In-situ spectroscopic ellipsometry for real-time characterization of the effects of high-flux helium plasmas on tungsten surfaces

In-situ analysis of surfaces during high-flux plasma exposure represents a long-standing challenge in the study of plasma-material interactions. While state-of-the-art materials characterization can provide rich structural and composition detail during post-mortem analyses, in-situ diagnostics offer the possibility of probing dynamic effects. In this study, we demonstrate the use of spectroscopic ellipsometry for real-time characterization of how tungsten surface morphology evolves during exposure to He plasmas (Γi = 3.5×10 16 He cms, ion energy = 92 eV.) The range of exposure conditions selected here is conducive to the growth of near-surface He bubbles, and at higher fluence, the formation of W nano-tendrils ranging between 50 – 100 nm in diameter. The evolution of these surface features was probed using a fixed-angle ellipsometer (280 – 1000 nm wavelength range) attached to an RF plasma source with direct line-of-sight to the sample. Over the parameter space explored here, changes in the two angles that define the polarization of the reflected light followed a distinct trajectory in (ψ, δ) space with increasing plasma fluence. Ex-situ ellipsometry of 15 additional tungsten specimens tested at a wide range of plasma fluences and temperatures mapped onto these in-situ results well. We used helium ion microscopy and focused ion beam profiling to provide a direct calibration of the ellipsometry measurements. Our results indicate that for a reproducible process such as the growth helium-induced surface morphologies, ellipsometry is a practical in-situ diagnostic to study how fusion plasmas modify materials. To study more general effects of plasmas on surfaces, including co-deposition and sputtering, different approaches to modelling the optical properties of the exposed surfaces are also considered.