CiPo - Beamline

Overview

The CiPo Beamline has been designed and constructed, in order to make available to the users the widest photon energy range (5 - 1000 eV), with high flux and control of the light polarization (circular and linear). The Beamline consists of two different monochromators (NIM and SGM),  which share the entrance and exit slits as well as the pre- and post-focusing optics, keeping fixed the position of the focus on the sample.

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.

Beamline Description

The CiPo Beamline has been designed and constructed, in order to make available to the users the widest photon energy range (5 - 1000 eV), with high flux and control of the light polarization (circular and linear). The Beamline consists of two different monochromators (NIM and SGM, i.e. G’ and G respectively),  which share the entrance and exit slits (S1 and S2) as well as the pre- and post-focusing optics (M0, M1 and M3) as sketched in the figure. The normal incidence monochromator (NIM, i.e. G’), covers the photon energy range of 5 - 35 eV using two different holographic gratings (Gold and Aluminum). The grazing incidence monochromator (SGM, i.e. G), covers the energy range 30 - 1000 eV, working in Padmore-type configuration and is equipped with a variable angle plane mirror (M2) and four spherical gratings (G1 to G4). Switching between the two monochromators is obtained by simply inserting or removing one mirror (M’2). The beamline accepts a maximum emission angle of (2 hor. x 1 vert.)mrad2 of the radiation that is focused in the vertical plane by means of plane (M0) and toroidal (M1) mirrors onto the entrance slit (S1). Both mirrors of the focusing optics have 2.5° grazing incidence and return the radiation parallel to the orbit plane. After the exit slit (S2) the radiation is focussed at the sample position with a toroidal mirror (M3).

Spherical Grating Monochromator (SGM)

The beamline SGM monochromator covers the energy range 40 - 1000 eV, working in Padmore-type configuration and is equipped with a variable angle plane mirror and four different spherical gratings (G1 to G4), that cover the following energy ranges:

• G1 grating (1050 l/mm) from 440 to 1000 eV

• G2 grating (1050 l/mm) from 300 to 700 eV

• G3 grating (1050 l/mm) from 120 to 400 eV

• G4 grating (550 l/mm) from 40 to 140 eV

The photon flux avaiable on the sample, as well as the photon energy resolution, is crucial for the proper evaluation of the feasibility of a given experiment. In the following figures the photon energy band width togheter with the corresponding value of the photon flux is plotted as a function of the entrance and exit slits width. The data reported have been measured at 245 (Ar L2,3 absorption threshold) and 867 eV (Ne K absorption threshold) were resolution has been determined from the Gaussian brodening obtained with a fitting procedure performed using Voight functions while the photon flux was determined by using the double ion chamber.

Grating #	Energy (eV)	Res. Power
G1	850	3400
G2	400	7600
G3	240	4400
G4	65	8100

Normal Incidence Monochromator (NIM)

The beamline NIM monochromator equipped with two different holographic gratings, covers the 5 - 35 eV energy range by means of two different gratings, as follows:

• Gold coated grating (2400 l/mm) from 10 to 35 eV

• Aluminum coated grating (1200 l/mm) from 5 to 15 eV

The photon energy resolution at relevant energies of fundamental absorption line of He (21.2 eV) and Neon (16.7 eV) gases shows a FWHM of  about 4meV at He line and 2meV at Ne absorption structure with 20 um slit apertures. The FWHM is influenced by the photon bandpass and the contribution from the gas, which is made of the natural linewidth plus the experimental parameters such as gas pressure, effective light-path etc.. A reasonable value for the gas contribution to the FWHM could be considered of the order of 2 meV for the He line and of about 1 meV in the case of  Ne. Using these consideration, the available resolving power is of about 6000 at 21.2 eV and 10000 at 16.7 eV.

Circular Polarization

Measurements of the polarization were made using a multilayer polarimeter at several photon energies between 92 and 573 eV. The measured linear and circular polarization rates as a function of the horizontal deflection parameter (i.e. EEW horizontal current I_h) for wiggler mode are shown in the figures. At 92.5 eV a comparison  between undulator and wiggler mode of operation is reported. In the undulator case a maximum circular polarization rate close to unit can be obtained with equal vertical and horizontal deflection parameters (K_x=K_y), whereas in the wiggler case smaller values of the circular polarization rate are obtained due to the ratio K_x/K_y always less than unit. In both cases reversing the horizontal magnetic field direction results in a change of the helicity of the radiation as expected. The photon energy range below 40 eV is available with the characteristic discrete line spectrum emitted by an undulator.
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.

Switching Mode

Because of the specificity of the CiPo Beamline to measure the dichroic behavior of materials, it was very important to provide a mode of operation that allows to minimize uncertainty introduced in the dichroic spectra by the random errors of monochromator repositioning. This is especially required at the edges of absorption of materials, where rapid change in the signal intensity may introduce false dichroic signal when scanning sequential absorption spectra alternating the right- and left–hand status of polarization of the photon beam. Handness switching of circular polarization is often important in the case of  XMCD measurements in magnetic materials where the reversing of the magnetic field in not always reproducible, but it is crucial in the case of PECD in which the effect is very small.
The EEW could produce alternatively a right- and left-handed circularly polarized radiation at a frequency of 0.1 or 0.05 Hz operating in wiggler or ondulatror mode. A square waveform is generated to drive the horizontal current power supply allowing to switch between +260 A and -260 A, producing the change of the polarization handness. After a switching time, a definite polarization state remain in the plateau of the square wave, during this steady state period the measurement occur before changing to the new handness and starting a new set of measurements. The sequence may be repeated for several times until the desired signal to noise ratio is reached. The timing of the measured signals to the polarization state is controlled by a suitable reference status voltage level from the EEW power supply. The measured signal from the sample is normalized to the incident photon flux detected through the photocurrent emitted by the last mirror gold surface or by a photodiode.