  Self-seeding schemes, consisting of two undulators with a monochromator in
between, aim at reducing the bandwidth of SASE X-ray FELs. We recently proposed
to use a new method of monochromatization exploiting a single crystal in
Bragg-transmission geometry for self-seeding in the hard X-ray range. Here we
consider a possible extension of this method to the soft X-ray range using a
cell filled with resonantly absorbing gas as monochromator. The transmittance
spectrum in the gas exhibits an absorbing resonance with narrow bandwidth.
Then, similarly to the hard X-ray case, the temporal waveform of the
transmitted radiation pulse is characterized by a long monochromatic wake. In
fact, the FEL pulse forces the gas atoms to oscillate in a way consistent with
a forward-propagating, monochromatic radiation beam. The radiation power within
this wake is much larger than the equivalent shot noise power in the electron
bunch. Further on, the monochromatic wake of the radiation pulse is combined
with the delayed electron bunch and amplified in the second undulator. The
proposed setup is extremely simple, and composed of as few as two simple
elements. These are the gas cell, to be filled with noble gas, and a short
magnetic chicane. The installation of the magnetic chicane does not perturb the
undulator focusing system and does not interfere with the baseline mode of
operation. In this paper we assess the features of gas monochromator based on
the use of He and Ne. We analyze the processes in the monochromator gas cell
and outside it, touching upon the performance of the differential pumping
system as well. We study the feasibility of using the proposed self-seeding
technique to generate narrow bandwidth soft X-ray radiation in the LCLS-II soft
X-ray beam line. We present conceptual design, technical implementation and
expected performances of the gas monochromator self-seeding scheme.
