Microscopic and macroscopic investigations on He bubble formation in W via <i>in situ</i> spectroscopic ellipsometry

Abstract In future fusion reactors, a significant flux of helium (He) ions will impinge on plasma-facing components, tungsten (W) being the current preferred material, inducing near-surface microstructure evolution. In this study, the near-surface evolution of W upon He plasma irradiation is followed using a non-contact optical diagnostic method, namely in situ spectroscopic ellipsometry. We exposed W to high-fluence ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>≃</mml:mo> <mml:mstyle scriptlevel="0"/> <mml:mstyle scriptlevel="0"/> <mml:mn>5</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mn>10</mml:mn> <mml:mrow> <mml:mn>23</mml:mn> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:msup> <mml:mrow> <mml:msup> <mml:mi>He</mml:mi> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msup> <mml:mstyle scriptlevel="0"/> <mml:mrow> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ), low-energy ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>≃</mml:mo> </mml:mrow> </mml:math> 80 eV) He ions at high temperatures (in the range 600–1000 K), which leads to the formation of He bubbles in the near-surface layer ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>≃</mml:mo> </mml:mrow> </mml:math> approximately the first 50 nm). In situ spectroscopic ellipsometry is exploited to measure the optical response of W (namely, reflectivity variation) during He plasma irradiation, varying different parameters such as the temperature and the initial W surface state. We used a time-dependent effective medium approximation model to simulate the porous He bubble layer and the Bennet–Porteus model to take into account He irradiation-induced roughness in order to extract the temporal evolution of significant physical quantities, such as the He bubble volume fraction across the layer (i.e. porosity) and the surface roughness. We compared and validated the in situ optical model with ex situ microscopy characterizations (direct measurements of the average size of He bubbles and of surface roughness). Finally, we estimated the kinetics of He bubble formation with cluster dynamics simulations (FEniCSx). We found that the increase of porosity and roughness with temperature and their temporal evolution behaves like a power law; this was partially confirmed by the FEniCSx simulations.