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TY - CONF AU - Schneidmiller, E. AU - Yurkov, M.V. ED - Petit-Jean-Genaz, Christine ED - Kim, Dong Eon ED - Kim, Kyung Sook ED - Ko, In Soo ED - Schaa, Volker RW TI - Reverse Undulator Tapering for Polarization Control at XFELs J2 - Proc. of IPAC2016, Busan, Korea, May 8-13, 2016 C1 - Busan, Korea T2 - International Particle Accelerator Conference T3 - 7 LA - english AB - 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 afterburner, but then one should have an efficient method to suppress powerful linearly polarized background from the main undulator. We describe a method for such a suppression: an application of the reverse taper in the main undulator*. 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. The scheme was successfully tested at LCLS** and is routinely used in user experiments. In this contribution we present the theoretical description of the method as well as the results of experiments with reverse taper at FLASH2. PB - JACoW CP - Geneva, Switzerland SP - 722 EP - 724 KW - undulator KW - FEL KW - resonance KW - radiation KW - bunching DA - 2016/06 PY - 2016 SN - 978-3-95450-147-2 DO - 10.18429/JACoW-IPAC2016-MOPOW008 UR - http://jacow.org/ipac2016/papers/mopow008.pdf ER -