• Y.A. Kot, V. Balandin, W. Decking, C. Gerth, N. Golubeva, T. Limberg
  DESY, Hamburg

The XFEL injector building has a length of 74.3 metres and is divided by 2.5 m long concrete shielding wall. The section upstream the shielding wall will have a length of 42.3 m and give place for the gun, accelerating module, 3rd harmonic section, laser heater and the beam diagnostics section. At its end the possibility for the beam dump is foreseen so that the tuning of the beam in the injector would become possible without any impact on the subsequent parts of the XFEL. Each of these components sets certain requirements on beam optics which may compete with each other. Downstream the shielding the beam will be vertically displaced by 2.75 m over the distance of 20 m by means of the so called dogleg - a combination of two four cell arcs (8 cell system). Since the vertical displacement takes place there it is important to optimize cells in such an order that the chromatic effects don't impact the beam quality noticeably. In this paper we describe the solution for the beam optics at the XFEL injector.

• 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.