(388a) Modeling and Simulation of 'fuzz' Formation Onset in Plasma-Facing Materials

Fusion energy is the most promising carbon-free and sustainable energy source for the 21st century and beyond. However, practical realization of economic fusion electricity on the grid is extremely challenging. One particularly challenging problem is the severe surface degradation in the form of nanometer-sized tendrils, or porous “fuzz”, observed following low-energy helium exposure of plasma-facing-component (PFC) tungsten in linear plasma devices and fusion tokamaks. Tungsten is the chosen PFC material for ITER (International Thermonuclear Experimental Reactor) due to its superior thermomechanical properties and relatively low sputtering yield. Helium produced in the deuterium-tritium fusion reaction is a natural impurity in the plasma and it is extracted from the plasma through the divertor in ITER. Experimental studies have revealed that high particle flux of this low-energy helium produces fuzz-like nanofibers at the tungsten divertor within the temperature range from 900 K to 2000 K (consistent with anticipated heat loads and temperatures expected at the divertor wall), which has adverse effects on the reactor performance.

Developing predictive models capable of accessing the spatiotemporal scales relevant to the fuzz formation process is essential for understanding the growth of such complex surface features. Here, we focus on the development of an atomistically-informed continuous-domain model, the outcome of a hierarchical multiscale modeling strategy over disparate spatiotemporal scales, for simulating the onset of fuzz formation in He plasma-irradiated tungsten and validating the model by comparing the simulation predictions with measurements from carefully designed experiments. The model accounts for the development of stress in the He-implanted tungsten layer due to formation of over-pressurized He bubbles in the layer, with large-scale MD simulation results used to parameterize the corresponding constitutive equations. Based on this model, we have conducted self-consistent numerical simulations of the He-implanted tungsten surface morphological evolution and compared the simulation results with experimental measurements. We have also carried out a parametric sensitivity analysis for the effects of the He concentration in the He-implanted tungsten and the He nanobubble size on our model predictions.

Another critical aspect of PFCs is their response to fluctuating heat loads, which can have a significant effect on the fuzz formation. We report a systematic study of the PFC tungsten surface morphological response over a temperature range between 753 K and 933 K to understand the temperature dependence of fuzz formation, under conditions similar to those at the divertor of fusion reactors. Temperature influences the onset of fuzz formation through the strongly temperature-dependent pressure of subsurface helium bubbles formed by implantation and surface diffusivity of tungsten, in addition to the thermal softening of the elastic moduli of tungsten. We explore the effects of helium concentration in the implanted tungsten and helium bubble size, report surface response diagrams predicted over the above temperature range, and interpret the trends exhibited in such diagrams. Our findings elucidate the role of plasma-irradiation duration in fuzz growth and demonstrate the possibility of fuzz formation at temperatures well below the range explored in experimental studies.

Furthermore, we present a cluster-dynamics model to simulate the sub-surface helium bubble evolution, which plays an important role in the surface morphological dynamics of He-implanted tungsten. Our cluster-dynamics model, implemented numerically in our code Xolotl, has been benchmarked based on large-scale MD simulation results and experimental measurements. For realistic predictions of the sub-surface He concentration, a simplified helium bubble bursting model is used in Xolotl to take into account the gas release occurring when a bubble is near the tungsten surface. Incorporating additional relevant physical processes into future versions of the model of PFC surface morphological response also is discussed.