27pB04
タングステンと炭素の表面混合層の形成によるタングステン中の重水素透過への影響
Effect of Tungsten and Carbon Surface Mixed Layer Formation
on Deuterium Permeation in Tungsten
PENG Han Yee、LEE Heun Tae、大塚裕介、上田良夫
Han Yee PENG, Heun Tae LEE, Yuusuke OHTSUKA, Yoshio UEDA
大阪大学大学院工学研究科
Graduate School of Engineering, Osaka University
In ITER, tungsten (W) will be used as a plasma
facing material in the divertor with carbon-fiber
composite at the strike plate. However, erosion andtransport of sputtered carbon (C) materials will resultin the simultaneous irradiation of W by hydrogenisotopes (H, D, T) and C impurities, leading toformation of a W-C mixed surface layer. In case of all
metal devices, C impurities will always be present as
impurities as observed in ASDEX-Upgrade [1].
The influence of such W-C mixed layer onhydrogen permeation through W has been scarcelyinvestigated. In our previous study, C/D mixedirradiation resulted in higher steady state permeation
flux in comparison to D-only irradiation [2]. However,whether the formation of W-C surface mixed layer
resulted in changes to diffusive or release properties
remained unclear. According to H transport theory ofDoyle and Brice [3], the dependence of permeation
flux on incident flux indicates certain transport regimes.
In this study, we have varied the incident flux andspecimen thickness to determine the transport regime
during C/D mixed irradiation, providing a moredetailed understanding of the impact of C on D
permeation.
Permeation experiments were performed using ahigh flux ion beam test device (HiFIT) coupled with apermeation device [4, 5]. The W specimens used were
thickness of 30 and 50 μm, with purity of 99.99%. TheD permeation flux was measured using a quadrupole
mass-spectrometer. The incident flux varied from 1019
– 1020 m-2s-1, and incident energy was 1 keV. The
specimen temperature range was 550 – 1050 K, and theC fraction in the incident flux was below 3%.
Previously, a peak enhancement factor of about
200 at 700 K was observed during C/D mixedirradiation in comparison to pure D irradiation [2].
Therefore, we measured the permeation flux
dependence on incident flux at 610, 710, and 850 K;
see Figure 1 At 850 K, the steady state D permeation
flux was proportional to the incident flux (n=1), and at
710 K the steady state D permeation flux was nearly
proportional to the square root of incident flux(n=0.59); where n indicates the exponent of the power

fit in Figure 1. These can be interpreted as diffusion
limited (DD) and surface recombination limited (RD)transport at 850 K and 710 K respectively [3].
However, physically unreasonable values of front
diffusivities are required to fit the observed increase inpermeation flux at this temperature At 610 K, noincident flux dependence was observed, which cannot
be explained by Doyle and Brice model.
We discuss further the effect of mixed W-C
surface on D permeation by considering the incident
flux dependency with new experiments on specimen
thickness dependence. We consider effects of changes
in diffusivity and the surface recombination coefficientof W-C surface mixed layer. This work contributes
towards quantifying the effect of mixed W-C material
formation on hydrogen transport and is valuable forestimating tritium permeation and safety in fusion
devices.

Figure 1: Steady-state D permeation flux plotted as afunction of incident flux at three temperatures. The
dash line corresponds to D-only result from Ref. [5].
[1] M. Mayer, et al., J. Nucl. Mater. 390 (2009) 538.
[2] H.Y. Peng, et al., Phys. Scr. T145 (2011) 014046.
[3] B.L. Doyle, B.K. Brice, Rad. Eff. 89 (1985) 21.
[4] Y. Ueda, et al., Fusion Eng. and Design 61-62(2002) 255-261.
[5] H.T. Lee, et al., J. Nucl. Mater. 415 (2011)
S696-S700.