(551d) On the Onset of ‘Fuzz’ Formation in Plasma-Facing Materials. II. Effects of Subsurface Bubble Dynamics, Temperature, and Elastic Moduli of Damaged Tungsten

Fuzz formation in helium-implanted tungsten (W) is a complex, multi-physics phenomenon resulting from the combined effects of driven surface diffusion, subsurface bubble dynamics, bubble bursting, loop punching, anisotropies of material properties, changes in material thermophysical properties in the damaged tungsten as a function of helium (He) content, and many more. Here, we focus on understanding the individual effects on the He-implanted W surface morphology of three such important physical phenomena and physical dependences, namely, subsurface bubble dynamics, temperature dependence of material properties, and changes in material properties due to helium implantation.

We report a systematic study of the subsurface He bubble dynamics based on a continuum-domain, spatially-distributed drift-diffusion-reaction cluster-dynamics model of He cluster and bubble evolution, as well as the retention of helium in low-energy He plasma-exposed W. Our cluster-dynamics simulator, Xolotl, has been benchmarked based on large-scale molecular-dynamics (MD) simulation results and experimental measurements available in the literature. For realistic predictions of the sub-surface He concentration, which reaches a steady state at an early stage of He-plasma exposure, a simplified helium bubble bursting model is incorporated into Xolotl to take into account the gas release occurring when a bubble is near the tungsten surface. A systematic comparison of the modeling predictions with experimental measurements will be presented and strategies for further improvement of the bubble bursting model implemented in Xolotl will be discussed.

We will also present a comparison of the predictions of our experimentally validated, continuum-scale model of surface morphological evolution for He plasma-exposed tungsten with experimental measurements under different temperature conditions. For this purpose, a set of ITER-grade W specimens were mechanically polished, stress relieved at 1000 °C, and then exposed to RF-generated He plasmas (incident ion energy of 75 eV and flux of 8.5×10²⁰ m^-2s^-1) at the same ion fluence (1.53×10²⁵ m^-2) but at temperatures varied between 472 °C and 786 °C. Following the plasma exposure, the W surfaces are characterized and compared with our computed morphologies under identical plasma exposure conditions.

Furthermore, we report the effects on the fuzz formation of the changes in the elastic moduli of the He-implanted W with respect to those of the undamaged tungsten matrix. The elastic moduli are determined through systematic MD simulations of straining tests on single crystalline tungsten containing nanopores filled with the proper amount of He to form helium nanobubbles, which are used as models of plasma-exposed tungsten in order to assess the effect of He-ion irradiation on tungsten’s elastic properties. These atomistically computed elastic moduli are incorporated into our surface evolution model to assess effects of the varying elastic properties of plasma-exposed tungsten on its surface morphology. A roadmap for the study of the combined effects of the three aforementioned physical phenomena and dependences on the predictions of our surface evolution model and a discussion on the incorporation into the model of other important physics that affects the fuzz formation process also will be presented.