24P070-P
The Orientation Dependence of Surface Damage
in He Ion Irradiated Mirror Materials
ヘリウムイオン照射したミラー材料における表面損傷の結晶方位依存性
Mitsutaka Miyamoto1, Hiromitsu Takaoka1, Satoshi Kumode1, Kotaro Ono1, Masayuki Tokitani2
宮本光貴1, 高岡宏光1，雲出 聡1，小野興太郎1，時谷政行2
1

Department of Material Science, Shimane University, Matsue, Shimane 690-8504, Japan2
National Institute for Fusion Science, Oroshi, Toki, Gifu 509-5292, Japan
島根大学総合理工学部物質科学科 〒690-8504 島根県松江市西川津町1060
自然科学研究機構核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6

The crystal orientation dependence on a surface damage in molybdenum mirror is investigated under theirradiation with 3 keV helium ions to a fluence of 1021 ions/m2. A strong correlation between the surface
roughening and the grain orientation was confirmed from SEM-EBSD analysis. It was found thatsignificant surface roughening including crater-like depressions is formed on grains with the direction close
to <001>, while relatively smooth surface remains on grains with the other directions. The grains which
stay smooth surface under the irradiation show rather large erosion than the strongly roughened grains.

1. Introduction
In an optical diagnostics system using firstmirrors, the degradation of reflectivity due tointeractions between plasmas and mirror surfaces
are becoming a problem regarding reliability andlong term usefulness of the system [1,2].
Therefore the reflectivity behaviors for some mirror
materials have been extensively investigated using
ion beams [3,4], fast neutrons [5] and plasma
confinement devices [6,7].
In our recent study, we reported that the level ofdegradation of the optical properties of a single
crystal molybdenum (Mo) mirror with a certain
orientation irradiated with low energy helium ionsis considerably lower than that of polycrystalline
Mo mirrors [8]. This indicates that the crystal
orientation of the mirror should significantly affectthe optical reflectivity. In this study, the orientation
dependence of a surface damage in helium ion
irradiated Mo mirror is investigated.
2. Experimental
The specimens used in the present study were
99.95% nominal purity polycrystalline Mo suppliedby Nilaco Corp. After the vacuum annealing at~1900K for 10 min. to facilitate recrystallization,
the samples were cut into sheets of a size of
10×10×1.0mm3
andthenmechanicallymirror-polished with alumina powder.
Before the irradiation, the crystal orientation on
the sample surface was analyzed using SEM-EBSD(HITACHI S-3100H) to avoid uncertainty in theorientation analysis of radiation damaged samples.
The subsequent irradiation was carried out with 3

keV-He+ ions to a fluence of 1×1022 He/m2. Afterthe irradiation, surface morphology was examinedconsidering its crystal orientation.
3. Results and discussion
Figure 1 shows a low magnification SEM imageof Mo irradiated with 3 keV-He+ to a fluence of
1×1022 He/m2. Some grains still have smoothsurfaces, while other surfaces are strongly
roughened by the irradiation. The clear dependenceof the surface roughening on the grain orientation
was confirmed from EBSD analysis. Figure 2
shows typical morphologies observed in each grainwith a specific crystal orientation in the irradiatedMo, and the corresponding stereographic triangle.
Here, the orientations are exhibited along thenormal direction to the sample surface in thetriangle. It is evident from this figure thatsignificant surface roughening including crater-like

50 µmFig. 1. Low magnification SEM image of re-crystallized
Mo irradiated with 3 keV-He+ ions to a fluence of 1×1022
He/m2.

depressions is formed on grains with the direction
close to <001>, while relatively smooth surface
remains on grains with the other directions. Figure
3 shows high magnification SEM images of thesurface on the grains with the direction close to (a)<001> and (b) <011> in the irradiated Mo.
Complex surface textures shown in Fig. 3 (a) seemto be caused due to localized flaking andnonuniform erosion. On the other hand, very fineasperous surface with tiny holes which should betraces of helium bubbles is uniformly observed inFig. 3 (b). From the surveillance of scratches which
was created during the sample polishing procedure(not shown in the figures), it is confirmed that thegrains remaining in smooth surface under theirradiation show rather large erosion depth than thestrongly roughened grains. This means that thegrain of the orientation with a high sputtering yieldshows the high smooth surface under the irradiation.
The several possible mechanisms of the dependenceof the crystal orientation on the surface roughness
will be discussed in the poster.

a100 nmb100 nmFig. 3. High magnification SEM images of the surface
on the grains with the direction close to (a) <001> and(b) <110> in Mo irradiated with 3 keV-He+ ions to afluence of 1×1022 He/m2.

4. Summary
The dependence of the orientation on the surface
damage in Mo mirror is investigated under the 3keV-He+ irradiation. As results of SEM-EBSD analysis,it was found that significant surface roughening
including crater-like depressions is formed on grains
with the direction close to <001>, while relatively
smooth surface remains on grains with the otherdirections. The grain of the orientation with a highsputtering yield shows the high smooth surface
under the irradiation.

References[1] A.E. Costley et al., Fusion Eng. Design, 55 (2001) 331[2] V.S. Voitsenya et al., J. Nucl. Mater. 329–333 (2004)
1476.
[3] M. Balden et al., J. Nucl. Mater. 329-333 (2004) 1515.
[4] M. Miyamoto et al., Plasma Dev. Oper. 17 (2009) 155.
[5] E.H. Farnum et al., J. Nucl. Mater. 219 (1995) 224.
[6] T. Akiyama et al., Rev. Sci. Instrum. 78 (2007) 103501[7] A. Litnovsky et al., J. Nucl. Mater. 363–365 (2007)
1395[8] K. Ono et al., Phys. Scr. T138 (2009) 014065.

500 nmFig. 2. Typical morphologies observed in each grain with a specific crystal orientation in Mo irradiated with 3 keV-He+
ions to a fluence of 1×1022 He/m2. The orientations are exhibited along the normal direction to the sample surface in thestereographic triangle.