Characterization of Critical Current Distribution in Roebel Cable Strands Based on Reel-to-Reel Scanning Hall-Probe Microscopy
We have characterized the local critical current distribution in 2-mm-wide Roebel cable strands using reel-to-reel scanning Hall-probe microscopy (RTR-SHPM). Roebel cable is an equally transposed cable formed from coated conductor architectures that is promising for large current applications. Local spatial critical-current uniformity becomes a crucial issue for the thermal stability of high-current windings. In general, the one-dimensional longitudinal critical-current distribution in coated conductors has been characterized by TAPESTAR as a de-facto standard method. However, it has proved to be difficult to use this method to achieve sufficient quality control of Roebel strands because of the unavailability of information on current flow across the width at a transposition. Furthermore, the lack of spatial resolution across the width becomes a problem for narrower strands. In this study, we applied RTR-SHPM for the characterization of Roebel strands. By scanning a Hall senor across the width of a conductor that was moving in a longitudinal direction, a high spatial-resolution two-dimensional distribution of magnetic field can be obtained for long-length conductors. This enabled us to estimate the local critical current, including the dependence on effective strand width, as a function of a longitudinal coordinate. This information is critical to the optimization of the fabrication processes and for nondestructive quality assurance of long-length Roebel strands.