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ABSTRACT: The minimum quench energy ( MQE ) and normal zone propagation velocity ( NZPV ) of three kinds of Nb<sub>3</sub>Al superconductors fabricated by the rapid heating, quenching and transformation (RHQT) process were measured under various conditions of applied magnetic field (10-14 T), temperature (7-11 K), and transport current (80-95% of the critical current), while cooled by a cryocooler for developing the over 20-T class cryogen-free magnet. As a result, MQE values were related to the critical current density ( J <sub>c</sub>); high MQE was obtained for low J <sub>c</sub>. It is assumed that J <sub>c</sub> has a stronger influence on the MQE than specific heat, thermal conductivity, resistivity, and other parameters of the composite superconductor including the matrix and the stabilizer. NZPV was mainly proportional to the transport current density varying with applied field and temperature. The second contribution to NZPV is assumed to be heat capacity depending on the wire configuration.
IEEE Transactions on Appiled Superconductivity 07/2009; · 1.04 Impact Factor
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ABSTRACT: The resistive type fault current limiter based on YBCO thin film with high critical current density was proposed and developed because of its fast recovery characteristics. We have been developing a magnetic field measurement system with hall sensor to study the critical current density distribution or magnetic properties of high-Tc superconductors such as thin film, tape conductor and coated conductors. In this study, the transport current distribution in straight line and U-corner region of meander type resistive fault current limiter using YBCO thin film was investigated experimentally. The both axial and radial self-magnetic field components according to the direct (DC) and alternating (AC) transport currents were measured by two-axis hall probe to estimate the current distribution. And also, the self-magnetic field at the corner of bispiral type fault current limiter was measured by two-axis hall probe with a high spatial resolution to compare with the meander type fault current limiter.
Physica C-superconductivity and Its Applications - PHYSICA C. 01/2008; 468(15):2059-2062.
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ABSTRACT: Pre-bending effects of Nb<sub>3</sub>Sn composite wires, enhancement in J<sub>c</sub> at the as-cooled condition and J<sub>c</sub> peak, J<sub>cm</sub>, against applied tensile strain, are well known in especially Cu-Nb reinforced Cu stabilized Nb<sub>3</sub>Sn wire. In an attempt to understand the effects, three directional strain analyses using FEM were studied by considering thermally-induced residual strain due to temperature difference between the reaction temperature and the cryogenic temperatures including the pre-bending process at room temperature for three types of Nb<sub>3</sub>Sn composite wires. To evaluate effects of change in three directional strain, the von Mises strain was introduced. As a result, the minimum von Mises strain against the applied tensile strain corresponded to J<sub>cm</sub>; low strain shows J<sub>cm</sub> enhancement by the pre-bending for Cu-Nb reinforced Nb<sub>3</sub>Sn wire. It was found that there is a direct correlation between the minimum von Mises strain and J<sub>cm</sub> related to the pre-bending.
IEEE Transactions on Appiled Superconductivity 07/2007; · 1.04 Impact Factor
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ABSTRACT: We have been developing a three-dimensional (3-D) superconducting actuator with HTS bulk and electromagnets. Proposed 3-D superconducting actuator consists of the stator constructed by two-dimensional arranged electromagnets and HTS bulk superconductor as a mover. To realize the 3-D superconducting actuator with high performance, we have to investigate the shape-dependences of electromagnets and HTS bulk on levitating characteristics. In this study, we fabricated the individually controlled eight-polarities electromagnets and the two kinds of bolted joints cap cores to improve the control ability and levitation characteristics. And also, we prepared the disk-shaped and ring-shaped (with 20mm inner diameter) HTS bulk with 60mm diameter, and evaluated how magnetic field distribution influences the levitating characteristics as functions of levitation force and height.
Physica C-superconductivity and Its Applications - PHYSICA C. 01/2007; 463:1346-1351.
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ABSTRACT: Superconducting properties of high-strength Nb<sub>3</sub>Sn wires with Ta-reinforced filaments were evaluated at various temperatures and magnetic fields. Mechanical properties were also investigated at 4.2 K. The critical current was measured at temperatures up to 14 K and in magnetic fields up to 15 T. The data were well scaled by using empirical scaling laws. The stress-strain curve showed that the stress corresponding to 0.5% strain was 284 MPa. The value is twice as large as that of conventional Nb<sub>3</sub>Sn wires, indicating that the high-strength Ta reinforcement enhances the mechanical property of the wire. Furthermore, the wire was sufficiently ductile. To evaluate the thermal stability, minimum quench energy and normal zone propagation velocity were measured under a cryocooled condition. The Ta reinforced Nb<sub>3</sub>Sn wire shows relatively high stability and large propagation velocity. The results suggest that the Ta reinforced Nb<sub>3</sub>Sn wire has sufficient applicability for cryocooled superconducting magnet windings.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
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ABSTRACT: It is known that current sharing effects of the stabilizer are limited in cryocooling conditions being different from poolboiling. Therefore we have studied stability under it, focusing on minimum quench energy (MQE) of Nb<sub>3</sub>Sn superconductors under various conditions of the applied magnetic field, temperature, and the transport current, for various volume fractions and configurations of Cu stabilizer. From the experimental results, the larger the external Cu volume fraction, the higher the MQE was obtained. All MQE data were summarized by only one line in characteristics of normalized MQE and normalized B(B/B<sub>c2</sub>). The obtained MQE behavior was to be characterized by only one empirical equation. It is thought that the obtained formula is useful for a standard of designing the Nb<sub>3</sub>Sn superconductor and its coil under cryocooled condition.
IEEE Transactions on Appiled Superconductivity 07/2005; · 1.04 Impact Factor
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ABSTRACT: A superconducting actuator with multiple motions is proposed and experimentally investigated for the development of microelectrical machines. The disk-shaped YBCO bulk was located above the electromagnets and cooled down in the field-cooling method, which enabled the flux pinning. The YBCO bulk superconductor was driven in vertical, horizontal and/or self-rotation by PC-controlled coil currents using multiple electromagnets. The most suitable levitation stability was obtained when we used four electromagnets and NNSS trapped field type. It was confirmed that the levitation force and horizontal moving force strongly depended on the coil current and the air gap length, respectively.
Physica C-superconductivity and Its Applications - PHYSICA C. 01/2005; 426:834-838.
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ABSTRACT: Quench properties of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8</sub>/AgMg tape were experimentally investigated under a cryocooled condition in high magnetic fields. The critical current (I<sub>c</sub>), quench current (I<sub>q</sub>), and minimum quench energy (MQE) were measured at various initial temperatures of 4.2 K, 10 K, and 20 K and at magnetic fields of 7 T and 9 T. MQE was the minimum disturbance energy enough to make thermal quench and was evaluated from the input energy by a resistive heater. It was found that MQE exhibited the minimum value around 20 K. The temperature margin, difference between the initial temperature and the thermal runaway temperature, decreased monotonously with increase at the operating temperature. Since the temperature dependence of MQE did not agree with that of temperature margin, we focused on the internal energy calculated by integrating specific heat of the sample. The temperature dependence of the internal energy was in agreement with that of MQE qualitatively.
IEEE Transactions on Appiled Superconductivity 04/2002; · 1.04 Impact Factor
