Local structure analysis of 300 keV He+ irradiated REBCO coated conductor using polarisation dependent Cu K edge EXAFS

Understanding irradiation damage of REBCO is increasingly of interest for compact tokamak fusion reactor designs, as these materials are critical for the proposed magnetic plasma confinement systems. Here commercially sourced samples of REBCO coated conductor are irradiated with 300 keV He+ ions to a damage level of 169 x 10-3 displacements-per-atom, to the point where superconductivity is no longer detectable, meaning these samples correspond to a non-functional end-of-life component in a fusion reactor context. Subsequent analysis of the crystal structure through a combination of x-ray diffraction and x-ray absorption spectroscopy measurements reveals a complex variation away from the as-grown structure. The local structure probed by the spectroscopy measurements is further observed to change as a function of the relative polarisation of the incident x-ray beam, indicating that within this damage regime the structural anisotropy of the REBCO unit cell plays a determining role in where defects accumulate within the material. Here the local structure measurements probing the a-b plane of the system vary significantly less than those probing the c-axis direction following irradiation, mirroring the observed trend in the x-ray diffraction data that the a:b ratio is preserved upon irradiation whilst the absolute values increase, whereas the c-axis parameter expands. These observations highlight the role of oxygen defect formation in driving the degradation of superconductivity within irradiated REBCO. These changes are observed to preferentially accumulate along the c-axis of the material, indicating a possible mechanistic signature of the degradation of the superconducting properties within these systems that are evident using a local structure probe such as extended x-ray absorption fine structure.