Rock Kil Ko

Fabrication of Bi-2223 HTS magnet with a superconducting switch

A test pancake magnet, equipped with persistent superconducting switch, was fabricated with Bi-2223/Ag-Mg tape. The magnet was operated in persistent mode at 20 and 77 K. For large operating currents that forces the magnet to operate in the flux flow range, the field decays is exponentially with time. A simple circuit model is used to compute field decay time constants. It was also found that Pb-Sn solder joints are unsuitable for persistent mode operation.

Study of Magnetomotive Force Control Type Superconducting Magnet Using BSCCO HTS Wire

In this study, a method for controlling the magnetomotive force using the current bypass of no-insulation magnets was developed, and the characteristics of 1G high-temperature superconductivity (HTS) no-insulation magnets were elucidated through experiments. The experiments were performed using a magnet designed with bismuth strontium calcium copper oxide HTS wires. Film-type heaters were used to control the MMF. The analysis of the experimental results was carried out in conjunction with an analysis using finite elements method.

Improvement in Stability and Operating Characteristics of HTS Coil Using MIT Material

This paper proposes a new method of achieving electrical stability and controllability of a high-temperature superconductor (HTS) coil. The proposed method applied a material with a metal–insulator transition (MIT) property to the turn-to-turn resistance of an HTS coil. As a result, the turn-to-turn resistance remained at the insulator level in the normal state, thus removing delay during charging and discharging. During transient state, the turn-to-turn resistance dropped to the metal level, and current bypass occurs similar to that in a noinsulation coil, thus preventing the coil from burning out. The validity of the MIT characteristic and the proposed method were verified through experiments.

Electrical Characteristic Analysis According to Contact Resistance Between Turns of HTS Coil

This paper presents our analysis of the output characteristics of a high-temperature superconductor (HTS) coil depending on the turn-to-turn contact resistance. Important design factors were investigated by taking into account the turn-to-turn contact resistance based on the analysis results. Analyses were performed on five self-fabricated coil models with different turn-to-turn contact resistances. To consider only the turn-to-turn contact resistance, we used a first-generation HTS BSCCO wire with simpler architecture and mechanical properties than the second-generation HTS wire. From this study, it was confirmed by experiment that as the turn-to-turn contact resistance decreases, the electrical stability increases but the output efficiency decreases compared with the input.

Oscillation and radiation of a superconducting ring in a Helmholtz coil

We discuss oscillation and radiation of a superconducting ring in a nonuniform magnetic field produced by a Helmholtz coil. This is a simple example of the oscillation of a superconductor between two magnets. Two oscillation modes (the parallel mode and the antiparallel mode) exist, depending on the directions in which the currents flow in the two coils of the Helmholtz coil. Calculations show the following facts. (i) A characteristic radius-to-height ratio of the Helmholtz coil where a maximum frequency can be produced exists. (ii) As the temperature increases, both the parallel and the antiparallel mode frequencies are fixed, but the parallel mode frequency starts to decrease and falls to 0 in a temperature region below the critical temperature, indicating an oscillation in the intermediate state. (iii) When the superconducting ring oscillates with trapped flux in it, as the trapped flux increases, the parallel mode frequency decreases, but the antiparallel mode frequency increases. When the frictional force is not present, radiation alone reduces the motion energy. We show that the oscillations in both modes stop as time approaches infinity. Over time, the amplitude decreases in the form of a root inverse for the parallel mode and decreases exponentially for the antiparallel mode. The half-lives of the amplitudes have been calculated as 2.1 × 1023 years for the parallel mode and 6.9 × 1023 years for the antiparallel mode.