24C03
Change of surface morphology and particle retention of the plasma facing
materials in the fusion devices under helium irradiation and formation of themixed-material deposits
ヘリウムプラズマ照射とMixed-material堆積層形成による核融合装置プラズマ対向材料の表面変質と粒子捕捉特性変化
Masayuki Tokitani1, Suguru Masuzaki1, Naoaki Yoshida2, Nobuaki Noda1, Akio Sagara1,Hiroshi Yamada1 and LHD experiment group1
1
1
時谷政行 ，増崎 貴 ，吉田直亮2，野田信明1，相良明男1，山田弘司1，LHD実験グループ1
1

2

National Institute for Fusion Science, 322-6 Oroshi, Toki, Gifu 509-5292, Japan
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasuga-koen, Kasuga, Fukuoka, 816-8580, Japan1
核融合科学研究所 〒464-0075 岐阜県土岐市下石町322-6
2
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
The effects of "radiation damages by helium for metals" and "formation of the mixed-material deposition
layer on the plasma facing components (PFCs)" to particle balance and dust generations in the large-sized
plasma confinement devices were studied by using nanoscale material analysis. During helium dischargesin Large Helical Device (LHD), heavy radiation damages such as helium bubbles and dislocation loops
were densely observed on the first wall materials (SUS316L). Furthermore, mixed-material deposition
layers composed by C, Fe and O have been formed on the divertor and first wall surface. Amount ofhydrogen retention in such layers were 5-6 times higher than that of bulk carbon materials. These two PWI
phenomena would be closely related to degradation of the materials, change of the retention property of thehydrogen isotopes and dust generation in the vacuum vessel. So we have to clearly understand them forachievement of the steady state plasma operations.

1. Introduction
Fusion engineering research has stepped into a
new stage of focusing on the technicalestablishment of a steady state reactor system. It is
one of the large phase changes for fusion researches,from the pulse operation experiment which has
been performed so far.
Understanding of the particle balance, tritium
inventory, degradation of the materials and impurity(dust) generation are four important issues to realize
the steady state plasma operations. Then, their four
issues would be basically controlled and influenced
by only following two phenomena.
First one is "radiation damages by helium formetals". It is known that injected helium atoms inmetals can aggregate by themselves and form
He-vacancy complexes and helium bubbles even if
the knock-on energy is insufficient for the
displacement damage [1]. It was reported that once
bubbles are formed, the material will become hardand brittle [2], and also, they could be acting as theeffective trapping site for hydrogen isotopes [3].
Second one is "formation of the mixed-material
deposition layer on the PFCs". During the longpulse discharges in Large Helical Device (LHD),
termination of the plasma sometimes occurs with an
unexpected sudden increase in impurity radiation[4], which is attributed to some kind of plasma-wallinteractions (PWI). It is thought that the impurities
originate from thin mixed-material deposition layeron the divertor plates. These are formed bystratified layers consisting of Fe and C elements [5].
If we can totally understand and control about
helium and mixed-material effects, we would beable to approach and reach the steady state plasma
operations. In this research, therefore, the effects of"radiation damages by helium for metals" and"formation of the mixed-material deposition layeron the PFCs" to particle balance and dust
generations in the large-sized plasma confinement
devices were studied.
2. Radiation damages by He for metals
The concentration of the helium in fusion plasma
would have been expected to be about one order
lower than that of hydrogen. So, the discussion
about the radiation effects of helium for metals had
not been made energetically until about 15 years
ago. However, recent years, strong radiation effects
such as formation of the dense bubbles anddislocation loops have been reported one after
another in the laboratory experiments. Especially,observation results of the transmission electron
microscope (TEM) have clarified the formation

mechanism of the severe radiation damages inmetals [1-3]. Fig. 1 shows the bright; (a) and dark;(b) field TEM images of the SUS316L exposed tohelium discharges (87 s in total) in LHD. Typicalcore plasma parameters were as follows: Ti =0.7–1.7 keV, ne = 0.3–8.1×1019 m-3. White dot
contras in (a) and (b) shows helium bubbles anddislocation loops, respectively. Although total
exposure time was only 87 s, very dense finehelium bubbles of about 1–2 nm in diameter andconsiderably large amounts of dislocation loops
were formed. The damages would be caused bycharge-exchanged (CX) helium atoms which are
not affected by magnetic field, and their energy andflux are sufficiently higher to create these defects.
As mentioned in section 1, these defects wouldchange the physical properties of the material
surface. Also, once these defects are created, they
are difficult to eliminate even up to the hightemperature (near the melting temperature of thematerials). In the future fusion devices, sequential
wall conditioning, for example, glow dischargecleaning (GDC), would not be conducted. Suchtypes of the helium radiation damages might
become a serious issue for plasma operations.

(a)metals (Fe, Ni, Mo) It is considered that thedeposition elements were piled up in turn, andfinally reached ~8 m thick in total. The reason
why such a stratified structure is seen is attributed
to the scheme of the operation sequence of LHD. In
general, thick deposited layer could be unstable andpossible source of impurity (dust) for plasma.
Especially, metallic dominant layers including
many types of the defects seem to be brittle andacting as the initiator of the exfoliation of the whole
deposition layer [6]. In addition, retained hydrogenatoms in the mixed-material deposition layer wereabout 5 times higher than that of the bulk graphite.
These two PWI phenomena would be closely
related to degradation of the materials, change ofthe retention property of the hydrogen isotopes anddust generation in the vacuum vessel.

(b)

Fig.

20nm
Fig. 1 (a) Bright field and (b) dark field TEM imageof SUS316L after exposed to the helium
plasma for 87 s at the first-wall position.

3. Formation of the mixed-material deposition
layer on the PFCs
The PFCs in fusion machines would be used not
only metal materials but also carbon relatedmaterials. The formed deposition layer could be themixed-materials including both elements. Fig. 2shows the cross-sectional TEM image of themixed-material deposition layer formed on thegraphite divertor tile used for 8 years in LHD. The
deposition layer has very fine and stratified
structures in the nanometer level. The bright layers
were mainly composed by C, and dark layers were

2 Cross-sectional TEM image of themixed-material deposition layer formed on
the graphite divertor tiles in LHD.

Acknowledgments
This research was partly supported by the Japan
Society for the Promotion of Science andGrant-in-Aid of Scientific Research from Japan
Ministry of Education, Culture, Sports, Science andTechnology.
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[3] H. Iwakiri et al., J. Nucl. Mater. 307–311 (2002) 135.
[4] T. Mutoh et al., Nucl. Fusion 47 (2007) 1250.
[5] R. Kumazawa et al., Fusion Sci. Technol. 58 (2010)524.
[6] M. Tokitani et al., J. Nucl Mater. 417 (2011) 668.