Analysis of Parameter Space of Soft X-Ray Free Electron Laser at the European XFEL Driven by High and Low Energy Electron Beam

Three undulator beamlines: SASE1 and SASE2 (hard X-ray), and SASE3 (soft X-ray) are in operation at the European XFEL serving six user instruments. Next stages of the facility development are installation of two undulator beamlines in empty tunnels SASE4 and SASE5 as middle term upgrade, and extension of the facility with the second fan of undulators as long term upgrade. Construction of soft X-ray beamlines is considered in both upgrade scenario. In the case of SASE4/SASE5 electron beam with energies 8.5 GeV - 17.5 GeV will be used in order to provide simultaneous operation of new undulator beamlines with existing SASE1-SASE3. One of the scenarios for a second fan of undulators involves using of low energy (2.5 GeV) electron beam. In this paper we analyze parameter space of soft X-ray SASE FELs driven by high energy and low energy electron beam, compare output characteristics, and discuss potential advantages and disadvantages