24P096-P
Evaluation of Carbon Erosion Process in Attached and Detached Deuterium
Plasmas
接触・非接触重水素プラズマ中での炭素損耗過程の評価
Masahiro Teraoka1, Noriyasu Ohno1, Shin Kajita2, Atsushi M. Ito3, and Hiroaki Nakamura3
寺岡正広1, 大野哲靖1，梶田信2，伊藤篤史3，中村浩章3
1

Department of Engineering and Science, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名大院工 〒464-8603 名古屋市千種区不老町
2
EcoTopia Science Institute, Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名大エコトピア 〒464-8603 名古屋市千種区不老町
3
National Institute for Fusion Science322-6, Oroshi-cho, Toki, Gifu, 509-5292, Japan
核融合研 〒509-5292 岐阜県土岐市下石町322-6

Carbon material, which is the candidate material of the divertor in the fusion reactor, would be damaged
significantly in a detached plasma since a carbon has high reactivity with a hydrogen isotope. It is ofimportance in the investigation of the fundamental characteristics of carbon-hydrogen interaction. In thisstudy, Highly Oriented Pyrolytic Graphite (HOPG) was exposed to the attached and detached deuterium
plasmas in the NAGDIS-T. By surface analysis using scanning electron microscopy (SEM) and Raman
spectroscopy, it was found that the HOPG irradiated with the detached plasma has anisotropic damages.
The anisotropic characteristics of graphite erosion are discussed on the basis of the results of thecomparison with molecular dynamics (MD) simulation.

1. Introduction
The interaction between carbon and hydrogenis one of the most important issues for nuclear
fusion research. Carbon materials are well known
to be beneficial for fusion device walls, such as
divertor plates. Because carbon has a highreactivity to a hydrogen isotope, carbon materials
also would be damaged heavily even in thelow-temperature plasma, so-called “detachedplasma”. It is of significant to investigate thefundamental characteristics of carbon erosion.
Thus, we focused the orientation of the graphitestructure and used HOPG, which has a multilayerof graphene structure. HOPG was irradiated withthe attached and detached deuterium plasmas in
the toroidal plasma divertor simulator NAGDIS-T
and analyzed by SEM and Raman spectroscopy toinvestigate the surface features. the light from theplasma was measured by spectroscopy. We
focused on the difference between the effect of
the attached and detached plasmas in this study.
2. Experimental Methods
HOPG, which reflects the two-dimensional
anisotropy of the graphene sheet strongly wasexposed to the attached and detached deuterium
plasmas in the NAGDIS-T and the light of CD
radicals was measured by spectroscopy toinvestigate amount of the carbon erosion. In theattached plasma, the typical electron density ne is1018 m-3 and the temperature Te is 2-5 eV. On theother hand, ne and Te are typically 1.5 × 1018 m-3
and less than 0.1 eV, respectively in the detachedplasma. After the plasma irradiation, the graphitetargets were analyzed by SEM and Raman
spectroscopy.
3. Results
Figures 1(a)-(f) were SEM micrographs of thecentral and edge ares of the HOPG samples before
irradiation and after the attached and detachedplasma irradiation, respectively. It was found that,after the irradiation with the detached plasma, thecenter of the HOPG target had no damage and theperipheral region of the target was damaged andshowed a staircase pattern, as shown in Figs. 1(e)and 1(f). This suggested that the edge area of theHOPG target was mainly damaged when theincident ion energy was low. The edge of thegraphene sheet is thought to be weaker than thecentral area for low-temperature deuterium plasma
irradiation. Figure 2 shows the Raman spectral
analysis results for the nonirradiated and irradiated
HOPG samples in terms of ID/IG and FWHMG[1]. A
part of surface of the HOPG sample irradiated withthe attached plasma became visually darker than its

margin and seemed to have more in-plane defects
than isotropic graphite. The dark area seemed to be
protrusion-like structure, as seen in the SEM
micrograph in Fig. 2.

Fig.1. SEM photographs of the surface of HOPG target.
(a) and (b) show the nonirradiated target in the centraland edge parts, (c) and (d) show the irradiated target withthe attached plasma in the central and edge parts, and (e)and (f) show the irradiated target with the detachedplasma in the central and edge parts.

Fig.2. FWHMG, which corresponds to disturbance of theinterlayer, as a function of the intensity ratio (ID /IG),
which corresponds to in-plane defects of nonirradiated,
and irradiated HOPG targets.

damaged by irradiation with the attached plasmacomparing with its margin. Figure 3 shows theenlarged diagram of Fig. 2 excluding the results ofthe dark area and isotropic graphite. The HOPG
irradiated with the attached plasma was observed tohave more in-plane defects than that irradiated withthe detached plasma. As for the irradiation with thedetached plasma, the edge region had particularly
in-plane defects. In the same manner as the resultsfrom SEM photograph, the HOPG target is likely to
be damaged from the edge of a graphene sheet at alow ion incident energy.
4. Discussion
The analysis of hydrogen-carbon interaction hasbeen conducted by MD simulation [3]. It was foundthat there is an energy barrier of 0.4 eV in the (0001)
plane of graphene sheet, while there is no suchpotential barrier in the edge region of graphenesheet by the energy potential calculation with an
MD simulation for a hydrogen atom to be adsorbed
onto the graphene sheet. In the attached plasma, theincident ion energy is much higher than the energy
barrier. However, the incident ion energy is lower
than 0.4 eV in the detached plasma. Thus, in theattached plasma, the incident deuterium ions canpass through the barrier, and most of the deuterium
ions might have been reflected before reaching thesurface in the detached plasma.
5. Conclusion
We have exposed HOPG specimens to attachedand detached plasma and measured by spectroscopy
and SEM and Raman spectroscopy. From SEM and
Raman spectral analysis results, it was found thatthe edge region of HOPG was severely damaged,
while damages were not observed in the central part.
From the results of MD simulation, it was foundthat there is the energy barrier in (0001) plane. It
was suggested that the prediction from the MD
simulation results qualitatively agrees with theexperimental results for the detached plasma.
Acknowledgments
This work was supported in part by a grant ofKnowledge Cluster Initiative (Tokai region)
Implemented by the Ministry of Education, Culture,Sports, Science and Technology (MEXT) and by
the NINS Program for Cross-Disciplinary Study.

Fig.3. Enlarged diagram of Fig. 5 except for the resultsof visually dark region and isotropic graphite.

Thus, the dark area seemed to be severely

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