SBIR-STTR Award

The remarkably superior current densities of RE-Ba-Cu-O (REBCO, RE = rare earth) tapes enable high power density and highly efficient electric machines that directly address the needs of the Navys Advanced Power Systems. At such high current densities sustained through just 1 m thick film, the superconductor is susceptible to localized heating at defective spots that are invariably present in a long tape. Since each tape turn of coils comprising a superconducting device is typically insulated, the hot spots do not dissipate easily which causes a thermal runaway, leading to a catastrophic failure. Since the normal zone propagation velocity in REBCO is extremely small, the hot spots cannot be detected in time to prevent thermal runaway, leading to a catastrophic failure. While long tapes with uniform critical current are highly desirable to avoid such hot spots, methods to manage local defects in REBCO tapes have to be investigated. Unlike metallic superconductors like Nb3Sn which consists of thousands of fine filaments in each strand, the wide geometry of REBCO tapes is not conducive for easy current sharing between tapes in a stack. The objective of the proposed project is to develop defect tolerant REBCO tape stacks that promote current sharing between tapes to bypass current around local defects to reduce the possibility of quench and potential failure. In the proposed project, AMPeers LLC will work with the University of Houston to design, develop, and test superconducting tape architectures that are defect tolerant and provide pathways to shunt current around defects. Through simulation and experiment, we will optimize the tape architecture and maximize its potential to shunt current around artificially created defects. Current sharing between tapes will be tested to confirm the efficacy of the new defect tolerant REBCO tape architecture. Additionally, innovative in-line quality control (QC) tools will be used for real-time monitoring of tape quality during the manufacture of tapes in this project. Such in-line QC tools will help enable early detection and correction to eliminate defective regions in the tape during manufacturing. Finally, we will assess the uniformity of current of our tapes not only by measurements at 77 K, 0 T but also by 2D mapping of critical current in a magnetic field up to 5 T, over 100% of their length, by a new reel-to-reel scanning hall probe microscope (SHPM).

Benefit:
The proposed project directly addresses recommendations of the Naval Power and Energy Systems (NPES) Technology Development Roadmap (TDR) to develop superconductors to enhance the efficiency for sustained warfighter operations and platform level energy storage and efficiency for propulsion advanced mission systems . Specifically, our project focuses on achieving superconducting state protection through superconducting tape processing and modification that are both specified in the NPES-TDR. As emphasized in the BAA, it is essential to protect superconducting devices that operate at very high current densities from uncontrolled quench. A defect tolerant tape is highly sought in nearly all high temperature superconductor (HTS) applications since it would address a major market need to protect expensive HTS devices during a quench event. A successful outcome of the proposed project will result in such a defect tolerant superconductor tape which will be transitioned to the ongoing superconductor projects at NSWC as well other applications such as fusion, accelerators, high-field magnets, superconducting magnetic energy storage, motors and generators.

Keywords:
defects, defects, superconductor, critical current, REBCO, current sharing, quench

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 Navys 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.

Benefit:
The Phase II project will lead to defect-tolerant REBCO superconductor tapes that are highly sought in nearly all High Temperature Superconductor (HTS) applications. These tapes would address a major market need to protect expensive HTS devices during a quench event. Our defect-tolerant REBCO tapes directly address the critical need of NSWC to self-protect its complex and expensive devices made of HTS coils from unanticipated quench and catastrophic failure. Currently, NSWC is using quench detection methods to quickly detect quench and avoid thermal runaway. However, such methods are not fully reliable, are sensitive to noises and vibrations, and have problems in integration in HTS coils. Our solution is elegantly simple in that it provides a built-in mechanism in the REBCO tape itself to shunt current around existing defects and self-protect the device. Used in conjunction with existing quench detection methods, it can be a low-risk method to protect the superconducting device. Our defect-tolerant REBCO tapes will greatly benefit a number of commercial applications such as compact fusion, accelerator magnets, superconducting magnetic energy storage systems, electric power equipment such as motors, generators, and transformers. In addition to defect tolerant superconductors, our research will lead to uniform, high-performance REBCO tapes that are needed in many applications, as well as new test methods to identify recessive defects in tapes that are not detected by state-of-the-art techniques.

Keywords:
manufacturing, critical current, superconductor, REBCO tape, magnet, coil, defect tolerant, quench