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Materials Science Beamline
The Materials Science Beamline (MSB) has been attached to the bending magnet exit 6.1 of the Elettra storage ring. The tuning range is from 35 to 950 eV with a theoretical spectral resolving power E/DE better than 4,000 in the whole tuning range. The design is based on a standard plane grating instrument (SX-700) as this permits wide scanning of the energy.
The first optical element is a toroidal mirror that focuses the light sagittally onto the entrance slit and tangentially onto the exit slit. The next element is a polarization aperture that is movable in the vertical direction and has variable opening to select photons of linear or elliptical polarization. The plane pre-mirror determines the angle of incidence of the beam on the plane grating, while the spherical mirror focuses the diffracted light onto the exit slit. The light beam is refocused to the specimen by means of a toroidal mirror that deflects the light beam in the vertical plane.
The theory of plane gratings has been covered adequately elsewhere. The present instrument works in fixed focus mode, with a spherical rather than the elliptical mirror, which was part of the original SX-700 optical concept. The coma term in the optical path has been minimised by setting the demagnification of the spherical mirror to 1.
The constant Cff was set to 2.4. This choice was based on diffraction efficiency optimization of the plane grating in a wide range of photon energies.
The optical parameters were tuned to optimize flux, focal spot size and resolution. Simulations took into account source brightness, geometrical losses, specular reflectivity of mirrors and diffraction efficiency of the grating. The expected beam spot at the sample has a size of 0.3×0.3 mm2 (sx×sz) approximately. The expected photon flux at the sample for a resolving power of 2,000 and 100 mA ring current is 1.9×1011 photons per sec at 100 eV.
The MSB monochromator contains three optical elements: the plane pre-mirror, plane grating and spherical mirror, all elements were placed in one chamber. This reduces the costs of chamber fabrication and pumping, and makes the design more compact.
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