DESY, Hamburg
Since last year, the prototype of a camera system, developed at KEK/Kyoto University, for the optical inspection of the inner surface of cavities is available at DESY. Its variable-angle illumination system provides sufficient contrast for an in-situ search for surface defects in high resolution. Such defects may limit the gradient when causing a quench. A large sample of prototype cavities for the European XFEL has been inspected and provides a large inventory of data for analysis. The comparison of features in the optical inspection and hotspots in the temperature mapping during RF-measurements give evidence for correlations. Consecutive inspections of cavities in different stages of the surface preparation process allow a monitoring of the evolution of surface defects.
DESY, Hamburg
INFN/LASA, Segrate (MI)
W.C. Heraeus GmbH COPY, Materials Technology Dept., Hanau
Few cavities of the 4th and 6th cavity generation treated accordingly the XFEL recipe have shown performance of ca. 15 MV/m caused by thermal break down without field emission. Effort to post purify some cavities with titanium, that was successfully applied for FLASH cavities, did not improve the performance. The T-map analysis detected the quench areas mainly close to the equator. Optical control by high resolution camera and non-destructive X-Ray radiography have been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to get more detailed information of defects some samples have been extracted from cavity and investigated by light microscope, SEM, EDX and Auger spectroscopy. The detected distinctions are discussed