The mechanical design, however, loses the advantage of a compact structure and also reduces the possibility to be used in superconducting magnets. A mechanical translation of one planar undulator in this configuration is therefore used to allow the LI mode . This configuration can also be used for high-field superconducting technology producing flexible polarizations except for the LI mode . A typical EM elliptically polarizing undulator is composed of two planar undulators and electrical current excitation in coils to generate two orthogonal magnetic fields with a π/2 phase shift . In contrast, an electromagnetic (EM) elliptically polarizing undulator allows fast polarization switching. Because of the narrow gap between the arrays, the motion must overcome large magnetic forces, resulting in slow polarization switching. It is worth noting, that a translation of the diagonal arrays in opposite directions is proposed to continuously rotate the linear polarization, also called the linear inclined (LI) mode . The device uses four permanent magnet (PM) arrays with a narrow gap between them and the polarization can be changed from horizontal linear (HL) through CP to vertical linear (VL) via mechanical translations of diagonal magnet arrays. Among them, the APPLE-II type, has been widely used due to its flexibility with respect to polarization changes and engineering considerations . To obtain circularly polarized (CP) light, numerous undulator configurations have been developed to generate elliptical magnetic fields . A typical undulator emits linearly polarized light from a sinusoidal magnetic field deflecting periodically the electron beam on a plane that is normal to the magnetic field. Over a half-century, the undulator concept has played a crucial role for the emission of intense quasi-monochromatic light from a synchrotron radiation facility or free electron laser (FEL). Developing a flexible polarized light source is therefore valuable and desired for scientific research. The experimental use requires to rotate linear polarized light with respect to the magnetization direction. In addition, an experimental technique called X-ray magnetic linear dichroism (XMLD) was developed to fill a growing research need on anti-ferromagnetism, which carries much promise for applications in nonvolatile logic and memory devices . Circularly polarized light, for example, has been widely used to probe electronic spin in ferromagnetic materials utilizing X-ray magnetic circular dichroism (XMCD) . Polarized light is a powerful tool to study atomic, molecular and electronic structures of materials .
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