In-situ measurement of deuterium retention in W under plasma exposure
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,where 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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Cited by (13)
Behavior of tungsten under irradiation and plasma interaction
2019, Journal of Nuclear MaterialsCitation Excerpt :In-situ diagnostics are required to measure the dynamically retained component. In-situ Nuclear Reaction Analysis (NRA) measurements have been made during plasma exposure [602], but these measurements are integrated over the entire depth range (approximately one micron) and required duration of 10 min of signal integration time. One would expect, however, the H isotope concentration to peak close to the stopping distance of the incoming plasma ions (typically a few nm) and for this concentration gradient to disappear quickly after the termination of the incoming ion flux.
Effect of neon on the hydrogen behaviors in tungsten: A first-principles study
2018, Journal of Nuclear MaterialsCitation Excerpt :However, W will be exposed in hostile environments such as extremely high fluxes of H isotopes, or helium (He) ions and 14.1 MeV high-energy neutrons in a fusion reaction, which results in blister formation and surface sputtering erosion and thus shortens the lifetime of PFMs [3,4]. In order to study the microscopic mechanisms of surface morphology variation, the interactions between H isotopes and W have been widely investigated with various experimental techniques [5–8]. For radioactive cooling, Ne and Ar are usually introduced to plasmas in tokamaks.
In situ NRA study of hydrogen isotope exchange in self-ion damaged tungsten exposed to neutral atoms
2016, Journal of Nuclear MaterialsCitation Excerpt :However, similar processes were observed also for tungsten exposed to plasma, where isotope exchange took place not only within the ion range but also deeper in the bulk [11,12]. Similarly, the isotope exchange has been recently studied in situ by NRA during plasma exposure of tungsten [13,14] and stainless steel [13]. Moreover, the efficiency of isotopic exchange when changing the feed gas in JET from one isotope to the other [15] and JET Ion Cyclotron Wall Conditioning discharge [16] was discussed and compared for JET carbon (JET-C) and ITER-Like wall (JET-ILW) configuration.
Dynamic fuel retention in tokamak wall materials: An in situ laboratory study of deuterium release from polycrystalline tungsten at room temperature
2015, Journal of Nuclear MaterialsCitation Excerpt :We note that this value is consistent with the recent report by Watanabe et al. [30] of a deuterium release on a sub-hour time scale as measured with in situ NRA. A decay time constant of τ ∼ 40 minutes was found at Tstorage ≅ 350 K in Ref. [30]. If we analyze this result along our proposition of a first order kinetic isothermal desorption, we extract an activation energy of Ea = 1.14 eV.
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Presenting author.