24P071-P
Simulation of Hydrogen Atom Injection into Amorphous Carbon Materials withChemical Reaction
アモルファス炭素材への化学反応を考慮した水素照射シミュレーション
1

Seiki Saito, 2Arimichi Takayama, 2Atsushi M. Ito and 1,2Hiroaki Nakamura1
斎藤誠紀，2高山有道，2伊藤篤史，1,2中村浩章
1

2Nagoya University; Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan.
National Institute for Fusion Science; 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan.
1
名古屋大学大学院工学研究科 〒464-8603 名古屋市千種区不老町
2
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

The behavior of incident hydrogen atoms in amorphous carbon materials is investigated by particle
simulation with taking both the physical and chemical reaction. The chemical reaction cannot be
neglected when the kinetic energy of projectile is low because a carbon atom makes four covalentbonds. The physical sputtering is often simulated by applying binary collision approximation (BCA).
However, BCA-based simulation cannot apply for the phenomena where chemical reaction isdominant. Molecular dynamics (MD) simulation with modified Brenner’s reactive empirical bond order
potential is possible to employ for phenomena with chemical reaction. In this paper, therefore, simulationof hydrogen injection into amorphous carbon materials is performed by BCA-MD hybrid simulation.

1. Introduction
Carbon materials are regarded as one ofcandidates of the divertor plate of nuclear fusion
reactor because carbon material has excellent
characteristics such as high melting point, low
atomic number, and high thermal conductivity.
However, carbon material has some problems thaterosion by chemical sputtering, co-deposition ofhydrogen isotope, and tritium retention in thebulk of carbon material.
The surface of divertor plate changes into
amorphous structure by the irradiation ofhydrogen plasmas. Moreover, carbon atoms
sputtered from divertor plates and they form
amorphous carbon materials on the vacuum
vesselwalls.
Carbonandhydrocarbon
redeposition onto plasma-facing components leadto long-term accumulation of large invessel
tritium inventories. To achieve the steady
operation of nuclear fusion device with carbon
divertor, it is necessary to investigate the physicaland chemical interactions between plasma andamorphous carbon materials.
In this paper, therefore, the behavior ofincident hydrogen atoms in amorphous carbon
materials is investigated by particle simulation
with taking both the physical and chemicalreaction.
2. Simulation Method
The physical sputtering is often simulated byapplying binary collision approximation (BCA)[1-3]. In the BCA-based simulations, multi-bodyinteractions
betweenatoms
inmaterials
approximate to consecutive two-body interactions
between a projectile and the nearest neighbor atom.
Therefore, the BCA-based simulation is notapplicable for the case where chemical reaction isdominant although the approximation significantly
reduces the computation time.
Molecular dynamics (MD) simulation [4] withmodified Brenner’s reactive empirical bond order
potential [5-6] is applicable for carbon andhydrogen system with taking chemical reaction
because the MD simulation calculates multi-bodyinteractions between atoms. However, MD
simulation needs longer computation time thanBCA-based simulation.
To research the motion of hydrogen atoms incarbon materials with chemical reaction inreasonable computation time, we developed hybrid
simulation combining BCA-based simulation usingAC∀T code [7] and MD simulation. As shown inFig. 1, the hybrid simulation [8-9] is performed by
the following way. A large material is treated byAC∀T. When the kinetic energies of atoms become
lower than a threshold value, the atoms and theirsurrounding atoms are calculated by the MD.
3. Preparation of Amorphous Carbon Material
BCA-based simulation has often applied forMonte-Carlo simulation to obtain statistical values.
In the Monte-Carlo simulation, the amorphous
structure is treated as atoms placed randomly. The
random structure is not applicable for the BCA-MD
hybrid simulation because strong forces suddenlyFig.1. Schematic diagram of the BCA-MD hybrid
simulation of hydrogen injection into a graphite [8].

work between atoms to reduce its potential energy
when the simulation method switches from BCA toMD. For the BCA-MD hybrid simulation, therefore,
it is necessary to prepare the amorphous carbon
material which is in the state of local minimum of
its potential energy.
We prepare a cubic amorphous carbon material
whose size of 20.19Å×20.86Å×20.0Å. The cubic
material has the periodic boundary in x, y and z
directions.
AsshowninFig.
2,asubmicrometer-scale amorphous carbon material isobtained by arranging the cubic material. Figure 3
shows the preparation. The process consists of threesteps. In the first step, an amorphous carbon
material is obtained by carbon deposition withperiodic boundary in x and y directions. In thesecond step, a cubic material whose size of
20.19Å×20.86Å×20.0Å is cut out from theobtained material by the first step. In the third step,
the cubic material is annealed at 2400 K in a box
which has periodic boundary condition in x, y andalso z directions.

Fig.2. Amorphous material
in submicrometer-scale
which is divided into
periodic boundary cells.

3. Research Approach
Hydrogen injection into the amorphous carbon
material, which is explained in section 2, isperformed by BCA-MD hybrid simulation. The
ranges and reflection rates of incident hydrogenatoms are extracted from the simulation results. The
sputtering yield is also investigated.

Fig.3. Simulation model for deposition [10].

Acknowledgments
This work is supported by the NINS Program for
Cross-Disciplinary Study, Grant-in-Aid for JSPS
Fellows (No. 23-340), and a Grants-in-Aid for
Scientific Research (No. 19055005) from the
Ministry of Education, Culture, Sports, Science andTechnology, Japan. This work is performed withthe support and under the auspices of the NIFS
Collaboration
Researchprograms(NIFS09KNSS002 and NIFS11KNTS008).

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