ILC-HiGrade cavities as a tool of quality control for the EXFEL and further SRF R&D

The order for superconducting radio-frequency (SRF) cavities for the European XFEL includes an additional set of 24 cavities which are part of the ILC-HiGrade program. Initially, these cavities serve as quality control (QC) sample extracted in parallel to the EXFEL cavities series production on a regular basis. These QC cavities include all processing steps of the EXFEL cavities and experienced identical treatment with the exception of mounting of the Helium vessel. After the normal acceptance tests at the cavity RF measurement facility, the cavities are removed from the production flow and subjected to further R&D studies and quality assurance (QA) tests. To maximize the information from these cavities, a temperature surface mapping technique is applied in the second cold RF test followed by a high-resolution optical inspection (OBACHT) of the inner cavity surface. These QA steps are carried out to improve the understanding of defects in close collaboration with the standing experts engaged in the EXFEL production. Correlations between cold RF tests, temperature mapping and optical surface quality help identifying limitations and provide valuable feedback for the production sequence. Results of the QC tests and R&D work will be presented and discussed