Developments on two lithium vapor-box linear test-stand experiments

The lithium vapor-box divertor is a possible fusion power exhaust solution. It uses condensation pumping to create a gradient of vapor density in a divertor slot; this should allow a stable detachment front without active feedback. As initial explorations of the concept, two test stands which take the form of three connected cylindrical stainless steel boxes are being developed: one without plasma at PPPL, to test models of lithium evaporation and flow; and one for the linear plasma device Magnum-PSI (at DIFFER in Eindhoven, The Netherlands) to test the ability of a lithium vapor cloud to induce volumetric detachment and redistribute the plasma power. The first experiment uses boxes with diameters of 6cm, joined by apertures with diameters of 2.2cm. Up to 1g of Li is placed in one box, which is heated to up to 600°C. The Li evaporates, then flows to and condenses in the two other, cooler boxes over several minutes. The quantity of Li transported is assessed by weighing the boxes before and after the heating cycle, and is compared to the quantity predicted to flow for the box at its measured temperature using a Direct Simulation Monte Carlo code, SPARTA. With good experimental conditions, the two values agree to within 15%. The experiment on Magnum-PSI is in the conceptual design stage. The design is assessed by simulations using the code B2.5-Eunomia. They show that when the H⁺ plasma beam, with n_e=4×10²⁰m⁻³, T_e=1.5eV, and r=1cm, is passed through a 16cm long, 12Pa, 625°C Li vapor cloud, the plasma heat flux and pressure on the target are significantly reduced compared to the case without Li. With the Li present, the plasma is cooled by excitation of Li neutrals followed by radiation until it volumetrically recombines, lowering the heat flux from 3.7MWm⁻² to 0.13MWm⁻², and the pressure is reduced by 93%, largely by collisions of H⁺ with Li⁰.