Electromagnetic Modelling and AC-Losses of Superconducting/soft-ferromagnetic wires and CORC cables
Creators: name: Fareed, Muhammad U.
Publisher: name: University of Leicester
Year: 2022
Subjects: Uncategorized

description: This thesis aims to understand the macroscopic electromagnetic behaviour of type-II superconducting (SC) wires when enclosed in a soft-ferromagnetic (SFM) sheath under self-field conditions. This means that only transport current is applied to the superconducting wire, which is aimed to be used under AC applications. Additionally, this thesis focuses on the electromagnetic modelling of conductor on rounded core (CORC) cables under external magnetic field conditions, which are expected to be used in the designing of ultra high field magnets. Thus, in the first part, a comprehensive analysis to understand the magnetic coupling between the SC and the SFM is performed, by presenting a semi-analytical numerical formulation which is validated with a variational approach of multiple functionals. Thus, the notorious electromagnetic characteristics of the SC-SFM heterostructures have been disclosed, demonstrating that the counterintuitive increment in the AC losses experimentally observed at self-field conditions, can be accurately explained under the framework of the general critical state theory. Then, the second part deals with the conception of a 3D finite element model for conductor on a rounded core (CORC) cables based on the so-called H-formulation. The model has been implemented into COMSOL Multiphysics to understand the electromagnetic properties of the cable, and to calculate its AC losses for determining possible paths of optimisation. Furthermore, a distinctive feature of our model is the use of an augmented multilayer thickness for the 1 micron superconducting film in the coated conductor, which tackles the aspect-ratio problem and numerical convergence when considering coated conductors with a width of 4-12 mm. This approach although conventional in the modelling of superconducting tapes, resulting in a renormalization of its critical current density, is performed in such way that the superconducting film thickness is not only increased from 1 micron to 50 micron, but it is simultaneously split in 5 to 10 layers, allowing therefore the existence of paths of current across the SC film and not only over its surface, i.e., the possibility to visualize the correct distribution of magnetization currents at a local level, and aspect that cannot be seen in other approaches where either the three-dimensionality of the coated conductor is reduced to a 2D approach, or when the depth size of the elements in the SC domain equals the renormalized thickness of the SC film. This approach has helped us to perform a systematic analysis of the AC losses of CORC cables by changing the twist pitch angle of the coated conductors, finding their optimal value which is crucial for the designing of CORC cables. In summary, the studies presented in this thesis aimed to understand the physical properties of the hybrid system SC-SFM and CORC cables, such as, the distribution of current density, the magnetic coupling of SC-SFM, and the AC-losses, such that research can be used as a practical benchmark to understand the electromagnetic features of heterostructure wires under self field conditions, and the inherent physical characteristics of CORC cables for engineering applications.