N. Kimura

S. Takada

National Institute for Fusion Science (NIFS-Repository)

  A 100 kA-class conductor is required to be used for the LHD-type helical fusion reactor, FFHR-d1, and the high-temperature superconducting (HTS) conductor is proposed as one of the options1). Using HTS, the reactor can be economically operated at an elevated operating temperature, e.g. 20 K. High cryogenic stability of the conductor is also obtained due to the increased heat capacity of metals equivalent to that of helium at 4 K. In addition, it is proposed that the fusion magnet can be constructed by connecting conductor segments2) (or coil segments3)). An HTS conductor having a 100 kA nominal current at 20 K, 12 T has been proposed and designed2). Development of such a conductor suitable for the helical fusion reactor is our final goal. We successfully fabricated and tested 30-kA class HTS conductors in FY20124, 5, 6)  A 100-kA class conductor sample was fabricated in FY2013 using GdBCO tapes (critical current: ~600 A at 77 K, self-field) simply stacked in a stabilizing copper jacket. The copper jacket was then installed in a rigid stainless-steel jacket assembled by using bolts. The conductor sample formed a one-turn short circuit with a race-track shape having two straight sections. One of the straight sections (A) had 54 GdBCO tapes and no joint, whereas the other side (B) had 42 tapes and a bridge-type mechanical lap joint7). These straight sections were surrounded by FRP jackets for a thermal insulation. The sample had no current-feeders and the current was induced by changing the background magnetic field generated by the 9-T split coils in the cryostat. Rogowski coils and hall probes were used for measuring the transport current of the sample. Figure 1 shows waveforms of the 100-kA conductor sample current and the bias magnetic field produced by the 9-T spit coils. The maximum current of 118 kA was attained in the sample at the temperature of 4.2 K and magnetic field of 0.45 T. A numerical analysis for examining the critical current of the 100-kA conductor sample was performed. In the calculation, the bundle of GdBCO tapes is regarded as one conductor, which is divided into current elements. The overall transport current of the sample was increased step by step, starting from a uniform current distribution. If the transport current of any element exceeds the critical current of that element which was obtained from the critical current characteristics of the GdBCO tape evaluated by a percolation model, the transport current is reduced to the critical current and the difference is equally distributed to other elements where the transport current is still lower than the critical current. This process is iterated, by developing a non-uniform current distribution, until the transport currents of all the elements reach the critical current. Then, the critical current of the conductor sample is defined as the summation of the transport currents of all the elements, and the iteration is finished. Figure 2 shows the distributions of the calculated magnetic field and critical current density when the sample reached the critical current at 4.2 K, 0.45 T. The critical current of the sample is dominantly determined by the perpendicular component (perpendicular to the tape surface) of the self-field at lower bias magnetic field. We consider that the sample was not deteriorated because the calculated critical current of 119 kA is higher than the measured value of 118 kA.   Fig.1 Temporal evaluations of the current of the 100-kA HTS conductor sample and the bias magnetic field of the 9-T split coils.  Fig.2 Calculated distributions of the magnetic field and critical current density in the 100-kA conductor sample at 4.2 K, 0.45 T.  [1] A. Sagara et al., Fus. Eng. Des., (2014) in press [2] N. Yanagi et al., Fusion Sci. Tech., 60 (2011) 648. [3] H. Hashizume et al., Fus. Eng. Des., 63 (2002) 449. [4] N. Yanagi et al., IEEE Trans. Appl. SC., 24 (2014) 4202805. [5] Y. Terazaki et al., IEEE Trans. Appl. SC., 24 (2014) 4801305. [6] S. Ito et al., IEEE Trans. Appl. SC., 24, (2014) 4602305.  [7] S. Ito et al., Plasma Fusion Res., 9, (2014) 3405086.   Adiabatic Demagnetization Refrigerator (ADR) is one of available refrigerators for extremely high sensitive detector under sub-Kelvin such as X-ray sensor using Transition Edge Sensor (TES) 1), 2).  Superconducting magnet is one of most important element in ADR. The superconducting solenoid magnet for ADR is required 3 features: large magnetic field, small current, rather large bore size of inner magnetic field area.  Thus the present superconducting magnet of bobbin-less was developed for the better efficiency of ADR system.  The thin multifilament superconducting wire of 0.127 mm in diameter was used for this magnet.  Because of difficulty of the tension control for this thin wire, the special winding machine was used to make the magnet. The machine can control small tension less than 3 N. The strength of magnet is kept with the epoxy resin (Nito fix) under cryogenic temperature condition. The epoxy resin was chosen due to easy handling, rather high thermal conductivity as shown in Fig.1.  The resin has rather high viscosity, but no void observed in the cross section of the test magnet shown in Fig.2.  The process of winding and fixing by the resin were done at the same time.  0.050.10.150.20.250.30.350.45 10 15 20 25Thermal conductivity (W / m K)Temperature (K)   Fig. 1 Thermal conductivity of the epoxy resin (Nitofix)   Fig. 2 A picture of cross-section of the bobbin-less magnet The specification of the present magnet is shown in Table.1.  This kind of bobbin-less magnet is the first trial for rather small magnet though the technique of bobbin-less magnet is familiar for rather large magnets3).       The measurement of center magnetic field is shown in Fig.3.  The experiments are conducted in liquid Helium about 4.2 K.  Center magnetic field 3T was recorded with no experience of training quench when electric current 6A was applied.     It can be concluded that the development is succeeded and the process of small bobbin-less magnet for ADR is established.     This work has been supported by Ken-ichi Tanaka, Iwao Murakami in cryogenic science center in KEK and Takumi Harada in Graduate school of University of Tsukuba.    Table. 1 Specification of the magnet Inner Diameter 31.0 mm Outer Diameter 47.4 mm Height  65 mm Turn 26531 turn Total length of superconducting wire 3026.4m Inductance 7.2 H 0.110.1 1 10Magnetic Field (T)Current (A) 	 Fig. 3 Center magnetic field measurement of the magnet  in LHe bath   1) Irwin K.,et al., IEEE. Trans. Appl. Supercond 5., 2690 (1995). 2) K. Shinozaki, et al., J. Plasma Fusion Res. SERIES7 (2006) pp.81-84 3) Makida, Y.,et.al., IEEE Trans. Appl. Supercond. 17(2007)1205 317§27. Development of a Small Bobbin-less Superconducting Solenoid Magnet of 3T Class for Adiabatic Demagnetization RefrigeratorTakada, S.,  Kimura, N. (KEK)

Development of a Small Bobbin-less Superconducting Solenoid Magnet of 3T Class for Adiabatic Demagnetization Refrigerator

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Development of a Small Bobbin-less Superconducting Solenoid Magnet of 3T Class for Adiabatic Demagnetization Refrigerator

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