R. Avril

Although modern fusion devices consider the use of lithium as a plasma-facing component, its transport mechanism remains under-investigated, being different from those of heavy impurities such as tungsten. For lithium and other light impurities, turbulent transport dominates over neoclassical contributions, motivating a dedicated study on the determination of turbulent diffusion, thermodiffusion, and convective pinch velocity. In this work, we benchmark a new method to extract these coefficients using the global full-f gyrokinetic GYSELA code. In the absence of a transport barrier, the method reproduces results consistent with helium, showing that thermodiffusion is comparable in magnitude to both diffusion and pure convection. Introducing an E×B shear layer to form a transport barrier modifies the radial structure of the transport coefficients. Transport levels decrease in the inner core, while within the barrier, convection becomes dominant, and all fluxes undergo sign reversals. Diffusive and thermodiffusive contributions are also found to partially compensate each other depending on local gradients. Finally, an evaluation of the peaking factor further demonstrates that the presence of a transport barrier significantly mitigates core impurity accumulation.

Compact nuclear fusion reactors will require advanced plasma-facing components capable of withstanding the higher heat and neutron fluxes characteristic of these machines. Liquid metal walls, based for instance on lithium, have emerged as a promising alternative to solid walls. This study investigates lithium redeposition on plasma-facing walls in a deuterium plasma environment using a self-consistent 1D3V Particle-in-Cell code. The model accounts for the interplay between lithium evaporation and the electric potential with a simplified but relevant physics. Four key parameters were analyzed for their impact on lithium redeposition: the ratio γs of the Li+ outflux over the D+ influx, the magnetic field inclination θ, the ionization mean free path Λiz, and the magnetic field intensity B. Results indicate that γs and θ significantly influence redeposition, leading to redeposition rates higher than 95%, while variations in Λiz and B had secondary effects. In general, an increase in one of these parameters drives to a lower redeposition, except for θ, where potential “bumps” formed under specific conditions, alters ion trajectories and results in a non-monotonic redeposition dependency. The role of lithium in flattening the potential profile and its evaporation rate highlight the need to take Li impurity into account in the calculation of the potential profile.