Masaichi Umeda

Cu1−xTlxBa2Ca3Cu4O12−y (Cu1−xTlx-1234) superconductor with Tc=126 K

Abstract The samples of Cu 1−x Tl x Ba 2 Ca 3 Cu 4 O 12−y (x=0∼0.5) system with low Tl concentration were prepared by a high pressure techniques. The Cu 1−x Tl x Ba 2 Ca 3 Cu 4 O 12−y (Cu 1−x Tl x -1234: x=0.4∼0.5) samples show Tc of 126 K and superconducting anisotropy of γ=4. Their Tc values are higher than those of parent materials of CuBa 2 Ca 3 Cu 4 O 12−y (Cu-1234) and TlBa 2 Ca 3 Cu 4 O 11 (Tl-1234) by 8 K and 4 K, respectively. The anisotropy of Cu 1−x Tl x -1234 system is close to the value (γ=3.5) of Tl-1234 and higher than that of Cu-1234 (γ=1.6.

AC loss measurement of HTS coils with ferromagnetic disks

Manufacturing techniques for low AC loss high-T/sub c/ superconducting coils are necessary to realize high efficiency superconducting power equipments. High-T/sub c/ superconducting tape conductors have anisotropic magnetic field dependence on critical current and AC magnetization loss. A magnetic field component perpendicular to the tape surface degrades HTS coil performance. We numerically investigated a magnetic field adjusting method and AC loss reduction effects by attaching ferromagnetic materials to both ends of HTS coils. We also measured total AC loss of an HTS coil comprising 10 HTS double pancake coils by the boil-off method. Total AC loss of the coil was dominated by mechanical loss due to vibration and movement of conductors and pancake coils. In this paper, methods to reduce total AC loss of HTS coils are discussed.

Fundamental Characteristics of Superconducting Fault Current Limiter Using LC Resonance Circuit

A new type of fault current limiter (FCL) utilizing series LC resonance circuit composed of a superconducting coil and a capacitor is proposed. The superconducting coil is wound by Bi-2223/Ag HTS tape conductor. The new FCL can suppress the rapid increase of a fault current by its energy storing capacity. Both theoretical analysis and experiments are carried out. The experimental result shows the current limiting ability of this FCL. The obtained experimental result agrees well with the theoretical one

Compressive strain and critical current properties of Nb/sub 3/Sn conductors reinforced by center-arranged and distributed Ta

We have fabricated two kinds of Nb/sub 3/Sn superconductors having the same diameter; one superconductor is reinforced by center-gathered tantalum (Ta) in a cross-section of the conductor, and the other is reinforced by distributed Ta in the cross-section. We applied compressive force to the conductors, and measured degradation of the conductors. When the conductors strain due to the compressive force to the conductors was zero, a critical current (I/sub c/) of the distributed Ta conductor was larger than that of the center-gathered Ta conductor. However, an I/sub c/ of the distributed-Ta type conductor rapidly decreased, and when the compressive strain became approximately 5 or 6%, the I/sub c/ of the center-gathered Ta conductor exceeded that of the distributed-Ta type. It was experimentally shown that the center-gathered Ta conductor was more tolerant than the distributed Ta conductor about the compressive strain to the conductor.

Numerical evaluation of AC loss in high temperature superconducting coil

An approximated method to estimate the electro-magnetic AC loss in high temperature superconducting coil was proposed. We applied this method to estimate the AC loss characteristics of the high temperature superconducting coil made by stacking pancake coils which are wound by Bi2223/Ag tape. The analytical AC losses of the HTS coil were compared with the measured AC losses before and after impregnation by epoxy resin obtained in the previous work by National Institute of Advanced Industrial Science and Technology. The analytically estimated electro-magnetic AC losses in the HTS coil were much smaller than the measured AC loss of the coil. The AC loss characteristics could not be explained by the method proposed in this paper. The main reason of this discrepancy was considered that the measured AC losses in the HTS coil contained large amount of the mechanical AC loss due to the AC vibration of the winding even if the HTS coil was impregnated.

Fiber-reinforced-superconductors for a 15 T-class high-field pulsed magnet and their conceptual design

The authors are developing a novel type of Nb/sub 3/Sn superconductor with high elastic modulus fibers for application to high field pulsed superconducting magnets, called the fiber-reinforced superconductor (FRS). They describe the conceptual design of a conductor for a pulsed magnet using FRSs and present superconducting characteristics for monofilamentary FRS samples. Each monofilamentary FRS supports hoop stress under operation of the magnet. Short lengths of monofilamentary FRSs using tungsten fiber were prepared by RF-magnetron sputtering. Critical current measurement up to 15 T and a uniaxial strain test at 10 T have been carried out. The data support the conceptual design of the 15-T-class pulsed superconducting magnet using FRSs.<<ETX>>

Experimental study on AC loss reduction of HTS coils by use of ferromagnetic disks

Manufacturing techniques suitable for fabricating low AC loss high-temperature superconductor (HTS) coils are necessary to realize high-efficiency power equipment. Usually, the AC magnetization loss of HTS tape conductors is anisotropically dependent on the magnetic field, with a magnetic field perpendicular to the tape surface degrading HTS coil performance. Therefore, decreasing the magnetic field perpendicular to the tape surface is an efficient method of reducing the total AC loss of HTS coils. In a previous study, the possibility of AC loss reduction through the use of ferromagnetic materials was shown numerically, and the total AC loss of HTS coils with and without attached ferromagnetic disks was measured. However, no obvious differences in AC loss were observed because the total AC loss was dominated by mechanical loss. In this study, a low mechanical loss, 30 A, 90 mH HTS coil is fabricated by winding with stainless-steel tape and employing epoxy impregnation. A clear reduction in AC loss is then observed by attaching the ferromagnetic disks.

Test of Resonance-Type Superconducting Fault Current Limiter

A resonance-type superconducting fault current limiter (FCL) using Bi-2223 superconducting coils was developed and tested in order to demonstrate its actual operation. Over-current tests of the FCL were carried out for 50 Hz steady-state current of up to 120 A with relevant combinations of a superconducting reactor, an arrester, a capacitance and a copper reactor. The test results showed that the currents during faults were controlled within low levels. It was also demonstrated that the FCL could promptly recover to the steady state after the faults were cleared

Heat generation by vibration and coil rigidity of Bi2223 pancake coil

AC losses of HTS pancake coil for the superconducting fault current limiter were investigated. The measured AC losses of pancake coil were found to be primarily heat generation by vibration. Therefore, the rigidity of coil was improved by epoxy impregnation, resulting in a 50% reduction in AC losses. Therefore, the total AC losses of HTS pancake coil are dependent on the rigidity of coil. In the present paper, the relationship between resonance frequency and heat generation by vibration is shown.

Mechanical properties of a niobium-tin superconductor reinforced by tantalum cores

We have been developing fiber-reinforced type of niobium-tin superconductor for large scale high field magnets. In this conductor, each niobium-tin filament has a tantalum core. Tantalum is selected as material for the core since it has good ductility with mechanical strength and hence a conventional drawing method can be applied to fabricate a conductor. Stress-strain characteristics and the irreversible strain limits of the conductor with different heat-treatment conditions are measured. Combining these results, discussions about the mechanical properties of the conductor are presented.