Clarification of current transport properties in YBCO coated conductors as a fundamental technology for power applications

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Current transport properties in YBCO coated high Tc superconducting (HTS) tapes have been investigated. Especially, we have succeeded for the first time to visualize flux flow dissipation under high magnetic field up to 5 T by developing novel high field type low temperature laser scanning microscope system (HF-LTLSM). It has been shown that the localized resistive region percolates into the sample as the bias current density is increased. We have also developed current visualization technique based on magnetic imaging by scanning SQUID microscopy (SSM). High sensitive magnetic imaging by variable temperature SSM allows us to estimate sheet current density in the superconducting layer. Combination of those measurements leads deep insights into current limiting mechanism in the coated conductor. Our results show that non-uniform current flow due to spatially distributed obstacles is responsible for the critical current, Ic, observed in macroscopic length scale, whereas grain connectivity in each YBCO grains is not the limiting factor. Typical period of those obstacles is several tens μm to hundreds μm. The relationship between local Ic distribution and global transport properties has also been investigated. Based on the local measurements as well as theoretical consideration, we developed a model to describe analytically the global current-voltage characteristics in the conductors. It allows us to predict the current carrying properties over broad conditions of temperature, magnetic field and electric field. The accuracy of the extrapolation has been verified experimentally in magnetic fields up to around 30 T. Those results are relevant as fundamental technologies for practical power applications of HTS based devices.