Abstract Body

Practical superconductor wires and cables have multifilament structure in which the superconductor filaments are electrically connected to each other through normal-conducting metal. When considering coil applications of superconductor wires or cables, their coupling time constant is an important parameter in determining the operating conditions. If the time constants at which the electromagnetic field distributions change at the location of the superconductor wires or cables are shorter than the coupling time constants, coupling currents are induced in the superconductor wires or cables, increasing ac losses and worsening stability. Especially when considering the application to ac equipment, the coupling time constant is directly related to the limitation of the operating frequency, so an experimental evaluation method for the coupling time constant of superconducting wires or cables is required. Furthermore, because the use of superconductor wires and cables at temperatures other than liquid helium and liquid nitrogen is currently being extensively investigated, the temperature dependence of the coupling time constants must also be evaluated in the measurements.

We have developed a system that can measure the coupling time constant of various superconductor wires and cables cooled with liquid or gas helium. The conceptual diagram of measurement system is shown in Figure 1. The system has a cryostat, which is filled with liquid or gas helium. By placing the sample in the narrow area at the bottom of the cryostat and inserting this area into the magnet as shown in Figure 1, a small amplitude ac magnetic field with various frequencies is applied to the sample, which is cooled by liquid or gas helium, to measure frequency dependence of magnetization loss. By using a small amplitude ac magnetic field, the hysteresis loss in the magnetization loss of the sample is reduced and the frequency dependence of the coupling loss is measured. The coupling time constant can be determined by fitting a Debye curve using the measured frequency dependence of the coupling loss. The cryostat is made of grass fiber reinforced plastic (GFRP). This is because if a metallic cryostat is used, eddy currents are induced in the cryostat when a magnetic field is applied to the sample from the outside, which causes the temperature of the gas helium to rise and hinders the application of the magnetic field to the sample.

Acknowledgment

This work was supported in part by JSPS Bilateral Program Number JPJSBP120221002, and in part by JST-Mirai Program Grant Number JPMJMI19E1, Japan.