Could Fusion Reactors Use Lithium Vapour Caves As Chamber Coolants?

If we are to see commercial fusion reactors soon then scientists and engineers have several hurdles to overcome. The primary challenge is to find an effective design with the right materials and processes to manage the extreme heat of the plasma within a confined space so that the reactor vessel survives and the energy produced can then serve to generate usable electricity.

Controlling hot plasmas created through the fusing of atoms has proven to be much harder than fusion itself. How do you keep hot plasmas stable and where do you keep them?

The most common designs include tokamaks that look like fat doughnuts, and stellarators that are like bagels wrapped in spaghetti. Then there is Vancouver-based General Fusion with a spherical chamber surrounded by giant hammers.

At the Princeton University Plasma Physics Laboratory (PPPL), scientists have proposed using a lithium vapour cave and porous plasma-facing wall within a tokamak. PPPL envisions adding a lithium vapour cave containing liquid lithium metal to fusion reactor designs. The liquid lithium would go through a phase change turning into vapour when exposed to the heat of the plasma contained within the reaction chamber.

A paper published in the journal IOPScience on July 2, 2024, describes the concept, calling it a lithium vapour cave with “a detached divertor design.” The cave keeps the liquid lithium contained within the boundary structure of the tokamak while being close enough to absorb the heat from the plasma causing lithium atoms to boil off and become vapour.

PPPL has done simulations testing two proposed locations: a“private flux region” and a “common flux region.” The former would be located in the bottom centre of the tokamak while the latter would encase the outer edge. Simulations of both have concluded that the private flux region would prove to be a better option. States Eric Emdee, Associate Research Physicist at PPPL, “You don’t want your core plasma to get dirty with lithium and cool, but you also want the lithium to do some heat mitigation before it leaves the cave.”

Originally, the lithium vapour cave was described as a full, four-sided metal box with a small opening at the top. It has since evolved (see illustration below) to be described as a cave with solid walls on top, bottom, and one side, with the opposite side facing the tokamak’s plasma chamber made of porous tile material. Liquid lithium would penetrate the porous wall and vaporize when exposed to heat, acting as a coolant.

The lithium liquid-to-vapour exchange could serve future fusion reactors as a solution for keeping their plasma cores from eroding or melting the tokamak’s walls. This type of heat management within fusion reactor containment vessels could accelerate the development of this zero-emission, zero-pollution sustainable energy source. Now, the PPPL team needs to translate the conceptual design to reactor scale to determine its practicality and maybe become the game-changer for commercial fusion energy to come of age sooner than later in the 21st century.