Beamline Optics

A document describing the optics geometry for the new setup for undulator radiation is described here. An important change has been effectuated in coincidence with the SLS2.0 ring upgrade. A third vertical mirror has been added in 2023 just before the start of the shutdown for the SLS2.0 upgrade. This mirror provides a very precise and reproducible horizontal focusing of the beam, that is significantly improved with respec to the formerly used option of sagittally focusing by bending the second crystal, especially as reproducibility and preserved monochromaticity, at the cost of a small side deviation (0.1 deg) of the beam path. The stations have al been realigned on the new position of the beam, that coincidentally was also displaced by a few cm because of the SLS2.0 ring upgrade.



 

Optics components sketch

The optical elements of the beamline and their functions are listed here.

ElementFunctionDistance from source [m]
High Power SlitsBeam defining11.662
SiC BPMPermanent beam position monitor12.44
FilterBeam attenuation17.806
Optics slitsBeam definition18.27
XBPM1Beam position monitoring18.341
First LN2 cooled DCM crystalMonochromatization18.758
Second DCM crystalMonochromatization18.8-19.1
XBPM2Beam position monitoring19.306
Mirror 1Vertical collimation20.13
Mirror 2Vertical focussing20.83
Mirror 3 (vertical)Horizontal focussing22.23
CVD screen 1Beam imaging and position monitor23.053
XBPM3Beam position monitoring23.326
Station shutterExtinction of experimental beam23.772
Bremsstrahlung blockerBlocking of direct beam and BS30.485
1D diffractometerExperiment32.79684
2D optical tableExperiment33 - 37 
3D diffractometerExperiment40.0865
Fixed XRE at rear wallBeam shaping43.28
The front-end safety system components of the MS Beamline
The new optics scheme. The beam goes left to right. In the left quadrant, the 2-crystal and 2-mirror optics elements in operation since 2010. In addition, a third vertical bending mirror has been added with the purpose of horizontal focussing. Also shown are some of the sensors (2x Diamond Fluorescent Screens, 1x Mythen III I0 module) used to keep the beam perfectly steady and to define specific focus and position conditions for the three experimental tables.
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The double crystal monochromator (DCM) consists of two Si 111 crystals which are precisely positioned and oriented in the X-ray beam.

View inside the DCM
View inside the DCM
DCM motor movements
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Two successive Bragg reflections, with an inherent energy resolution of 0.014 %, direct photons of the desired energy parallel to the incoming beam direction, but offset downward (out of the direct Bremsstrahlung beam) by 15 mm. Changes in photon energy require changes in the mirror angle and hence in the height of the DCM. 

The polychromatic incoming beam (shown here as white) is monochromated by the first Si-crystal. The second Si-crystal is nominally set at the same angle and can be moved along a translation stage such that the outgoing beam (here, blue) is 15 mm lower in height than the incoming beam. 

_An aperture 90 cm downstream allows the monochromated beam to pass, but blocks any high-energy Bremsstrahlung transmitted at the original height of 1400 mm. The large heat load absorbed by the first DCM crystal (up to 1 kW) causes it to deform, and a mechanical bending apparatus, ("TORII", developed at Hasylab), dynamically corrects this curvature._ The second crystal provides sagittal focusing; it is a ribbed crystal, cylindrically bent to a variable curvature radius (Rmin=1 m) in a flexure-hinge fixture developed at the ESRF. 

The DCM motors are controlled by EPICS channel widgets. A schematic of the motor movements and a table listing their functions are shown here. 

The outgoing beam is shifted up by 20 mm compared to the incoming beam. See Table below for the motor functions.

EPICS channelDescription
First crystal 
MOX1:ROXrotation (pitch) of the first crystal stages around X axis
MOX1:ROYrotation (roll) of the first crystal stages around Y axis
MOX1:TRYtranslation of 1st crystal stage
Second crystal 
MOX2:ROXfine adjustment of Bragg angle of 2nd crystal
MOX2:ROYroll of 2nd crystal
MOX2:ROZyaw of 2nd crystal
MOX2:TRXyaw of second crystal around its normal
MOX2:TRYvertical movement
MOX2:TRZhorizontal translation of 2nd crystal
MOX2:TRYA...Dflexing of 2nd crystal to produce sagittal (horizontal) focussing
1st and 2nd Mirror

The beamline optics are shown below. The first mirror (MI1) provides vertical collimation and removes high-order harmonics. The mirror consists of a 1 m long Si blank, Rh-coated (K-edge: 23 keV), which can be tilted up to 5 mrad and bent (5 < R < 30 km) as required for the desired photon energy. The surface of the mirror has been polished to a rms roughness less than 0.5 nm and to a rms residual slope error less than 2.5 m rad. 

The second mirror (MI2) is similar to MI1. A variable curvature permits vertical focusing at the experimental station in use. Since this focusing introduces a vertical divergence, which is undesirable, for example, in high-resolution powder diffraction, the user may choose to either set the radius of curvature to infinity or to remove MI2 from the beam altogether, the latter option causing an inclined beam. 
 

The horizontal focussin function has been devolved to the newly installed mirror 3 (vertical), that shows the same excellent performance in precision and reproducibility as the vertical focussing by mirror 2. 

Last revision A. Cervellino, July 2025