A. Sagara

H. Hashizume

K. Yuki

M. Kondo

S. Ebara

S. Ito

T. Kunugi

T. Muroga

T. Nagasaka

T. Yokomine

National Institute for Fusion Science (NIFS-Repository)

  LHD has provided good particle confinement and a high β with quasisteady-state operation. Currently, the operation of LHD is performed with hydrogen and/or helium gases. To obtain a higher-performance heliotron plasma and a prospect of reactor based on LHD concept, deuterium experiments are in plan on LHD. One of key physics subjects in the LHD deuterium experiment is to enhance the understanding of fast ion-related physics such as a ripple transport, anomalous transport of fast ions caused by Alfvénic modes or turbulences. Because most of neutrons will be produced by so-called beam-plasma reactions in a neutral beam (NB)-heated LHD plasma, neutron diagnostics will play an important role not only in measuring fusion output but also in assessing global confinement property of beam ions. Deuterium experiments also will provide a new opportunity to investigate confinement of MeV ions isotropic in the velocity space in LHD plasmas. A list intended to satisfy the needs for optimization of fusion output measurement and energetic-ion physics in LHD includes 1) ex-vessel neutron flux monitors (NFMs) and activation foil system, 2) neutron profile monitor, 3) 2.5 MeV neutron energy spectrometer (NES), 4) 14 MeV neutron diagnostic, and 5) prompt γ-ray diagnostic [1]. To work on development of diagnostics mentioned above, the joint fusion neutron team has been organized from 2008 in the framework of NIFS collaboration (see Fig.1) and has performed a feasibility study and a proof of principle (POP) experiment for neutron detection system promising for LHD. An outline of representative results of our activities from FY2008 to 2011 is described below. The neutron transport calculation is essential in a design stage of NFM. To perform Monte Carlo calculations for LHD, we have been made a great effort so as to model a complicated LHD structure on the MCNP-4C2 code for these four years. As a result of our efforts, in addition to the fluence rate and the energy spectrum of neutron near the LHD, the response function of the candidate neutron flux monitor for the DD neutrons and those from a 252Cf neutron calibration source can be now calculated. We have investigated a) neutron shielding effects by a pair of helical coils, b) neutron energy spectra for a DD and 252Cf neutron sources at the candidate position of NFM, c) the calibration process and the uncertainties associated with measurement of total neutron yield, d) the influence of the liquid helium for cooling the superconducting coils that will not exist during the calibration process, and so on. The obtained results are summarized in Refs. 2 and 3 in detail. A novel DD NES based on associated-particles coincident counting technique has been developed. Because fuel ions are no longer Maxwellian in NB-heated plasmas, a neutron spectrum mainly reflects velocity distribution of beam ions. Neutron spectrometry can therefore contribute to the study of energetic-ion behavior in deuterium plasmas. The POP experiment indicated that our NES has the energy resolution of ~6% for DD neutrons [4,5] and this value is comparable with that of NES employed in JET.  For a neutron emission profile monitor, we have made an effort to enhance n-γ discrimination performance of stilbene detector by use of digital signal processing technique [1,7,8]. In addition, we have also proposed a neutron pinhole camera with state-of-the-art nuclear emulsion technique for imaging of the emission profile of DD neutron produced by beam ions [6]. Recoiled proton due to elastic scattering of incident neutron passing though the pinhole makes a track in the nuclear emulsion. The incident neutron energy can be estimated by the energy of recoil proton and scattering angle that are derived from the track length and an angle between the track and the incident direction of neutron, respectively. We made preliminary designs on the pinhole collimator size and arrangement. The estimated spatial resolution would be better 100 mm with tungsten alloy collimator for the application to emission profile imaging of DD neutron in LHD plasma.  Development of artificial diamond as a 14 MeV neutron spectrometer has been carried out [9]. To suppress stress resulting from unconformity of lattice spacing between a substrate and a grown layer, an off-axis (001) surface fabricated on HP/HT type IIa diamond single-crystal substrate was adopted. Energy resolution of 0.6% for alpha particles was obtained by one CVD diamond single crystal.   Fig. 1. Structure of the joint team for development of fusion neutron diagnostics in the framework of NIFS collaboration.   1) Isobe, M. et al.: Rev. Sci. Instrum. 81 (2010) 10D310. 2) Nishio, N. et al.: Rev. Sci. Instrum. 81 (2010) 10D306. 3) Nishio, N. et al.: Plasma Fus. Res. 6 (2011) 2405115. 4) Iwai, H. et al.: IEEE Nucl. Sci. Symposium/Medical Imaging Conf., Conference Record, N25-49 (2009). 5) Tomita, H. et al.: Rev. Sci. Instrum. 81 (2010) 10D309. 6) Nomura, Y. et al.: Plasma Fus. Res. 6 (2011) 2402148. 7) Ishii, K. et al.: Rev. Sci. Instrum. 81 (2010) 10D334. 8) Ishii, K. et al.: Plasma Fus. Res. 5 (2010) S1043. 9) Kaneko, J.H. et al.: Diamond & Related Materials 26 (2012) 45. Flibe blanket using molten salt Flibe as breeding material as well as a coolant, is one of the advanced liquid blankets for fusion DEMO reactors and its conceptual design is in progress for LHD-type fusion reactor, FFHR, to date. Although the blanket has many strong points, e.g.,MHD pressure drop is negligibly small because of low electric conductivity of Flibe, there are still several issues to be solved. High melting temperature of Flibe (459 deg. C) is one of the issues because a temperature window of the blanket design is very limited due to the upper limit of temperature of the structural material (RAFS) of about 550 deg. C. In addition, it is very important from engineering point of view to demonstrate a heat removal of 1 MW/m2corresponding to the heat flux assumed to be imposed on the first wall of FFHR.Regarding the melting temperature, previous studies revealed that composition change in Flibe, which was achieved by changing ratio of BeF2 to LiF or by adding another salt such as CsF to Flibe, was effective to make itlower. In these cases, thermal and nuclear properties of the composition-changed Flibe are necessary to estimate. As for the high heat flux removal, this has been accomplished by using a sphere-packed pipe (SPP) in which a molten salt HTS as a simulant of Flibe flows. However, the detailed heat transfer characteristic of the SPP under high heat flux condition has yet to be clearly elucidated. In this study, heat transfer experiment using Tohoku-NIFS Thermal loop (TNT loop) and molten salt HTS was conducted to make an advanced heat transfer correlation available for the high heat flux case. Moreover, mixed Flibe was tested to obtainits thermal and nuclear properties.The form of a heat transfer correlation equation is assumed to be expressed as follows.Nu Re Pr                         (1)zyw x b sd dw fkCk          where Nud and Red correspond to Nusselt and Reynolds numbers based on the diameter of the sphere packed in a pipe as the length scale, respectively, and Pr is Prandtl number using thermal properties at bulk temperature. The symbols of b and w are viscosities at bulk and wall temperatures, respectively, and ks and kf are thermal conductivities of the packed sphere and fluid, respectively.In the experiment, the diameter of the packed spheres was ahalf of the pipe. In the equation, the term of ( b / w ) yexpresses the influence of local temperature difference induced by high wall heat flux and ( ks / kf ) z expresses the fin effect, i.e., effect of high thermal conductivity of solid spheres to heat transfer in the SPP. By using different material of the packed spheres, SS304 and Al, whose thermal conductivities were largely different, it was found that the fin effect hardly appeared. On the other hand, the effect of viscosity change clearly appeared in the experiment. And as shown in Fig. 1, the index was determined as 2.2.As for the index of Pr, change in Pr in the experiment was too small (24-27) to determine a reliable value. For this reason, the index was determined as 0.3106 by using previous experimental data obtained from heat transfer experiments using three kinds of silicon oils with largely different Pr (23-150). The remaining index and constantwere determined by regression analysis, and the resultant expression is shown as follows.0.220.7595 0.3106Nu 0.2424Re Pr        (2)bd dw    Figure 2 shows the correlation equation together with the experimental data obtained when two values of wall heat flux, 0.2 and about 1.0 MW/m2 were imposed. Theequation shows good agreement with the data whether the wall heat flux is high or low.0.1 0.22.362.382.42.42log(µb/µw)[-]log(Nu d)[-]log(Nud)=0.218log(µb/µw)+2.36Fig. 1 Influence of viscosity change on the heat transfer characteristic1000 2000 3000406080100120Red [-]Nud/Pr0.3106 /(µ b/µw)0.22[-]SS304(0.9㹼1.1 MW/m2)SS304(0.2 MW/m2)Al(0.9 MW/m 2)Al(0.2 MW/m 2)Maximum error : 8.5%Average error  :  2.4%Correlation  eq.Fig. 2 Heat transfer characteristic expressed by a heat transfer correlation equation219§20. Heat Removal Demo-research for Flibe Blanket DevelopmentHashizume, H., Ebara, S., Ito, S. (Tohoku Univ.),  Kunugi, T., Yokomine, T. (Kyoto Univ.),  Yuki, K. (Tokyo Univ. Sci., Yamaguchi),  Kondo, M. (Tokai Univ.),  Sagara, A., Muroga, T., Nagasaka, T.

Heat Removal Demo-research for Flibe Blanket Development

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Heat Removal Demo-research for Flibe Blanket Development

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