Session PM9: Mini-Conference: Plasma Surface Interactions Involving Helium (SciDAC and Beyond) III

Energetics of large helium-vacancy clusters in tungsten

The divertor in a fusion reactor is subject to intense, low energy (1-100 eV) hydrogen isotope and helium bombardment from the plasma. Tungsten is the leading candidate material for the divertor plates. Helium in a material can cause changes in thermal and mechanical properties, such as swelling, change in ductile to brittle transition temperature, bubbles and nanofuzz formation. Molecular dynamics (MD) simulations is a valuable tool for studying energetics, structures and many radiation damage phenomena that happen on short time and length scales. Using MD simulations we have studied a wide range of size and composition of He bubbles in tungsten. By annealing a bubble and calculating the formation energy, a clear trend is found, which can be fit to a semi-empirical expression and thus providing the energetics of a bubble of any size. As helium is added to a bubble and the bubble pressure grows, W interstitials are punched out to relieve pressure, depending on bubble size and temperature. As nanofuzz formation has only been observed for helium plasma exposure we also compare the bubble formation mechanisms with neon based on ab initio data and a new W-Ne potential.   

SEAKMC results of small He clusters near free surfaces in tungsten

The behavior of small helium and helium vacancy clusters is believed to crucial to understanding the plasma-surface interaction that gives rise to was has been called ``fuzz'' formation on tungsten metal surfaces exposed to low-energy helium plasmas at elevated temperatures. Detailed characterization of this behavior requires a computational approach capable of capturing the atomistic physical properties while reaching times well beyond those typically accessible by molecular dynamics. The recently developed self-evolving atomistic kinetic Monte Carlo (SEAKMC) method has been applied to investigate small He clusters in tungsten at a range of relevant temperatures, and as a function of distance from a free surface. A strong effect of the free surface on cluster behavior was found for clusters within about 4 lattices parameters of the surface. In some cases, cluster migration to the surface leads to the production of tungsten adatoms associated with Frenkel pair formation, resulting in atomic-scale surface roughening. The resulting He-vacancy cluster is immobile, and can be a stable trap for other He atoms. Capture of additional He atoms can lead to formation of additional Frenkel pair and adatoms. The possible role of these mechanisms in surface fuzz formation will be discussed.   

Cluster dynamics simulations defining end of dilute limit for fusion relevant conditions

In fusion reactors, plasma facing components (PFC) and in particular the divertor will be irradiated with high ion fluxes of low energy (100 eV) helium and hydrogen. Tungsten is one of the leading candidate divertor materials for ITER and DEMO fusion reactors. However, the behavior of tungsten under high dose, coupled helium/hydrogen exposure remains to be fully understood. The aim of this study is to understand and predict primary defect production and defect diffusion, clustering and interaction of tungsten surface exposed to low energy helium irradiation at high fluences (10$^{\mathrm{26}}$ He/m$^{\mathrm{2}})$ and temperatures (1000 K). We report results from a spatially-dependent cluster dynamics model based on reaction-diffusion rate theory. The model was improved to be able to include very large helium vacancy clusters expected to form under these irradiation conditions. We also focus on defining the validity of the assumptions of the model (dilute limit assumption, surface morphology changes). We find good agreement between the model and analytical work. We also compare results with molecular dynamics simulations and existing experimental results. We identify the regime in which the dilute limit approximation breaks down and/or the surface morphology changes are dominant and discuss potential solution to overcome these difficulties.   

Energetics and Dynamics of Mobile Helium Clusters in Near-Surface Regions of Plasma-Exposed Tungsten

The implantation of helium (He) atoms has significant implications for the surface morphological evolution and the near-surface structural evolution of plasma-facing components in nuclear fusion reactors. In tungsten (W), such interstitial He atoms are very mobile and aggregate to form clusters; the smaller of these clusters are mobile and their migration mediates the evolution of the surface morphology and the near-surface microstructure of the plasma-exposed material. In this presentation, we report results of a systematic atomic-scale analysis of the energetics and dynamics of mobile He clusters in near-surface W regions based on carefully parameterized many-body potentials. The analysis combines molecular-statics computations of the energies of structurally relaxed He-cluster configurations as a function of their distance from the surface with molecular-dynamics simulations of mobile cluster migration in the near-surface region. The cluster size n (ranging from 1 to 7 He atoms) and the surface crystallographic orientation are important parameters in the study. This atomic-scale analysis also is extended to the migration of small mobile He clusters near sinks other than surfaces, such as grain boundaries (GBs), with emphasis on GBs in near-surface regions.   

Intricacies of helium effects on displacement cascades in tungsten

MD simulations were performed to study the effect of He on displacement cascades in W. The ability of He clusters to displace W atoms (kick-out mechanism) was studied with thermalization of interstitial He for 200 ps. The minimum cluster size to initiate kick-out decreases from 7 (1025 K) to 5 (2050 K). No kick-out was found at 300 K even at 4600 appm. Effects on damage were studied with 75 keV PKA for 55 ps with no initial He clustering. At 1025 K, data with 100{\%} interstitial He shows the number of surviving SIAs remains unchanged: 81 (pure), 82 (2300 appm), while vacancy count drops to \textless 50 at 2300 appm due to He filling the vacancies. Further study was done for 460 appm at several T. For 100{\%} interstitial, the number of He-filled vacancies is 7 (300 K), 8 (1025 K) and 15 (2050 K) due to increased mobility. For 50{\%} substitutional, the number of vacancies is minimum at 1025 K (65) compared to 87 (300 K) and 73 (2050 K) indicating there is an interplay between increased He mobility to find a vacancy vs decreased He$+$vacancy stability. The 100{\%} substitutional data confirms the observation. The seemingly decreased stability of He$+$vacancy cluster may be due to increased mobility of SIAs and overtake vacancies from those clusters. More studies are planned to elucidate the competition.   

Flux effects on helium accumulation in tungsten

The growth process of helium bubbles in tungsten under flux rates spanning five orders of magnitude was investigated using direct molecular dynamics and parallel replica dynamics. We show clear differences in the evolution of bubbles as a function of the growth rate; in particular, we show that the critical size before bursting is overestimated at the high flux accessible to standard molecular dynamics simulations. These results have deep implications for multiscale and continuous modeling of plasma-facing materials under operating conditions.   

Molecular Dynamics Investigation of H/He Behaviors in W

Tungsten (W) and W alloys are regarded as the most promising candidates for PFMs because of their good thermal properties, such as high melting temperature and low sputtering erosion, which will be widely used in the next generation of fusion reactors. However, blistering in W-PFM induced by extremely high fluxes of low-energy hydrogen (H) and helium (He) ions irradiation will seriously influences the plasma stability and limits the lifetime of PFM. Based on the W-H-He potential developed by ourselves, we systematically investigate the interaction between H/He and different kinds of defects in W using MD calculations. We have demonstrated the physical origin of H-H repulsion and He-He attraction in W, and given the binding energy dependence of H/He, vacancy and self-interstitial atom to the H/He-vacancy cluster on H/He-vacancy ratio. The formation and growing process of H-vacancy clusters and He-vacancy clusters have been demonstrated, respectively. However, higher H concentration is needed to form the H-vacancy clusters, while the He-vacancy clusters tend to form spontaneously.