Understanding radiation damage mechanisms in high temperature superconductors for fusion applications

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REBCO (rare-earth barium copper oxide) 2nd generation coated conductor tapes are a top contender for the magnet material in compact tokamak fusion reactors because of their superior current carrying/magnetic field generating properties. Radiation introduces various types of defect to the REBCO crystal lattice and these changes in microstructure are known to affect key superconducting properties such as critical current density and critical temperature. Initially, at low doses, irradiation tends to improve the critical current density of the superconductor slightly because the defects introduced act as efficient pinning sites for magnetic flux lines, enabling higher currents to be carried before resistance is generated. However, as the irradiation dose increases and the defect concentration increases, the superconducting properties degrade rapidly, eventually resulting in the complete loss of superconductivity. The main aim of this project is to improve understanding of how changes in microstructure induced by irradiation with energetic particles affects flux pinning in commercial REBCO coated conductors. This is vital engineering information for the designers of future fusion magnets. The project will involve comparing samples irradiated with neutrons at the new NNUF facilities in Birmingham with proxy samples ion-irradiated at the Dalton Cumbrian and Surrey Ion Beam Facilities. Detailed characterisation of the superconducting properties pre- and post-irradiation will be carried out using a state-of-the-art Physical Properties Measurement System situated in the Materials Research Facility at Culham Centre for Fusion Energy. This is the only facility in the UK where the electrical and magnetic properties of active superconductor samples can be measured over a wide range of temperatures and magnetic fields, and the results obtained will be of direct relevance to the design of magnets for the STEP programme. A combination of advanced microstructural characterisation techniques including atomic resolution transmission electron microscopy, X-ray diffraction and X-ray spectroscopy (at Diamond Light Source), will also be used to deduce the nature of the defects introduced by irradiation and correlate irradiation damage with changes in superconducting properties. Key parameters of practical relevance that will be investigated are the effects of irradiation temperature (including irradiation at cryogenic temperatures) and post irradiation annealing, as well as the type of projectile for assessing the suitability of ion-irradiation as a proxy for expensive neutron damage experiments. This project spans the EPSRC Energy Theme (Magnetic Fusion, Materials for Energy, Manufacturing Technology) and the Engineering Theme (Manufacturing Technology, Materials Engineering, Engineering Design). The project is in collaboration with Dr Holly Campbell at the UK Atomic Energy Authority (UKAEA), Culham Centre for Fusion Energy, with 50% of the funding being provided by UKAEA.This is a 4-year 'Fusion CDT' Studentship (part of the course fee paid from Oxford Materials funds)