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Beamline Description

The beamline


The optical layout of the BEAR beam line is based on the PMGM (Plane Mirror Plane Grating) configuration [see G. Naletto et al., Pure Applied Optics 1, 357, 1992]. Any movement of the beam is monitored by a photon beam position monitor (BPM) installed at the beginning of the beamline. After this, the beamline is equipped with a device for selecting the polarization of light. The PMGM configuration is based mainly on a first parabolic mirror, collimating the light emitted by the BM source without any entrance slit, a monochromator working in parallel light at variable deviation angle, a second parabolic mirror focusing the dispersed beam onto the exit slit, and finally an elliptical mirror focussing the beam on the sample. The layout has been conceived to be theoretically aberration free and to have the non plane optics in sagittal focusing. This configuration reduces the slope error aberration effects on the dispersion plane of a factor 1/cos. The polarisation effects introduced by the sagittal focusing of the non plane optics are compensated by the tangential focusing of the monochromator plane optics. The bending magnet light is collimated on the monochromator by a platinum coated mirror, working in sagittal focusing. The mirror is placed at 12059 mm from the source. Eventual movements of the source are monitored by a Photon Beam Position Monitor. The monochromator is based on two different channels working in parallel light: the grazing incidence channel working in the 40-1600 eV energy range, and the normal incidence channel working in the 3-50 eV energy range. Both channels are mounted on the same mechanical stage, whose horizontal translation allows to select the different channel. A second paraboloidal mirror P2 collects the radiation coming from the monochromator focusing it onto the exit slits. The monochromatized beam is focused into the exit slits. The beam is focused on the sample by an ellipsoidal mirror (REFO). The intensity of the light on the sample is monitored on the monitor section.


Layout of the BEAR beamline


 


The beam position monitor
 

 A photon beam position monitor (BPM) is installed at the the beginning of the beamline. The device is based on four insulated Mo photoemitting plates, installed upstream of the beamline optics at 12 meters from the source. The current drained from the plates, typically of the order of some tens of uA, is read by four floatable ammeters. The BPM is currently used in differential mode operation, i.e. reading the normalized difference between up and down plates S=(Iup-Idown)/(Iup+Idown). This mode features a sensitivity of the order of about 1 um. A quantitative model connecting the energy shift with beam height was deduced (see A. Giglia et al., The beam position monitor of the BEAR beamline,  Rev. Sci. Instrum. 76, 063111 (2005).


Picture of the Photon BPM


 

BEAR is installead at the 8.1 exit of Elettra. The optical layout is not based on entrance slits. The front end is equipped with a pinhole copper mask at 2.6 meters from the synchrotron source. The left pinhole (see picture) corresponds to the Bear exit.


Picture of the pinhole



 


The monochromator

The grazing incidence channel is a based on a monochromator working in parallel light with variable deviation angle. The grazing channel is composed by a plane mirror and two plane laminar gratings G1200 and G1800, respectively with 1200 and 1800 ll/mm. The mirror collects the plane radiation from the first paraboloidal mirror. The monochromator works in the internal first order configuration. The PMGM optical layout allows to deliver the same wavelength following different Deviation Angles (the Deviation Angle is the angle between the incident and the reflected beam from the mirror), for the optimization of the flux or for higher order rejection. The first curve optimizes the experimental flux on the sample, the second one is the curve that gives the maximum flux, assuming that the calculated ratio given by the higher orders and the first order is less than 1%. According to the PMGM scheme, the plane mirror rotates around an axis far from the centre of the mirror and at different positions of the mirror the deviated ray doesn’t reach the same position on the grating surface. This fact suggests not to use the mirror rotation for energy scansion, but it’s better to place M1 in the optimal position and only rotate gratings. The two gratings rotate around an axis placed in the centre of its optical surface. The plane laminar grating G1200 works in the energy range 40–1400 eV, while G1800 is for the energy range 600 – 1400 eV.
The normal incidence channel works in the 2.7–50 eV spectral range. The channel is composed by a plane mirror and a plane grating GNIM with 1200 ll/mm. The normal incidence channel works also in parallel light but at fixed deviation angle of 30° (that is, the radiation illuminates the plane mirror M2 with an incidence angle equal to 15°). Then, differently from the grazing incidence, this channel has only one degree of freedom, that is the rotation of the grating around the centre of its surface. The grating is optimised to work at 40 eV with the incidence angle of 16.10°.

 
 

The grazing incidence channel

 
Layout of the NIM channel

The normal incidence channel


The monitor section
 

News!


The monitoring of the beam intensity now is routinely done by reading the current from the last Zerodur platinum coated REFO mirror. The current can be read or directly from the mirror or from two +100 V Mo plates placed in front of the mirror. The comparison between the REFO current and the Gold mesh shows that in the soft x-rays we gained about two orders of magnitude in the signal intensity. Moreover, the removal of the mesh improves the quality of the focused beam in particular relation to the diffraction introduced by the mesh in particular in the UltraViolet range.      




    

The monitor section is placed between the refocusing chamber and the experimental endstation. It's destinated to the monitoring of the light intensity, but it is also equipped with a gold coated mirror for single experiment brenchline (for experiments with E<150 eV). The monitoring of intensity is done mainly by reading the current from a gold mesh placed transversally to the beam (usually measurements are done biasing the mesh at -100V in saturation mode). The current of the mesh can be used also as an absolute intensity monitor, using the calibration law obtained recording it simultaneously with a silicon detector AXUV-100.
In case of low signal, the emission current from the mesh is read by CEM (SJUTS model KBL10RS) placed laterally respect to the mesh. In the Visible-UV range, the monitor section is also equipped with a beam splitter, based on a CaF2 flat window and a silicon detector IRD SXUV-300. This detector can be used also as intensity monitor for the branchline.

 

    

Gold mesh parameters
Nominal aperture 0.25 mm
wires/inch 82x82
open area 65%
wire diameter 0.06 mm

   

 

This curve shows the calculated ratio between the photon flux and the mesh current. Click here to access numerical values.


Estimated Photon flux for typical values of mesh current

 
   


Picture of the monitor section (right view)

  


Picture of the monitor section (left view) with the branch line

    


The photon flux
 

 

The photon flux on the sample has been measured with a calibrated silicon diode AXUV IRD-100. The photon flux as a function of energy has been measured for two limit cases: Maximum flux and Higher order rejection. For energies greater than 100 eV (G1200 grazing incidence channel), the maximum flux condition is obtained following the deviation angle curve that maximizes the diode current intensity, while Higher order rejection corresponds to the deviation angle that maximizes the photon flux, but with higher orders contribution less than 1%. Below 100 eV, the curves ‘High spectral purity’ are obtained inserting the appropriate filters. The beamline is in fact also equipped with a carousel for choosing the appropriate filter and window for delivering high purity radiation. 



Experimental Photon Flux on  the sample
(E=2.4 GeV, Elettra current 100 mA, Vertical slits = 100 um)


Monochromator working curves used to optimize flux or high purity radiation


 

The windows and filters for high purity radiation

 

Available windows and filters
(*: filters supported by Ni mesh with nominal transmittance of 0.86)

Filter/Window (Thickness)

Energy
range (eV)

Wavelength
range (nm)
B270 window (1 mm) 2.7-4 310-459.2

Pyrex window (4 mm)
(Refo Valve)

2.7-4

310-459.2

Fused silica (quartz)
SiO2 window(1 mm)

3.9-7.8

159-317.95

LiF window (0.5 mm)

5.6-11.2

110.7-221.4

In filter (0.1 μm)*

12-17

72.94-103.3

Sn filter (0.15 μm)*

15-24

51.7-82.7

Al filter (0.2 μm)*

36.25-72.5

17.1-34.2

Si filter (0.5 um)*

50-100

12.4-24.8

Ag filter(0.6 μm)

200-400

3.1-6.2

Ag filter(1.0 μm)

200-400

3.1-6.2


                           Nominal transmittance of available windows and filters


The resolution
 


The resolution of the monochromator can be experimentally measured by measuring the broadening of gas absorption lines by means of a Samson type ionization chamber, placed between the exit slit stage and the refocusing chambers. The resolution is calculated as the ratio between the photon energy E and the FWHM width of the Gaussian curve, shown in the Figures below. The resolution of the monochromator E/DE for the grazing incidence channels at a given energy depends on the vertical slits aperture and on the deviation angle, while for the GNIM depends only on the slit aperture. The experimental resolution has been experimentally determined at about 400 eV by measuring the K-shell vibrational states 1s->p* of biatomic nitrogen, at about 867 eV  by the K-shell excitations of atomic Ne, at 60 eV the autoionizing states of double excited He for the grazing incidence channels, the Rydberg states of atomic He at about 20 eV for the GNIM channel.


Experimental resolution of G1200 channel (High flux configuration)


Experimental resolution of G1200 channel (Higher order rejection configuration)

Experimental resolution of G1800 channel (High flux configuration)
 


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

 
 


The end stations
 

The experimental apparatus is mainly based on two end stations and the transfer stage. The spectroscopy chamber is mainly dedicated to the synchrotron analysis of samples, the preparation chamber to the preparation of samples. The transfer stage allows to insert samples and to transfer them between the two chambers, always remaining in UHV environment (P=1e-10 mbar). The end stations are equipped with these instruments:

 


Picture of the whole experimental station: insertion stage, preparation chamber and experimental endstation


 


 

Last Updated on Thursday, 12 October 2017 18:25