• A. Meseck, M. Abo-Bakr, J. Bahrdt, B.C. Kuske
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministrium für Bildung, und Forschung, the state Berlin and the Zukunftsfonds Berlin

BESSY proposes a soft X-ray free electron laser (FEL) multi-user facility. It will consist of three undulator lines, each based on cascaded High-Gain Harmonic-Generation (HGHG) scheme delivering photons in energy range of 24 eV to 1 keV. Start-to-end Simulations including error sources from the injector, and linac structure have been performed to provide realistic information about the expected radiation field (B. Kuske, FEL2005). However, the beamline designer needs to know the exact location and the size of the photon beam waist to maximize the brightness at the sample. This information can be derived from results of longitudinal propagation of the electric field distribution, which can be extracted from simulation results using the code GENESIS. The results of the Start-to-End simulation are used for realistic prediction of the photon beam properties of the BESSY Soft X-ray FEL.

• A. Meseck, J. Bahrdt
  BESSY GmbH, Berlin

Funding: Funded by the Bundesministrium für Bildung, und Forschung, the state Berlin and the Zukunftsfonds Berlin

Wide-range wavelength-tunability is one of the key aspects of proposed FEL facilities. Once the electron beam energy and undulator period length is given, the span of the available K-values determines the achievable wavelength range, according to the resonance condition. As the usable range of the K-values is limited by technical considerations like minimum acceptable gap or permanent magnet technology etc, alternatives to enlarge the output wavelength range are of high interest. Using revolver-undulator design, different magnetic structures can be incorporated in the same undulator segment. Thus it is possible to switch between different undulator periods, covering a wider wavelength range at a given FEL-line. Because of the transverse-positions dependency of the magnetic field, the alignment reproducibility of the revolver-undulators is of concern, in particular for the APPEL type devices. Simulation studies have been performed taking the BESSY FEL-lines as examples to investigate the alignment tolerances of these devices, to reveal their limits of applicability.

• J. Bahrdt
  BESSY GmbH, Berlin

The design of beamlines for VUV and x-ray FEL facilities requires a detailed knowledge of the coherent radiation source. Time dependent simulations with FEL codes like GENESIS provide the electric field distribution at the end of the FEL which represents the complete information. Ray tracing codes used to transform the light from the source to the sample are generally based on geometrical optics and do not include directly the coherent properties of the FEL radiation. On the other hand Fourier optic techniques are usually applied to the propagation across normal incidence optics. We present an algorithm based on physical optics which permits the propagation of wavefronts across grazing incidence optics including interference effects, diffraction, polarization variation and pulse lengthening. Some examples are given for the proposed BESSY soft x-ray FEL.