Quenching the intense heat of a fusion plasma may require a well-placed liquid metal evaporator

Inside the next generation of fusion reactors, called spherical tokamaks, scientists at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have imagined a hot region flowing with liquid metal, reminiscent of an underground cave. The researchers say that the evaporation of the liquid metal could protect the interior of the tokamak from the intense heat of the plasma. The idea goes back decades and is tied to one of the lab’s strengths: working with liquid metals.

“PPPL’s expertise in using liquid metals, particularly liquid lithium, to enhance fusion performance is helping to refine ideas on how best to deploy them inside a tokamak,” said Rajesh Maingi, PPPL’s head of experimental tokama science and co-author of a new paper in Nuclear fusion detailing the lithium vapor cave.

Recently, researchers ran computer simulations to find the best location for a lithium vapor “cavern” inside the fusion reactor. To achieve commercial fusion, each part of the doughnut-shaped tokamak must be precisely placed. The idea behind a lithium vapor cavern is to keep lithium in the boundary layer away from the hot molten plasma of the core, but close to the excess heat.

An evaporator, a heated surface to boil lithium atoms, moves the lithium vapor particles along the ideal trajectory to where most of the excess heat tends to accumulate. The scientists considered three possibilities for the location of the cave. The lithium vapor cave could be located at the bottom of the tokamak near the central stack, in an area known as the private flux region; it could be on the outer edge, which is known as the common flux region; or the lithium vapor could come from both regions.

The results of several computer simulations determined that the best location for the lithium vapor cave is near the bottom of the tokamak, near the central stack. The new simulations provide additional insight: They are the first to account for collisions between neutral particles, which have neither a net positive nor a net negative charge.

“The lithium evaporator doesn’t really work unless it’s placed in the private flux zone,” said Eric Emdee, a research physicist at PPPL and lead author of the new paper. When lithium evaporates in the private flux zone, the particles become positively charged ions in a region with lots of excess heat, shielding nearby walls. Once the lithium particles are ionized, they obey the same magnetic fields as the plasma, spreading and dissipating the heat so that it hits a larger area of ​​the tokamak and reducing the risk of component meltdown.

The private flow region is also the ideal target for evaporated lithium because it is separated from the central plasma, which needs to stay hot. “You don’t want your central plasma to get dirty with lithium and cool down, but you also want the lithium to dampen the heat a little bit before it leaves the cave,” he said.

Holding Lithium: Box vs. Cavern

Researchers initially thought that lithium would be best housed in a “metal box” with an opening at the top. The plasma would flow into the gap, allowing the lithium to dissipate heat from the plasma before reaching the metal walls. Now, researchers say that a cave—geometrically just the inside half of a box—filled with lithium vapor would be simpler than a box. The difference isn’t just semantics: it affects where the lithium goes and how effectively it dissipates heat.

“For years, we thought we needed a solid box with four sides, but now we know we can make something much simpler,” Emdee said. Data from new simulations pointed them in a different direction when the research team realized they could contain the lithium just as well by cutting their box in half. “Now we call it the cave,” Emdee said.

In the cave configuration, the device would have walls at the top, bottom, and side closest to the center of the tokamak. This optimizes the lithium evaporation path, putting it on a better trajectory to capture the most heat from the deprived flow region while minimizing the complexity of the device.

Consider a capillary porous system to suck up lithium

Another approach proposed by the PPPL scientists in the new paper could achieve the same thermal quenching effect without radically changing the shape of the tokamak walls. In this approach, liquid lithium flows rapidly under a porous wall facing the plasma. This wall would be located where excess heat impacts the tokamak the most: at the divertor.

The porous wall allows lithium to penetrate the surface directly in front of the plasma heater, so that liquid lithium is delivered exactly where it is needed most: in the area where the thermal intensity is highest. This capillary porous system is explained in a previous article published in the journal Plasma physics.

The study’s lead author, Andrei Khodak, a senior engineering analyst at PPPL, said he preferred the idea of ​​using a plasma-facing porous wall alone, in the form of tiles embedded in the tokamak. “The advantage of the plasma-facing porous wall is that you don’t have to change the shape of the containment vessel. You can just change the tile,” Khodak said. Khodak was also a co-author of the new study, along with former lab director Robert Goldston.

Lithium evaporation on the divertor surface leads to a strong coupling between the plasma edge and the plasma-facing component in terms of heat and mass transfer, because heating of the plasma will cause lithium evaporation, which in turn will change the heat flux from the plasma to the plasma-facing component to liquid lithium. A new model, described in a paper by the same authors in IEEE Transactions on Plasma Scienceexplains this strong bidirectional coupling. PPPL scientists and engineers will continue to test and develop their ideas as part of their core mission of making fusion an important part of the power grid.