Speaker
Description
In fusion devices like WEST, ASDEX Upgrade, EAST and ITER, tungsten (W) has been chosen as the plasma facing material for the divertor, where the heat and particles fluxes are the most intense. In particular, helium (He) irradiation leads to the formation of nano-sized bubbles in the subsurface area, which increase hydrogen isotopes retention. More dramatically, W-fuzz may form, presumably caused by the repeated bursting of bubbles reaching the surface. Understanding bubble migration and diffusion is therefore of prime importance.
We have used a nanoscience approach to characterize the bubbles in order to control separately the multiple factors met in a tokamak environment such as surface conditions, temperature evolution, microstructure, pre-existing defects and He irradiation. Well prepared single crystals were implanted below and above the displacement threshold of W using 400 eV or 2 keV He ions. Bubbles were characterized by Grazing Incidence Small Angle X-ray Scattering (GISAXS) at the ESRF synchrotron. This technique provides statistical information on the sample morphology. At ITER relevant temperature (1273 K), GISAXS measurements have revealed that bubbles are facetted. The equilibrium shape has been precisely described and consists of a truncated rhombic dodecahedron composed of {100} and {110} facets [1]. The growth kinetics of bubbles has been measured in real time during implantation by in-operando measurements. The growth mechanism has been identified as migration-coalescence [2].
We will present the latest results addressing the migration of bubbles. In complementary to in-operando GISAXS measurements, in-situ Transmission Electron Microscopy (TEM) measurements were performed. Thin lamellae extracted from implanted samples were heated from room temperature to 1100 °C, enabling direct visualization of bubble evolution events. Brownian diffusion of bubbles through the W matrix, coalescence leading to bubble growth and bursting events at the surface have been captured. The diffusion coefficient of bubbles was measured between 900°C and 1100°C for bubbles ranging from 1 to 4 nm in diameter by in-situ TEM and from 4 to 10 nm in diameter by in-operando GISAXS. In parallel, kinetic Monte Carlo simulations were conducted to confront experimental results. The diffusion coefficient decreases exponentially with the bubble diameter, which is consistent with a diffusion process hindered by the nucleation of a ledge on the bubble facets.
[1] L. Corso et al., Nucl. Mater. Energy 37, 101533 (2023).
[2] L. Corso et al., Nucl. Mater. Energy 42, 101894 (2025).