M. Ueyama

Preliminary tests of a 500 kVA-class oxide superconducting transformer cooled by subcooled nitrogen

The authors have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in sub-cooled nitrogen. The primary and secondary windings are three- and six-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. In the parallel conductors, the strands are transposed several times for uniform current distribution among them. A transformer, cooled by liquid nitrogen of 77 K, was steadily operated with a 500 kVA secondary inductive load. The efficiency in full operation at 77 K was 99.1 %, even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. They installed the transformer in a continuous flow system of sub-cooled nitrogen as a fundamental step for compact superconducting transformers operating in sub-cooled nitrogen with a single-stage refrigerator. Short-circuit tests of the transformer were also performed in a region of temperature below 70 K. The transformer was operated with no quenching up to a level of critical current at 66 K, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3 % in the sub-cooled nitrogen.

Bismuth superconducting wires and their applications

Abstract The combination of a bismuth high T c phase ( T c = 110 K) and powder-in-tube processing technology enables the fabrication of superconducting wires with high critical current density, mass producibility and flexibility. The maximum critical current density in liquid nitrogen reached 53 700 A cm −2 in zero magnetic field, 42 300 A cm −2 at 0.1 T and 12 000 A cm −2 at 1 T. J c and J c − B enhancements were obtained with finely dispersed non-superconducting phases and clean grain boundaries. Various prototypes were made to clarify their feasibility, such as 114 m long wires ( J c ≈ 10 000 A cm −2 at 77.3 K), large current conductors ( I c = 2300 A at 77.3 K), a 0.21 T coil at 77.3 K, a 20.35 T coil at 20.3 K and a 23.37 T coil at 4.2 K.

Development of Ag-sheathed Bi-2223 superconducting wires and their applications

Silver-sheathed multifilamentary BiPbSrCaCuO 2223 superconducting wires with long length and high Jc of over 3/spl times/10/sup 4/ A/cm (77.3 K, 0 T) were developed by using the powder-in-tube method. For future industrial applications, high Jc with high Ic wires, high strength wires and low AC loss wires were also developed for large scale magnet applications and electric apparatus for AC use. In the case of AC conductor, twist effect was investigated in order to reduce the wire AC loss, PVF coating technology was applied to high strength wire. PVF coating technology made it possible to obtain amperage parallel conductor with low AC loss, because each strand is electrically insulated. As a progress of technology, we could fabricate many application prototypes. In the high amperage conductor application, current leads and cable conductors were developed. In the magnet application, pancake magnets cooled by GM refrigerator and operated at around 20 K were developed, We have also developed the transformer as an AC application operated in the liquid nitrogen.

Transport current properties of silver-sheathed BiPbSrCaCuO wire and coil

Abstract High J c silver-sheathed BiPbSrCaCuO superconducting wires have been developed. The maximum critical current densities at 77.3 K were 4.7 × 10 4 A cm −2 in zero magnetic field, 3.1 × 10 4 A cm −2 at 0.1 T and 1.1 × 10 4 A cm −2 at 1 T. At 4.2 K, the maximum critical current density was 1.03 × 10 5 A cm −2 at 23 T. The anisotropy of critical current density for different magnetic field directions was only 30% at 4.2 K, even at 23 T. Long wires, for example a 60 m wire, were fabricated and different types of coils and conductors were made and tested over a wide temperature range from 4.2 to 77.3 K. This revealed that one could obtain coils generating over 1000 G, conductors transporting over 600 A at 77.3 K operation, current leads transporting over 1500 A with small helium consumption and superhigh field coils generating over 20 T at 4.2 K operation.

Status of Bi-2223 tapes performance and development

We are developing Bi-2223 superconducting tapes on a wide viewpoint from fundamental superconducting property to application usage. In basic characterization of superconductivity, the behavior of critical current density (J/sub c/) in terms of both temperature and magnetic field was studied in detail and a new scaling law which can estimate the value of J/sub c/ in multiple conditions is defined. There were significant improvements in the long length fabricating process. The 1 km order length tape having J/sub c/ at 77 K over 20 kA/cm/sup 2/ can be produced reproducibly. The distribution of J/sub c/ through the whole length is very uniform. These results in both basic characterization and production progress made it possible and easy to design a high field magnet. Then, we have successfully developed a magnet using a refrigerator.

Electromagnetic properties in parallel conductors composed of Bi2223 multifilamentary wires for power transformer windings

The authors study, theoretically, the AC losses in superconducting parallel conductors exposed to a transverse alternating magnetic field in relation with transposition among the strands. They obtained an analytical expression of the additional coupling loss for the deviation of transposition from an optimum condition. The AC losses for the alternating external field were measured in parallel conductors composed of Bi2223 and NbTi multifilamentary wires. The additional losses can be well explained by the theoretical expression in a wide range of the amplitude of external field. They also estimated, experimentally, the current distribution among the strands in solenoidal coil of the parallel conductor and discussed the effect of transposition on the current distribution.

Filament decoupling and magnetization loss of multifilamentary Bi2223 superconducting tapes

Several types of multifilamentary Bi2223 superconducting tape were fabricated to study the filament decoupling and its effect on the magnetization loss reduction. First, twisted and untwisted multifilamentary Bi2223 tapes with pure silver matrix were prepared. The spacing between filaments was increased to ensure the suppression of the inter-filament bridging. Therefore, the engineering current density decreased, while the critical current density was kept at 150 A/mm/sup 2/. Measured magnetization loss of the tapes indicates that filaments are decoupled by twist in the parallel magnetic field, and the hysteresis loss is reduced. Second, the inter-filament resistive barriers were introduced. Several materials were tested as the barrier material to study their compatibility with the formation of Bi2223 phase during the fabrication process. The tapes with resistive barrier were fabricated successfully with a couple of meters of piece-length. DC magnetization of the fabricated tapes was measured by a SQUID magnetometer. The magnetization-temperature curves indicate that Bi2201, not superconducting at 77 K, and the Al/sub 2/O/sub 3/ do not affect the formation of Bi2223 superconducting phase. The transverse resistivity of a Bi2201-barrier tape, estimated from the measured coupling time constant, is 4.7/spl times/10/sup -8/ /spl Omega/m. Results of AC magnetization loss measurement indicate that the filaments can be decoupled by the Bi2201 barrier.

High-Ic HoBCO coated conductors by PLD method

Ho/sub 1/Ba/sub 2/Cu/sub 3/O/sub x/ (HoBCO) coated conductors by using Pulsed Laser Deposition (PLD) have been developed on textured Ni-alloy substrates. Hetero-epitaxial growth of CeO/sub 2//YSZ/CeO/sub 2/ was developed to introduce in-plane alignment for buffer layers. CeO/sub 2/ seed and cap layers were deposited by RF sputtering. YSZ diffusion layer was deposited by PLD. The most advantage of buffer layers are high growth rate and large deposition area by using in-plane alignment of textured substrate. This was demonstrated by a high production speed of buffer layers. CeO/sub 2/ and YSZ buffer layers show a high production speed of 7 m/h and 8 m/h, respectively. As for the HoBCO deposition, a production speed of 10 m/h was demonstrated by using high growth rate of 4 /spl mu/m/min in our PLD process. Ic of the short sample reached 357 A/cm-width at 77 K, 0 T, and 2,423 A/cm-width at 30 T, 4.2 K. Long length HoBCO conductors up to 35 m were performed by using reel to reel tape transfer system of each process.

Angular dependence of AC transport losses in multifilamentary Bi-2223/Ag tape on external DC magnetic fields

AC transport losses, in multifilamentary BSCCO-2223 silver sheathed tapes, as a function of the applied external magnetic field and its direction with respect to the plane of the samples, are reported. The measurements were carried out at 77 K and at frequency of 80 Hz. The AC transport current values through the tapes, in the range of 10 to 70 percent of their DC critical current values, were kept constant, while rotating the external magnetic field. Also, in a similar way, DC critical current angular characteristics were measured. The transport losses, as a function of the angle and the magnetic field, scale when using the analogous DC critical current relations. Obtained results are compared with theoretical models for the self-field losses.

Development of 200 meter HoBCO Coated Conductors on Ni-Alloy Textured Substrates

(Ho1Ba2Cu3Ox) (HoBCO) coated conductors on textured Ni-alloy substrates were developed by using pulsed laser deposition (PLD). Hetero-epitaxial growth of CeO2/YSZ/CeO2 was used to introduce in-plane alignment for buffer layers. CeO2 seed and cap layers were deposited by RF sputtering. YSZ diffusion layer was deposited by PLD. The most advantage of these buffer layers are high growth rate and large deposition area by using textured substrate. As for the HoBCO deposition, a production speed of 20 m/h with growth rate of 2-3 mum per minute was demonstrated in our PLD process. By optimizing the deposition temperature, lc and Jc of the short samples reached 265 A/cm-width and 5.6 MA/cm2 at 77 K, 0 T, respectively. Long length HoBCO conductors over 200 m were performed by using reel to reel tape transfer system of each process.