TESLA-FEL 2004-05 The Potential for Extending the Spectral Range Accessible to the European X-ray Free Electron Laser in the Direction of Longer Wavelengths

The baseline specifications of European XFEL give a range of wavelengths between 0.1 nm and 2 nm. This wavelength range at fixed electron beam energy of 17.5 GeV can be covered by operating the SASE FEL with three undulators which have different period and tunable gap. A study of the potential for the extending the spectral range accessible to the XFEL in the direction of longer wavelengths is presented. The extension of the wavelength range to 6 nm would cover the water window in the VUV region, opening the facility to a new class of experiments. There are at least two possible sources of the VUV radiation associated with the X-ray FEL: the ”low (2.5 GeV) energy electron beam dedicated” and the ”17.5 GeV spent beam parasitic” (or ”after-burner”) source modes. The second alternative, ”afterburner undulator” is the one we regard as the most favorable. It is possible to place an undulator as long as 80 meters after 2 nm undulator. Ultimately, VUV undulator would be able to deliver output power approaching 100 GW level. A beam from this device could be run in pump-probe mode with the X-ray FEL.