O. Miura

Excess magnetization due to interfilamentary proximity coupling in NbTi multifilamentary wires

Abstract It is shown theoretically that the excess magnetization of a multifilamentary superconducting wire caused by interfilamentary proximity coupling, ΔMp, is proportional to the twist pitch of the wire, I. It is also shown that this theoretical prediction can be confirmed experimentally for Nb - Ti multifilamentary wires. The critical current density of the superconducting shielding current flowing through the matrix, which becomes a weak superconductor due to interfilamentary proximity coupling, is given by J cp = J 0 ( B B c2p ) −1.4 × [1 − ( B B c2p )] 1.4 with J 0 ∝ [ cosh ( d N 2ξ N )] −2 , where B is the magnetic flux density, and Bc2p, ξN and dN are the upper critical flux density, the effective coherence length and the width of the weakly superconducting matrix, respectively. It is concluded that ΔMp can be estimated quantitatively by measuring the I dependence of total magnetization, and also that the most effective method of decreasing ΔMp is to decrease ξN to the dirty limit.

Achievement of high current density in Nb Ti superconducting multifilamentary wires by introducing designed artificial pins

Abstract It is shown theoretically that a further increase in the overall critical current density of Nb  Ti multifilamentary wire is possible by introducing artificial pinning centres into Nb  Ti filaments in a specified fashion. As the first step towards experimental confirmation of theoretical predictions of the summation of individual forces, Nb  Ti monofilamentary wires were fabricated, in each of which were embedded fine filamentary Nb rods with a given diameter and spacing, to act as artificial pinning centres. These wires show higher critical current densities than have been reported so far, at fields lower than ≈ 2 T. Although the quantitative agreement between the theoretical and observed results is not yet very satisfactory, further investigation of the pinning characteristics using various artificial pins leads to a new stage where it is possible to design the wire parameters for obtaining the required characteristics of critical current density.

66 kV-2 kA peak load test of high-Tc superconducting model cable

Abstract The superconducting power transmission cable is one of the most feasible applications for oxide superconductors. One case of introducing superconducting cables into power cable systems is as a replacement type in cable ducts, so that it is necessary to develop a compact superconducting cable which can be installed in existing ducts of diameter 150 mm. For this purpose, long-length Bi-2223 phase multifilamentary silver-sheathed wires with high critical current densities, such as over 7000 A cm −2 , have been developed and the effects of mechanical strain were investigated. As the next step, we have designed a 66 kV-2 kA superconducting cable of outer diameter 124 mm and developed a 5 m long model cable having a flexible conductor assembled by winding the foregoing wires in layers around a former. Furthermore, two different types of termination have been developed for a simultaneous current and voltage withstand test. As a result, we have been able to successfully perform a 66 kV-2 kA withstand test on an oxide superconducting power cable. The AC loss of the cable measured by the four-probe method was about 35 W m −1 . Moreover, generation of partial discharge in the cable was not observed at the test voltage.

Pinning characteristics in multifilamentary NbTi superconducting wires with submicrometre filaments introduced artificial pinning centres

Multifilamentary NbTi superconducting wires for a.c. use were developed by introducing ultrafine Nb island-type artificial pinning centres. The maximum Jc value obtained for wire with a filament diameter of 0.4 μm reached as high as 1.07 × 1010 A m−2 at 1 T and 4.44 × 109 A m−2 at 3 T, which are more than twice the Jc values for conventional wires for a.c. use. For these wires, the constants of the scaling rule for global pinning force, Fpαbγ(1−b)δ, became γ = 0.8 and δ = 1.6. This indicates that the pinning characteristics of the present wires were shifted to the oversaturation-type by the introduction of artificial pinning centres, while they are of the saturation-type for conventional a.c. wires. As a result, the global pinning force at low and medium magnetic fields can be explained fairly well by the direct summation model which is available only for strong pins. A helpful guideline in designing Jc values for a.c. wires is also discussed.

Magnetic properties of proximity-induced superconducting matrix in multifilamentary wires

Critical current densities Jcp of proximity-induced superconducting matrices in NbTi multifilamentary wires are estimated from measured twist-pitch dependence of magnetization. The values of Jcp are 2–4 orders of magnitude smaller than those of the superconducting filaments and decrease rapidly by raising temperature T and magnetic field Be. The interfilamentary spacing dN of submicrons results in zero screening length at measured temperatures ranging 4.5 to 8 K and the induced superconductivity is suggested to be type-II. The upper critical field Bc2p is obtained by applying the scaling law to Jcp data. Bc2ps at 4.5 K are 1.2–3.0 T for dN = 0.20–0.59 μm. The Cooper pair penetration length KN−1 estimated from the temperature dependence of Bc2p shows the clean limit characteristic as T−1 and 0.32 μm for dN = 0.59 μm wire at 4.5 K. KN−1 estimated from the temperature dependence of Jcp agrees consistently with those from Bc2p(T).

Development of high-field a.c. superconducting magnet using ultrafine multifilamentary Nb-Ti superconducting wire with designed Nb artificial pins

Abstract A 2.5 T/100kVA a.c. superconducting magnet has been developed using Nb-Ti superconducting wire with artificially introduced pins. Investigations have been conducted regarding the suitability of critical current density J c design using artificial pins and the effectiveness of magnets using wires with high J c . Studies on pin design suggested that the maximum pinning force density F p would be reached at a magnetic field of 2.5 T. As anticipated, the maximum F p was achieved at 2.5T, and a high J c of 4.7 × 10 9 A m −2 was obtained. Using an a.c. magnet fabricated from this wire, current operation at 60 Hz and 4.2 K was effected, and steady state operation at a capacity of 104.8 kVA, a current of 105.8 A rms and a voltage of 991.2 V rms was achieved. At the same time, a central magnetic field of 2.5 T was obtained. In addition, it was demonstrated that the total a.c. losses were 4.8 W at a central magnetic field amplitude B 0 of 2 T, a mere 0.007% with respect to the capacity of 69 kVA and much lower than in a.c. magnets of the 100 kVA class made using conventional a.c. wires. This confirmed the effectiveness of reducing magnet size through the use of wires having high current density.

Flux Pinning Property of Artificially Introduced Nb in Nb-Ti Superconductors

The elementary pinning force of thin Nb layers introduced as artificial pinning centers in Nb-Ti is theoretically investigated in the relatively low field region using the Ginzburg-Landau theory. It is found that the pinning energy comes from the kinetic energy due to the induced superconductivity in the Nb layers by the proximity effect. Hence, these layers act as repulsive pinning centers, while normal layers such as α-Ti do as attractive ones. The elementary pinning force depends sensitively on the coherence length in the Nb layers and is larger for the longer coherence length. This value is predicted to be larger than that of α-Ti layers in commercial Nb-Ti wires even in the case of short coherence length.

Development of high Jc Nb-Ti multifilamentary superconducting wires for a.c. use by artificially introduced pins☆

Abstract To achieve further increases in the critical current densities of Nb-Ti ultrafine multi-filamentary superconducting wires for a.c. applications, wires were fabricated in which Nb island-type artificial pinning centres had been designed and introduced into filaments according to flux pinning theory. The changes in pin parameters which accompanied the diameter reduction increased the pinning force density F p over a wide range of magnetic field, and oversaturation phenomena were observed, with the position of the F p peak shifting towards the high-field side. High critical current densities J c were achieved: 1.42 × 10 10 A m −2 at 1 T for 0.1 μm filaments, and 8.7 × 10 9 A m −2 at 2 T and 6.1 × 10 9 A m −2 at 3 T for 0.4 μm filaments. Furthermore, it was determined that where the correlation length I 66 and fluxoid interval a f are such that 2 I 66 a f , the magnetic field at the F p peak is determined by the pin interval d s . This discovery has given us useful pointers for the design of Nb-Ti superconducting a.c. wires, in terms of J c , through the introduction of artificial pins.

Flux creep in proximity-induced superconducting matrix in NbTi multifilamentary wires

Abstract In NbTi multifilamentary wires, the critical current densities J cp of proximity-induced superconducting matrices, as estimated from the measured twist-pitch dependence of magnetization, decreased rapidly on raising temperature T and magnetic field B e . The pinning potential for flux creep U cp in the proximity-induced superconducting matrices was also estimated from measurement of the twist-pitch dependence of the creep rate R c . The observed values of U cp were 14–860 meV at T = 4.5 K and B e = 0.2-1.0 T and showed the characteristics of strongly pinned superconductors with a low pin concentration.

Measurement of AC Penetration Depth in Superconducting Wires with Fine Filaments

The critical state model of the Bean-London type predicts that the AC hysteresis loss energy density in the superconducting filaments of the diameter d f is given by W = 4μoH/3J c d f for the AC field amplitude H m smaller than the penetration field, H p = J c d f/2. However, this prediction does not agree with experimental results for multifilamentary wires with fine superconducting filaments for AC use. That is, the AC loss energy density is much smaller than the above prediction and the region of H m in which W is proportional to H m 3 extends up to much larger value than H p. This deviation was found to be caused by the reversible fluxoid motion inside the pinning potential well.1