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 keV Si(111) & Si(311) Interpolation error reduced by phasing and averaging 4 readheads 1 st Axle

I20 QCM Stability ± 150 nrad Pk-Pk at 60 & 100 Hz 2 nd Axle

Specification Fixed exit Energy Scanning Energy Range keV Single Crystal Si(111) Pair Vertical Beam Offset mm Liquid Nitrogen Cooled Nominal Heat Load 54W

Main Design Changes Side Cooled Crystal: Improved cooling geometry LN 2 Flow Enhancers: Increase cooling efficiency Simplified Water Circuit: Reduce complexity Large Scattered Radiation Shields: Improve thermal stability Cool 2 nd Crystal From 1 st : Remove extra manifold & pipes Non-flexible LN2 Manifold Lines: Raise resonant frequency Stiffen 1 st Crystal Support: Improve stability Rib 2 nd 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] 1 st to 2 nd 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 Motion 1.5 µrad Rocking Curve Excellent crystal cage stability & repeatability 3 µrad Rocking Curve

In-House DCM – Crystal Mount 1 st crystal no clamping distortion under beam foot print 2 nd 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

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 FFT Prior to bolting granite to floor Flooring Bearing Support Piezo LN2 Pipes Servo Motor Crystal Cage

5 Reflection Autocollimator Test The pitch variation as measured by an autocollimator due to gravitational deflection with Bragg rotation

DLS DCM – Summary The 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