Title: 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.