General guiding principles of introducing the pinning centers for the achievement of high current density are discussed in metallic superconducting composite wires. It is pointed out that, among various methods satisfying these guiding principles, the use of artificial pinning centers is more advantageous than other existing methods for the achievement of high current density. The present status on the development of the NbTi composite wires using artificial pinning centers are reviewed. The flux-pinning mechanisms of various types of artificial pinning centers are also discussed.
To develop a small, light, and highly reliable cooling system for a Nb-based SQUID (Superconducting QUatum Interference Device) magnetometer, we measured the magnetic noise produced by a Gifford-McMahon (GM) cycle cryocooler containing magnetic regenerative material, Er3Ni, and calculated the magnetic field around the regenerator by integral computation. Both results showed that the GM cryocooler containing magnetic regenerative material such as Er3Ni was able to be used for SQUID systems for biomagnetic measurement. The GM cryocooler with Er3Ni was installed in a GFRP cryostat. Two SQUID magnetometers were mounted in this cryostat and the magnetic noise produced by the GM cryocooler was measured in a magnetically-shielded room. Intensity of the magnetic noise was about 40pT p-p. It was about six times the value expected from the results measured under the earth magnetic field. So we eliminated the magnetic noise by digital filtering and magnetocardiogram (MCG) and auditory evoked magnetoencephalogram (MEG) were successfully measured.
It is shown that the observed value of the AC loss of a cylindrical BSCCO bulk superconductor with the diameter of 9.6mm, carrying the AC transport current with the frequency of 60Hz, deviates noticeably from the theoretical values estimated from the critical state model. It is clarified that this deviation results from the fact that the internal distribution of flux density, B(r), is quite different from that predicted by the critical state model, when the AC transport current with the frequency, f, of 60Hz is applied to the sample with a large diameter, R, and with a low n-value. This conclusion is based on the numerically simulated results of B(r) obtained by solving the Maxwell equations using the observed Jc=Jc(B) and E(J)=E(0)(J/Jc)n characteristics for the DC electric field, E, the DC transport current density, J, and the DC critical current density, Jc, determined by the criterion of 0.1μV/cm. Finally, a general numerical chart of the AC loss against fR2Jc is presented for the various n-values, for the convenience of the design of the high-Tc current leads.
A novel multiple staging configuration for the pulse tube refrigerator is described for increase in its performance and simplicity. To decrease both regenerator loss and pulse tube loss, a regenerative tube was developed for operating the pulse tube from room temperature to liquid helium temperature. With this configuration, 3.6K of lowest temperature and 119mW of cooling capacity at 4.9K were achieved by a three-stage pulse tube refrigerator. Test results and refrigeration performance of this refrigerator are presented.