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Description
In the context of nuclear fusion, the inner walls of the reactor are exposed to extremely high ion fluxes, particularly at the divertor, which collects most of the particles escaping from the plasma. Tungsten (W) is used for this component because of its high melting point, erosion resistance, low hydrogen retention and good thermal conductivity[1]. However, helium implantation induces the formation of bubbles, which can alter the material properties. Moreover, tungsten oxidizes easily, even at room temperature, and this oxidation is enhanced in fusion environments due to oxygen impurities and extreme temperature[2]. The formation of a surface oxide layer can reduce the thermal conductivity and change the He retention [3]. It is therefore essential to understand how the presence and thickness of a thin oxide layer influence helium bubble formation and evolution.
In this work, we investigate helium implantation in W(110) single crystals covered with high-purity tungsten oxide thin films of controlled thickness. Oxide layers ranging from 7 to 50 nm were produced by thermal oxidation under low oxygen pressure (10⁻⁴ Torr) and characterized prior to implantation using grazing-incidence X-ray techniques. Helium implantation was performed at room temperature at 400 eV, below the displacement threshold of tungsten atoms, enabling us to isolate the effects induced solely by helium. The structural and morphological evolution of the samples was monitored in situ at the ESRF BM32 beamline.
During implantation, grazing incidence small angle X-rays scattering GISAXS measurements show a progressive broadening of the specular rod. Implantation was stopped at a fluence of 3 × 10²⁰ m⁻². After implantation, GISAXS patterns exhibit a diffuse low-q halo, indicating the formation of non-faceted helium bubbles. Post-mortem transmission electron microscopy reveals helium bubbles in the sample coated with the thinnest oxide layer (7 nm). In this case, the bubbles are homogeneously distributed throughout the WO₃ (~3 nm) layer and are larger than those observed in the tungsten substrate (~1 nm). In W, bubbles are mainly located at the W/WO₃ interface and remain detectable up to ~10 nm in depth. These results demonstrate that oxide thickness plays a decisive role in helium bubble formation, and is therefore a key parameter for understanding the behaviour of oxidized tungsten under fusion-relevant conditions.
Bibliography
[1] G. Laval, Nuclear Fusion: EDP Sciences, 2007.
[2] C. V. Ramana and al., J. Phys. Chem. B, vol. 110, pp. 10430–10435, 2006.
[3] A. W. Kleyn and al., Vacuum, vol. 80, pp. 1098–1106, 2006.