Three different approaches have been proposed so far for production of highly
monochromatic X-rays from a baseline XFEL undulator: (i) single-bunch
self-seeding scheme with a four crystal monochromator in Bragg reflection
geometry; (ii) double-bunch self-seeding scheme with a four-crystal
monochromator in Bragg reflection geometry; (iii) single-bunch self-seeding
scheme with a wake monochromator. A unique element of the X-ray optical design
of the last scheme is the monochromatization of X-rays using a single crystal
in Bragg-transmission geometry. A great advantage of this method is that the
monochromator introduces no path delay of X-rays. This fact eliminates the need
for a long electron beam bypass, or for the creation of two precisely
separated, identical electron bunches, as required in the other two
self-seeding schemes. In its simplest configuration, the self-seeded XFEL
consists of an input undulator and an output undulator separated by a
monochromator. In some experimental situations this simplest two-undulator
configuration is not optimal. The obvious and technically possible extension is
to use a setup with three or more undulators separated by monochromators. This
amplification-monochromatization cascade scheme is distinguished, in
performance, by a small heat-loading of crystals and a high spectral purity of
the output radiation. This paper describes such cascade self-seeding scheme
with wake monochromators. We present feasibility study and exemplifications for
the SASE2 line of the European XFEL