Electrical-thermal characteristics of HTS winding applications

Abstract

Superconductivity is considered as one of the great scientific discoveries of 20th century. With the advantages of the properties of superconducting materials with zero electrical resistance and perfect diamagnetism. There is a wide range of innovation and developing applications in the field such as large magnets for Nuclear Magnetic Resonance, Magnetic Resonance Imaging, and large scale accelerators for multiphysics. At the end of 1980s, the discovery of High Temperature Superconducting (HTS) materials such as Yttrium Barium Copper Oxide (YBCO) and Bismuth Strontium Calcium Copper Oxide (BSCCO) made it possible to use liquid nitrogen to cool down HTS windings for various electrical power systems, instead of using more expensive liquid helium. Other advantages for using HTS windings instead of copper windings include the ability to manufacture smaller and lighter devices with greater energy saving. In reference to designing an HTS winding from long lengths of superconducting wire, (which is presently unavoidable without including joints), the electrical contacts themselves must have similar performance. Since the electrical contact resistance of the HTS joint has a significant impact on the thermal stability of the entire system during the operation, a reproducible soldering methodology is required for manufacturing small electrical contacts between current leads and superconductor and or superconductor to superconductor. A potentially attractive nanobond technique was applied for the first time to HTS tapes at Oxford Instruments for making joints with very low contact resistance, and experimentally characterised at University of Southampton. The nanobond process that used a patented nanofoil offered a possible solution for making reliable and repeatable HTS joints. In parallel, a new methodology was developed for joining 2G-tapes joints in the construction of a long HTS coil. The contact resistance measurements from a series of nano-bonded soldered joints, demonstrated the potential to apply the same technique to make in-situ and reproducible joints of tapes that could be replicated for much longer scale HTS windings. The 2G YBCO Roebel cable is a promising option for high energy accelerator magnets and Tokamak devices due to its high current carrying capability and compactness. However, its structure and configuration complicate how its current shares, and therefore it is difficult to apply traditional superconductor quench measurement methodologies to characterise Roebel cables in an adiabatic condition. This work reports upon a new methodology developed to study quench of a 2G YBCO Roebel cable pancake coil. The geometrical design was such, that it could be fully immersed in liquid nitrogen (LN2) to perform the experimental measurements. The initial results from a series of trial quench measurements demonstrated the robustness of the Roebel pancake coil under quench conditions and good agreement between experimental and modelled results to assist in the understanding of current sharing. The full scope of work presented in this thesis carried out measurements to obtain the thermal, mechanical and electrical properties of nanofoil at cryogenics temperatures. The work progressed to study and developed a new method of formed HTS joints by nanobonds. Finally, the critical current, current injection electrical contacts, and quench characteristics of a Roebel cable were performed and investigated in LN2 at 77 K

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ORCID for Yifeng Yang: orcid.org/0000-0002-3874-6735

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