It is important to control the layer current distributions of coaxial multi-layer HTS cables because homogeneous layer current distribution decreases AC loss and can supply the largest operational current. In a previous paper, we proposed a theory that can control current distribution based on the concept of flux conservation between two adjacent layers, and demonstrated the theory is in good agreement with experiment results. The theory was effective for an operational current less than the critical current of the cable. It is important to investigate current distribution under the condition of operational current more than the critical current of the cable because the cable experiences fault currents. We have extended the theory to treat the operational current more than the critical current by considering V-I nonlinear characteristics of HTS tapes including flux flow resistance and contact resistance between the cable and terminals. In order to verify the extended theory, we have fabricated a two-layer cable with the same twisting layer pitch, and hence caused inhomogeneous current distribution. It was observed that almost all of operational current less than the critical current flowed on the outer layer because of its lower inductance. When the operational current increased above the critical current of the second layer, the flux flow resistance appeared and distorted the current waveform with phase deviations. Finally, in the case of operational current more than the critical currents of both layers, flux flow resistance strongly affected current waveforms, and thereby the currents of both layers were determined by flux flow resistance. The extended theory simulated the layer current distribution waveforms and demonstrated good agreement with the experimental results under all operational current regions.
The refrigeration capacity of a conventional GM refrigerator was studied over a temperature range of 7-25 K by employing a second-stage regenerator stacked in series using different Ag-based regenerator materials, RAg (R=Pr, Er, Ho), together with pure metals, Pb, Cu and Ag. The combinations of PrAg/HoAg and PrAg/Pb stacked as columns in series could reach the lowest temperature of 7.0 K in contrast to that of 7.3 K reached with the conventional Er3Ni compound, as far as the present refrigerator is concerned. These combinations are in possession of specific heats over the temperature range of 7-25 K, consistently higher than that of Er3Ni. Moreover, we revealed that, if the amount of PrAg is increased, the replacement of Pb with Ag or Cu at the high-temperature end of the regenerator is possible without affecting the refrigeration capacity of reaching the lowest temperature of 7.0 K. Therefore, we conclude that combinations of PrAg/Ag and PrAg/Cu stacked in stratified form can serve as environment-friendly regenerator materials in the temperature range below 25K.