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Beamline Description

The polarization
 


 

The source of BEAR is the ELETTRA bending magnet 8.1 (at the ring energy E = 2 GeV the critical energy is 3200 eV, at E = 2.4 GeV the critical energy is 5500 eV). The source properties are related to the electronic beam properties and to the natural divergence of the synchrotron radiation beam, that depends on the ratio between the critical energy and the photon energy. The polarization properties of the radiated field from bending magnet are also dependent on the photon energy and the vertical emission angle respect to the plane orbit. Ideally, the synchrotron radiation light in the orbit plane is linearly polarized, while is circularly polarized at a infinite distance from the orbit plane (respectively right circular and left circular over and below the orbit plane). A double vertical slit stage (see the figure on the right) can be used to deliver alternatively linear, right or left circular polarization light.

 

The bending magnet source


The polarization state of the BEAR beam has been measured at different photon energies. The polarization state of the light can be selected by moving the polarization selector, a double slit device that can be moved vertically to define the vertical acceptance of the bending magnet source. The acceptance is defined by the position of the slits and is given by two parameters: Zmin related to the position of the lower edge, and Delta Z related to the position of the guilliottine edge. The measurements have been performed in linear polarization configuration, i.e. with the polarization selector centred respect to the beam source vertical position, in elliptical (right or left) polarization configuration, i.e. with the polarization selector shifted respect to the beam source vertical position, or in full beam configuration.  The measurements have been done  by a polarimetric scan, i.e. by measuring the reflectivity of mirrors as a function of the  angle between the experimental chamber (PsiC) and the orbit plane, in correspondence of the Brewster angle where the polarizing power of the mirrors Rs/Rp is maximum. In the UV range, we used as a mirror a   LiF thick slab,  while in the EUV-soft-x-ray range measurements have been made using multilayer interferential mirrors (for example Mo/Si in the EUV range). The least square fitting analysis  of experimental results allowed to determine the Stokes parameters of the beam S0, S1 and S2 representing respectively the absolute incoming intensity, the degree of linear polarization of light in the planes at 0° and 45°, while the absolute value of S3, representing the degree of circular polarization, wass calculated assuming the beam completely polarized. The degree of linear polarization LP is given by
LP=(S12+S22)0.5/S0, while the degree of circular polarization CP is given by CP=S3/S0.
Samples for polarimetry measurements are listed here;
E<11 eV: LiF window
E=150 eV: samples Pd/Y (ref. Jonnard)
E=285 eV: sample W/Si (ref. Kortright)
E=400 eV: sample 09057 (ref. Kortright)
E=650 eV: sample Cr/B4C  MP05108 (Institute d'optique - ref. Delmotte)


multicols} 


Layout of the polarization selector

The polarization selector

 

Linear polarization
Energy (eV) Wavelength (nm) Aperture (mm) Zmin DZ S1/S0 S2/S0 DCP=|S3|/S0 DLP E=Ey/Ez
4 310                
5 240                
7.9 156.96 12 60 42 0.89 0.01 0.44 0.89 4.3
10.3 120.39 12 60 42 0.85 0.08 0.51 0.86 3.6
100 12.39 4 64 50 0.97 -0.02 0.24 0.95 8
285 4.35 4 64 50 0.96 0.01 0.29 0.96  
288.5 4.30 4 64 50 0.93 0.1 0.35 0.93  
650   4 65 50 0.97 0 0.23 0.97 8.3

 

Circular polarization
Energy (eV) Wavelength (nm) Aperture (mm) Zmin DZ S1/S0 S2/S0 DCP=|S3|/S0 DLP E=Ey/Ez
680 1.82 40 68 14.5 0.61 -0.02 0.79 0.61 2.02

 

Full beam
Energy (eV) Wavelength (nm) Aperture (mm) Zmin DZ S1/S0 S2/S0 DCP=|S3|/S0 DLP E=Ey/Ez
11 112.7 40 45.8 14.5 0.71 0.02 0.70 0.71 2.43
100 (Si filter) 12.39 4 45.8 14.5 0.6 -0.02 0.79 0.6 2.02
100 (no filter) 12.39 4 45.8 14.5 0.6 -0.02 0.79 0.6 2.0

 
 

Last Updated on Thursday, 12 October 2017 18:25