• A. Brinkmann, W. Singer
  DESY, Hamburg

For more than 10 years DESY has been operating a high resolution eddy-current scanning installation with rotating table for nondestructive flaw detection on niobium sheets for SRF cavities. More than 2000 sheets have been examined up to now, several types of defects have been detected and identified using different supplementary methods such as EDX, X-ray fluorescence, neutron activation analysis etc. In order to scan Nb-sheets needed for XFEL-cavity production, new scanning devices have to be build. One option of the eddy-current installations could be an application of SQUID-sensors due to much higher sensitivity instead of conventional probes. A SQUID based scanner system was built and is in evaluation at DESY. A status report will be given.

• P. Pierini, A. Bosotti, N. Panzeri, D. Sertore
  INFN/LASA, Segrate (MI)
• H.T. Edwards, M.H. Foley, E.R. Harms, D.V. Mitchell
  Fermilab, Batavia
• J. Iversen, W. Singer, E. Vogel
  DESY, Hamburg

The third harmonic cavities that will be used at the injector stage in the XFEL to linearize the rf curvature distortions and minimize beam tails in the bunch compressor are based on the rf structures developed at FNAL for the DESY FLASH linac. The design and fabrication procedures have been modified in order to match the slightly different interfaces of XFEL linac modules and the procedures followed by the industrial production of the main (1.3 GHz) XFEL cavities. A revision of the helium vessel design has been required to match the layout of the cryomodule strings, and a lighter version of the tuner has been designed (derived from the 1.3 GHz ILC blade tuner activities). The main changes introduced in the design of the XFEL cavities and the preliminary experience of the fabrication of three industrially produced and processed third harmonic rf structures are described here.

• W. Singer, I. Jelezov, A. Matheisen, X. Singer
  DESY, Hamburg
• G. Ciovati, P. Kneisel, M. Morrone
  JLAB, Newport News, Virginia

Funding: This manuscript has been partially authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
The technology of forming multi-cell seamless niobium cavities has been developed at DESY within the European CARE (Coordinated Accelerator Research in Europe) program. Three cell units have been manufactured successfully and a 9-cell cavity has recently been completed from three sub-sections and will be tested in the near future. Meanwhile, we have equipped two 3-cell units – one center unit of a 9-cell cavity and one end-unit – with niobium beam pipes, have tuned these units and carried out cryogenic rf tests after standard bcp surface treatments had been applied to these cavities. In addition, we will take temperature maps with Jlab's two-cell thermometry system; since in cavities fabricated by 'standard' methods such as deep drawing of half cells and electron beam welding cavity performance limitations have often been found at or near equator welds. It will be of particular interest to compare the seamless cavity quench locations to those from standard cavities. This contribution will report about the cryogenic test results and the T-mapping findings.

• E. Vogel, W. Decking, M. Dohlus, M.G. Hoffmann, M. Hüning, J. Iversen, K. Jensch, G. Kreps, T. Limberg, A. Matheisen, W.-D. Möller, K. Rehlich, A. Schmidt, J.K. Sekutowicz, W. Singer
  DESY, Hamburg
• A. Bosotti, P. Pierini, D. Sertore
  INFN/LASA, Segrate (MI)
• H.T. Edwards, E.R. Harms, T.N. Khabiboulline
  Fermilab, Batavia

Ultra short bunches with high peak current are required for the creation of high brilliance coherent light in the VUV and X-ray range in undulators. At the Free Electron Laser in Hamburg (FLASH) and the European X-ray free electron laser (XFEL) they are obtained by a two stage bunch compression scheme based on acceleration off the rf field crest and transverse magnetic chicanes. The deviation of the rf field's sine shape from a straight line leads to long bunch tails and reduces the peak current. This effect can be eliminated by adding a third harmonic rf system. The paper gives an overview on the actual status of the beam dynamical examinations and as well on the development of the third harmonic sub-systems like modules, cavities and radio frequency systems for FLASH and the XFEL.