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