25pC07
Deuterium retention behavior for tungsten exposed in LHD during
the 18th plasma experimental campaign
Cui Hu1, Hiroe Fujita2, Kenta Yuyama2, Yuki Uemura2, Shodai Sakurada2, Keisuke Azuma1, Suguru Masuzaki3,
Masayuki Tokitani3, Naoaki Yoshida4, Yuji Hatano5, Takumi Chikada2, Yasuhisa Oya2
1

Faculty of Science, Shizuoka Univ.; 2 Graduate School of Science, Shizuoka Univ.; 3 National Institute for Fusion Science; 4Institute for Applied Mechanics, Kyushu Univ.; 5 Hydrogen Isotope Research Center, Univ. of Toyama

Abstract: Tungsten specimens were placed in the four positions of the plasma facing wall, namely PI (typical PWI area), DP(deposition dominated area), HL (higher heat load area) and ER (erosion dominated area) in the Large Helical Device (LHD)
and were exposed to hydrogen plasma in the 18th experimental campaign. Microstructure and chemical composition of thespecimens were analyzed by SEM, TEM and XPS. Additional deuterium implantation was performed for these specimens andtheir deuterium retention behavior was evaluated by TDS. Results showed that a carbon-dominated mixed-material layer wasformed on the all of the specimens except for ER sample. The thickness of this layer has increased as compared with that of
17th experimental campaign. The major deuterium desorption temperature was correspondingly shifted toward the hightemperature for DP and PI samples as well.
Keywords: LHD, Tungsten, Deuterium retention, TDS
1.

Introduction
Tungsten (W), possess high energy threshold for
sputtering, low ability to hydride formation and lowhydrogen solubility level, has been selected as the most
likely candidate of plasma facing materials (PFMs) for
future fusion reactors [1]. However, it is well known that theirradiation damages and impurity layer will be inevitably
formed on W in the actual fusion reactor. So, theinvestigations about the influence of damages and impurity
layer on hydrogen isotope retention of W have been studied
with interest using various heavy ion and ion irradiation,
respectively [2]. For study the hydrogen isotope retention
behavior in W exposed in plasma, the W specimens wereplaced in LHD and were exposed to long-term hydrogenplasma experiment. The deuterium retention behavior of theexposed W for the different LHD experiment campaign was
systemically investigated [3]. Our previous works showedthat the thickness of carbon-dominated mixed-material layer
with impurities of oxygen and Nitrogen etc. deposited on W
has increased year by year because of the usage of graphite
in closed divertor to enhance the plasma performance.
Additional deuterium ion implantation and thermal
desorption spectroscopy (TDS) experiment results provedthat the hydrogen isotope would be accumulated in thiscarbon-dominated mixed-material layer. In addition, thehelium (He) desorption behavior has been observed for 2013
experiment campaign due to long-term He plasma discharge
was performed to achieve high plasma performance. As thedevelopment of the system and the improvement of theplasma performance, the composition and microstructure of
the surface impurity layer will be changed. So, we willcontinue to investigate the deuterium retention behavior fortungsten which was exposed in LHD during the latest (18 th)experimental campaign in the present work.
2. Experimental
The mirror polished W samples with size of Φ10 mm ×5 mm were annealed at 1173 k for 30 min under vacuum
circumstance and then were placed in the four uniquepositions of the plasma facing wall in the LHD. The sample
labels are corresponding to their positions in LHD, which
are named as PI, DP, HL and ER, respectively. After thehydrogen plasma experiment, the samples were picked up
and transported to Shizuoka University with air exposure.
Then, the 1.0 keV deuterium ion (D2+) were additionallyimplanted into these samples with the flux of 1.0 × 1018 D+

m-2 s-1 and fluence of 5.0 × 1021 D+ m-2, respectively. Thedeuterium retention behavior of the samples was evaluatedusing the TDS with high resolution quadrupole mass
spectrometer, which can distinguish the D2 from He.
Chemical composition and microstructure of the samples
were examined by XPS, SEM and TEM.
3. Result and discussion
Figure 1 shows the D2 TDS spectra for D2+ implanted
tungsten exposed to LHD 18th experiment campaign. It canbe seen that the major deuterium desorption of ER sample
was located at low temperature less than 600 K. However,there are two deuterium desorption stages at higher
temperature around 600-700 K and 900-1000 K for DP, PI
and HL samples. As compared with that of 17th experimental
campaign, the major deuterium desorption temperature wascorrespondingly shifted toward higher temperature for DPand PI. It was suggested that the deuterium desorption attemperature at around 900 k should be caused by desorptionof deuterium which was trapped by thick carbon-dominatedmixed-material layer as C-D bond. So, the surface
carbon-dominated mixed-material layer should be an
important factor on the deuterium retention enhancement onW, especially for the DP and PI area in LHD.

Fig.1 D2 TDS spectra for D2+ implanted tungsten exposed to 18thexperiment campaign

4. Reference[1] A.El-Kharbachi, etc., Int. J. Hydrogen Energy, 39 (2014)
10525-10536[2] Ryo Miura, etc., Fusion Eng. Des. , 88 (2013) 1827-1830[3] Y. Oya, etc., Fusion Eng. Des. , 89 (2014) 1091-1095