Defect-Tolerant High-Temperature Superconductor for Coil Applications

The remarkably superior current densities of RE-Ba-Cu-O (REBCO, RE = rare earth) superconductor tapes enable high power density and highly-efficient electric machines that address the needs of the Navy’s Advanced Power Systems. But, at such high current densities, the superconductor is susceptible to localized heating at defective spots which causes a thermal runaway, leading to a catastrophic failure. Uniform, long tapes with minimal defects are desirable to avoid these hot spots. The critical current (Ic) of long REBCO tapes is tested for uniformity only at 77 K in zero applied magnetic field. But there is strong evidence that even tapes that exhibit uniform Ic at 77 K, 0 T can have inconsistent Ic in a magnetic field at lower temperatures. Consequently, sections of a long tape that have lower Ic in a magnetic field at lower temperatures – that are not detected by Ic measurements at 77 K, 0 T – can be a location of quench and onset of failure. While long tapes with uniform Ic are highly desirable to avoid hot spots, methods to manage local defects in REBCO have to be investigated. Unlike metallic superconductors like Nb3Sn which consist of thousands of fine filaments, the wide geometry of REBCO tapes is not conducive for easy current sharing between tapes in a coil or cable. The objective of the proposed project is to develop defect tolerant REBCO tapes that promote current sharing between tapes to bypass current around local defects to reduce the possibility of quench and potential failure. In the Phase I effort of the proposed project, AMPeers worked with the University of Houston to design, develop, and test REBCO architectures that are defect tolerant and provide pathways to shunt current around defects. In addition, we worked to minimize defects in tapes using in-line quality control during fabrication. Further, we developed a reel-to-reel (R2R) Scanning Hall Probe Microscope (SHPM) measurement of Ic at 65 – 77 K in magnetic fields up to 5 T. Using the in-field R2R SHPM, we detected recessive defects that are not obvious in Ic measured at 77 K, 0 T, that can possibly be an origin for quench. In the Phase II project, we will evaluate all approaches for defect-tolerant REBCO tapes in terms of efficacy of current sharing, scalability to manufacturing, and impact on other properties relevant to applications; select the best choice and evaluate quench characteristics of meter-long tapes. We will then scaleup the best approach for defect-tolerant REBCO tape to 50 meters and deliver long lengths of defect-tolerant tape to Navy for independent evaluation. Additionally, we will design and construct a coil with long lengths of defect-tolerant REBCO tape and demonstrate its effectiveness in quench mitigation.