Elsevier

Journal of Nuclear Materials

Volume 463, August 2015, Pages 1049-1052
Journal of Nuclear Materials

In-situ measurement of deuterium retention in W under plasma exposure

https://doi.org/10.1016/j.jnucmat.2014.12.011Get rights and content

Abstract

The time dependence of the deuterium (D) retention in tungsten (W) was investigated by conducting successive D and hydrogen (H) plasma exposure using the plasma surface dynamics with ion beam analysis (PS-DIBA) device, which can conduct in-situ D retention measurement. At the sample temperature of 450 K, D retention rapidly dropped after the termination of D plasma exposure. On the other hand, at the sample temperature of 350 K, a slow decay of D retention after termination of D plasma was observed. The results indicate that the sample temperature has a strong influence on the dynamic behavior of D retention in W. Furthermore, by successive H plasma exposure to D retained W, D retention in W decreased exponentially in time. The result suggests that successive H plasma exposure is one of effective methods to remove tritium in W.

Introduction

Tungsten (W) is one of the candidate materials for future reactor and has chosen as a plasma facing material (PFM) in the ITER divertor region because of its high melting temperature, high thermal conductivity, and low yield of physical sputtering by neutral particle [1]. In particular, hydrogen isotope retention in W is a critical issue for ITER, because it can affect fuelling efficiency, plasma density control, and tritium inventory. Therefore, hydrogen isotope-tungsten interactions have been widely investigated with various experimental parameters [2], [3], [4], [5], [6]. However, under the conditions relevant to divertor plasmas in fusion devices, few studies have attempted to investigate the time evolution of D retention under plasma exposure, due to the difficulty of the measurement.

In addition, since tritium is radioactive material, tritium inventory in vacuum vessel should be limited to less than acceptable amount [7]. According to past studies, in carbon materials, it was observed that D retention clearly decreased by irradiation of hydrogen (H) plasma [8]. The experimental result indicated that successive H plasma exposure is effective method for reducing the tritium inventory in PFM.

In this study, the time evolution of D retention in W under successive deuterium and H plasma exposure has been investigated using a PS-DIBA (plasma surface dynamics with ion beam analysis) device [8], [9], [10]. In the device, the in-situ ion beam analysis of D retention under high density plasma exposure can be carried out.

Section snippets

Experimental

A schematic view of the PS-DIBA device is shown in Fig. 1(a). The typical plasma parameters and characteristics of the PS-DIBA device are shown in Refs. [8], [9], [10]. The amounts of retained D were measured by nuclear reaction analysis (NRA) using the reaction D(3He, p) α. To determine the D retention, the cross section of D(3He, p) α nuclear reaction in Ref. [11] was utilized. A 3He ion beam with 1.0 MeV generated by Van de Graaff accelerator was injected in the sample and produced protons

Experimental results

Fig. 2(a–c) shows the time-dependent change of deuterium retention in W. In Fig. 2(a) and (b), the experiments were conducted under successive D and H plasma exposure, while in Fig. 2(c), only D plasma irradiation was conducted. In the hatched regions, the samples were irradiated with D or H plasma, while the sample temperature was controlled by electron heating in the other regions. The sample temperature of Fig. 2(a)–(c) was fixed at 450, 350 and 500–350 K, respectively, and controlled by a

Discussion

In Fig. 2(a) and (b), after the termination of D plasma, D retention decreased exponentially in time. The decay time τ of the D retention is determined by fitting the data in Fig. 2(a) and (b), and τ is found to be approximately 500 s (2578 s) at a sample temperature of 450 K (350 K). From our perspective, the difference of τ in each figure is caused by the difference of diffusion coefficient of D in W. The diffusion distance Δd of D in W during the decay time can be described as,Δd=2Kdτwhere Kd is

Conclusion

An in-situ measurement of the time evolution of the deuterium (D) retention in tungsten (W) was investigated by conducting successive D and hydrogen (H) plasma exposure in the PS-DIBA device. At the sample temperature of 450 K, it was observed that D retention rapidly dropped after the termination of D plasma exposure. On the other hand, at the sample temperature of 350 K, slow decay of D retention after termination of D plasma has been observed. The results indicate that the dynamic behavior of

Acknowledgements

This work was supported by JSPS KAKENHI Grant Number 23656573, and partially supported by NIFS/NINS under the project of Formation of International Scientific Base and Network. One of co-authors (N. O) would like to thank Dr. Oya in Shizuoka Univ. for providing tungsten samples.

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