  The magnetic gap of the baseline XFEL undulators can be varied mechanically
for wavelength tuning. In particular, the wavelength range 0.1 nm - 0.4 nm can
be covered by operating the European XFEL with the SASE2 undulator. The length
of the SASE2 undulator (256.2 m) is sufficient to independently generate three
pulses of different radiation wavelengths at saturation. Normally, if a SASE
FEL operates in saturation, the quality of the electron beam is too bad for
generation of SASE radiation in the subsequent part of undulator which is
resonant at a few times longer wavelength. The new method of SASE
undulator-switching based on the rapid switching of the FEL amplification
process proposed in this paper is an attempt to get around this obstacle. Using
mechanical SASE shutters installed within short magnetic chicanes in the
baseline undulator, it is possible to rapidly switch the FEL photon beam from
one wavelength to another, providing simultaneous multi-color capability.
Combining this method with a photon-beam distribution system can provide an
efficient way to generate a multi-user facility.
