W.C. Heraeus GmbH, Materials Technology Dept., Hanau
Plansee Metall GmbH, Reutte
TU Darmstadt, Darmstadt
DESY, Hamburg
The RRR value of high pure Nb is showing strong relations to the individual production steps. Mainly the different kind of internal stresses caused by the several production steps are resulting in the variation of the RRR value. This work shows the RRR values along the complete production chain from the molten Ingot till to the finished cavity. The influence of the RRR value caused by stresses and the release of that stresses by vacuum annealing is shown.
DESY, Hamburg
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).
DESY, Hamburg
INFN/LASA, Segrate (MI)
The preparation phase for the European XFEL cavity production includes a number of actions. Material issues: qualification of high purity niobium vendors, verifying of large grain material as a possible option, construction of the scanning device for the niobium sheets. Mechanical fabrication issues: accommodation of the TESLA cavity design to the XFEL demands, device construction for RF measurement of components, integration of the helium tank and it's welding to the cavity into the fabrication sequence, documentation and data transfer, application of a new high resolution camera for inspection of the inside surface. Treatment and RF measurement: establishing the XFEL recipe, in particular the final surface treatment (final 40 μm EP or short 10 μm Flash BCP), and the cavity preparation strategy (vertical acceptance test with or without helium tank welded, with or without assembly of HOM antennas), construction of the cavity tuning machine. About 50 prototype cavities are produced at the industry, treated (partially in industry and partially at DESY) and RF-tested at DESY. The XFEL requirements are fulfilled with a yield of approx. 90%.
