Polarization Based Out-Coupling for Cavity Based X-ray FELs

Cavity Based X-ray Free-Electron Lasers (CBXFELs) promise fully 3D coherent, very brilliant and shot-to-shot stable X-ray pulses. For CBXFELs the X-ray radiation is trapped in an X-ray optical cavity, which is formed using Bragg-reflecting crystal mirrors. As is the case for laser systems in the optical regime, a major question for the CBXFELs is how to couple out the radiation from the cavity. Possibilities range from employing semi-transparent thin crystals, over manipulation of the electron phase space density to cavity dumping schemes. In this work, making use of the strong polarization of Bragg reflection shall be studied for out-coupling. As the radiation does not change its polarization during Bragg reflection, reflection in the direction of the polarization vector is suppressed. By adjusting the 3D orientation of the crystals with respect to the polarization axis of a linearly polarized undulator, or vice versa, the transmission through the crystals can be tailored to some degree independently of the crystal thickness.