscent of a subterranean cave, that could potentially revolutionize the field.
The concept of using evaporating liquid metal to shield the inside of the tokamak from the intense heat of the plasma is not only cutting-edge but also rooted in decades of research and the Lab’s expertise in working with liquid metals.
“PPPL’s expertise in using liquid metals, particularly liquid lithium, for enhanced fusion performance is helping refine ideas about how it can best be deployed inside a tokamak,” said Rajesh Maingi, PPPL’s head of tokamak experimental science and co-author of a new paper in Nuclear Fusion detailing the proposed placement of the lithium.
Recently, groundbreaking research has been focused on pinpointing the optimal location for a lithium vapor “cave” within the fusion vessel using advanced computer simulations. To achieve the goal of commercial fusion, every aspect of the doughnut-shaped tokamak must be meticulously placed.
The ingenious concept behind the lithium vapor cave is to strategically position the lithium in the boundary layer, away from the intensely hot fusing core plasma, yet close to the excess heat. By utilizing an evaporator, a heated surface designed to vaporize lithium atoms, the lithium vapor particles are directed toward the ideal location where the majority of the excess heat accumulates.
After considering three potential placement options, the scientists have determined, through multiple computer simulations, that the optimal location for the lithium vapor cave is near the bottom of the tokamak by the center stack. These new simulations represent a significant advancement as they are the first to take into account collisions between neutral particles, which do not possess a net positive or negative charge.
“The lithium evaporator really does not work unless it is placed in the private flux region,” said Eric Emdee, an associate research physicist at PPPL and lead author of the new paper.
When the lithium is evaporated in the private flux region, it transforms into positively charged ions, creating a shield of heat-absorbing particles that safeguard the nearby walls from excessive temperatures. The ionized lithium particles then align with the magnetic fields of the plasma, distributing and dissipating the heat over a larger area within the tokamak. This process significantly reduces the risk of components melting.
The private flux region is strategically targeted for the introduction of evaporated lithium to maintain the purity of the core plasma, which needs to remain at high temperatures. This approach ensures that the core plasma remains uncontaminated while allowing the lithium to effectively mitigate heat before dispersing into the surrounding environment.
The researchers initially believed that containing lithium in a “metal box” with an opening at the top would be the most effective approach. This setup was designed for the plasma to flow into the gap, allowing the lithium to dissipate the heat of the plasma before reaching the metal walls.
However, the researchers now propose that a cavity, essentially just the inner half of a box filled with lithium vapor, would be a simpler and more impactful solution. This adjustment goes beyond mere terminology; it significantly influences the path of lithium and its heat dissipation efficiency.
“For years, we thought we needed a full, four-sided box, but now we know we can make something much simpler,” said Emdee. Data from new simulations pointed them in a different direction when the research team realized they could contain the lithium just as well if they cut their box in half. “Now we call it the cave,” Emdee said.
In the proposed cave configuration, the device features walls strategically positioned to optimize the path for evaporating lithium, allowing it to capture the maximum amount of heat from the private flux region while simplifying the overall device design.
However, PPPL scientists have introduced another innovative approach that could achieve the same heat-quenching effect without the need for significant modifications to the tokamak’s wall shape. This alternative method involves the rapid flow of liquid lithium under a porous, plasma-facing wall located at the divertor, where the excess heat impacts the tokamak the most.
The porous wall facilitates the direct penetration of the lithium to the surface facing the plasma heating, ensuring that liquid lithium is delivered precisely to the area of the highest heat intensity. This capillary porous system, detailed in an earlier paper published in the journal Physics of Plasmas, presents a compelling solution to effectively manage heat in tokamak devices.
The lead author of that groundbreaking paper, PPPL Principal Engineering Analyst Andrei Khodak, strongly advocates for the use of a porous plasma-facing wall as an independent solution rather than embedded tiles in the tokamak. Khodak emphasized the advantage of the porous plasma-facing wall, highlighting that it eliminates the need to alter the shape of the confinement vessel, offering a more flexible and efficient approach. This perspective was further reinforced by Khodak and former Lab Director Robert Goldston, who co-authored the new paper.
Furthermore, the introduction of lithium evaporation on the diverter surface establishes a compelling link between the plasma edge and the plasma-facing component, significantly impacting heat and mass transfer. The intricate interplay between plasma heating and lithium evaporation leads to a dynamic alteration in the plasma heat flux to the liquid lithium plasma-facing component.
This pivotal insight is meticulously explained in a recent paper by the same authors, published in IEEE Transactions on Plasma Science, providing a comprehensive account of this formidable two-way coupling.
As PPPL scientists and engineers forge ahead, their relentless pursuit of testing and refining these innovative concepts underscores their unwavering commitment to advancing fusion as a vital component of the power grid.
Journal reference:
- E.D. Emdee, R.J. Goldston, A. Khodak and R. Maingi. Optimization of lithium vapor box divertor evaporator location on NSTX-U using SOLPS-ITER. Nuclear Fusion, 2024; DOI: 10.1088/1741-4326/ad57d2
Leave a Reply