Investigation on the fine tuning of different performance parameters of High Temperature Superconductors

Abstract

Nuclear fusion can be considered as a viable source of energy in the future when considering the high depletion rate of conventional energy sources and the increase in energy demands. Superconducting magnets being an integral part of any nuclear fusion reactor, wires and cables that can carry a huge amount of current without any loss and with minimum cost are to be developed. The high-temperature superconductors (HTS) are considered to be cost-effective when compared to low-temperature superconductors (LTS). However, the development of HTS-based wires or cables requires much research, mostly because majority of them are brittle. The second-generation HTS of the rare-earth barium copper oxide (REBCO) family are potentially promising because of their large current density and low hysteresis losses. However, being brittle in nature, they must be first transformed into tapes before being converted into superconducting cables and wires for use. REBCO conductors also have many desirable properties required for various applications, such as strength, flexibility, and strain tolerance for winding, and current densities at high to very high magnetic fields even at liquid nitrogen temperatures (77 K). However, they can degrade and lose their superconducting property unless they are kept within certain limits (known as critical limits) in terms of temperature, current density, magnetic field, and structural strain. It is necessary to determine these critical limits for any given type, geometry and size of superconductor when they are subjected to mechanical forces/stresses. HTS tapes and cables being expensive, the experimental investigations for determining these critical limits are costly and also challenging. This is mainly due to their brittle nature, high current density, and cryogenic operating temperatures. Therefore, the use of simulation studies employing FEM software is found to be a viable option. This study, using simulation, attempts to understand the degradation of the REBCO tapes under different mechanical loading (tensile, bending, torsion, cyclic and winding loads) and also by varying the geometric parameters of the tape. The geometry parameters considered are the thickness of the constituting layers (mainly those made of Hastelloy and copper), width of the tape, winding angle, central core diameter, etc. Parametric studies carried out by varying the type and magnitude of the forces applied on them will help to identify the relative involvement of each geometric parameter on performance of the superconducting tape. The model for the simulation studies has been developed using COMSOL Multiphysics and have been compared with the iv experimental and simulation studies reported in the literature. The results are obtained under tension, bending, torsion winding loads and also under the fatigue type of loading. The results obtained reveal that Hastelloy and copper thicknesses have a significant influence on the development of intrinsic strain under tensile and bending loads; but found to be not much significant in the case of torsional load. The tape width is found to be the most crucial parameter in case of torsional loads. By decreasing the tape width, the critical limits can be pushed further, giving more flexibility for the manufacturers to accommodate combined tensile and torsional loads. Decreasing the tape width by 75 %, increased the maximum allowable angle of twist by 326 %. The winding study of simple superconducting CORC cable revealed that 45o is the best angle for winding, and also found that, the intrinsic strain in the REBCO layer is increased with the decrease in the central core diameter. The fatigue loading studiesshowed that the bending load is the most severe one of the three types of loading. Also, it is found that, upon fatigue type loading, the Hastelloy layer fails first, followed by the bottom copper layer, the silver layer, and then the top copper layer for the given tape configuration and loading conditions. It is found that the role of each tape parameter is different depending on the type of load applied. Under the tensile type of load, the thickness of the Hastelloy and copper layers is more significant than other tape parameters. Whereas, under bending, the thickness of the Hastelloy layer has more important role. Under the torsional load, the width of the tape is the most determining parameter. The thickness of the Hastelloy and copper layers is significant in the case of fatigue type of loading for tension, bending and torsion. The various results obtained are expected to help the manufacturers and researchers to develop better HTS REBCO tapes and cables when they are subjected to tension, bending, torsion, cyclic and winding loads as well as under fatigue type of loading