Dependence of the critical current density of Bi-2223 superconducting tape on the angle of external magnetic field is investigated theoretically and experimentally. The model proposed here is based on an effective coherence tensor model which can deal with the anisotropy of the coherence length, and has been derived so as to be applicable for the arbitrary angle of the external field. The resulting mathematical expression of the angular dependence of the critical current density shows that the critical current density is dependent only on the normal component of the magnetic field to the tape surface (i.e., the component perpendicular to the ab plane). The measurements of critical current density were made based on the four-point DC method, and the results show the validity of the theoretical considerations.
We have designed and constructed a 500kVA-class oxide-superconducting power transformer. The windings are cooled by liquid nitrogen or subcooled nitrogen in a G-FRP cryostat of 785mm in diameter and 1, 210mm in height, that has a room-temperature space for an iron core with the diameter of 314mm. The primary and secondary windings are three-strand and six-strand parallel conductors of a Bi-2223 multifilamentary tape with silver sheath, respectively. The strand of 0.22mm thick and 3.5mm wide has 61 filaments with no twisting. The ratio of superconductor is 0.284. In the parallel conductors, the strands are transposed five times in each layer for a uniform current distribution among them. It was proved that the transformer has the rated capacity of 500kVA by means of two-hours short-circuit test and half-hour no-load test in liquid nitrogen of 77K. The efficiency is estimated as 99.1% from a core loss of 2.3kW and a thermal load of 2.2kW in coolant. The latter is composed of AC losses in windings and heat leakage from the cryostat and current leads, and is multiplied by a refrigeration penalty of liquid nitrogen, 20. Load test was also performed up to 500kVA. The transformer was furthermore operated in subcooled nitrogen at 66K with no quenching up to a critical level, that is equivalent to 800kVA. The efficiency estimated was improved to 99.3% in subcooled nitrogen. Measured AC loss in both windings are well explained by a theoretical prediction with the ‘critical state model.’ We also discuss prospective applications of the parallel conductors composed of advanced HTS multifilamentary tapes to AC windings with large current capacity.
The electric field and current (E-I) characteristics of Bi2Sr2CaCu2O8+δ/Ag (Bi2212/Ag) superconducting tapes in a flux flow state have been investigated in order to clarify the quenching mechanism for oxide superconductors. The measured E-I curves were compared with those for a NbTi/CuNi superconducting wire. The n value, obtained from the slopes of log E-log I curves, for Bi2212/Ag decreased with the increase in electric field, while the n value for the NbTi/CuNi wire remained constant. This is explained by the fact that a part of the current transfers to the lowresistivity Ag substrate. The intrinsic n value for Bi2212 was found to be constant in the range of the electric field measured in this study. A mechanism of quenching for NbTi/CuNi superconductors in liquid helium causes an abrupt temperature rise induced by a transition from nucleate boiling to film boiling without a current transfer to the matrix. Oxide superconductors in liquid nitrogen are considered to be quenched by a mechanism similar to that for NbTi/CuNi superconductors in liquid helium, if the substrate resistivity or the critical current density is two orders larger than that of the present level.
Highly strengthened and stabilized Nb3Sn wires have been developed utilizing CuNb. We demonstrated the high-tension-winding superconducting model coil by the “React & Wind” method using newly developed CuNb/(Nb, Ti)3Sn wire. The mechanical and superconducting properties under a state of high electromagnetic stress were measured for the model coil. The maximum hoop stress was about 280MPa. The tensile strain of about 0.51% for transverse direction and compression strain of about 0.1% for longitudinal direction were obtained. A detailed analysis of the quenching behavior suggests that coil quenching occurred around current terminals.
The heat transfer in ‘parallel channels’ filled with pressurized He II (He IIp) has been investigated to clarify the mechanism of the characteristics as compared to behavior in the Gorter-Mellink channel. The parallel channel with a rectangular cross-section consists of three insulator walls and a copper surface distributed along the channel length. Unlike the sudden rise in temperature in the G-M channel, the subcooled He I layer spread on the heated surface restrains abrupt transition at the critical heat flux, qλ, at which superfluidity is broken at the center of the parallel channel. The metastabilization of a superconducting coil cooled with He IIp has been also discussed by taking the specific heat transfer in the parallel channel into account.
The superfluid shock-tube facility has been developed as a versatile tool for general research of low-temperature thermo-fluid dynamic phenomena. The shock tube was designed to be operated with the He II-filled test section immersed in superfluid helium. A gasdynamic shock wave impinging onto a He II-free surface generates a transmitted compression shock wave (of first sound origin) and thermal shock wave (of second sound origin) as the genuine temperature wave in He II. The target physical phenomena in the He II are measured using pressure transducers and superconductive temperature sensors, by applying a laser beam refraction method and with the aid of some optical visualization methods. In this study, the general superfluid thermodynamic performance of shock waves in the facility is investigated to verify the validity of the facility under a wide range of experimental conditions.