• X. Singer, S. Aderhold, A. Ermakov, W. Singer, K. Twarowski
  DESY, Hamburg
• M. Hoss, F. Schoelz, B. Spaniol
  W.C. Heraeus GmbH, Materials Technology Dept., Hanau

Performance of XFEL prototype cavities fabricated at the industry and treated at DESY demonstrates big scattering from 15 to 41 MV/m. Most cavities satisfy the XFEL specification. Few cavities with low performance (15-17 MV/m) are limited by thermal break down without field emission. The T-map analysis detected the quench areas mainly close to the equator. Optical control by high resolution camera has been applied and allowed to monitor the defects in some cases with good correlation to T-map data. In order to understand the cause of reduced performance and get more detailed information of defects origin some samples have been extracted from two cavities and investigated by light microscope, 3D- microscope, SEM, EDX and Auger spectroscopy. Several surface flaws with sizes from few μm to hundreds of μm were detected by microscopy. The defects can be separated in two categories. The first category of defects indicates foreign elements (often increased content of carbon). Inclusions with increased content of carbon adhered on the surface and presumably have a hydrocarbon nature. Deviation from smooth surface profile characterizes the second type of defects (holes, bumps and pits).