A. Takayama

A.M. Ito

H. Nakamura

S. Saito

National Institute for Fusion Science (NIFS-Repository)

We have investigated inﬂuences of plasma irradia-tion on plasma facing materials, such as sputtering ornano-structure formation, by use of simulations withatomic scale. The principal target was a material com-posed of carbon and hydrogen isotopes, and it was sim-ulated by molecular dynamics (MD) with Brenner’s re-active empirical bond order (REBO) potential, binarycollision approximation (BCA), or a BCA-MD hybridmodel. In the present study, simulation target mate-rial is extended to metallic materials such as tungsten.In order to understand phenomena occurring in plasmafacing materials of metallic elements, ﬁrst-principles cal-culations are carried out and potential model for MDsimulation are also developed.First-principles calculations are carried out with the‘OpenMX’ code package, which is designed for nano-scalematerial simulations based on density functional theories(DFT), developed by Dr. Ozaki at Japan Advanced In-stitute of Science and Technology1). We improve thecode package to accelerate computation speed. The im-provement enable us to calculate more wide conﬁgura-tional and parametric space.Systems where noble gas atoms, such as helium, aretrapped within a tungsten monovacancy are considered.In the ﬁrst-principles calculations, the generalized gradi-ent approximation (GGA) with Perdew-Burke-Ernzerhof(PBE) functional is used for exchange-correlation poten-tial. Linear combinations of pseudo-atomic localized or-bitals, norm-conserving pseudo-potentials, and projec-tor expansions are employed for core Coulomb poten-tial. The ‘OpenMX’ uses numerical pseudo-atomic or-bitals (PAOs) as basis function to expand one-particleKohn-Sham wave functions. We employ the 3 s-, 2 p-,2 d-, and 1 f-state radial functions with cut-oﬀ radius of7.0 a.u. for tungsten, and the 2 s-, 2 p- and 1 d-stateradial functions with cut-oﬀ radius of 8.0 a.u. for helium.The Brillouin zone with 6×6×6 k-points sampling usingthe Monkhorst-Pack method is employed.The lattice constant is obtained by minimizing thecohesive energy Ec, and is 3.172 A˚ in the present cal-culation parameter set. We employ a lattice constantof 3.172 A˚ for present calculations, although this valueis slightly large than that in the literature, 3.165 A˚ (at298K).Iterative calculations of self-consistent ﬁeld (SCF)convergence and molecular dynamics (MD) accordingto the resulting force are performed, and a relaxationstate is obtained. For the geometry optimization, theBroyden-Fletcher-Goldfarb-Shanno (BFGS) method isemployed. The ﬁnal state with the lowest total energyfor each sample of the same number of helium atoms isregarded as the most stable state. The conﬁgurationsobtained by the above mentioned method are used forthe calculation of binding energy.Figure 1 depicts calculated values of the binding en-ergy EB as a function of the number of helium atomstrapped within a tungsten monovacancy. When thebinding energy EB takes a negative value, the conﬁg-uration is less stable than the conﬁguration whose num-ber of trapped He atoms is less by one. It is inferredfrom Fig. 1 that a tungsten monovacancy can contain atleast nine helium atoms. That is, helium has a propertythat it tend to gather in a tungsten monovacancy ratherthan stay as an interstitial atom. This tendency may berelated to helium bubble generation, which will lead tonano-structure formation.First-principles calculations require high computa-tion cost, thus it is hard to be applied to a system hav-ing wide spatial scale. Moreover it is limited to a steadyand ground state. Molecular dynamics (MD) or kineticMonte Carlo (kMC) simulation is suitable for a widescale system and can treat dynamics or kinetics. A po-tential model to tungsten-helium-argon coexistence sys-tem required for MD simulation has been developed withﬁrst-principle calculation in the present study. Prepara-tion of MD simulation and development of kMC simula-tion code is progressing, and they will be carried out inthe next ﬁscal year.Fig. 1: Binding energy as function of the number of Heatom trapped within a W monovacancy2).1) http://www.openmx-square.org/2) A. Takayama, A. M. Ito, et al., “First-principles In-vestigation on Trapping on Multiple Helium Atomswithin a Tungsten Monovacancy”, Jpn. J. Appl.Phys., (2013) vol. 52, 01AL03.417§14. Simulation Approach to Understanding Phenomena Occurring in Plasma Facing MaterialsTakayama, A., Ito, A.M., Nakamura, H.,  Saito, S. (Nagoya Univ.)

Simulation Approach to Understanding Phenomena Occurring in Plasma Facing Materials

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