Spatial homogeneity is one of the most important issues for practical applications of REBCO based magnets. Reel-to-reel Ic measurement is now widely used for the investigation of local Ic variation, however, these measurements are limited at low magnetif field around self-field and at 77 K in most cases, whereas practical operation condistions of superconducting magnets are higher magnetic field, B, and lower temperature, T. Furthermore, the relationship among the positional Ic variation, transport Ic and n-value in the long REBCO tapes is not yet fully understood.
In this study, we carried out spatially resolved Ic measurement in IBAD-PLD processed REBCO tapes under external magnetic field up to 5 T and down to 35 K by combining reel-to-reel contimuous measurement and in-field scanning Hall probe microscopy. We have also succeeded in measuring local n-value in the tape continuously with a spatial resolution of 1 mm. These measurements allow us to obtain deep insight into the controlling factors of B, T and position dependences of the current transport properties of REBCO tape. We confirmed good correlation between the measurements at low-field, 77 K and at in-field lower temperatures. Also, the positional dependence is almost independent from measurement conditions such as B and T. This strongly suggests that the B and T dependences are dominated by uniform flux pinning in a mesoscopic scale while the local Ic variation is mainly originated from current blocking obstacles in a macroscopic scale. Based on this understanding, we also propsed a multi-scale model to describe global current-voltage characteristics, i.e., transport Ic and n-value of long length REBCO tapes analytically. This is useful for describing properties in a long length magnet winding under practical operation condisions.
This work was supported by JSPS KAKENHI Grant Number 16H02334 and the New Energy and Industrial Technology Development Organization (NEDO).