24P056-P
Influence of crystal orientation on damages of tungsten
exposed to helium plasma
ヘリウムプラズマ照射によるタングステンの損傷に結晶方位の与える影響
Yuki Hirahata1, Noriyasu Ohno1, Masato Yamagiwa1, Shin Kajita2, Naoaki Yoshida3,
Reiko Yoshihara3, Tomonori Tokunaga4 and Masayuki Tokitani5
平畑佑樹1, 大野哲靖1, 山際正人1, 梶田信2, 吉田直亮3, 吉原麗子3, 徳永知倫4, 時谷政行5
Graduate School of Eng.,Nagoya Univ.1, EcoTopica Sci. Inst. Nagoya Univ.2, RIAM Kyushu Univ.3,IGSES Kyushu Univ.4, NIFS5
1
名大院工 , 名大エコ2 , 九大応力研3 , 九大総理工4, 核融合研51Graduate School of Engineering, Nagoya University, Furo-Cho, Chikusa-Ku, Nagoya 464-8603, Japan
2
EcoTopia Science Institute, Nagoya University, Furo-Cho, Chikusa-Ku, Nagoya 464-8603, Japan
3
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka 816-8580, Japan
4
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University 6-1, Kasuga-koen, Kasuga, Fukuoka
816-8580, Japan
5NIFS, 322-6 Oroshi-cho, Toki, Gifu 509-5292, Japan1

名古屋大学大学院 工学研究科 〒464-8601 名古屋市千種区不老町
名古屋大学 エコトピア科学研究所 〒464-8601 名古屋市千種区不老町3
九州大学 応用力学研究所 〒816-8580 福岡県春日市春日公園6-1
4
九州大学 総合理工学府 〒816-8580 福岡県春日市春日公園6-1
5
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

2

To figure out the mechanism of the morphology change of tungsten (W), which is the candidate material
for divertor in ITER, we investigated the influence of crystal orientation on damages of tungsten exposedto helium plasma. ITER grade W was used for the specimens. The specimens were polished to mirror
finish and heated by electron for crystal growth. After helium plasma exposure, the surface of specimens
was analyzed by scanning electron microscope and orientation imaging microscopy. The difference in thesurface morphology was observed on different crystal grains. The regular wavy structure and helium
bubbles were observed on grains of {001}, {101}, {111} surface, but there is little change and helium
bubbles on the grain of {102} surface. It is thought that the area density of grains is important to thesurface morphology change of tungsten.

1. Introduction
Because divertor in ITER is exposed to high heatand particle load, material for divertor needs tohave high thermal property and low sputtering yield.
From those point of view, tungsten is regarded on
one of the candidate material for divertor in ITER
[1,2].
However, by helium plasma irradiation, it has
been observed that helium bubbles, holes andfiberform nanostructure are formed on the tungsten
surface even when the incident ion energy was lessthan the threshold energy of sputtering [3,4]. It is
likely that these surface morphologies lead to thereduction of the durability to particle and heat load
[5,6].
Recently, the difference in surface morphology
change was observed on different crystal grains [7].
It is speculated that the difference in the helium
induced structure can be attributed to the differencein the orientation of crystal structure.
In this study, we will report that influence ofcrystal orientation on damages of tungsten exposedto helium plasma, which including bubbles, holes,and fiberform nanostructure.
2. Experiment
ITER grade W was used for the specimens. The
specimens were polished to mirror finish and heatedby electron bombardment in plasma for crystal
growth so that crystal grains grew about 100μm.
Helium plasma irradiation to the specimens wasconducted in the linear divertor plasma simulator
NAGDIS-II [8]. The surface temperature of thespecimens was measured with a radiation
pyrometer. After helium plasma irradiation, thesurface of specimens was analyzed by scanningelectron microscopy (SEM), and crystal orientation
was analyzed by orientation imaging microscopy

(OIM).
3. Results and discussion
Figure 1 shows the ITER grade W exposed tohelium plasma at the surface temperature of 1700 K,
and the incident ion energy and fluence were 25 eV
and 5.3×1026 m-2 respectively. Figure 2 is expanded
images of Fig. 1. We analyzed crystal grains ofwhich the crystal orientations were {001}, {101},{102}, and {111} surfaces of bcc crystal structure.
The {101} face is the highest area density surface inbcc crystal structure.
As shown in Fig. 2, the difference in the surface
morphology was observed on different crystalgrains. The regular wavy structure and helium
bubbles were observed on crystal grains of {001},{101}, and {111} surface, and the size of wavy
structures were different at different crystal
orientations. However, there are little change andfew helium bubbles on the {102} surface.
These differences show that crystal orientation has
an effect on helium bubbles and the density ofcrystal face may be important because theformation of helium bubbles is originated with theinteraction between the thermal vacancies andhelium atoms. Moreover, the results indicate that it
is more difficult to form the helium bubbles when
the density of crystal face is lower. The area densityof {102} face is lower than the other, it may beharder for defects to be migrated. Therefore lesshelium bubbles were observed on {102} surface
than the other surfaces. On the other hand, since{001}, {101}, and {111} surfaces have higher area
density of crystal face, the defects could be easily
migrated, and, consequently, more helium bubbles
were formed compared with {102} surface.
{101} face is slip face which has the highest area
density face in bcc crystal structure, and binding
energy of tungsten atoms in this face is stronger
than that of the other faces. Taking account of this,the pressure from helium bubbles in specimen
increases. Consequently, {101} face may be moved
in the slip direction and the wavy structure isformed. Thus, it is speculated that this regularity of
the wavy structure is associated with the angle
between {101} face and surface of crystal grain.
4. Conclusions
The difference in the tungsten surface morphology
was observed on different crystal grains exposed tohelium plasma. Concerning the relationshipbetween helium bubbles and crystal orientation, thedensity of crystal face seems to be important. Thesurface morphology change is likely to relate toformation process of fiberform nanostructure. Sowe will research the relationship between regularityof direction on wavy structure and crystal
orientation.

Fig.1. SEM and OIM images of helium irradiated
ITER grade W

Fig.2. SEM images of grains in helium irradiated
ITER grade W
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