Atsushi M. Ito

Molecular dynamics and Monte Carlo hybrid simulation for fuzzy tungsten nanostructure formation

For the purposes of long-term use of tungsten divertor walls, the formation process of the fuzzy tungsten nanostructure induced by exposure to the helium plasma was studied. In the present paper, the fuzzy nanostructures formation has been successfully reproduced by the new hybrid simulation method in which the deformation of the tungsten material due to pressure of the helium bubbles was simulated by the molecular dynamics and the diffusion of the helium atoms was simulated by the random walk based on the Monte Carlo method. By the simulation results, the surface height of the fuzzy nanostructure increased only when helium retention was under the steady state. It was proven that the growth of the fuzzy nanostructure was brought about by bursting of the helium bubbles. Moreover, we suggest the following key formation mechanisms of the fuzzy nanostructure: (1) lifting in which the surface lifted up by the helium bubble changes into a convexity, (2) bursting by which the region of the helium bubble changes into a concavity, and (3) the difference of the probability of helium retention by which the helium bubbles tend to appear under the concavity. Consequently, the convex-concave surface structure was enhanced and grew to create the fuzzy nanostructure.

First-principles investigation of possible clustering of noble gas atoms implanted in bcc tungsten

Using first-principles calculations, we have determined key properties of Ar and Ne in bcc W that relate to the possible bubble formation of implanted Ar and Ne atoms. The most stable interstitial site of Ar/Ne in bcc W is the tetrahedral site, as in the case of H and He. An interstitial atom causes the substantial strain of surrounding W atoms and the low electron-density region. The calculated migration energy of both Ar and Ne is very small, less than 0.2 eV of H. We calculated the binding energies of nearby interstitial atoms and found a strong dependence of atomic species. The attractive interaction between Ne atoms is the largest. Owing to such a small migration energy and a significant interaction between Ar/Ne atoms, interstitial atoms can act as traps for additional atoms and tend to form clusters in the absence of previous damage such as vacancies. The results presented here can explain the possible clustering of implanted atoms in bcc W via self-trapping.

Hybrid simulation between molecular dynamics and binary collision approximation codes for hydrogen injection into carbon materials

Abstract Molecular dynamics (MD) simulation with modified Brenner’s reactive empirical bond order (REBO) potential is a powerful tool to investigate plasma wall interaction on divertor plates in a nuclear fusion device. However, the size of MD simulation box is generally set less than several nm because of the limits of a computer performance. To extend the size of the MD simulation, we develop a hybrid simulation code between MD code using REBO potential and binary collision approximation (BCA) code. Using the BCA code instead of computing all particles with a high kinetic energy for every step in the MD simulation, considerable computation time is saved. By demonstrating a hydrogen atom injection into a graphite by the hybrid simulation code, it is found that the hybrid simulation code works efficiently in a large simulation box.

Formation of large-scale structures with sharp density gradient through Rayleigh-Taylor growth in a two-dimensional slab under the two-fluid and finite Larmor radius effects

Two-fluid and the finite Larmor effects on linear and nonlinear growth of the Rayleigh-Taylor instability in a two-dimensional slab are studied numerically with special attention to high-wave-number dynamics and nonlinear structure formation at a low β-value. The two effects stabilize the unstable high wave number modes for a certain range of the β-value. In nonlinear simulations, the absence of the high wave number modes in the linear stage leads to the formation of the density field structure much larger than that in the single-fluid magnetohydrodynamic simulation, together with a sharp density gradient as well as a large velocity difference. The formation of the sharp velocity difference leads to a subsequent Kelvin-Helmholtz-type instability only when both the two-fluid and finite Larmor radius terms are incorporated, whereas it is not observed otherwise. It is shown that the emergence of the secondary instability can modify the outline of the turbulent structures associated with the primary Rayleigh-Taylor instability.

First-Principles Investigation on Trapping of Multiple Helium Atoms within a Tungsten Monovacancy

We examine the binding energy of helium trapped in a tungsten monovacancy using first-principles calculation based on density functional theory (DFT) and investigate the trapping of multiple helium atoms within a tungsten monovacancy. Calculation shows that a tungsten monovacancy can contain at least nine helium atoms. We find that six monovacancy-trapped helium atoms form a kind of a cluster structure with an octahedral configuration, and the cluster structure is tightly bound around a monovacancy located at the center of a W cube.

Comparison between helium plasma induced surface structures in group 5 (Nb, Ta) and group 6 elements (Mo, W)

Group 5 elements (niobium and tantalum) and group 6 elements (molybdenum and tungsten) were exposed to helium plasma, and the resulting surface structures were observed by electron microscopy. Group 5 elements showed hole structures, where the size of the holes ranged from several tens of nm to a few hundred nm in diameter, while group 6 elements showed fiber-like structures. As a first step in understanding such differences, the difference in helium agglomeration energies and changes in the stress tensor as a function of the number of He atoms at interstitial sites were investigated for each element using density functional theory. The calculations revealed that helium atoms prefer to agglomerate in both of these groups. However, helium in group 6 elements can agglomerate more easily than group 5 elements due to higher binding energy. These results indicate a possible correlation between the shape of helium plasma induced surface nanostructures and the atomic level properties due to helium agglomeration.Group 5 elements (niobium and tantalum) and group 6 elements (molybdenum and tungsten) were exposed to helium plasma, and the resulting surface structures were observed by electron microscopy. Group 5 elements showed hole structures, where the size of the holes ranged from several tens of nm to a few hundred nm in diameter, while group 6 elements showed fiber-like structures. As a first step in understanding such differences, the difference in helium agglomeration energies and changes in the stress tensor as a function of the number of He atoms at interstitial sites were investigated for each element using density functional theory. The calculations revealed that helium atoms prefer to agglomerate in both of these groups. However, helium in group 6 elements can agglomerate more easily than group 5 elements due to higher binding energy. These results indicate a possible correlation between the shape of helium plasma induced surface nanostructures and the atomic level properties due to helium agglomeration.

Molecular Dynamics Simulation of Chemical Vapor Deposition of Amorphous Carbon: Dependence on H/C Ratio of Source Gas

By molecular dynamics simulation, the chemical vapor deposition of amorphous carbon onto graphite and diamond surfaces was studied. In particular, we investigated the effect of source H/C ratio, which is the ratio of the number of hydrogen atoms to the number of carbon atoms in a source gas, on the deposition process. In the present simulation, the following two source gas conditions were tested: one was that the source gas was injected as isolated carbon and hydrogen atoms, and the other was that the source gas was injected as hydrocarbon molecules. Under the former condition, we found that as the source H/C ratio increases, the deposition rate of carbon atoms decreases exponentially. This exponential decrease in the deposition rate with increasing source H/C ratio agrees with experimental data. However, under the latter molecular source condition, the deposition rate did not decrease exponentially because of a chemical reaction peculiar to the type of hydrocarbon in the source gas.

Theoretical Mn K-edge XANES for Li2MnO3: DFT + U study

Spectral features of Mn K-edge x-ray absorption near-edge structure (XANES) for Li2MnO3 were calculated using the first-principles full projector augmented wave method with the general gradient approximation plus U method. We demonstrated that the U parameter affects the spectral features in the pre-edge region while it does not affect those in the major absorption region. From the comparison with the experimental spectra and those of reference compounds, we showed that the spectral features of Mn K-edge XANES and the differences in the valence state can be reproduced well.

Reaction between graphene and hydrogen under oblique injection

The reaction between a graphene sheet and an incident hydrogen atom was clarified through a classical molecular dynamics simulation based on the modified Brenner’s reactive empirical bond order potential under the NVE condition, in which the number of particles (N), volume (V), and total energy (E) are conserved. The energy dependence of three types of reaction (i.e., adsorption, reflection, and penetration) for the oblique injection of a hydrogen atom into a graphene sheet was investigated. The reaction depends on the energy and two angular parameters, namely, the polar angle θ and the azimuthal angle φ of the incident hydrogen atom. The reflection and adsorption rates were found to strongly depend on θ. This dependence is caused by the three-dimensional structure of small potential barriers that cover adsorption sites (i.e., a local minimum point of the potential energy). The θ dependence of the penetration rates was observed. The penetration rates are proportional to cos2θ. The φ dependence of the pene...

Anisotropic Bond Orientation of Amorphous Carbon by Deposition

In general, diamond-like carbon is classified on the basis of the sp2, sp3, and hydrogen contents, otherwise known as the ternary phase diagram of amorphous carbon. In this study, however, it has been found that there is still some structural difference even though amorphous carbons may be located at the same point on the ternary phase diagram. Bond orientations of two types of amorphous carbon, arising in the deposition and annealing processes, are investigated, and it is shown that the bond orientations are totally different from each other even though the sp2, sp3, and hydrogen contents are the same.