Analysis of Degraded Cavities in Prototype Modules for the European XFEL

In-between the fabrication and the operation in an accelerator the performance of superconducting RF cavities is typically tested several times. Although the assembly is done under very controlled conditions in a clean room, it is observed from time to time that a cavity with good performance in the vertical acceptance test shows deteriorated performance in the accelerator module afterwards. This work presents the analysis of several such cavities that have been disassembled from modules of the prototype phase for the European XFEL for detailed investigation like optical inspection and replica.