Polarization properties of a crossed planar undulator

Instead of using a helical undulator, another method to generate circularly polarized radiation in an FEL is to utilize two crossed planar undulators. The main benefit of the crossed undulator over the helical option is that its structure is simpler so that it is technically and economically more convenient. Unlike a helical undulator, two orthogonal polarizations are respectively generated from two crossed undulators and their combination comprises circularly polarized radiation. Therefore, the overlap and resemblance of the two polarizations determine the quality of the composed radiation field. The 3D simulations have been performed with parameters from the European XFEL. Polarization has been studied in detail and optimization of the crossed undulator length has also been done.