  Baseline design of a typical X-ray FEL undulator assumes a planar
configuration which results in a linear polarization of the FEL radiation.
However, many experiments at X-ray FEL user facilities would profit from using
a circularly polarized radiation. As a cheap upgrade one can consider an
installation of a short helical (or cross-planar) afterburner, but then one
should have an efficient method to suppress powerful linearly polarized
background from the main undulator. In this paper we propose a new method for
such a suppression: an application of the reverse taper in the main undulator.
We discover that in a certain range of the taper strength, the density
modulation (bunching) at saturation is practically the same as in the case of
non-tapered undulator while the power of linearly polarized radiation is
suppressed by orders of magnitude. Then strongly modulated electron beam
radiates at full power in the afterburner. Considering SASE3 undulator of the
European XFEL as a practical example, we demonstrate that soft X-ray radiation
pulses with peak power in excess of 100 GW and an ultimately high degree of
circular polarization can be produced. The proposed method is rather universal,
i.e. it can be used at SASE FELs and seeded (self-seeded) FELs, with any
wavelength of interest, in a wide range of electron beam parameters, and with
any repetition rate. It can be used at different X-ray FEL facilities, in
particular at LCLS after installation of the helical afterburner in the near
future.
