タングステン材料に対するレーザー照射表面溶融と EUV相互作用による、重水素輸送、表面改

質への影響に関する研究

Laser power induced surface melting and laser driven EUV light interaction with mixed He+W material and its

impact on D transport and W surface morphology.

H.T. Lee a, N. Tanaka b, H. Nishimura b

a Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, Suita, Osaka 565-0871 b Institute of Laser Engineering, Osaka University, 2-6 Yamada-Oka, Suita, Osaka 565-0871, Japan

INTRODUCTION

ITER, the next magnetic fusion device, is moving forward with plans to start operations with a full tungsten divertor, raising the risk of W surface melting due to unmitigated power loads. Recent efforts have focused on the consequence of W melting on both plasma performance and material lifetime due to ejection, evolution and re-solidification of W melt layer [e.g. 1]. What has not been investigated in any detail so far is the impact such W melt layers may have on hydrogen transport, and to what degree surface release or diffusion behavior is affected.

To address this issue, we have performed ion driven permeation experiments with samples whose surface has been melted by laser irradiation. SEM microscopy showed that the melted surface layer thickness was limited to less than 1 μm for the 30 μm thick samples; see Figure 1. Significant surface morphological changes resembling filamentary structures and increased surface roughening was observed. The melted side was irradiated using a D ion beam and permeation fluxes measured as a function of sample temperature (500 < T < 1000 K).

In comparison to a non-melted sample, a factor of two increase in the steady state permeation was observed at T < 750 K; see Figure 2. Furthermore, differences in the permeation transients were observed, where the rise to steady state could be fitted using two diffusivities, as summarized in Figure 3. Our results suggest that the impact of W melting may potentially increase both tritium permeation rates and total inventory.

Fig. 1. SEM cross-section images of melted and non-melted surface structure of W specimens.

Fig. 2. Steady state permeation flux during D-only irradiation for specimens with melted surface (red triangle) in comparison to non-melted surface (blue circle and black square). The data has been scaled to correct for the different thicknesses of the specimens.

Fig. 3. Transient permeation flux fitted by two diffusivities and their summary as function of irradiation temperature.

REFERENCE(S)

[1] J. Coenen, et al., J. Nucl. Mater. 415 (2011) S78.

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2014/06/02 12th Hydrogen Workshop, Toyama, Japan 11

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7 2014/06/02 12th Hydrogen Workshop, Toyama, Japan

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6 2014/06/02 12th Hydrogen Workshop, Toyama, Japan

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