Speaker
Description
Tungsten (W) plasma-facing components in magnetic-confinement fusion are simultaneously exposed to intense helium (He) implantation and to high-velocity dust remobilization events. While each driver is known to degrade W surfaces, their combined impact remains insufficiently constrained because He implantation generates a spectrum of subsurface defect morphologies (e.g., bubbles, bubble arrays, and platelets) that can fundamentally alter how impact energy is dissipated and how material is removed. Here, we use large-scale molecular dynamics simulations (up 300 million particles) to isolate how He defect topology governs impact induced cratering and ejecta production in fusion relevant tungsten.
We simulate normal incidence high velocity impacts of a nanoscale W projectile onto single crystal W targets containing representative near-surface He morphologies: (i) pristine W, (ii) sparse bubble arrays, (iii) dense bubble arrays, (iv) a population of isolated bubbles with varied sizes and depths, and (v) a He platelet. By tracking crater evolution, cumulative ejected mass and fragment statistics, helium redistribution, and depth resolved dislocation activity, we identify distinct deformation and failure regimes that are controlled primarily by topology rather than He inventory alone.
In all cases, an early shock dominated stage is followed by a slower relaxation stage. However, the late time response diverges strongly with He morphology, yielding a robust hierarchy in retained crater volume and ejecta: pristine < bubble arrays < isolated bubbles ≪ platelet. Bubble arrays act as a mechanically compliant, gas-venting layer that promotes localized plastic accommodation at the implanted depth, sustaining steady production of fine debris without catastrophic opening. Isolated bubbles produce intermittent, spatially localized micro-collapse events that lead to step-like crater and ejecta evolution. In contrast, the platelet morphology triggers a qualitatively different failure mode: progressive decohesion culminating in delayed delamination and a sudden, large burst of ejecta, consistent with opening-dominated damage.
Ejecta kinetics for all non platelet cases are well described by a stretched exponential law, whereas the platelet requires an additional discrete “burst” term to capture delamination driven mass loss. Dislocation density signatures (a narrow spike for arrays versus a broader near surface elevation for platelets) provide a mechanistic diagnostic linking subsurface He topology to macroscopic erosion outcomes. These results support helium aware erosion and lifetime models for W plasma facing materials and identify He platelet as a critical topology to mitigate.
Keywords: tungsten; plasma-facing materials; helium bubbles; helium platelets; dust impact; molecular dynamics.