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In magnetic confinement nuclear fusion reactors, the tungsten (W) divertor (the main plasma-facing component) is exposed to extreme fluxes of helium (He) and hydrogen isotopes as well as high thermal loads. Despite tungsten’s strong thermo-mechanical properties (e.g. high melting point and high erosion resistance), the low solubility of He in W leads to the formation of near-surface He nano-bubbles [1]. These bubbles severely damage the microstructure, particularly when they burst [2], potentially creating what it is referred to as “fuzz” [3] and consequently affecting the properties and lifetime of the material [4]. The aim of this work is to study the formation and growth of He bubbles on W samples, through measurements of the size and spatial distribution of the bubbles formed near the surface, as well the estimation of the density of He trapped in each bubble, thus allowing us to calculate the pressure inside these bubbles.
In this study, two sets of samples were analyzed: Post-mortem W monoblocks extracted from the WEST divertor after the C4 He plasma campaign performed in 2019 [5], as well as W polycrystalline samples that were irradiated in the laboratory using a He plasma. The laboratory irradiations were performed at a constant fluence of 4.5.1023 m-², flux around 1.1019 m-²s-1, incident ions energies of 79 eV and at temperatures of 823 K or 973 K.
Bright-Field Transmission Electron Microscopy images proved the existence of sub-nanometer sized bubbles in the first 20 nm bellow the surface in the WEST samples, as well as showing an increase in overall bubble size as a function of the temperature in the laboratory samples. On the other hand, the He K-edge was acquired using STEM-EELS for various bubbles, resulting in the estimation of the He density in bubbles, using the framework developed by Walsh et al. [6]. The evolution of the He density as a function of the bubble size and the He K-edge energy shift, as well as the He pressure calculations will then be discussed and compared to previously published work.
[1] H. Iwakiri et al. JNM 283 – 287 (2000), 1134 – 1138
[2] M. Alfazzaa et al. NME 42 (2025), 101883
[3] K. Saito et al. NME 42 (2025), 101859
[4] S. Kajita et al. Nuclear Fusion 49 (2009), 095005
[5] E. Tsitrone et al. Nuclear Fusion 62 (2022), 076028
[6] C. A. Walsh et al. Philosophical Magazine A 80 (2000), 1507-1543