The potential for extending the spectral range accessible to the European XFEL down to 0.05 nm

Specifications of the European XFEL cover a range of wavelengths down to 0.1 nm. The baseline design of the European XFEL assumes standard (SASE) FEL mode for production of radiation i.e. only one photon beam at one fixed wavelength from each baseline undulator with tunable gap. Recent developments in the field of FEL physics and technology form a reliable basis for an extensions of the mode of operation of XFEL facilities. This paper explores how the wavelength of the output radiation can be decreased well beyond the European XFEL design, down to 0.05 nm. In the proposed scheme, which is based on the use "fresh bunch" technique, simultaneous operation at two different wavelengths possible. It is shown that one can generate simultaneously, in the same baseline undulator with tunable gap, high intensity radiation at 0.05 nm at saturation, and high intensity radiation around 0.15 nm according to design specifications. We present a feasibility study and we make exemplifications with the parameters of SASE2 line of the European XFEL.