Session NM11: Mini-Conference: Measuring and Modeling of Plasma Material Interactions II

Cluster dynamics modeling of He accumulation kinetics in W exposed to low-energy He plasma exposure

We report a hierarchical multi-scale modeling study of implanted helium segregation to surfaces of tungsten, considered as a plasma facing component in nuclear fusion reactors. We employ a hierarchy of atomic-scale simulations based on a reliable interatomic interaction potential, including molecular-statics and molecular dynamics simulations to understand the origin of helium surface segregation. The near-surface cluster dynamics found in these simulations have significant effects on the surface morphology, near-surface defect structures, and the amount of helium retained in the material upon plasma exposure. We integrate the findings of such atomic-scale simulations into a properly parameterized and validated spatially-dependent, continuum-scale reaction-diffusion cluster dynamics model, capable of predicting implanted helium evolution, surface segregation, and its near-surface effects in tungsten. This cluster-dynamics model sets the stage for development of fully atomistically informed coarse-grained models for computationally efficient simulation predictions, toward optimal design of plasma facing components.   

Analysis of Helium Cluster Dynamics near Grain Boundaries of Plasma-Exposed Tungsten

We report results of a systematic atomic-scale analysis of the kinetics of small mobile helium clusters near a model symmetric tilt grain boundary (GB) in tungsten (W). The small mobile helium clusters migrate toward the GB region by Fickian diffusion and drift due to an elastic interaction force that drives GB segregation. As the clusters migrate toward the GB, trap mutation (TM) reactions are activated at rates higher than those away from the GB and are the dominant kinetic processes for 4-member and larger mobile helium clusters. Each TM reaction produces a W interstitial atom on the GB, in the form of an extended interstitial configuration, and an immobile helium-vacancy complex with the W vacancy located at a short distance from the GB. These reactions are identified and characterized in detail based on analysis of a large number of molecular-dynamics trajectories. The mobility of the extended W interstitial on the GB depends on the location of the helium-vacancy complex. The identified cluster reactions are responsible for important structural, morphological, and compositional features in plasma-exposed tungsten.   

Accelerated Molecular Dynamics studies of He Bubble Growth in Tungsten

Understanding how materials respond to extreme environments is critical for predicting and improving performance. In materials such as tungsten exposed to plasmas for nuclear fusion applications, novel nanoscale fuzzes, comprised of tendrils of tungsten, form as a consequence of the implantation of He into the near surface. However, the detailed mechanisms that link He bubble formation to the ultimate development of fuzz are unclear. Molecular dynamics simulations provide insight into the He implantation process, but are necessarily performed at implantation rates that are orders of magnitudes faster than experiment. Here, using accelerated molecular dynamics methods, we examine the role of He implantation rates on the physical evolution of He bubbles in tungsten. We find that, as the He rate is reduced, new types of events involving the response of the tungsten matrix to the pressure in the bubble become competitive and change the overall evolution of the bubble as well as the subsequent morphology of the tungsten surface. We have also examined how bubble growth differs at various microstructural features. These results highlight the importance of performing simulations at experimentally relevant conditions in order to correctly capture the contributions of the various significant kinetic processes and predict the overall response of the material.   

On He cluster dynamics in W

The results of our recent simulations of the nano-bubble evolution show that the growth of nano-bubble is accompanied by rather significant distortion of the lattice in the vicinity of the bubble, the formation of the dislocations, and even vacancies, which can serve as the He traps and lead to the nucleation of new nano-bubbles. Therefore, we can have an avalanche effect, which can strongly facilitate the nucleation and growth of nano-bubbles, reduce the penetration of He atoms into the bulk of the sample and, therefore, decrease the width of the layer of nano-bubble near the surface. In this presentation we discuss theoretical model, based on 1D reaction-diffusion equations, describing spatiotemporal dynamics of He clusters in W, which takes into account He trap generation related to the growth of He clusters.   

Deciphering gas implantation rate effects on bubble nucleation in tungsten

We use the object kinetic Monte Carlo code KSOME to study the sub-surface helium clustering behaviour in tungsten at various conditions relevant to plasma exposure of divertor surfaces. In particular, we have investigated helium implantation fluxes from 10$^{\mathrm{20}}$ to 10$^{\mathrm{27}}$ m$^{\mathrm{-2}}$s$^{\mathrm{-1}}$ at temperatures from 400K to 1600K for 100-eV helium ions implanted below tungsten surfaces as a function of pre-existing vacancy concentration. For these conditions, the helium retention rate, the surface areal density of adatoms, and the number density of clusters are analysed. A phase diagram is constructed to summarize the results, which maps the ratio of self-trapped helium to vacancy-trapped helium with respect to the helium flux, the target temperature, and the concentration of pre-existing vacancy. According to the phase diagram, the boundary between the self-trapping dominant regime and the vacancy-trapping dominant regime can be distinguished. In general, pre-existing vacancies are dominant in trapping helium atoms for low fluxes and high temperatures, while self-trapping is dominant for high fluxes. These results provide important insight into the mechanisms of helium clustering for plasma facing components in fusion reactors.   

Analysis of Helium Segregation on Surfaces of Plasma-Exposed Tungsten

We report a systematic theoretical and atomic-scale computational study of implanted helium segregation on surfaces of tungsten, which is considered as a plasma facing component in nuclear fusion reactors. We employ a hierarchy of atomic-scale simulations, including molecular statics to understand the origin of helium surface segregation, targeted molecular-dynamics (MD) simulations of near-surface cluster reactions, and large-scale MD simulations of implanted helium evolution in plasma-exposed tungsten. We find that small, mobile helium clusters (of 1-7 He atoms) in the near-surface region are attracted to the surface due to an elastic interaction force. This thermodynamic driving force induces drift fluxes of these mobile clusters toward the surface, facilitating helium segregation. Moreover, the clusters' drift toward the surface enables cluster reactions, most importantly trap mutation, at rates much higher than in the bulk material. This cluster dynamics has significant effects on the surface morphology, near-surface defect structures, and the amount of helium retained in the material upon plasma exposure.   

Development of nanostructures on plasma facing components

Exposure to low temperature helium plasma, with parameters similar to tokamak edge plasmas, have been found to induce the growth of nanostructures on tungsten. These nanostructures results in an increase in the effective surface area, and will alter the physical properties of the components. Although this has several potential applications in the industrial scenario, it is an undesired effect for fusion reactor components, and is hence necessary to understand their growth mechanisms in order to figure out suitable remedial schemes. Work done using a high density, low temperature helicon discharge plasma source with a resistively heated tungsten wire immersed in the discharge as the substrate have demonstrated the well-defined stages of the growth as a function of total fluence. The required fluence was attained by extending the exposure time. Extensive research work has also shown that a variety of other materials are also prone to develop such structures under similar conditions. In the present work, the effect of the experimental conditions on the various stages of structure development will be presented and a comparison between the structures developed on different types of substrates will be shown.   

The role of lithium thin-film coatings on W surface morphology evolution under high-fluence and high temperature He irradiation

Tungsten is the candidate plasma-facing component material for the ITER divertor due to its high sputter threshold, high melting temperature, and excellent thermal conductivity. However, when exposed to He ions with E = 0.01-1.0 keV and high fluences $>$10$^{26}$ m$^{-2}$ [1], as those expected in a burning plasma fusion tokamak divertor, the damage to the surface can include the creation of bubbles, holes and tendril-like fuzz morphology. Recent studies show that adding low-Z impurities (C and Be) to a He plasma can inhibit the growth of fuzz. In other applications, lithium (Li) as a PFC coating in multiple tokamaks has improved plasma performance, yet its interaction with high-Z materials (i.e. W) and its role inhibiting fuzz formation is not well understood. We investigated the effect of a thin $\sim$1000 nm Li coating on formation of W surface defect morphology under high fluence and temperature conditions. Samples were exposed with fluxes of $\sim$10$^{24}$ m$^{-2}$s$^{-1}$ and T$_{surf}$ $\sim$1100 $^{\circ}$C. After irradiation, the surfaces of the samples were characterized with SEM. These results are presented along with XPS and SIMS results elucidating the persistence of Li coatings under these conditions. $^{1}$O. El-Atwani, et al., Nucl. Fusion 54 (2014) 083013.   

He bubble growth and interaction in W nano-tendrils

Tungsten plasma-facing components (PFCs) in fusion devices are exposed to variety of extreme plasma conditions, which can lead to alteration of tungsten micro-structure and degradation of the PFCs. In particular, it is known that filamentary nano-structures called fuzz can grow on helium plasma exposed tungsten surfaces. However, mechanism of the fuzz growth is still not fully understood. Existing experimental observations indicate that formation of helium nano-bubbles in tungsten plays essential role in fuzz formation and growth. In this work we investigate mechanisms of growth and interaction of helium bubbles in fuzz-like nano-tendrils using molecular dynamics simulations with LAMMPS code. We show that growth of the bubbles has anisotropic character producing complex stress field in the nano-tendrils with distinct compression and tension regions. We found that formation of large inter-bubble tension regions can cause lateral stretching and bending of the tendrils that consequently lead to their elongation and thinning at the stretching sites. The rate of nano-tendril growth due to the described mechanism is also evaluated from the simulations.