22D11
Impact of Surface Modification of Tungsten on Deuterium Retention
タングステンの重水素吸蔵に対する表面改質効果
Mizuki Sakamoto1, Aleksandr Rusinov2, Ryouhei Ohyama2, Naoaki Yoshida3, Koichiro Honda2,Naoko Ashikawa4, Masayuki Tokitani4, Mitsuki Miyamoto5, Tatsuo Shoji6
坂本瑞樹1、A. Rusinov2, 大山亮平2、吉田直亮3、本多耕一郎2、芦川直子4、時谷政行4、宮本光貴5、庄司多津男612

Plasma Research Center, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan3-1-1-4F,
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, kasuga, Fukuoka 816-8580, Japan
3
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
4
National Institute for Fusion Science, Toki, Gifu, 509-5292, Japan
5
Department of Material Science, Shimane University, Matsue, Shimane 690-8504, Japan
6
Department of Engineering, Nagoya University, Nagoya, Aichi, 464-8601, Japan1
筑波大学プラズマ研究センター 〒305-8577 茨城県つくば市天王台1-1-1
2
九州大学総合理工学府 〒816-8580 福岡県春日市春日公園6-1
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
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-6
5
島根大学総合理工学部 〒690-8504 島根県松江市西川津町1060
6名古屋大学工学部 〒464-8601 愛知県名古屋市不老町
Effects of surface modification by He bubble formation and by tungsten deposition on hydrogen isotope
retention in tungsten have been studied in the compact PWI simulator APSEDAS. Low-energy deuterium
plasmas (~28 eV, ~2.2x1025 m-2) are exposed to a W sample with He bubbles with the size of ~10 nm to~200 nm beneath the surface and a W sample with a tungsten deposited layer (thickness ~30 nm) as well asa bulk W sample without surface pre-treatment at a surface temperature of ~500 K, and then D retention isdetermined by a TDS method. The diffusion of D in W is a dominant process of D retention in W. The
modification layer of the surface may play a role of the diffusion barrier.

1. Introduction
Tungsten (W) is an important candidate
material in the divertor region in ITER, because it
has good properties such as a high energetic
threshold for physical sputtering, a high melting
temperature and a low retention of hydrogen (H)isotopes. However, surface condition of a plasma
facing material continues to change during
plasma operation due to plasma-wall interaction(PWI) such as erosion, redeposition, radiation
damage, bubble formation and so on. Such a
surface modification could affect the physicalproperties of W as well as fuel retention.
Codeposition is an important mechanism for thefuel retention of the metal wall as well as carbon .
A He irradiation effect in W is another criticalproblem for H isotope retention, becauseradiation damage such as bubbles and dislocation
loops, which are created by He irradiation, acts asboth a strong trap site and a diffusion barrier of H
isotope [1-5]. It is important to investigate themechanism of surface modification and its effecton fuel retention.
In the compact PWI simulator APSEDAS, W
samples with a deposited W layer and withhelium (He) bubbles by pre-implantation wereexposed to low energy and high flux deuterium(D) plasma to investigate surface modificationand its impact of H isotope retention.
2. Experimental Results
Three samples of polycrystalline tungsten with
0.1 mm thickness provided by Nilaco Co. wereused in this work. The quoted purity was 99.95 %.
The grain sizes were estimated to be in the range of~1 nm to 10 nm. A sample W-1 was pre-annealed at1170 K in vacuum for 1 h. A sample W-2 was not
pre-annealed but was exposed to low energy He
plasma at the surface temperature of ~1770 K for
50 min. Many fine irregularities were produced on
the W-2 surface. Figure 1(a) shows a micrograph ofa scanning electron microscope (SEM) of the W-2
surface. Many holes and fine irregularities wereproduced by the He plasma exposure. Figure 2(b)shows a cross-sectional micrograph of W-2 byusing a focused ion beam (FIB) process and atransmission electron microscope (TEM). It isfound that bubbles with the size of ~10 nm to ~200nm were generated beneath the surface. A sample
W-3 was mechanically polished and pre-annealed at

(a)2Total Retention(D/m )15µm(b)

2

21

1x10

3457
6

20

1x101024

φ 0.5210251026
2Fluence(D/m )Fig.1. (a) SEM micrograph and (b) cross-sectional
micrographs of the tungsten sample W-2.

1170 K in vacuum for 1 h and then a tungsten
deposited layer was produced on the surface by aPVD method. The thickness of the deposited layer
was ~30nm, which was estimated from a data of aquartz microbalance monitor (QMB), which wasinstalled below the sample holder, and thegeometric arrangement of the sample and the QMB.
Each sample was exposed to the low energy D
plasma in APSEDAS. The exposed area was 8 mmφ
and the surface temperature during the D plasma
exposure was about 500 K. Typical parameters
during the D plasma exposure in this work were ne~2.4 x 1017 m-3, Te ~ 8eV and Vs ~ 28 V (Γi ~ 3 x1021 m-2 s-1). Although the plasma parameters of theHe plasma exposed to the sample W-2 could not bemeasured directly, ion energy and fluence areconsidered to be in a range of 20 to 40 eV and ~4 x1025 m-2 from the measurement of another similar
plasma in APSEDAS, respectively. After D plasma
exposure, D retention was measured using thermal
desorption spectroscopy (TDS) with a quadruple
mass spectrometer (QMS). During TDS, the sample
was heated linearly at 1 K/s to 1127 K.
Figure 2 shows the fluence dependence of thetotal retention of W-1, W-2 and W-3 samples. In thecase of the sample without the surface modification(i.e. W-1), the retention increased proportional to
0.52th power of the fluence. This means that
diffusion is a dominant process in D retention in W.
The retention of the W sample with the depositedlayer significantly reduced to one-fifth of that ofW-1. The deposited layer may play a role ofdiffusion barrier.
As for the W sample with He bubbles (i.e. W-3),
the retention in the low fluence region was muchhigher than that of W-1, but it decreased slightlywith the fluence and saturated. This decrease may

Fig.2. Fluence dependence of total retention of the bulk
W without surface modification (triangle), W withhelium bubbles (circle) and that with W deposited layer(square). The number next to the symbol indicates theorder of the D plasma exposure.

be attributed to the characteristic change of the He
bubble layer due to the repetition of the D plasma
exposure. The decrease in the retention of 6th and7th exposures is considered to be caused by thesame reason. The high retention at the low fluenceis attributed to a number of trap sites created by theHe bubble. And the saturation of the retention isconsidered to be caused by suppression of thediffusion of D due to the He bubble layer.
3. Summary
Effects of surface modification of tungsten onhydrogen isotope retention have been studied in thecompact PWI simulator APSEDAS. The D
retention is in good agreement with a square-root
dependence of the fluence, indicating that the Ddiffusion is a dominant process for the retention.
The He bubble layer has a substantial potential ofhydrogen isotope retention, and the retention
decreases with fluence and saturate. The retentionof W with a W deposited layer is significantly
reduced to one-fifth of that of W without surface
modification. Both surface-modified layers of theHe bubbles and W deposition may play a role of adiffusion barrier.
References[1] H. Iwakiri et al., J. Nucl. Mater. 307-311 (2002) 135.
[2] D. Nishijima et al., J. Nucl. Mater. 337-339 (2005)927.
[3] H. T. Lee et al., J. Nucl. Mater. 363-365 (2007) 898.
[4] M. Miyamoto et al., Nucl. Fusion 49 (2009) 065035.
[5] Y, Ueda et al, J. Nucl. Mater. 386-388 (2009) 725.