BESSY is proposing a Soft X-ray FEL user facility in Berlin, delivering short and stabile photon pulses in the wavelength range of 62 nm < λ < 1.2 nm by applying up to four cascaded High Gain Harmonic Generation (HGHG) stages. For optimization of the FEL performance of the cascaded HGHG stages extensive Start-to-End (S2E) simulations have been carried out. In order to test the quality of the chosen configuration with respect to the sensitivity against various error sources tolerance studies from the injector to the linac end have been performed. Procedures and results of these studies will be presented.

BESSY is proposing a linac-based High Gain Harmonic Generation FEL multi user facility covering the VUV to soft X-ray spectral range. A photoinjector and a superconducting 2.3 GeV CW linac will feed three independent multi-stage HGHG lines consisting of APPLE II type undulators. Tolerance studies have been performed for the FEL process, including variations in the electron beam parameters like current, emittance, and energy spread. The influence of undulator field errors has been investigated and start to end calculations, taking varying conditions for each slice into account, have been performed. The investigations show, that the behaviour in the presence of errors is quite different from the SASE case, and depends heavily on the specific design and optimization of the different stages.

The BESSY soft X-ray FEL is planned as a High Gain Harmonic Generation (HGHG) FEL multi-user facility covering the VUV to soft X-ray spectral range. In the HGHG scheme, the properties of the radiation output are dominated by the characteristics of the laser seed. In this connection, the influence of the laser pulse shape on the output, in particular on the output spectrum is of interest. Simulation studies for the BESSY-HGHG-FELs are presented and the output performance for different shapes of the laser pulse is discussed.

BESSY plans a linac based high gain harmonic generation FEL user facility with three FEL lines [1]. The modulators and most of the radiators are planar pure permanent magnet undulators. The last radiator and the final amplifier produce radiation of linear polarization with arbitrary orientation as well as elliptically or helically polarized light. They will have a modified APPLE II design which provides higher fields compared to a conventional APPLE II [2]. Detailed calculations for this design will be presented. FEL 3D-calculations provide information about the radiation field distribution at the end of the undulator. A beamline designer needs the information about the effective source size at the waist and the location of the waist. The electric fields calculated by GENESIS have been propagated and the source characteristics have been derived for various FEL parameters. The FEL process requires tight gap tolerances within and between modules. We present a new gap measurement system with a gap positioning accuracy of 1.5 microns.