Optical Surface Properties and their RF Limitations of European XFEL Cavities

Abstract: The inner surface of superconducting cavities plays a crucial role to achieve highest accelerating fields and low losses. The industrial fabrication of cavities for the European X-Ray Free Electron Laser (XFEL) and the International Linear Collider (ILC) HiGrade Research Project allowed for an investigation of this interplay. For the serial inspection of the inner surface, the optical inspection robot OBACHT was constructed and to analyze the large amount of data, represented in the images of the inner surface, an image processing and analysis code was developed and new variables to describe the cavity surface were obtained. This quantitative analysis identified vendor specific surface properties which allow to perform a quality control and assurance during the production. In addition, a strong negative correlation of $\rho= -0.93$ with a significance of $6\,\sigma$ of the integrated grain boundary area $\sum{\mathrm{A}}$ versus the maximal achievable accelerating field $\mathrm{E_{acc,max}}$ has been found.