Continuum-scale Modeling of Surface Morphological Response of Plasma-Facing Tungsten

Under fusion reactor operating conditions, implantation of helium ions is responsible for the formation of a fragile fiber-like crystalline nanostructure on the plasma-facing component (PFC) tungsten surface, known as “fuzz”, with detrimental effects on the reactor performance. Numerous atomistic simulation studies have provided insights into this complex surface nanostructure formation. However, such atomic-scale studies cannot access the spatiotemporal scales of micrometers and hours relevant to fuzz formation.

Here, we present a hierarchical, continuum-scale modeling framework for the surface morphological response of PFC tungsten, capable of accessing the length and time scales characteristic of surface evolution in PFC tungsten. The model accounts for curvature-driven surface diffusion, stress-driven surface transport due to over-pressurized helium bubbles forming in the near-surface region of PFC tungsten during He irradiation, and defect fluxes toward the PFC surface. The model employs properly parameterized, atomistically-informed equations of state for He in over-pressurized bubbles and constitutive equations for the mechanical state of the near-surface region of PFC tungsten and is implemented computationally using Fourier spectral methods and semi-implicit front tracking techniques.

We report results of numerical simulations and stability analyses based on the above model, which predict the onset of fuzz formation in the form of nanotendrils growing from the PFC tungsten surface with an incubation time in agreement with experimental data. We also explore the effects on the surface morphology and growth kinetics of the surface temperature, the elastic softening of the near-surface region, the helium accumulation kinetics, and the formation of nanometer-scale holes on the PFC surface due to helium bubble bursting. The simulation predictions are compared with experimental data and provide fundamental interpretation to experimental observations.