24P088-P
Simulation Study of Tungsten Materials Exposed to Helium / Neon / Argon Gas
Based on Binary Collision Approximation Model
タングステン材へのヘリウム・ネオン・アルゴン入射に対する二体衝突近似モデルによる評価
Arimichi Takayama, Seiki Saito, Atsushi M. Ito, Noriyasu Ohno, and Hiroaki Nakamura
髙山有道1, 斎藤誠紀2，伊藤篤史1，大野哲靖2，中村浩章1,2
1

National Institute for Fusion Science; Oroshi-cho 322-6, Toki,Gifu 509-5292, Japan
2
Nagoya University; Furo-cho, Chikusa, Nagoya 464-8603, Japan1
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
2
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

It has been experimentally observed that nanostructured tungsten is formed due to exposure of helium
plasma, and that the formation of nanostructure differs from irradiation nuclear species, for example,
helium, neon, or argon. We applied a binary collision approximation (BCA) based simulation to the noble
gas plasma exposure to tungsten material and found that the BCA model could qualitatively explain
differences of nanostructure formation between irradiation nuclear species.

1. Introduction
Tungsten is promising for plasma-facing
materials in future nuclear fusion reactors. So it
has been intently investigated experimentally andtheoretically. It is experimentally observed thatnanostructured tungsten is formed due toexposure of helium plasma [1-3], and that theformation of nanostructure or damage on thesurface differs from irradiation nuclear species,
for example, helium, neon, or argon [3].
To clarify the experimental observations, weapply a binary-collision-approximation (BCA)
model [4, 5] to the noble gas plasma exposure totungsten material.
2. Simulation Model
2.1 Binary-collision-approximation model (BCA)
We consider solving behaviors of injected
atoms into a material in atomic scale bynumerical simulation. When kinetic energy oftraced particles is relatively high, it is justified tosimplify interactions between material elements
including injected particles and reduce them to asequence of binary collisions.
Each binary collision under a given interatomic
potential can be solved analytically [4, 5]. In eachcollision process, momentum and energy of
projectile is transferred to the collision pair oftarget particle. When the transferred energy isless than a given energy, which corresponds tobinding energy, the target material is considered
not to be flicked and the position is kept
unchanged.
Furthermore, the trajectories of particles areapproximated as the asymptotes of them. Thus we
can obtain linkages of straight-line segments asparticle trajectories. This is a basic concept ofbinary collision approximation (BCA) model. The
BCA model is rather simple and requires lesscomputing resources than molecular dynamics(MD) model, which solves equations of motion
for all particles. By use of the BCA model,
parameter surveys can be conducted.
2.2 Simulation conditions
We employ the BCA-based simulation code
ACAT [4], which stands for atomic collisions inamorphous target. As it is indicated by the name,the ACAT code can treat only amorphous
structure, not crystalline. Is should be also notedthat the temperature effect of target materials is
not considered and is assumed to be zero.
Amorphous tungsten is set up in a simulation
domain as a target material, and a projectile ofhelium, neon, or argon atom is repeatedly injected
into the material. Incident energy varies from 20eV to 10 keV and incident angle is 0, that is,normal to the material surface.
For each atomic species and fixed incident
energy, 105 trials of particle injection into theamorphous tungsten are performed. The Molière
approximation to the Thomas-Fermi potential isemployed as the interatomic potential.
3. Results and Discussions
A part of incident particles is reflected out of
the target material, and the others remain and aredistributed in the target material. Fig.1 shows theFig.1. Dependence of mean depth of incident particles
on incident energy. Solid line with circles (), dashedline with triangles (), and dotted line with squares ()indicate the cases for helium, neon, and argon injection,respectively.

mean depth of incident particles as a function ofincident energy. It is found that the mean depth is
proportional to the square root of the incident
energy, which implies that mean depth is notdetermined by the incident energy, but by thevelocity or momentum of projectiles.
Fig.2 depicts the dependence of the sputtering
yields on incident energy. It is obvious thatsputtering does not occur below the thresholdincident energy determined for each projectile
species.
In experiments [3], temperature of the target
material of tungsten is kept in the range of1000-2000 K and incident energy of projectile isin the range of 20-200 eV. In the incident energy
range, sputtering does not exist, or at least, isrelatively small. Fig.1 shows that the mean depth
for helium injection at 20 eV is about 1 nm,
which corresponds to 3 layers of tungsten crystal.
On the other hand, the mean depth for neon orargon injection is 0.2-3 nm corresponding to thethickness of single layer. The depth is shallower
than that of experimental results. This difference
could be compensated by thermal diffusion
process in target material. Furthermore, incident
particles ceasing near the target material surface,
whose depth corresponds to the thickness ofsingle layer, may be escaped from the surface dueto the thermal diffusion, rather than diffuse within
the material. If it is the case, our simulation
results could qualitatively explain experimental
observations of damages on tungsten surface
exposed to noble gas plasma [3], especiallyFig.2. Sputtering yields dependence on incident energy.
Meaning of the symbols is same as Fig.1.

differences between irradiation nuclear species.
4. Conclusion
We performed binary-collision-approximation(BCA) based simulation to the noble gas plasma
exposure to tungsten material. It is found that theBCA model could qualitatively explain differencesof nanostructure formation between irradiation
nuclear species.
Acknowledgments
This work is supported by the National Institutefor Fusion Science Collaborative ResearchPrograms NIFS11KNTS008 and NIFS09KNSS002,
NINS Program for Cross-Disciplinary Study, and aGrant-in-Aid for Scientific Research (No.
19055005) from the Ministry of Education, Culture,Sports, Science and Technology, Japan. A part ofnumerical simulation was carried out by use of thePlasma Simulator at NIFS.

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