30E28P
希ガス照射によるタングステンナノ構造形成理解のための第一原理シミュレーション
First Principles Investigation on Noble Gas Embedded Tungsten
高山有道1, 伊藤篤史1, 中村浩章1,2TAKAYAMA Arimichi1, ITO Atsushi M. 1, NAKAMURA Hiroaki1,21

核融合科学研究所, 2名古屋大学
1
National Institute for Fusion Science, 2Nagoya Univ.
Introduction
It is reported that tungsten, which is candidate
material for the divertor plate in a thermonuclear
fusion reactor, is cracked by helium irradiation. The
cause of the crack is that irradiated helium creates
the bubble structure called ‘helium bubble’. Since
the crack of tungsten material becomes problem of
the use in the reactor, extensive tests of helium
irradiation were performed. Moreover, it wasexperimentally found that helium irradiation generates fuzzy structure in/on tungsten surfaces via
helium bubble under the specific conditions of surface temperature, influence and irradiation energy[1]. The fuzzy structure has a potential for functional nano-material beyond the nuclear fusion
research field.
Although the condition to generate bubble andfuzzy structures by helium irradiation was madeclear experimentally, their formation mechanismsare not clarified. One expectation from experiments
is that a key point of bubble structure creation is
trap phenomenon of helium atoms into thermalvacancy of tungsten lattice structure. In this context,
the binding energies of helium atoms in tungsten
material are evaluated by use of first principle
calculation based on density functional theory(DFT) [2].
MethodFirst-principles calculations of the binding energies
between noble gas atoms and mono-vacancies oftungsten are performed with the ‘OpenMX’ code
package, which is designed for nano-scale material
simulations based on DFT [3].
The generalized gradient approximation (GGA)with Perdew-Burke-Ernzerhof (PBE) functional isused for exchange-correlation potential. Linear
combinations of pseudo-atomic localized orbitals,
norm-conserving pseudopotentials, and projector
expansions are employed for core Coulomb potential. The Brillouin zone with 6x6x6 k-points sampling using the Monkhorst-Pack method isemployed.
Iterative calculations of self-consistent field (SCF)
convergence and molecular dynamics according to
the resulting force are performed to obtain arelaxation state. For the geometry optimization, theBroyden-Fletcher-Goldfarb-Shanno method is employed.
Results
Binding energies and total binding energy of 3x3x3
super-cell consisting of 53 tungsten atoms withmono-vacancy and helium atom(s) are evaluated. Itshows that tungsten mono-vacancy can contain atleast 9 helium atoms. It is also found that 6
mono-vacancy trapped helium atoms form a kind ofcluster structure of octahedral configuration, andthe cluster structure is tightly bounded around amono-vacancy located at center of tungsten cube.
In this presentation, calculated binding energies forthe cases of neon and argon will be shown as well.
Acknowledgments
This work is partly supported by the National Institute for Fusion Science Collaborative Research
Programs (NIFS12KNTS015, NIFS12KNSS026),
the National Institutes of Natural Sciences (NINS)
program for cross-disciplinary study, and a Grant-in-Aid for Scientific Research (No.19055005)
from MEXT. A part of this work was carried out
using the HELIOS supercomputer system at
Computational Simulation Centre of International
Fusion Energy Research Centre (IFERC-CSC),
Aomori, Japan.
References[1] S. Kajita, W. Sakaguchi, N. Ohno, N. Yoshida,and T. Saeki: Nucl. Fusion 49 (2009) 095005.
[2] A. Takayama, A. M. Ito, S. Saito, N. Ohno, andH. Nakamura: to be published in Jpn. J. Appl. Phys.
[3] OpenMX web-site:
http://www.openmx-square.org/