  The possibility of producing highly circularly polarized X-ray radiation,
especially in the soft X-ray region, is an important asset at XFELs. However,
the baseline of the European XFEL, including the soft X-ray SASE3 line,
foresees planar undulators only. The lowest-risk strategy for implementing
polarization control at SASE3 involves adding an APPLE II-type undulator at the
end of the planar undulator, to exploit the microbunching from the baseline
FEL. 5m-long APPLE II undulators are standard devices at synchrotrons. However,
the choice of a short helical radiator leads to the problem of background
suppression. The driving idea of our proposal is that the background radiation
can be suppressed by spatial filtering. Slits can be inserted behind the APPLE
II radiator, where the linearly-polarized radiation spot size is about 30 times
larger than the radiation spot size from the helical radiator. The last 7 cells
of the SASE3 undulator are left with an open gap in order to provide a total 42
m drift section for electron beam and radiation. The linearly-polarized
radiation spot size increases along the drift, and the linearly polarized
background radiation can be suppressed by the slits. At the same time, the
microbunch structure is preserved. 100GW level coherent radiation is emitted in
the helical radiator. The filtering setup consists of a pair of water cooled
slits for X-ray beam filtering and of a 5m-long magnetic chicane, creating an
offset for slit installation immediately behind the helical radiator. Electrons
and X-rays are separated before the slits by the chicane, and the electron beam
can pass by the filtering setup without perturbations. Based on start-to-end
simulations we present calculations from the SASE3 undulator entrance up to the
radiator exit including the modulated electron beam transport by the FODO
focusing system in the low charge (20pC) mode of operation.

Accelerator Physics