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XFELs provide X-ray pulses with unprecedented peak brightness and ultrashortduration. They are usually driven by planar undulators, meaning that the outputradiation is linearly polarized. For many experimental applications, however,polarization control is critical: besides the ability to produce linearlypolarized radiation, one often needs the possibility of generating circularlypolarized radiation with a high, stable degree of polarization. This may beachieved by using a first part of the XFEL undulator to produce bunching andthen, by propagating the the bunched beam through an "afterburner" - a shortundulator with tunable polarization, where only limited gain takes place. Oneof the issues that one needs to consider in this case is the separation of thecircularly polarized radiation obtained in the radiator from the linearlypolarized background produced in the first part of the FEL. In this article wereview several methods to do so, including the inverse tapering technique. Inparticular, we use the Genesis FEL code to simulate a case study pertaining tothe SASE3 FEL line at the European XFEL with up-to-date parameters and weconfirm that a high degree of circular polarization is expected. Moreover, wepropose to further improve the effectiveness of the inverse tapering techniqueeither via angular separation of the linearly polarized radiation or stronglydefocusing it at the sample position. In this way we exploit the uniqueflexibility of the European XFEL from both the electron beam and the photonbeam optics side.

Circular polarization opportunities at the SASE3 undulator line of the European XFEL