Description
Although modern fusion devices consider the use of lithium as a plasma-facing component [1], its transport mechanisms remain under-investigated, as they differ from those of heavy impurities such as tungsten. For lithium and other light impurities, turbulent transport dominates over neoclassical contributions [2], which motivates a dedicated study to determine the turbulent diffusion, thermodiffusion, and convective pinch velocity.
In this work, we benchmark a new method to extract these coefficients using the global flux-driven gyrokinetic code GYSELA [3,4]. The method is applied in the framework of adiabatic electrons, in a plasma turbulence regime dominated by Ion-Temperature Gradient (ITG) instabilities, one of the main micro-instabilities responsible for turbulent transport in tokamaks. Radial profiles of diffusion and thermodiffusion coefficients, as well as pinch velocities, are derived [5].
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 ExB 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.
As tungsten and tin are the main solid and liquid alternatives to lithium, we also propose a comparison between tungsten, tin, and lithium, highlighting the impact of ion mass and charge on the turbulent to neoclassical flux ratio and the relative magnitudes of the transport coefficients.
[1] T.W. Morgan et al. Plasma Phys. Control. Fusion, 60(1):014025, 2018
[2] K. Lim et al. Nucl. Fusion, 61(4):046037, 2021
[3] V. Grandgirard et al. Comput. Phys. Commun., 207:35-68, 2016
[4] G. Lo-Cascio et al. Nucl. Fusion, 65(5):056021, 2025
[5] R. Avril et al. Submitted to Physics of Plasmas, 2026