24P087-P
Deuterium and carbon accumulation in tungsten coating
exposed to JT-60U divertor plasmas
JT-60Uダイバータプラズマを照射した被覆タングステン中の重水素および炭素の蓄積
Masakatsu Fukumoto1, Tomohide Nakano1, Kiyoshi Itami1,Takaaki Wada2, Yoshio Ueda2 and Tetsuo Tanabe3
1
福本正勝 , 仲野友英1，伊丹 潔1，和田隆明2，上田良夫2，田辺哲朗3
1

Japan Atomic Energy Agency, 801-1 Mukoyama, aka, Ibaraki 311-0193, Japan
Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
3
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University,
6-10-1 Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan.
1
日本原子力研究開発機構 〒311-0193 茨城県那珂市向山801-1
2
大阪大学大学院工学研究科 〒565-0871 大阪府吹田市山田丘 2-1
3
九州大学大学院総合理工学研究院 〒812-8581 福岡県福岡市東区箱崎6-10-1

2

In the tungsten coating exposed to JT-60U divertor plasmas, graphitic and disordered carbon and tungsten
subcarbide (W2C) were formed within ~2 µm in depth. Retained amount of the graphitic and disorderedcarbon in tungsten coating exceeded the solution of carbon in tungsten lattice. These findings indicate thatthe irradiated carbon migrated to deep region through grain boundaries and pores in the tungsten coating,and that the carbon atoms penetrated and diffused into tungsten grains from grain boundaries and pores andformed W2C.

1. Introduction
Tungsten coating on the blanket first wall isplanned to reduce in-vessel tritium inventory andsputtering erosion of structural materials, whilecarbon based materials will be used for high heatload components such as the tiles facing neutralbeam injection systems. In the case that thedeuterium retention in the tungsten coating wasincreased since the carbon ions, which wassimultaneously injected with deuterium ions,migrated to deep region and this carbon retained
the incident deuterium ions [1,2]. Hence, carbon
migration mechanisms in tungsten coating areexpected to play an important role for deuterium
retention. However mechanisms of carbon
migration in tungsten coating are still unknown.
2. Experiment
In JT-60U, the tungsten-coated carbon fiber
composite (CFC) tiles were installed in the upper
part of the outer divertor plates. Tungsten with athickness of ~50 µm was produced by vacuum
plasma splaying (VPS) on the ~15 µm thickrhenium/tungsten multi-inter layer, which wasdeposited by physical vapor deposition on CFCtiles.
About 1100 discharge experiments wereperformed. Total discharge time was ~8420 sec.
The maximum surface temperature during plasma

discharges was ~700 K, which was evaluated byfinite element methods (FEM) based on the tile
temperature. After the experimental campaign, twotungsten-coated tiles were taken out from JT-60U
for the present post mortem tile analysis. Samples
with a size about 8 × 8 × 1 mm were cut out fromthe tungsten-coated tiles. Chemical state of thecarbon in the tungsten coating was investigated by
X-ray photoelectron spectrometer (XPS) with Mg
Kα radiation. The binding energies were calibrated
with the Au 4f7/2 peak at 84.0 eV. In order toinvestigate the depth profiles of the carbon
chemical compositions in the tungsten coating, thesample surface was sputtered by a 3 keV argon ion
beam.
3. Results
The C 1s spectrum measured by XPS wasdecomvoluted by Gauss-Lorentz functions toexamine the chemical compositions of the carbon inthe tungsten coating. Since the chemical state of thecarbon in tungsten is expected to be graphitic anddisordered carbon, tungsten carbide (WC), andtungsten subcarbide (W2C). Hence, the C 1sspectrum was decomvoluted by four curves with thefixed peak energies with 284.2, 285.2, 283.1, and283.6 eV for graphitic carbon, disorderedcarbon, WC, and W2C, respectively [4]. Figure 1
shows the C 1s spectrum measured at the depth of

~6 µm (see Fig. 2) and the results of thedecomvolution. The C 1s spectrum was best fit withthree curves, disordered and graphitic carbon andW2C. Atomic concentration of these carbon
compositions based on the areal ratio of each peaks
was ~11.3, ~5.8, and ~14.2% for disorderedcarbon, graphitic carbon, and W2C, respectively.

4Intensity (x10 cps)

1.5Graphiticcarbon
1.0

C1s spectrum
Total fit

W 2C

Disorderedcarbon

0.5

0.0
288

286
284
282
Binding energy (eV)280

Fig.1. Carbon 1s spectrum in the tungsten coating
measured by XPS with the best fits for the measured
spectrum. The gray points show C 1s measured spectrum.
The three colored lines represent the curvesdeconvoluted by Gauss-Lorentz functions. The black line
shows total fit of the deconvolution curves.

Figure 2 shows depth profiles of chemicalcompositions of the carbon in the tungsten coating
measured by XPS. The carbon composition for each
depth was investigated by the methods mentionedabove. Atomic concentration of the graphitic andthe disordered carbon increased with depth andreached ~10 % at ~0.6 µm in depth. After that, theatomic concentration of these carbon compositions
was decreased with increasing depth to ~1 % at thedepth of ~2.2 µm. The W2C concentration was
Atomic concentration (%)

100
Tungsten

806040

Disordered Graphiticcarbon
carbon

200
0.0

0.5

1.0
1.5
Depth (µm)

2.0

W 2C

2.5

Fig.2. Depth profiles of chemical compositions of thecarbon in the tungsten coating measured by XPS.

increased with depth and saturated at the depth
deeper than ~1 µm (16-17 %).
4. Discussion
According to the carbon diffusion coefficient forpolycrystalline tungsten obtained from the 14C
diffusion experiments [4], the calculated carbon
diffusion length in tungsten lattice was less than 10nm provided that the surface temperature and thedischarge duration were 700 K and 8420 s,respectively. As shown in Fig. 2, the migration
length of the injected carbon atoms was much
deeper than this calculated diffusion length. This
discrepancy suggested that the dominantmechanisms of carbon migration in the tungsten
coating are different from diffusion of carbon atoms
in the tungsten lattice.
As shown in Fig. 2, more than one percent of thegraphitic and the disordered carbon wereaccumulated in the tungsten coating within thedepth of ~2 µm. This indicates that most of thegraphitic and the disordered carbon existed at grainboundaries and pores, since the solubility of carbon
in tungsten grains is less than 0.3 at.% [5].
From above findings, the carbon migration
mechanism in tungsten coating is expected asfollows. The injected carbon migrated to deep
region through tungsten grain boundaries and pores
as a form of carbon atoms or gaseous molecules
such as CDx. The carbon atoms penetrated fromgrain boundaries and pores and diffused into
tungsten grains, and most of the carbon reacted withtungsten atoms, forming W2C
5. Conclusion
Carbon migration mechanisms in tungsten
coating irradiated by JT-60U divertor plasmas werediscussed based on the chemical composition of thecarbon. The injected carbon migrated to deep region
through grain boundaries and pores. After that, thecarbon atoms were penetrated and diffused into
tungsten grains and formed W2C.
References[1] M. Fukumoto et al.: J. Plasma Fusion ResearchSERIES 9 (2010) 369-374.
[2] M. Fukumoto et al.: Accepted to J. Nucl. Mater.
[3] J. Luthin and Ch. Linsmeier: Surface science454-456 (2000) 78.
[4] L.N. Aleksandrov and V. Ya. Shchelkonogov:
Poroshkovaya Metallurgiya 4 (1964) 28.
[5] H.J. Goldschmidt and J.A. Brand: J. Less-Common
Metals 5 (1963) 181.