CiPo - Beamline

Insertion Device

The development of insertion devices able to produce circularly polarized radiation in the vacuum ultraviolet and soft-X ray regions open the possibility of a large variety of experiments, since both the absorption and the photoemission spectra may depend on the polarization state of the incident light. Classical experiments have been performed on magnetic materials but new results may involve also non-chiral and non-magnetic compounds. The panorama of the possible experiments allowed with circularly polarized soft-x ray radiation include magnetic and natural dichroism as well as magnetic extended X-ray absorption fine structure, spin-resolved photoemission, vacuum ultraviolet ellipsometry, gas phase spectroscopies.
The Electromagnetic Elliptical Wiggler (EEW) was designed to provide linearly and circularly polarized radiation over a wide range of photon energies, 5 - 1000 eV, using both undulator and wiggler modes of operation. The request for helicity switching led to a fully electromagnetic design which combines the horizontal and vertical periodic magnets into one open-side structure.
Under wiggler conditions the EEW operates for different values of the vertical and horizontal magnetic field providing an elliptically polarized radiation and a continuum soft x-ray emission spectrum extending from 40 eV to more than 1000 eV. In addition to the continuum, the EEW emits a spectrum of harmonics in the low photon energy side (<40eV), whose fundamental energy is at 4.5 eV when ELETTRA operates at 2.0 GeV. The degree of circular polarization changes with photon energy in the range from 40% at 8.5 eV to 80% at 575 eV. Full circular polarization condition (90%) is achieved when the EEW works as a pure circular undulator, i.e. when the horizontal and vertical magnetic fields are equal. Under these conditions the available first harmonic lies in the photon energy range between 50 eV and 150 eV.
The undulator mode allows to produce also linearly polarized radiation with the polarization vector parallel or perpendicular to the orbit plane. In fact switching off the horizontal magnetic field, linearly polarized radiation parallel to the orbit plane is produced, while switching off the vertical coils, the remaining field drive the stored electrons to oscillate in the direction perpendicular to the orbit plane emitting vertical linearly polarized radiation. In plane linear polarization of the emitted radiation is also available in wiggler mode.