The beam can be focused by inserting Beryllium compound refractive lenses in the beam path. The focal length can be adjusted for a given X-ray energy by selecting an appropriate number of individual lenses (up to 13) and stacking them. One can switch from one stack to another (up to three) remotely. Such a unit is located 3.852 meters upstream from the sample. The focusing unit has the capability to be translated longitudinally ±0.15m. This allows some tunability in the beam size at the sample when not working at the focus.

The lenses are fabricated by B. Lengele group in RWTH Aachen (http://www.physik.rwth-aachen.de/en/institutes/institute-iib/group-lengeler/)

The current configuration and nominal position of the Be lens stacks are listed in the file xppconfig (from xpp-control terminal) not updated!

Updated inventory can be found in g.drive here.

We have sets of Be lenses with thicknesses: 50 um (rarely used because small aperture, only for really hard x-rays), 100 um, 200 um, 300 um, 500 um, 1000 um, 1500 um and 2000 um.

The rayleigh length of this lenses is approx. 3 cm and the focus doesn't blow up that fast far from focus (approx. 1 um bigger at the sample per 1 cm motion of the lenses or 1 um bigger for 1 cm upstream/downstream the sample)

CALCULATE FWHM:

To calculate the number and type of Be lenses required for certain beamsize and energy we use the following function in XPPPython:

Distance from the lenses to the sample is 3.852 m at XPP.

Focal length for certain lenses configuration and energy:

LensFocalLength(LensEffectiveR((200e-6, 300e-6),(2,1)),10,'Be') [LensFocalLength(LensEffectiveR((lensesthickness1,...), (number1,...)), Energy,ID]

Beamsize for certain lenses configuration and energy at a given distance:

ub.calcBeamFWHM(9,(3,100e-6,1,200e-6),distance=3.852+-0.15)

[ub.calcBeamFWHM(Energy,(number1,lensthick1,number2,lensthick2....),distance=distance)

Make sure the focus is not too upstream if there is risk of melting beamline components (usually we set it downstream)

Now we can also use: crl.planSet(E,size), e.g. crl.planSet(9.5,20e-6), which will propose various set of lenses for that size (specifying the achievable range) for downstream focus.

*Note that the C* crystal on the LODCM introduces additional divergence

For the old TT which was downstream the lenses:

and to check if we have enough power on the time tool:

ub.calcBeamFWHM(9,(3,100e-6,1,200e-6),distance=3.852-0.7) (the tt was 0.7 m upstream)

and put this number as FWHM in:

TTfactor('Si3N4',2e-6,1e-3,E=9,FWHM=200e-6)

TTfactor = 1 means 10% jump, it has to be close to 1 (at least 5% so at least 0.5)

Basic Be lenses alignment.

pack 1 is around 9.5 mm, pack 2 is around 36 mm, pack3 is around 62 mm, out is around 93 mm (m. Be_ypos)

How to clean dust particles from Be lenses (from Bruno Lengeler):

• • Pressurized (oil free) air, distance to lens 1-2 cm

  • Crosscheck in stereo microscope, immediately before mounting into holder

  • Don’t try brush or gel cleaning!