Understanding Radiation Damage of High Temperature Superconductors Under Relevant Operating Conditions for Fusion Magnets

High temperature superconductors (HTS) in the form of REBa₂Cu₃O₇ (REBCO) coated conductors are an enabling technology for the next generation of compact nuclear fusion reactors that require higher magnetic fields than Nb₃Sn can provide. However, in operation, the superconducting magnet windings will be exposed to a flux of fast neutrons which will introduce structural damage at cryogenic temperatures. Many previous studies using both fission spectrum neutrons and ions at room temperature (or slightly elevated temperatures) have shown that the evolution of superconducting properties with irradiation follows a universal pattern: an initial increase in critical current density (J_c) as a result of the generation of defects that improve flux pinning, is followed at higher doses by a severe degradation of the properties and eventually complete loss of superconductivity. Transition temperature (T_c) is found to decrease monotonically with increasing dose, strongly suggesting that radiation-induced defects occur throughout the entire crystal lattice, even at relatively low doses. Here we will report on two facets of the work being carried out by the Oxford group to improve understanding of radiation damage of high temperature superconductors. The first concerns innovative in situ ion irradiation experiments to assess radiation damage of HTS coated conductors at cryogenic temperatures relevant for operation in a fusion magnet. These are crucial experiments because early work has shown that excursions to room temperature lead to an evolution of the defect landscape and a partial recovery of the superconducting performance which could be exploited to extend magnet lifetime.<br/>Secondly, we will report results of our recent studies aimed at understanding the nature of the lattice defects that cause the degradation in T_c using light ion irradiation as a proxy for neutrons. Atomic resolution electron microscopy studies comparing pristine and He⁺ irradiated coated conductors show that, even when the irradiation dose is high enough for T_c to have dropped to below 20 K, the cation lattice is still intact. Therefore, to study the defects that are responsible for the loss of superconductivity, polarisation dependent high energy resolution x-ray absorption spectroscopy (HERFD-XAS) has been used for the first time to probe small changes in the bonding environment of the copper ions. The enhanced energy resolution of this technique means that we can see unprecedented detail in the near edge spectra. We have successfully modelled the experimental spectra using DFT core hole calculations, allowing the origin of the various spectral features to be identified. We find that irradiation with He⁺ ions significantly affects the environment of the plane site copper ions. The nature of the defects responsible have been investigated by calculating spectra from a range of defect structures and comparing with the experimental results. Our results challenge conventional wisdom that it is the formation of oxygen vacancies in the chain sites that leads to a T_c reduction in irradiated REBCO. By comparing our preliminary He⁺ irradiation results with samples irradiated by other ions and neutrons we aim to determine to what extent ion irradiation can serve as an easy and safe proxy for understanding neutron damage in these complex materials.