The Harmonic Susceptibility Series of High Field Superconductors for Fusion Energy Applications

The magnetic phase diagram of superconductors is a rich and diverse research landscape in which the properties of vortex matter relate closely to the maximum lossless current density which can flow through the superconductor, JC(B, T). The growing requirement for high field magnets in academic and industry environments, especially in applications such as fusion energy where the superconducting magnets and local magnetic fields intersect at oblique angles, means we need to understand how the complex pinning landscape of high field superconductors impacts the achievable transport currents, JC. This requires a detailed understanding of vortex matter and anisotropy in High Temperature Superconducting (HTS) materials, due to both the intrinsic anisotropy of the Cuprate superconductors and the geometric anisotropy of the high aspect ratio superconducting layer in these tapes. This thesis presents detailed measurements of the irreversibility transition in HTS tapes using an harmonic AC susceptibility methodology in fields up to 35 T perpendicular to the tape normal and in oblique fields up to 24 T. A numerical procedure for modelling the susceptibility of high aspect ratio superconducting tapes is presented and used to extract JC(B, T) for a Superpower 2G HTS tape without artificial pinning. An extension to this model which includes the effect of anisotropic pinning forces in superconductors with oblique fields is presented and compared to the measured susceptibility in fields up to 24 T to extract JC(B, T, θ). This work provides fresh insight into the behaviour of vortex matter in 2G HTS tapes in high, oblique fields and suggests how improvements can be made to inductive measurements methodologies in superconductors with very high critical current densities