(332b) Hierarchical Multiscale Modeling of Surface Morphological Response of Plasma-Facing Tungsten

One of the key impediments in harnessing fusion energy for the electrical grid is to develop suitable wall materials for the fusion reactor which can withstand the extreme reaction environment, especially under the conditions expected for plasma-facing components (PFCs) in linear plasma devices and fusion tokamaks. Tungsten is the PFC material for ITER (International Thermonuclear Experimental Reactor) and under the expected typical reactor operating conditions, implantation of helium ions produced in the fusion reaction is responsible for the formation of a fragile fiber-like crystalline nanostructure on the PFC tungsten surface, known as "fuzz", with detrimental effects on the reactor performance.

Fuzz formation in PFC tungsten is a complex multi-physics phenomenon resulting from the combined effects of driven surface diffusion, subsurface bubble dynamics, bubble bursting and surface crater formation, dislocation loop punching, anisotropies in material properties, changes in material thermophysical properties in the damaged tungsten, and many more. Developing a predictive model capable of capturing the formation and evolution of the fuzz-like complex surface morphology mediated by the underlying dynamical processes that are characterized by disparate spatiotemporal scales has been a major theoretical and computational challenge. 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 (He) bubbles forming in the near-surface region of PFC tungsten during He irradiation, and defect fluxes toward the PFC surface [1]. The model employs a properly parameterized, atomistically-informed equation of state (EOS) for He in over-pressurized bubbles as well as 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. One highlight of this hierarchical multiscale framework is the calculation of the bubble-matrix (He-W) interfacial free energy per unit area, a key parameter in the EOS for the He density in the bubble [2]. Based on this hierarchical modeling framework, we report results of numerical simulations that 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 [3], the elastic softening of the near-surface PFC region [4], the helium accumulation kinetics [4], and the formation of nanometer-scale holes on the PFC surface due to helium bubble bursting [5]. The simulation predictions are compared with experimental data and provide a fundamental interpretation to experimental observations.

Furthermore, we have used a cluster-dynamics model to simulate the subsurface helium bubble evolution and bubble bursting and develop a coarse-grained model for He concentration evolution in PFC tungsten, which can be coupled with the surface evolution model. Our cluster-dynamics model, implemented numerically in our code Xolotl, has been parameterized based on large-scale MD simulation results and validated by comparison of its predictions with experimental measurements.

[1] D. Dasgupta, R. D. Kolasinski, R. W. Friddle, L. Du, D. Maroudas, and B. D. Wirth, Nucl. Fusion 59, 086057 (2019).

[2] K. D. Hammond, D. Maroudas, and B. D. Wirth, Sci. Rep. 10, 2192 (2020).

[3] D. Dasgupta, D. Maroudas, and B. D. Wirth, Surf. Sci. 698, 121614 (2020).

[4] C.-S. Chen, D. Dasgupta, A. Weerasinghe, B. D. Wirth, and D. Maroudas, Nucl. Fusion 61, 016016 (2021).

[5] C.-S. Chen, D. Dasgupta, A. Weerasinghe, B. D. Wirth, and D. Maroudas, J. Appl. Phys. 129, 193302 (2021).