Speaker
Description
Plasma-facing components (PFCs) primarily made of tungsten face significant lifetime limitations due to neutron embrittlement, dust formation, local melting and cracking, which threatens the high availability required for future fusion power plants [1,2]. Liquid-metal (LM) based PFCs have emerged as a promising route to overcome these limitations thanks to their intrinsic replenishment capability, providing so-called self-healing behaviour. Two main LM concepts have been explored since the 1970s: (i) direct flowing LM along plasma-facing walls [3], particularly in the divertor, and (ii) Capillary Porous Systems (CPS), where LM is retained inside a porous network by capillary forces to prevent splashing [4]. Both approaches rely on low-melting-point metals such as Li or Sn, while CPS additionally requires a refractory matrix, typically tungsten.
However, plasma–surface interactions may still lead to sputtering or evaporation of both low-Z and high-Z species, resulting in possible plasma contamination. The net impact depends on the fraction of emitted impurities that are redeposited on the surface, a process strongly influenced by the grazing magnetic field and the electric field structure within the sheath [5]. Quantifying this redeposition fraction, and distinguishing the physics associated with sputtering-driven versus evaporation-driven emission, is therefore essential for assessing LM-based PFC viability.
In this work, different edge-plasma conditions in density and temperature are simulated using a home-developed 1D–3V Particle-in-Cell code that self-consistently computes the electric potential in the sheath and pre-sheath. Impurity test particles (W, Sn, Li) are injected at the wall and their trajectories are tracked until redeposition on the emitting surface or loss toward the plasma core.
The simulations enable a systematic comparison of redeposition rates as a function of (i) the emission process—sputtering versus evaporation, which produce different initial velocity distributions and therefore ionization mean free paths, (ii) plasma conditions, (iii) incident particle energy, and (iv) the ion species responsible for sputtering. This analysis will make it possible to identify the combinations of plasma edge parameters and emission mechanisms that favour impurity return to the surface rather than escape, thereby helping to determine the operating windows in which LM-based divertor components can function without unacceptable plasma contamination.
[1] R. Pitts et al, Nucl. Mater. Energy 20, 100696 (2019)
[2] Y Corre, the WEST team et al. Phys. Scr. 96, 124057 (2021)
[3] L.E. Zakharov, Phys. Rev. Lett. 90, 045001 (2003)
[4] J.G.A. Scholte et al, Nucl. Mater. Energy 37, 101522 (2023)
[5] RJ. Guterl et al, Nucl. Mater. Energy 47, 100948 (2021)