S-P5-2
Molecular Dynamics Simulation Approach to Plasma-Wall Interaction
プラズマ－壁相互作用の分子動力学シミュレーション研究
Atsushi M. Ito1, Seiki Saito2, Arimichi Takayama1, Sachio Yonemura2, and Hiroaki Nakamura1,2
伊藤 篤史1, 斎藤 誠紀2, 高山 有道1, 米村 幸朗2, 中村 浩章1,2
1) National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan.
2) Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan.
1) 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 〒464-8602 愛知県名古屋市不老町
In the investigation of integrated simulation for plasma-wall interaction (PWI) to evaluate tritium retention
and exhaust, the role of molecular dynamics (MD) simulation was to clear the mechanism of chemicalsputtering. The results of the MD simulations implied that the chemical sputtering is composite
phenomenon of "chemical desorption" and "thermal desorption" processes. The former is the moleculardetachment by using not only incident energy but also adsorption energy of incident atoms and ions. The
latter means that molecules are thermally desorbed from local fragile structures created by amorphization,i.e. chemical erosion. In this paper, furthermore, new extensions of PWI simulation are introduced.

1. Introduction
In the "tritium science and technology for fusion
reactor" project, the Ohya simulation group has
developed an integrated simulation code for PWI inorder to estimate accurately tritium retention andexhaust in fusion reactors. The key point of thisintegrated simulation code was how to connect
different models of scrap-off layer (SOL)/divertor
plasma and plasma facing material (PFM). Here,the EDDY[1] based on binary collision
approximation (BCA) was employed as asimulation code for the particle dynamics in PFM
because the BCA can be quickly executed. The
effect of chemical sputtering, which cannot betreated by the BCA, was compensated with theparameters evaluated by the MD. However, when
the project started, the chemical sputtering was notwell understood. Therefore, the first goal of the MD
was to clarify the mechanism of the chemical
sputtering.
In the present paper, at first the chemical
sputtering is explained with the result of the MD.
Next, new extensions of the MD and BCA for
future problem of PFM are introduced.
2. Understanding of the Chemical Sputtering
The word of "chemical sputtering" had been used
as meaning of the molecular desorption from PFM
which cannot be explained by physical sputtering.
Researches with the MD made progress inunderstanding the chemical sputtering. As a result,the chemical sputtering can be classified into thefollowing two processes.
The first process is that atoms on a PFM surface
are desorbed by chemical reaction with an incident
hydrogen atom/ion in a short time of pico-second.

In this “chemical desorption process”, although theincident energy is lower than displacement energy,the atoms on a PFM surface can be detached byadding energy generated from the adsorption of anincident particle onto the surface, which is called
adsorption energy. The chemical desorption process
can be clearly explained by the MD in which anhydrogen atom was injected onto a diamond (111)
surface[2]. On the diamond (111) surface, atoms or
molecules are sputtered if an incident energy was
higher than 5 eV. Figure 1 shows the ratio ofspecies of the sputtered atoms and molecules. From
this figure, it is understood that the CH molecule ismainly desorbed for incident energy of lower than
20 eV, while the C atom is dominantly sputtered inhigher incident energy. Moreover, from many
simulations varying incident position, it was foundthat the incident position causing the CH desorption
corresponds to that causing the adsorption of theincident hydrogen atom. This fact confirms theexpectation that hydrocarbon molecules aregenerated by the chemical desorption mainly
whereas the physical sputtering means to knock out
a single atom by high energy incidence.
It is expected that the chemical desorption
process easily occurs in also the surface ofamorphous graphite. However, we has difficulty inunderstanding that the chemical desorption process
hardly occurs in the center region of a surface of“graphene” because the structure of the graphene is
too stable to be sputtered by low energy incidenceof an hydrogen atom[3,4].
Desorption from graphitic materials, which arecomposed of the graphenes, e.g. carbon fiber
composite (CFC), is explained by the second
process in a long time. Through incident andadsorption energies are not enough to detachhydrocarbon molecules from the surface of thegraphene, they are enough to reconnect covalentbonds. Namely, the graphitic material isamorphized by the chemical reaction with theincident hydrogen atoms/ions, which is calledchemical erosion. By the chemical erosion, local
fragile structures are generated with the increase of
the H/C ratio. The hydrocarbon molecules aredetached from the local fragile structure by using
thermal energy during long time vibration, which iscalled “thermal desorption process” here.
In the MD, this thermal desorption process canbe observed by using thermostat method[5]. TheMD with the Langevin thermostat showed thedominant molecular species emitted by the thermal
desorption process was hydrogen rich hydrocarbon
molecules such as CH4 and CH3, and their
sputtering yields agreed with experimental
results[6] in the dependence on surface temperature.
The important prediction from the MD is that
almost all hydrocarbon was detached in a state of
CH residue firstly and then it grew to CH4 while
drifting in the void region of eroded amorphous
carbon. Direct detachment in the state of CH4 andCH3, which has been considered as a main process
for a number of years, hardly occurred. Moreover,the quantum chemical calculation result that thestate of CH2 residue is unstable in formationenergy[7] supports this prediction from the MD.
In this manner, "chemical sputtering" we has
been called is classified into the chemicaldesorption process in a short time and the thermal
desorption process in a long time. The deference
between the chemical and thermal desorption
processes is that in the former process, the desorbedhydrocarbon molecule is accompanied by the last
incident hydrogen atom.

Fig.1. The ratio of species of the sputtered atom andmolecules from the diamond (111) surface as afunction of the incident energy of an hydrogen atom.

3. Development of New Simulation Methods
The problems about the PFM cannot be solved
only by the current simulation codes. Therefore, wehave developed theory and simulation methods,
where we stick to atomic dynamics.
One of developed methods for high energy
incidence is modified BCA code AC∀T[8] which
can deal with arbitrary material structure, while theoriginal ACAT[9] and ACOCT[10] codes can treat
only random (not amorphous) and crystal structures,
respectively. Using the AC∀T code, amorphizationfrom a crystalline material by the continuous
injection of atoms and ions can be simulated[11].
Furthermore, we have developed BCA-MD hybrid
simulation code[12] in which while the kinetic energy
is high, atomic trajectory is quickly calculated by theAC∀T, and after the kinetic energy becomes low,region around cascade particles is locally simulated by
the MD. The BCA-MD hybrid code enables to treat
chemical and physical processes in micro- scale PFM.
4. Summary
By the MD simulation, the chemical sputtering isclarified as combination phenomena of the chemicaldesorption and thermal desorption processes. In
order to achieve nuclear fusion, new simulation
methods for the PWI and PFM problems should
continue to be developed.
Acknowledgments
The present works were supported by KAKENHI,MEXT (No. 19055005 and No. 23710135), NIFS
Collaboration Research program (NIFS09KNSS002
and NIFS11KNTS008), and NINS Program for
Cross-Disciplinary Study.
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