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.