The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force.
Japanese researchers connected a superconducting coil and persistent current switch using rare earth HTS wire materials and achieved a persistent current operation of 100A at 20K, and verified a persistent current operation that maintained a magnetic field of 3,500 Gauss for 10 hours.
At present, superconducting MRI scanners use a superconducting coil made from metallic superconductive wire and cooled with liquid helium to an ultralow temperature (minus 269 ℃). By making a closed circuit containing several such superconducting metallic coils and persistent current switches connected by superconductive wires, we have produced a persistent current maintaining a constant magnetic field for a long time (more than one year) with no external current supply. By obtaining a stable high magnetic field with low running power, producing a persistent current has promoted the broader use of MRI in medical practice.
Since HTS wires enable superconductivity at liquid nitrogen temperature (minus 196 ℃), they show promise in MRI scanners that would not use helium, a scarce resource, and the AMED project has been conducting the development of an MRI coil using rare earth HTS wire materials. Rare earth superconducting wires do not facilitate persistent current operation because they do not enable superconductive connections, and so a continuous electrical current flow from a power supply was always required. To obtain clear images, a special high-stability power supply also had to be developed.
Furukawa Electric has produced a test superconducting coil and a persistent current switch using the rare earth HTS wire materials of SuperPower Inc., a subsidiary of Furukawa Electric in the U.S., and connected them with superconductors to manufacture a superconducting coil device. At Tohoku University, researchers cooled this superconducting coil device to 20 K (minus 250℃) using a compact refrigerating machine before applying a 100 A current from a power source with the persistent current switch in the open position to generate a magnetic field of 3,500 Gauss. They then closed the persistent current switch to isolate the device from the external power source and verified that a current of 100 A flowed for more than 24 hours. When magnetic field attenuation had stabilized, they verified from 10 hours of magnetic field attenuation measurements that the superconducting connection had a of 10-12 Ω connection resistance.
Furukawa Electric will continue working on the development of persistent current technology based on the results of this technical development, such as lowering resistance by improving the critical current of superconducting connections. The development of persistent current technology is expected to contribute toward the realization of devices based on high-temperature superconducting (HTS) wire materials other than MRI scanners, such as nuclear magnetic resonators (NMR) and superconducting levitation devices.