  The LCLS baseline includes a planar undulator system, producing linearly
polarized light in the range 0.15-1.5 nm. Polarization control in the soft
X-ray region from linear to circular is highly desirable. Several schemes using
helical undulators have been discussed for the LCLS. One consists in replacing
three of the last planar undulator segments by APPLE III. A second proposal,
the 2nd harmonic helical afterburner, uses short, crossed undulators tuned to
the second harmonic. This last scheme is expected to be the better one. Its
advantages are a high and stable degree of circular polarization and a low
cost. Its disadvantage is a small output power and a narrow wavelength range.
We propose a novel method to generate 10 GW level power at the fundamental
harmonic with 99% degree of circular polarization from the LCLS baseline. Its
merits are low cost, simplicity and easy implementation. After the baseline
undulator, the electron beam is sent through a 40 m long straight section, and
subsequently passes through a short APPLE II radiator. The microbunch structure
is easily preserved, and intense coherent radiation is emitted in the helical
radiator. The background radiation from the baseline undulator is suppressed by
letting radiation and electrons through horizontal and vertical slits upstream
the helical radiator, where the radiation spot size is about ten times larger
than the electron bunch transverse size. Using thin Beryllium foils for the
slits the divergence of the electron beam halo increases by Coulomb scattering,
but the beam will propagate through the setup without electron losses. The
applicability of our method is not restricted to the LCLS baseline setup. LCLS
II or the European XFEL may benefit from this work as well, due to availability
of sufficiently long free space at the end of undulator tunnel.
