Seiki Saito

Yamagata University | YU · Graduate School of Science and Engineering

Human resource development program in nuclear engineering has been developed and carried out for national college students in Japan. It consists of (a) remote lectures and e-Learning contents on radiation and nuclear related subjects, and (b) on-site workshops and visits at nuclear related facilities or Nagaoka University of Technology. In approxim...

Binary-collision-approximation-based (BCA-based) simulation is performed for the investigation of the effect of a polycrystalline structure on the penetration depth, absorption rate, and sputtering yield of tungsten materials irradiated by helium plasma. To clarify the possibility of suppressing the generation of helium bubbles in tungsten material...

Tungsten is popular to use as a divertor tile in plasma equipment. However there are not enough information of tungsten at high temperatures. Recently, fiber-form nano-structured tungsten, which is called fuzz structure, was discovered. We modeled this fuzz structure to three types models, i.e., convex, concave and bubble models. We calculated the...

Binary-collision-approximation simulation of hydrogen isotope (i.e., hydrogen, deuterium, and tritium) and noble gas (i.e., helium, neon, and argon) injections into tungsten materials is performed. Three tungsten structures (i.e., amorphous, bcc crystalline, and helium bubble-containing structures) are prepared as target materials. Then, the trajec...

Using the binary-collision approximation simulation with atomic collision in any structured target code AC∀T, we calculated sputtering yield, range, and retention rate for tungsten with a rough surface under argon atom irradiation. The simulation revealed the sputtering yield decreases and the retention rate increases as the surface becomes rougher...

Binary-collision-approximation-based (BCA) simulation is performed for the investigation of bubble formation and the influence of the growth of bubbles on the characteristics of tungsten as a plasma-facing material. The BCA simulation provides the time evolution of the surface modification, the sputtering yield of tungsten atoms, and the absorption...

It is necessary to evaluate the endurance of tungsten (W) under plasma irradiation to realize the nuclear fusion reactor. The tungsten that has flat surface and amorphous structure is often assumed in the experiment and the simulation in plasma-wall-interaction study. However, from experiments, it has been reported that the sputtering yield for the...

In the field of nuclear engineering, national colleges have been carrying out important roles with about 10 % of graduates obtaining jobs in nuclear related companies. Although national colleges do not have special courses and classes for nuclear engineering, we have been trying to educate our students in cooperation with universities and industrie...

Carbon is a candidate material for divertor plates in nuclear fusion devices. To investigate the erosion and hydrogen retention in carbon at the atomic scale, we performed molecular simulations of hydrogen injection into single-crystal graphite considering the thermal vibration of the target atoms, the diffusion of incident atoms in the target mate...

The generation of tungsten fuzzy nanostructure by exposure to helium plasma is one of the important problems for the use of tungsten material as divertor plates in nuclear fusion reactors. In the present paper, the formation mechanisms of the helium bubble and the tungsten fuzzy nanostructure were investigated by using several simulation methods. W...

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 nanostructure's formation has been successfully reproduced by the new hybrid simulation method in which the deformation of the tungsten material...

The investigation of the erosion process of plasma facing material under plasma irradiation is an important issue for the achievement of the steady-state operation of nuclear fusion devices. Although binary-collision-approximation-based (BCA) simulation is a powerful tool for the investigation of plasma-material interaction, there is a difficulty t...

Many different carbon materials are developed for diverse applications for industrial and scientific uses. It is known that the amount of hydrogen retained in carbon materials under plasma irradiation strongly depends on the structural characteristics of the material, such as grain size. To clarify the cause of the difference in the amount of hydro...

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 bubbl...

A number of experiments show that helium plasma constructs filament (fuzz) structures whose diameter is in nanometer-scale on the tungsten material under the suitable experimental condition. In this paper, binary-collision-approximation-based simulation is performed to reveal the mechanism and the conditions of fuzz formation of tungsten material u...

To reveal the possibility of fuzz formation of tungsten material under noble gas irradiation, helium, neon, and argon atom injections into tungsten materials are performed by binary-collision-approximation-based simulation. The penetration depth is strongly depends on the structure of the target material. Therefore, the penetration depth for amorph...

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 dispersiv...

Under bombardment of hydrogen plasmas whose thermal energies are higher than the binding energy of target material, the structure of the surface of single-crystalline graphite dynamically turns into hydrogenated amorphous carbon via the chemical and physical reactions of incident hydrogen atoms. This structural change affects the processes of reten...

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-...

By using molecular dynamics simulation, formation mechanisms of amorphous carbon in particular sp${}^3$ rich structure was researched. The problem that reactive empirical bond order potential cannot represent amorphous carbon properly was cleared in the transition process from graphite to diamond by high pressure and the deposition process of amorp...

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. Bon...

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 type...

We have investigated plasma--surface interactions by performing molecular dynamics (MD) simulations. However, such simulations have a high computation cost and are limited to simulations of materials of nanometer order. In order to overcome this limitation, a complementary model based on the binary collision approximation (BCA) can be established....

We demonstrate the chemical interaction of hydrogen atoms with single-crystal diamond by performing classical molecular dynamics (CMD) simulations based on the modified Brenner reactive empirical bond order (REBO) potential. Erosion of the diamond structure is shown not to occur for hydrogen injection energies Ein < 5 eV, which is consistent with Y...

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...

We have investigated plasma–surface interactions by performing molecular dynamics (MD) simulations. However, such simulations have a high computation cost and are limited to simulations of materials of nanometer order. In order to overcome this limitation, a complementary model based on the binary collision approximation (BCA) can be established. W...

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...

In this article, we have analyzed the measured probe characteristics of microwave-sustained Ar and He plasma jets at atmospheric pressure using the high-pressure probe theory developed by Talukder et al. in order to determine the generated plasma parameters (electron temperature, Te, electron density, Ne, plasma potential, φp). The calculation show...

The ratio of the electron and ion saturation currents in single probe I-V characteristics for microwave-sustained plasma jets at atmospheric pressure are found to be much smaller than the value expected from the standard high-pressure single probe theory providing an over estimation of electron temperatures. By assuming that the single probe charac...

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, MD simulation box's size is less than several nm for the performance of a computer. To extend the size of the MD simulation, we de...

We have investigated plasma-surface interactions with molecular dynamics (MD) simulations. It, however, is high cost computation and is limited to simulations for materials of nanometer order. In order to overcome the limitation, a complementary model based on binary collision approximation (BCA) can be established. We employed a BCA-based simulati...

Examination of Temperature Dependence of Chemical Sputtering on Graphite by Comparing the Langevin and Berendsen Thermostats

Fluid mechanical properties of microwave discharge jet for argon and helium plasmas at atmospheric gas pressure is investigated. Kelvin-Helmholtz (K-H) instability due to shear flow of working gas with surrounding air is seen to reflect on the plasma structure. Pulsation phenomenon for helium plasma jet is found not to have a counterpart in gas flu...

By a classical molecular dynamics (CMD) simulation with a modified Brenner's reactive empirical bond-order (REBO) potential, we found that graphite with zigzag (1010) and armchair (1120) edge states is destroyed more easily than other structures, i.e., graphite with the (0001) surface, and diamond with the (100), (111), (120), and (110) surfaces. E...

Incident angle dependence of reactions between graphene and hydrogen atoms are obtained qualitatively by classical molecular dynamics simulation under the NVE condition with modified Brenner reactive empirical bond order (REBO) potential. Chemical reaction depends on two parameters, i.e., polar angle theta, and azimuthal angle phi of the incident h...

We have been developing a molecular dynamics (MD) simulation code, a binary collision approximation (BCA) based simulation code, and a BCA-MD hybrid simulation code. In this paper, an overview of the codes is presented. BCA-based simulation results of hydrogen isotopes injection into graphite target are also shown.