Experimental and modeling studies for the development of the lithium vapor-box divertor

Abstract

The lithium vapor box is a concept for a divertor that can handle the extreme heat fluxes in future fusion reactors. Within a slot lined with capillary-porous material, Li vapor induces plasma detachment by cooling the plasma until it volumetrically recombines, lowering the heat flux on the divertor surfaces. The vapor is localized within the slot by condensation on walls near its entrance and by friction with the plasma. The concept is early in its development.

Two experiments are described—one completed, one proposed—in support of the lithium vapor-box divertor. They use a linear geometry for reduced size and complication, rather than being integrated into a tokamak. They each take the form of three connected cylindrical stainless steel boxes, one of which is heated to 900 K to evaporate Li.

The first studies the evaporation and flow of vapor without plasma, in order to demonstrate that it is possible to create a cloud of lithium vapor of specified density within an open box. We measure the temperature of the heated box, initially loaded with 1 g of Li, during a heating cycle, and measure the vapor mass flowing out during the cycle by weighing the box before and after. We compare the measured mass with a value calculated from simulations using the code SPARTA incorporating the measured temperatures in the boundary conditions. In runs with good experimental conditions, the two agree to within 15%. The technology developed for this first experiment is to be used in the second.

The second, not yet built, studies the interaction of a 4e20/m³, 1.5 eV, 1 cm radius, magnetized plasma beam, supplied by the Magnum-PSI device, with a 16 cm long Li vapor cloud. Its goal is to demonstrate that Li vapor can induce recombination within the box and cause the beam's power to flow to the box walls rather than to the target. In simulations with the code B2.5-Eunomia, a 12 Pa vapor cloud reduces the plasma pressure at the target by 93%, largely via ion-neutral collisions, and the heat flux there is reduced from 3.7 MW/m² to 0.13 MW/m² by plasma recombination and cooling via Li excitation.