Optical elements
distance from source
First mirror
two strips Si and Rh
~=26.6 m
Monochromator
pseudo channel-cut type
~=28.3 m
Second mirror
two strips Si and Rh
~=33.1 m
focusing type

 

vertical collimation

 

dynamical sagital focusing vertical focusing
horizontal acceptance
spectral range
2 mrad
5.0-35 KeV
flux at sample 1012ph/s
(beam current 0.2 A)
nominal beam
size at
A1 A2
(H x V) ~=0.1 x 0.1 mm2 ~=0.1 x 0.1 mm2


 The optics (see figure) accept the 2 mrad horizontal aperture and produce a small focal spot at different sample positions. The beam is first vertically collimated by a one meter long cylindrically bent mirror of fixed meridional radius. The horizontal focusing is achieved by the second monochromator crystal through a sagittal cylindrical bending. By a second vertically bendable mirror the beam can be focused to different sample positions 40-45 meters away from the source. The XAS station is allocated at the sample position A1 (40m), while the HRPD station is at the sample position A2 (45 m). All optical components of the beamline operate under vacuum. A fixed Be (500 microns) window at the front end is used to separate vacuum between the beamline and the electron ring. Therefore the effective cut-off energy is around 4 keV. In order to reduce the influence in the region of low energy (increasing the real cut-off energy) vacuum gate valves with a beryllium disk are used to separate the different components. In some special cases, if necessary and if the vacuum conditions are good the Be windows can be opened for maximum flux. The first mirror (collimating mirror) with a constant curvature radius is coated with Rh and Si. The angle of incidence (glancing angle) is set at 2.5 mrad, which corresponds to a cut-off energy of 26.8 keV for the Rh coating and 12.4 keV for Si. Due to the high power loads of the ESRF beams (~ 150 Watts) cooling of the mirror is mandatory.

 Branch A

Schematic representation of beam path for the configuration of Branch A. Dimensions are given in millimeters

 

   The monochromator is of the pseudo channel-cut type with two fixed Si(111) crystals moved together by a simple goniometer circle, in the (-n,+n) configuration. The possibility to change both crystals on the monochromator according to resolution requirements (i.e. Si(311)) has been considered. The first monochromator crystal is water cooled while the second is kept at room temperature. The second crystal is equipped with a piezoelectric driver that allows to change very slightly the Bragg angle (pitch adjustment) in order to reduce the harmonic content of the beam, if necessary, and to keep the transmission of the monochromator constant during long time intervals. Also, a bender curves sagittally the second crystal in order to dynamically focus the beam at the sample positions A1 and A2 (see figure). The position and dimension of the focused beam are kept constant during a ~ 1 keV energy scan, standard in a EXAFS measurement.