Speaker
Description
Recent developments in advanced nuclear fusion reactors consider the use of solid plasma-facing components (PFCs), typically made of tungsten. However the emergence of new compact nuclear fusion reactor concepts, presented as more viable for commercial applications, can lead, due to the reduced plasma wetted surface, to heat fluxes much higher than the 15MW/m$^2$ estimated for the ITER divertor. Consequently, the periodic replacement of PFCs will become more frequent due to faster structural degradation and erosion. To overcome this issue, several concepts consider the use of liquid metal, generally lithium (Li) or tin (Sn), as a plasma-facing material.
The physics of liquid lithium PFCs differs significantly from that of solid tungsten. Simulations performed with a 1d3v Particle-in-Cell code show that a high evaporated flux of lithium from the wall, once ionized and interacting with the magnetized plasma, can substantially modify both the sheath and presheath structures. These modifications alter the potential drop and influence in-sheath redeposition. The study further identifies four key parameters governing these boundary conditions: (i) the ratio of lithium influx to hydrogen-isotope outflux, (ii) the ionization mean free path, (iii) the magnetic-field strength, and (iv) the magnetic-field inclination.
If Li impurities are not redeposited due to the sheath and presheath electromagnetic field, they enter the Scrape-Off layer and are subjected to a competition between parallel (to B) and perpendicular transport. Therefore lithium can cross the separatrix and penetrate the core plasma. As a light species, Li is mainly transported by turbulence, with neoclassical contributions playing only a minor role. Using the global gyrokinetic code GYSELA, a new method was developed and benchmarked to determine impurity turbulent-transport coefficients, enabling the separate evaluation of diffusion, thermodiffusion, and pure convection. In the absence of a transport barrier at the core edge, the study finds impurity-transport trends similar to those observed for helium, with a much stronger outward thermodiffusive flux than in the case of tungsten. Introducing a transport barrier, however, reduces turbulence intensity, and thus turbulent transport, while generating a dominant inward convective contribution inside the barrier due to strong $E \times B$ shear. An analysis of the peaking factor shows that steady-state lithium accumulation in the core results from a balance between pure convection and thermodiffusion, underscoring the crucial role of the transport barrier in limiting Li contamination of the core plasma.