The Magnetic Field, Temperature and Strain Dependence of Jc in Narrow Superconducting Tracks of (RE)BCO Coated Conductor

The dependence of the critical current density (Jc) on an applied magnetic field (B), temperature (T), angle of the field to the tape normal (θ) and strain (ε) for narrow tracks of Rare Earth Barium Copper Oxide (REBCO) coated conductor (CC) with a width down to 5 µm is presented. Our motivation is to investigate the properties of REBCO CC tracks and the viability of this superconductor for future Tokamak magnet systems. We found the dependence of Jc on an applied strain for 100 µm chemically etched tracks to be inverse parabolic, similar to that of full-width 4 mm tapes. However the Jc(ε) dependence for 5 µm focused ion beam milled (FIB) tracks was found to be much more sensitive to strain, with a shift in the peak of the Jc(ε) curve to tensile strains from 0 % to above 0.3 %. After fitting the data to the bimodal chain model, and imaging the REBCO layer using electron backscatter diffraction (EBSD), we conclude that FIB milling changed the microstructure of the narrow REBCO tracks causing them to be richer in A-domains. The Jc dependence on field, temperature and angle for the chemically etched and FIB tracks at 0 % strain was measured in detail and found to be the same as that for a full-width REBCO CC tape. Detailed measurements of Jc(B, T, θ) data were obtained down to 10 K and up to 0.7 T by developing a Cold Head cryocooler system for measuring the 5 µm FIB tracks. A preliminary investigation into the Jc(ε) dependencies for different orientations of current and applied strain in the REBCO tapes was also completed. We found that the Jc response to strain was weaker when the current and strain were orthogonal, and strongest when the current and strain were parallel. We also found that Jc was more sensitive to strain when the current and strain were parallel and along the tape length rather than across it.