(551b) On the Onset of ‘Fuzz’ Formation in Plasma-Facing Materials. I. Surface Morphological Evolution Model

Tungsten (W) and tungsten alloys are widely viewed as the most promising plasma-facing material (PFM) candidates for divertor and first-wall systems in a nuclear fusion reactor. However, experiments aiming to investigate PFM performance under continuous operation and high first-wall temperature have shown that a nanostructure with a fuzz-like morphology develops on the W surface under the operating conditions of temperature, helium (He) impact energy, and He flux expected for ITER’s divertor. Formation of such fuzz nanostructure may adversely influence the reactor performance and operation.

Here, we focus on obtaining a fundamental understanding of the initial stage of fuzz formation by modeling the surface morphological evolution of helium-ion-irradiated tungsten considered as a PFM based on an atomistically-informed, continuous-domain predictive model that we have developed. The constitutive equations of the model are parameterized based on analysis of large-scale molecular-dynamics (MD) simulation results; for the He-implanted W layer, the stress level is calculated using heterogeneous elastic inclusion theory and the state of stress in this region is taken to be biaxial compression consistent with MD simulations of the deformation state of a He-implanted W region. According to this model, we have conducted self-consistent numerical simulations of the dynamics of the He-implanted W surface morphology and compared the simulation results with experimental measurements. Under the experimental conditions of exposure of ITER-grade W at 1113 K to a medium-flux RF plasma source (∼ 3 × 10²⁰ m^−2s^−1) of 75 eV He, the predicted surface morphology shows a good qualitative and quantitative agreement with the experimental data for a He plasma exposure time of 80 min. The strong quantitative agreement of the mean and width of the height distribution functions for the experimental and computed W surface morphologies establishes that our model can predict successfully the initial growth rate of nanotendrils emanating from the surface, which is the precursor to fuzz formation. In addition, we have conducted a linear stability analysis to examine the morphological stability of the planar surface of the helium-implanted tungsten and the simulation results are consistent with the predictions of the linear stability theory. We also present a parametric sensitivity analysis for the effects on our model predictions for the average nanotendril spacing and the nanotendril growth rate of three key model parameters, namely, the He concentration in the He-implanted tungsten, the He nanobubble size, and the elastic moduli of the damaged W in the nanobubble region. Furthermore, we have investigated the dependence of the nanotendril width and arrangement on the surface temperature for He-ion energy below (75 eV) and above (250 eV) the sputtering threshold, and compared the simulation results with measurements from carefully designed experiments.