Arimichi Takayama

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.

Molecular dynamics simulation of a helium bubble bursting on tungsten surfaces

The bursting and expansion of helium bubbles near the surface of a tungsten material were investigated by using a molecular dynamics (MD) simulation. These helium bubble processes are considered to be important in the formation mechanism of fuzzy tungsten nano-structures. The phase diagram of the occurrence of bursting and expansion of helium bubbles was obtained by our MD simulation. The results of the simulation indicate that a helium bubble with a radius of 1.0 nm needs a high pressure of several tens of GPa to burst near the surface and to expand the bubble structures under the surface to the scale of ten nanometers. Moreover, from the viewpoint of the dynamics, the results of the MD simulation imply that the concavities and convexities observed on the surface in the early stage of the formation of a tungsten fuzzy nano-structure are caused by the bursting of the helium bubble.

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.

Comparison of Damages on Tungsten Surface Exposed to Noble Gas Plasmas

Tungsten was exposed to pure Ar or Ne plasmas over 1550 K at several incident ion energies. Even under the irradiation condition that the tungsten nanostructure is formed by He plasma irradiation, holes/bubbles and fiberform nanostructures were not formed on the surface by exposure to Ar or Ne plasmas. In addition, the results from energy dispersive X-ray spectroscopy supported the facts that Ar and Ne did not remain in the sample. We will discuss the reason for the differences in the damage to the tungsten surface exposed to noble gas plasmas.

Suppression of nonlinear interchange mode by zonal counterstreaming flow generation

Nonlinear evolution of interchange mode produces both vortex flow and shear flow in a scrape‐off layer (SOL) plasma with unfavorable magnetic curvature. When the Rayleigh number becomes the order of 105, the shear flow exceeds the vortex flow and the zonal counterstreaming flow appears in the perpendicular direction to the ambient magnetic field. Simultaneously, the fluctuation level decreases and the associated cross‐field transport becomes almost classical. However, since reduction of the saturation level weakens the shear flow generation, an intermittent oscillatory behavior appears and continues. Transport due to the vortex flow measured with a Nusselt number may depend on the Prandtl number for a given Rayleigh number.

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.

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.

Nonlinear Interchange Mode in Edge Plasma with Poloidal Shear Flow

Nonlinear evolution of two dimensional pressure-driven interchange mode produces relaxation oscillations in the presence of poloidal shear flow for Reyleigh number R a ∼10 4 . Here the flow velocity has a symmetric profile with respect to the center of slab plasma where the velocity shear disappears. There is a tendency that the velocity shear becomes weak after the saturatioin. When the velocity shear almost disappears, fluctuations grow rapidly. Then the velocity shear is again generated to suppress the growth of fluctuations. This kind of relaxation oscillation may be related to ELM [Edge Localized Mode] in the H-mode of tokamak.

Optimization of a corrugated millimeter-wave waveguide and a miter bend by FDTD simulation

We estimate the transmission efficiency of the electromagnetic wave through the system composed of waveguide and miter bend by Finite-Difference Time-Domain (FDTD) simulation. As the fisrt approach of this estimation, we choose the case that the input wave is TE11 mode. In this case, the efficiency is estimated as 99.65 % for the system without grooves and 76.48 % for the system with grooves (which is called as corrugate). Comparing the distributions of the input electric field with that of the output electric field, the effect of the grooves is found as follows: Because the TE11 mode has an anisotropy, its shape is changed by the miter bend. This property appears in the corrugated system more strongly than the non-corrugated system.

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.