Radiation damage of superconducting materials for fusion application
Keywords: REBCO
superconductivity
cold irradiation
ion irradiation
coated conductor

Superconducting wires are a key enabling technology for generating electricity from fusion reactions as they can carry high current densities in high magnetic fields, allowing them to make better use of the limited space available in a tokamak and improve its power density. This thesis focused on REBCO coated conductors (CC) as, despite its documented deterioration during neutron irradiation, it is the superconductor of choice for several tokamak power plant (TTP) concepts due to reliable products with appropriate performance becoming increasingly commercially available. During their operation in TPP, REBCO CC will carrying large currents whilst being neutron irradiated, subjected to high applied fields and maintained at their operating temperature. An appraisal of the previous literature shows that experiments designed to determine REBCO’s suitability for fusion applications use various irradiation methods, but all apply it to samples maintained at room temperature. This thesis, therefore, aims to get closer to the actual operating conditions within a TPP by investigating the response of REBCO to irradiation when irradiated at its operating temperature.
After a thorough discussion of the previous literature and a description of the experimental methods used herein, this thesis presents 4 experimental chapters before summarising and drawing conclusions. The first (chapter 4) uses computer simulations to evaluate how REBCO is affected by fusion spectrum neutrons and compares that with published data of how it responds to the fission neutron spectrum. Discussion of these results includes an analysis of this comparison, determination of the expected lattice damage and impurity build-up in REBCO during its operating lifetime in a TPP, and determination of an appropriate ion and energy combination to be used as a proxy for fusion neutron irradiation. The second (chapter 5) describes an experiment to determine how the chosen ion-energy combination effects REBCO samples when they are maintained at room temperature, using magnetometry to track superconducting properties between irradiation steps. This experiment allowed distinctions between the samples to be made, and allowed one sample, Fujikura’s 2018 unenhanced GdBCO CC, to be chosen to go forward to cold irradiation experiments. The third (chapter 6) presents a description of the design process behind the Cold Irradiation Experiment (CIE) – an apparatus capable of maintaining a REBCO CC at a set temperature below its critical temperature (Tc), performing electrical testing to determine superconducting properties and allowing the sample to be ion irradiated, although the later 2 were not done simultaneously during this work. This culminated in the successful building of the CIE which is used in the fourth experimental chapter (chapter 7) to generate data on how the temperature of a REBCO sample effects how its superconducting properties change with increasing irradiation dose. The key results of this experiment were that sample temperature does not affect how the REBCO superconducting properties deteriorate with increasing irradiation dose but does affect what happens if the samples are left to anneal at room temperature for extended periods. Although the exact cause of this change is not definitively established during this work, various hypotheses are discussed and several avenues of further work that would allow the hypotheses to be tested are presented.