  This paper describes a novel single-bunch self-seeding scheme for generating
highly monochromatic X-rays from a baseline XFEL undulator. Previous schemes
made use of a four-crystal fixed-exit monochromator in Bragg geometry. In such
monochromator the X-ray pulse acquires a cm-long path delay, which must be
compensated. For a single-bunch self-seeding scheme this requires a long
electron beam bypass, implying modifications of the baseline undulator
configuration. To avoid this problem, a double bunch self-seeding scheme based
on a special photoinjector setup was recently proposed. At variance, here we
propose a new time-domain method of monochromatization exploiting a single
crystal in the transmission direction, thus avoiding the problem of extra-path
delay for the X-ray pulse. The method can be realized using a temporal
windowing technique, requiring a magnetic delay for the electron bunch only.
The proposed setup is extremely simple and composed of as few as two simple
elements. These are the crystal and the short magnetic chicane, which
accomplishes three tasks by itself. It creates an offset for crystal
installation, it removes the electron micro-bunching produced in the first
undulator, and it acts as a delay line for temporal windowing. Using a single
crystal installed within a short magnetic chicane in the baseline undulator, it
is possible to decrease the bandwidth of the radiation well beyond the XFEL
design down to 10E-5. The installation of the magnetic chicane does not perturb
the undulator focusing system and does not interfere with the baseline mode of
operation. We present feasibility study and exemplifications for the SASE2 line
of the European XFEL.
