Double Crystal Monochromator Workshop

Development of a DLS Double Crystal Monochromator

May 2014

Jon Kelly

Project Aim

I18 Focus Crystal Cage

&

I20 4 Bounce Mono Axle

I20 QCM• Fixed exit • Energy Scanning• Energy Range 6 - 32keV• Si(111) & Si(311)

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Bragg1 B1:ENC2 (1-12) B1:ENC3 (1-6) B1:ENC4 (1-9) B1:ENC34 (1-3)

Interpolation error reduced by phasing and averaging 4 readheads

1st Axle

I20 QCMStability ± 150 nrad Pk-Pk at 60 & 100 Hz

2nd Axle

Specification

• Fixed exit • Energy Scanning• Energy Range 2.1 - 25keV• Single Crystal Si(111) Pair• Vertical Beam Offset 15 - 18mm• Liquid Nitrogen Cooled• Nominal Heat Load 54W

Main Design Changes• Side Cooled Crystal: Improved cooling geometry• LN2 Flow Enhancers: Increase cooling efficiency• Simplified Water Circuit: Reduce complexity• Large Scattered Radiation Shields: Improve thermal stability • Cool 2nd Crystal From 1st: Remove extra manifold & pipes• Non-flexible LN2 Manifold Lines: Raise resonant frequency• Stiffen 1st Crystal Support: Improve stability• Rib 2nd Crystal Cage Plates: Increase stability & dynamic properties• Mount Mechanism & Vessel On Air Pads: Proved stiff X adjustment• Tonic Encoders: Improved performance & simplified assembly

Design Inspired Tests & Papers• I09 DCM stability, repeatability & crystal strain tested on B16 using a

channel cut crystal and rocking Si(555) reflection at 20keV [1]• The crystal mounting was tested and optimised using the DLS Nanometer

Optical Metrology instrument (NOM) [2]• 1st to 2nd Crystal Braids not sufficiently flexible: Investigate other options

settling on annealed Cu 50 µm foils [3]• Eddy Current Damping

[1] Design, Build & Test of a DCM, SRI 2012, Journal of Physics Conference Series[2] Measurement & Minimization of Mount Induced Strain, SRI 2012, Journal of Physics

Conference Series[3] Measurement of Flexible Cooling Link Conductance for X-Ray Monochromator Applications,

MEDSI 2012 Conference Proceedings

In-House DCM – B16 Tests• Excellent Bragg Motion1.5 µrad Rocking Curve

• Excellent crystal cage stability & repeatability3 µrad Rocking Curve

In-House DCM – Crystal Mount

• 1st crystal no clamping distortion under beam foot print• 2nd crystal bottom mounting optimised for minimum strain via 3 point contact

In-House DCM – Cooling Links

• Foils improvement upon braids• Optimised for low strain & mechanical influence but may be changed easily• PEEK thermal isolation effective & stiff but large differential thermal effects

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In-House DCM – Direct Drive

• Direct drive stages must be well balanced – 18 kg counter weight added to enable the motor phasing upon start-up• Eddy current damping added to LN2 pipes

In-House DCM – Direct Drive

Pros• 0.2 µrad step size• 1°/second• ± 0.1µrad Bragg Stability

Cons• Requires accurate balancing• Sensitive to pipe supports• Controls much more difficult to optimise• Ferro-fluidic rotary seal• Large noisy torque motor

B21 DCM

I09 DCM -Stability

I09 Beamline Commissioning: • 10.4 µm FWHM (2 θ) at 28.3 m gives 78 nrad RMS pitch stability

• Stability now improved by bolting down granite but value yet to be measured

I23 DCM -Stability

I23 Beamline Commissioning: • 1.75 µm sigma (2 θ) at 18 m gives 49 nrad RMS effective pitch stability

• 5 µm PK-PK (2 θ) at 18 m gives 140 nrad PK-PK pitch stability

In-House DCM – Beam FFTPrior to bolting granite to floor

Flooring

Bearing Support

Piezo

LN2Pipes

Servo Motor

Crystal Cage

5 Reflection Autocollimator Test

The pitch variation as measured by an autocollimator due to gravitational deflection with Bragg rotation

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parasitic pitch before cooldown

parasitic pitch after cooldown

parasitic pitch after cooldown_rpt

DLS DCM – SummaryThe pitch stability measured on I23 of 49 nrad RMS is a significant improvement on the I18, 80 nrad RMS. There was also a significant improvement in the parasitic pitch and roll upon rotation.

Summary of key design lessons• Solid Interface to concrete – bolt or grout to slab• Side cooled crystals – low strain, high heat transfer• Cooling foil stacks – high flexibility & efficient• Balanced mechanism essential for direct drive phasing• Direct drive control system tuning is key to stability• Ferrofluidic seals exhibit pressure bursts• Lowest vibration mode due to fabricated support

Many thanks to Armin Wagner & Tien-Lin Lee