A. Nishimura

C. Konno

K. Idemitsu

K. Ochiai

M. Inoue

S. Imagawa

T. Kiss

National Institute for Fusion Science (NIFS-Repository)

  The diagnostics of plasma facing materials (PFMs) is a primary issue for maintenance of the high performance plasma in fusion devices. In order to evaluate PFMs conditions which alternate continually during long duration discharges, in-situ and real-time diagnostic methods of PFMs are highly desired as an alternative to the existing postmortem methods. In this study, optical reflectivity measurement is proposed as a convenient in-situ diagnostics of the radiation induced microstructure change and its applicability is evaluated.  The in-situ reflectivity measurement was performed in LHD with a retro mirror and a super continuum white laser to verify applicability of this diagnostic method in a large plasma confinement device. Fig. 1 shows the evolution of reflectivity spectra obtained under the Ne (left) and H (right) glow discharge in LHD. The reflectivity is normalized by the initial value. For both discharge cases, significant changes are observed in the reflectivity spectra. The reduction in the reflectivity under the exposure to the Ne glow discharge seems to be caused by radiation damages formed in the sub-surface region of the mirror since a similar reduction are observed in the ion irradiated sample in the laboratory experiments [1]. On the other hand, the drastic reduction of the reflectivity and the peculiar wavelength dependences observed under the He glow discharge cannot be reproduce for ion irradiated samples. Because under the He glow discharge the chemical sputtering of carbon materials and their re-deposition have been observed, the drastic reduction is attributed to the formation of the deposition layer on the mirror. Due to the growth of the deposition layer, time dependence of the reflectivity under the He glow discharge shows a specific oscillation depending on the thickness of the deposition layer. Fig.2 shows the evolution of the reflectivity obtained under the H glow discharge. The oscillation of a reflectivity associated with an increase of a deposition thickness had been reported and apprehended as an optical interference phenomenon [2]. In this figure, the calculated reflectivity of the light with O = 780 nm is also plotted as a function of the deposition thickness. From the comparison of the frequency in the oscillation, the deposition thickness can be roughly estimated if the homogeneous deposition of known optical constant is formed on the mirror surface. These results indicate that optical property measurement is considered to be a possible method for convenient in-situ diagnostics for PFMs in spite of the limited application. For more accurate diagnosis, a combination of several diagnostics methods and an accumulation of comparative basic data are required. In this study, a transparent conductive film (GZO: gallium doped zinc oxide thin film) are also suggested as a new material for diagnostics. Fig. 3 shows optical transmittance (left) and electrical resistivity (right) for GZO films irradiated with 3 keV-He+. By using GZO films, optical and electrical properties can be measured simultaneously, which is considered to be one of possible methods with high accuracy for in-situ diagnostics.           Fig. 1 Evolution of reflectivity spectra obtained under the Ne (left) and H (right) glow discharge in LHD.                  Fig. 2 Exposure time dependence of the reflectivity under H glow (bottom), and calculated dependence of the reflectivity on the deposition thickness (upper).          Fig. 3 Optical transmittance and electrical resistivity for GZO films irradiated with 3 keV-He+.  1) Miyamoto, M. et al.: Ann. Rep. NIFS (2011-2012) 229. 2) Sagara, A.: Report of the Grant-in-Aid for Scientific Research (no. 16560726) (2005).   Purpose of this study is to investigate the influence of neutron irradiation on superconducting properties such as critical currents and flux pinning, and explore its fundamental mechanism. The influence of 14 MeV neutrons generated by the Deuterium-Tritium (D-T) reaction on superconducting material is one of the most important issues for the design of fusion devices using D-T burning plasma. Although there are some reports on the neutron irradiation effect on macroscopic superconducting property like a critical current density, the effects on the flux pinning in a mesoscopic scale has not been fully understood yet. In this study, we have carried out direct observation and analysis of trapped fluxoids by using scanning SQUID microscopy (SSM) to investigate the influence of neutron irradiation on flux pinning behavior. We adopted a pure Nb thin plate as a sample in order to investigate in a simple system. By using Fusion Neutronics Source (FNS) at Japan Atomic Energy Agency (JAEA), we irradiated the samples with fast neutron of 14 MeV. The neutron fluence was about 1019 n/m2. As shown in Fig.1, we have successfully observed the trapped fluxoids in the Nb thin plate, where white dots correspond to the trapped fluxoids. We can clearly see that the peak height of each fluxoids become more regular after the irradiation, whereas the number of anti-vortex and multi fluxoids structure can be seen in the initial sample. Variation of the statistical distribution of the peak heights of each fluxoiods can also be confirmed. This result strongly suggests that fluxoids are more stably pinned after neutron irradiation because of the introduction of nanoscale defects due to irradiation even though the fluence is relatively low, 1019 n/m2. It is hardly possible to detect such variation with this level of fluence from standard measurements of critical current or magnetization in a macroscopic scale. We also investigated magnetic field profiles of single fluxoid precisely. As show in Fig.2, we confirmed that the pinned flux profile has not changed after the irradiation. From this analysis, we can estimate magnetic penetration depth  around the fluxoid, then can obtain superfluid density in the matrix. Present results show that the neutron irradiation with a flence of 1019 n/m2 doesn’t cause deterioration of superconductivity in the matrix. Furthermore, temperature dependence of the superfluid density can well be explained by the theory of s-wave gap symmetry with no degradation of Tc as shown in Fig. 3.    100m(a)  100m(b) Fig. 1 Trapped fluxoids visualized by the scanning SQUID Micriscioy (a) before irradiation and (b) after irradiation.  01 10-62 10-63 10-64 10-65 10-66 10-67 10-68 10-6-3 10-5 -2 10-5 -1 10-5 0 1 10-5 2 10-5 3 10-5Exp.: non-irradiationExp.: after irradiationTheoryB(x) [T]x [m]  Fig. 2 Magnetic field profile of single fluxoid. Solid curve indicates theoretical calculation with the penetration depth of 140 nm.  00.20.40.60.811.20 2 4 6 8 10s-wave theoryExp.: after irradiation(0)2  / (T)2T [K]  Fig. 3 Temperature dependence of superfluid density estimated from that of field profile of single fluxoid as shown in Fig. 2. Solid curve is the result from s-wave gap symmetry. 255§20. Study on Mechanisms of Superconductivity Change by Neutron IrradiationKiss, T., Inoue, M. (Dept. of Electrical Eng., Kyushu Univ.),  Idemitsu, K. (Dept. of Appl. Quantum Phys. and Nuclear Eng., Kyushu Univ.),  Ochiai, K., Konno, C. (JAEA FNS),  Nishimura, A., Imagawa, S.

Study on Mechanisms of Superconductivity Change by Neutron Irradiation

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Study on Mechanisms of Superconductivity Change by Neutron Irradiation

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