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NanoESCA beamline description

Insertion device

The NanoESCA beamline  shares the insertion device with the Nanospectroscopy beamline. Based on the Sasaki Apple II scheme, it consists of two identical undulator sections (Elettra insertion devices 1.1 and 1.2) and a phase modulation electromagnet arranged in an optical klystron configuration. This enables the two undulators to be properly phased, thus effectively doubling the undulator length and the useful flux. The insertion device is able to provide elliptically polarized light (circular left and right as well as linear horizontal and vertical as special cases) in a spectral range extending from 50 eV to 1000 eV, with high brilliance (using the first, third and fifth undulator harmonic). We emphasis the undulator capability of helicity inversion, which allows performing XMCD (X-Ray Magnetic Circular Dichroism) measurements. The optimum phase for measurements requiring circular polarisation can be found here.

Beamline Layout

The light source is the middle-point (4) between the undulator two sections (1) and (2) with phase modulator electromagnet (3).At 10 m from the source middle position, the pinhole (5) sets the beamline angular acceptance and stops unwanted radiation from the undulator.The toroidal mirror (6) demagnifies the source by a factor of 8 in the horizontal plane and 5.3 in the vertical. The entrance slits are located at the horizontal (7) and vertical (8) foci of the toroidal mirror. The slit (7) becomes the virtual source for all of the following mirrors in the horizontal plane. The light is then dispersed by the monochromator (9),which also determines a further vertical demagnification by a factor 1.7. After the exit-slit (10), a retractable plane mirror (11) allows switching operation between the two branches of the beamline. The refocusing mirrors are two bendable elliptical cylinder mirrors arranged in a Kirkpatrick-Baez geometry. They are located in dedicated vacuum chambers (12). The SPELEEM microscope (13). The NanoESCA  photoemmission microscope (14). On the beamline first branch, the demagnification introduced by these elements is 11.5 in the horizontal direction and 5 in the vertical direction. For the NanoESCA (second branch), the demagnification factors are 13.9 and 7.6, for the horizontal and the vertical direction respectively.


Monochromator

The beamline monochromator uses the Variable Line Space (VLS) plane grating architecture. This solutions allows a large energy range can be covered with a relatively small change in the resolving power. Only two gratings are used to cover the energy range from 50 eV to 1000 eV. This is achieved by coupling the rotation of the grating with preceding plain mirror. The first grating has 200 l/mm and covers the energy range from 50 to 250 eV, while the second grating (400 l/mm) covers the range from 200 to 1000 eV. The calculated resolving power for the two VLS plane gratings is reported on the left hand side of the figure below. The full line is for the 200 l/mm grating and the dotted one for the 400 l/mm grating. The graph shows the measured spectrum of the nitrogen 1s to 1π* absorption from which we deduce a resolving power of 4000 at 400.8 eV, in agreement with the calculations.


Beamline flux

The experimental flux curves for a storage ring energy of 2.0 GeV are shown below for linear horizontal and circular polarisation (corresponding to the undulator phase set to 0 mm and 35 mm, respectively), normalized to 200 mA ring current. The data were collected with a photodiode inserted after the exit slit, which was opened to 10 μm (best energy resolution). In the case of the horizontal polarisation, the beamline delivers more than 1012 ph/s in an energy range extending from 50 to 600 eV. The maximum value of 1.8 x 1013 ph/s is reached at 145 eV. When considering the photon flux on the sample, the above figures have to be decreased by a factor of 0.7 for the first branch and 0.5 for the second one.


Refocussing optics

The micro spot size on the NanoESCA has been first measured and calibrated using an imaging setup consisting of a YAG crystal, a microscope objective and a visible CCD camera. At 140 ev, the total demagnication factor of the beamline for this branch is 110 and 70 for the horizontal and the vertical direction respectively. The best beamspot measured has a FWHM of 7.2x3.5 μm (HxV). Note that due to grazing incidence of the light onto the sample in the PEEM microscope, the effective beamspot size is 7x10 μm. The beamspot can be enlarged by a factor 2, without noticing a drastic change in the shape. Above that, the beamspot starts to be very inhomogeneous and affected by striations.


 

Last Updated on Tuesday, 10 January 2012 17:35