(332g) Onset of Fuzz Formation in Plasma-Facing Tungsten As a Surface Morphological Instability

Tungsten is the plasma-facing component (PFC) material for the divertor region of the International Thermonuclear Experimental Reactor (ITER) due to its exceptional thermomechanical properties, such as high melting point, mechanical strength, and thermal conductivity, as well as its low sputtering yield, low erosion, and low tritium retention. A large body of experimental evidence has established that PFC tungsten suffers severe surface degradation as a result of exposure to high fluxes of helium (He) and extreme heat loads. Specifically, experimental studies have shown that, under typical operating conditions expected in ITER, He implantation above a threshold of incident ion energy of approximately 35 eV causes formation of a fuzz-like nanostructure on the tungsten surface over the temperature range from 900 K to 2000 K. The formation of such surface nanostructure has adverse effects on the thermomechanical behavior and structural response of PFC tungsten as well as on the reactor performance.

Here, we demonstrate that the onset of formation of the “fuzz” surface nanostructure in PFC tungsten is the outcome of a stress-induced surface morphological instability. Our study is based on numerical simulations and theoretical analysis according to an atomistically informed continuum-scale model for the surface morphological response of PFC tungsten in nuclear fusion devices. Specifically, we show that formation and growth of nanotendrils emanating from the exposed surface of PFC tungsten, a precursor to fuzz formation, is caused by a long-wavelength surface morphological instability triggered by biaxial compressive stress in the PFC near-surface layer due to over-pressurized helium (He) bubbles forming in this near-surface region through implantation of low-energy He ions from the plasma. Using linear stability theory (LST), we predict the onset of surface growth in response to low-amplitude perturbations from a planar PFC surface morphology and calculate the average spacing between nanotendrils growing from the PFC surface. The LST predictions are in excellent agreement with self-consistent dynamical simulations of surface morphological response according to the fully nonlinear surface evolution model starting with random fluctuations from the planar surface morphology that result in nano-scale surface roughness. In addition, we examine the morphological response of the PFC surface to low-amplitude perturbations of very long wavelengths from its planar morphology and interpret fully the simulation results on the basis of a weakly nonlinear tip-splitting instability theory, which predicts a post-instability nanotendril pattern formation with nanotendril separation consistent with the LST predictions regardless of the initial surface perturbation. Finally, we compare our simulation predictions for surface nanotendril growth with experimental measurements of fuzz layer growth on the exposed surface of PFC tungsten. Our simulation results are in very good agreement with the surface growth measurements at the early stages of fuzz growth, further establishing the onset of fuzz formation in PFC tungsten as the outcome of a stress-driven PFC surface morphological instability.