(323a) On the Onset of ‘Fuzz’ Formation in Plasma-Facing Materials: A Hierarchical Multiscale Modeling Approach

Tungsten (W) and tungsten alloys are widely viewed as the most promising plasma-facing material (PFM) candidates for divertor and first-wall systems in nuclear fusion reactors. 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 plasma-exposed W surface under the operating conditions of temperature, helium (He) impact energy, and He flux expected for ITER’s divertor. Formation of such fuzz surface nanostructure in tungsten may adversely influence the reactor performance and operation.

Fuzz formation in plasma-facing component (PFC) tungsten is a complex multi-physics phenomenon resulting from the combined effects of processes that include driven surface diffusion, subsurface He bubble dynamics, He bubble bursting, and dislocation loop punching, as well as material property anisotropies and changes in material thermophysical properties in the damaged tungsten. The development of a predictive model capable of capturing the formation and evolution of the fuzz-like complex surface morphology mediated by the various underlying dynamical processes that are characterized by disparate spatiotemporal scales is particularly challenging. While large-scale molecular-dynamics (MD) simulations have been used to successfully capture and explore many of the aforementioned phenomena [1], simulating the experimentally relevant spatiotemporal scales (mm-hr) for surface nanostructure formation in PFC tungsten is well beyond the scope of such atomistic simulations. Therefore, we followed a hierarchical multiscale modeling paradigm to develop a continuum-scale model for simulating the onset of fuzz formation in He plasma-irradiated tungsten [2].

In our modeling framework, large-scale MD simulation results are used to parameterize the constitutive equations required for the closure of the continuum-scale model for the surface morphological response of the plasma-facing material and targeted MD simulations are used to determine the thermophysical properties [3, 4] of He-implanted tungsten. Based on this atomistically-informed continuum-scale model, we have conducted self-consistent numerical simulations of the He-implanted tungsten surface morphological evolution and validated the model by comparing the simulation predictions with measurements from carefully designed experiments [2]. Furthermore, we have explored the effects on the PFC surface morphology and growth kinetics of the surface temperature [5], the elastic softening of the near-surface PFC region [6], the formation of nanometer-scale holes on the PFC surface due to He bubble bursting [7], and the formation of surface adatom-vacancy pairs under low-energy He implantation [8]. The simulation predictions are compared with experimental data and provide a fundamental interpretation to experimental observations.

References: [1] K. D. Hammond, S. Blondel, L. Hu, D. Maroudas, and B. D. Wirth, Acta Mater. 144, 561-578 (2018); [2] D. Dasgupta, R. D. Kolasinski, R. W. Friddle, L. Du, D. Maroudas, and B. D. Wirth, Nucl. Fusion 59, 086057 (2019); [3] A. Weerasinghe, B. D. Wirth, and D. Maroudas, ACS Appl. Mater. Interfaces 12, 22287 (2020); [4] A. Weerasinghe, B. D. Wirth, and D. Maroudas, J. Appl. Phys. 132, 185102 (2022); [5] D. Dasgupta, D. Maroudas, and B. D. Wirth, Surf. Sci. 698, 121614 (2020); [6] C.-S. Chen, D. Dasgupta, A. Weerasinghe, B. D. Wirth, and D. Maroudas, Nucl. Fusion 61, 016016 (2021); [7] C.-S. Chen, D. Dasgupta, B. D. Wirth, and D. Maroudas, J. Appl. Phys. 129, 193302 (2021); [8] C.-S. Chen, D. Dasgupta, A. Weerasinghe, K. D. Hammond, B. D. Wirth, and D. Maroudas, Nucl. Fusion 63, 026033 (2023).