MRI systems in the medical field are widely used, and it is already well known that great progress has been made in the development of superconducting technologies. Superconducting magnets are also being used for growing high-quality silicon single crystals. This is one example of the industrial application of superconductivity, illustrating that superconducting technologies have been able to meet many customer requirements. In this report, we present the history of the technological developments for these industrial applications of superconducting magnets.
In appreciation of a time relaxation behavior of critical currents as a result of flux creep in type-II superconductors, the possibility has been investigated that superconducting magnets may quench even in the persistent current mode. Because the time relaxation behavior of critical currents has been not observed directly, it has been thought as “the shadow, ” i.e., an implicit phenomenon. If a quench in the persistent current mode is experimentally confirmed, “the substance” of the time-dependent critical current will be grasped and then it appears explicitly. Experimental methods that can meet with quenches in the persistent current mode are proposed.
We have constructed a microwave resonator module using High Tc superconducting (HTS) thin films and a dielectric sapphire rod to measure microwave surface resistances of the HTS thin films. In this method the surface resistance is calculated from the measured unloaded Q values of the resonator. For precise measurements of surface resistance, therefore, the microwave loss not generated by HTS thin films must be minimized. By reducing the parasitic Q factors of the resonator module, we obtained Q values of as high as 1.2×107 at 17.3K and 12GHz. From the high Q values measured in this improved resonator, the surface resistance of the YBa2Cu3Oy superconducting thin films on MgO substrates was calculated and found to be 0.064 mohm at 20.2K and 12GHz.
To develop low cost Nn3Sn coils, we examined coil fabrication through the react and wind (R&W) technique by narrowing down the target to relatively big bore magnets for industrial applications. We first carried out various strain evaluations for the annealed Nb3Sn wire. Grasping the characteristics, we set up the allowable strain condition for handling the annealed wire. Using our internal-tin route wire, which has high cost performance, the wire was primarily reacted, then we coated it with cheap formal insulation while controlling the strain applied to the wire and then wound it to Nb3Sn coils. As a result of evaluating them, we succeeded in passing a current of 284A for the 23 layers of small-scale coil, which is equivalent to the case of short-length wires, and achieved a maximum field of 6.7T. We also succeeded in generating a maximum field of 9.5T for a commercial scale coil. As a result, we were able to demonstrate that the wire doesn't degrade and we were able to fabricate the coil through the R&W technique if the tensile and bending strain were securely controlled.
Because Nb3Sn superconducting wires have strong strain-sensitivity, it is desired that we can grasp Ic degradation rate analytically when bending and tensile strains are applied to these wires. We fabricated Nb3Sn coils through the react and wind technique in which the wires, processed by our internal-tin method, were primarily reacted, then coated with formal insulation and finally wound to a coil, and we also succeeded in passing the current, which is equivalent to the case of short length wires. An estimate of the Ic degradation rate is needed when bending and tensile strains are applied to the wire for examining the above-mentioned experimental result. Therefore we theoretically analyzed it by considering the shift of neutral axis when the wire was bended, taking the plastic deformation of composite materials into account, and also numerically calculated it. The results were that the neutral axis shifts from the center of the wire toward 72μm inside; the strain range of the Nb3Sn filaments are from 0.40% of maximum value to -0.06% of minimum one when winding the coil; the strain of the filaments increases 0.14% when the coil is cooled to 4.2K; and the additional Hoop's strain of 0.033% is applied to the wire for passing current to the coil. It was estimated that the Ic degradation rate based on the Ic-strain characteristics of the wire becomes 8.5%. This value agrees substantially with the experimental data of our small-scale coil; thus the validity of the calculation on Ic degradation rate can be verified.