2016/08/29 by Svitozar Serkez, Gianluca Geloni, Serkez, Svitozar +5
Engineering · Physics and Astronomy · #Particle Accelerators and Free-Electron Lasers #Advanced X-ray Imaging Techniques #Crystallography and Radiation Phenomena
paper · pdf · doi:10.48550/arxiv.1608.08175
XFELs provide X-ray pulses with unprecedented peak brightness and ultrashort\nduration. They are usually driven by planar undulators, meaning that the output\nradiation is linearly polarized. For many experimental applications, however,\npolarization control is critical: besides the ability to produce linearly\npolarized radiation, one often needs the possibility of generating circularly\npolarized radiation with a high, stable degree of polarization. This may be\nachieved by using a first part of the XFEL undulator to produce bunching and\nthen, by propagating the the bunched beam through an "afterburner" - a short\nundulator with tunable polarization, where only limited gain takes place. One\nof the issues that one needs to consider in this case is the separation of the\ncircularly polarized radiation obtained in the radiator from the linearly\npolarized background produced in the first part of the FEL. In this article we\nreview several methods to do so, including the inverse tapering technique. In\nparticular, we use the Genesis FEL code to simulate a case study pertaining to\nthe SASE3 FEL line at the European XFEL with up-to-date parameters and we\nconfirm that a high degree of circular polarization is expected. Moreover, we\npropose to further improve the effectiveness of the inverse tapering technique\neither via angular separation of the linearly polarized radiation or strongly\ndefocusing it at the sample position. In this way we exploit the unique\nflexibility of the European XFEL from both the electron beam and the photon\nbeam optics side.\n