Transcript Jon Kelly: Development of a DLS Double Crystal

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)
Bragg1
B1:ENC2 (1-12)
B1:ENC3 (1-6)
B1:ENC4 (1-9)
B1:ENC34 (1-3)
2.4
2.1
1.8
1.5
1.2
Diff
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•
•
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0.9
0.6
0.3
0.0
-0.3
0.2
0.4
0.6
0.8
1.0
Pos
Interpolation error reduced by
phasing and averaging 4 readheads
1st Axle
I20 QCM
Stability ± 150 nrad Pk-Pk at 60
& 100 Hz
2nd Axle
Specification
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•
•
•
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•
•
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
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•
•
•
•
•
•
•
•
•
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 Motion
1.5 µrad Rocking Curve
• Excellent crystal cage stability & repeatability
3 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
250
AC measured roll angle [urad]
200
150
100
50
0
12:00
12:15
12:30
12:45
13:00
time
13:15
13:30
13:45
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
LN2
Pipes
Piezo
Servo
Motor
Flooring
Bearing
Support
Crystal
Cage
5 Reflection Autocollimator Test
parasitic pitch before cooldown
parasitic pitch after cooldown
80
parasitic pitch after cooldown_rpt
Parasitic pitch [urad]
60
40
20
0
-20
-40
-60
5
10
15
20
25
30
35
40
45
Bragg angle [deg]
The pitch variation as measured by an
autocollimator due to gravitational deflection with
Bragg rotation
50
55
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