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Thermal Environment & Mechanical Support. Phase and Trajectory Tolerances Foundation Considerations Thermal Distortions Support Design. Phase error tolerance implications. 2 micron rms trajectory tolerance (perfect undulator) Segment to segment strength variation of 1.5 x 10 -4

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Thermal environment mechanical support

Thermal Environment & Mechanical Support

Phase and Trajectory Tolerances

Foundation Considerations

Thermal Distortions

Support Design

J. Welch, SLAC

Phase error tolerance implications
Phase error tolerance implications

  • 2 micron rms trajectory tolerance (perfect undulator)

  • Segment to segment strength variation of 1.5 x 10-4

    • Temperature coefficient of NdFeB is 0.1%/C

    • Undulator compensation via Ti/Al assembly) magnet temperature tolerance ~ +-0.2 C

  • Vertical undulator alignment 50 mm causes 10 degrees of additional slippage

2 mm deviation from straight over 10 m is about the average curvature of the Earth’s surface

J. Welch, SLAC

Path length increases due to bumps
Path Length Increases due to Bumps

  • LCLS: A < 3.2 mm

  • LEUTL: A < 100 mm

  • VISA: A < 50 mm

from H-D Nuhn

J. Welch, SLAC

Alignment and stability strategy
Alignment and Stability Strategy

  • Three layers of defense against trajectory errors

    • Beam based

      • fast orbit feedback for launch errors

      • full BBA with multiple beam energies to measure BPM and Quad offsets.

    • Wire Positioning System and Hydrostatic Leveling System

      • HLS systems have shown good long term stability

      • WPS system have shown good short term stability

    • Make foundation and supports as stable as possible

      • thermal stability, geotechnical, and support mechanical design

J. Welch, SLAC

Beam based alignment
Beam Based Alignment

If errors are too big they must be fixed rather than “corrected for”

  • BBA is the fundamental LCLS tool to obtain and maintain ultra-straight trajectories over long term.

  • Corrects for

    • BPM mechanical and electrical offsets

    • Field errors, (built-in) and stray fields

    • Field errors due to alignment error

    • Input trajectory error

    • Does not correct undulator alignment errors

  • Establishes a best fit straight line electron trajectory

  • Procedure

    • Take orbits with three or more very different beam energies, calculate corrections

    • Move quadrupoles and/or adjust steering coils to correct orbit

  • Disruptive to operation

offsets don’t depend on energy

1/month is ignorable, 1/day is intolerable

J. Welch, SLAC

Bpm and quad stability requirements
BPM and Quad Stability Requirements

  • After BBA, changes of BPM offsets will be seen erroneously as orbit errors

  • Stability of BPM mechanical and electrical offsets determine trajectory stability

    • need BPM stability of ~ 2 mm rms

  • BPM’s have to be mechanically more stable than all other components

  • Known BPM motions are taken out in software

Quad stability requirements are more like 5 microns

J. Welch, SLAC

Foundation instability
Foundation Instability

J. Welch, SLAC

Settlement implications for lcls
Settlement Implications for LCLS

  • Expect settlement of order

    • ~ 300 - 1000 mm / year = 1 - 3 mm / day,

    • not well correlated with location

    • Good alignment lasts only a day or so

  • Mover range cannot accommodate much of the drift; need another mechanism with plenty of range and periodic realignment

J. Welch, SLAC

Foundation design guidance
Foundation Design Guidance

  • Uniformity of construction along length

    • avoid fill areas which settle much faster

    • try to avoid kinks, gentle bends are more tolerable

  • Strong thick floor

    • ~ 3 ft, essentially monolithic

  • Buried/tunneled

    • research yard has poor stability

    • good thermal insulation

  • Water table considerations

    • desire either wet or dry all year

    • keep sandstone wet between exposure and concrete

J. Welch, SLAC


  • Normally vibration amplitudes are much less than 1 micron, typically 10 - 100 nm.

    • ~10 nm measured on top of berm.

  • Possible areas of concern

    • air handling units

    • passage of vehicles over undulator hall tunnel.

  • Pointing sensitivity ~ 10-7 radians (1/10 angular divergence)

    • e.g. 10 Hz -> yrms ~ 1 micron

    • Q factors for equipment can be 100’s, supports need to be checked

J. Welch, SLAC

Thermo mechanical instabilities
Thermo-Mechanical Instabilities

  • Dilatation (ordinary thermal expansion)

  • Warp caused by thermal gradients (heat flux)

J. Welch, SLAC


Support column height from (fixed?) bedrock 3+ meters.

Temperature coefficient for Anocast 12 ppm/C

Temperature change for 1 micron vertical motion is 0.03 C

--> BBA re-measure at 0.06 C change

-->stability during BBA procedure 0.03 C/ 8 hr, (~1 degree/week)

J. Welch, SLAC

Warping from heat flux
Warping from Heat Flux

  • Long beams bend easily if there is a heat flux across them.

  • Heat fluxes can arise from

    • Temperature differences between walls and radiant heat transfer

    • Air temperature differences

    • Contact with supports or other materials

  • It is easy to show the bar goes to “average” temperature



J. Welch, SLAC

Heat flux example
Heat Flux Example

  • Heat flow a the bar for 1 degree temperature difference

J. Welch, SLAC

Heat flux distortions
Heat Flux Distortions

  • Bar Warp


L = 3 m, titanium

3 W/m2 -> 2 micron warp for an undulator segment

2 microns is the walk-off tolerance,

-> Max wall temperature difference is ~1 degree C

J. Welch, SLAC

Thermal environment
Thermal Environment

  • Air temperature in both time and space

  • Surface temperatures

  • Heat sources and sinks

J. Welch, SLAC

Air temperature illustration
Air Temperature Illustration

Air Temperature

Match MMF temperature

J. Welch, SLAC

More temp specs
more temp specs

J. Welch, SLAC

Girder concept
Girder Concept

If the girder is truly stable, linearly correlated motion along the girder can be identified and corrector for. The longer the girder the better

  • Stability of bedrock is not good (1-3 mm/day)

  • Long girder to provide good relative alignment stability

  • Length > gain length ( ~ 5 m)

  • Reduce the number of supports req’d

J. Welch, SLAC

Girder concept1
Girder Concept

J. Welch, SLAC

Why granite

Good overall long term stability

common choice for metrology and magnet measurement benches

Large thermal mass

averages temperature fluctuations, good passive stability

Low thermal expansion coefficient

~ 1/2 cte of steel, similar to ceramics

Reasonable cost in large sizes

~ $40,000 for 12 x 0.8 x 0.8 m, finished and delivered (enough for 3 undulator segments)

Low thermal conductivity

sensitive to heat fluxes

Variable mechanical properties

Doesn’t take a tap

hard to add features

Not ductile

handle with care


Why Granite?



J. Welch, SLAC

Other girder options

Aluminum tubes with temperature stabilization

Steel or cast iron girders

Engineered stone (Anocast)

Carbon reinforced plastic tube trusses

Specialized concrete

NLC technology

SiC girders!

Other Girder Options

J. Welch, SLAC

Support assembly concept
Support Assembly Concept

J. Welch, SLAC

Earthquake bracing
Earthquake bracing

J. Welch, SLAC

Support in tunnel
Support in Tunnel

J. Welch, SLAC

Support r d

Testing a 6 m piece from Barre Vt for long term stability - start this summer

does it slowly sag?

how much does it warp with temperature and humidity changes in the surrounding tunnel?

What does sealing do?

does insulation help? how much?

thermal stabilization time?

Prototype mounting schemes for adjustable support platform and kinematic supports

Support R&D

J. Welch, SLAC

Schedule cost

Granite manufacture and shipping time 10 weeks for first item

don’t know at what rate they can be produced, need at least 11.

Quarry closed Jan - Mar

Stabilization time ~ 2 months, before ready to measure

Integration into installation schedule under development

Granite beams ~ $500,000

Other support costs ~ $500,000

Thermometry, kinematic supports, insulation, tubes, plates, eq bracing, etc

Schedule & Cost

J. Welch, SLAC

Extra slides
Extra Slides item

J. Welch, SLAC

Temperature specs
Temperature specs item

J. Welch, SLAC

Basic tolerance requirements from simulations
Basic Tolerance Requirements from Simulations item

  • Saturation length (86 m) increases by one gain length (4.7 m), for the 1.5 Angstrom case if there is:

    • 18 degree rms beam/radiation phase error

    • 1 rms beam size ( ~ 30 mm) beam/radiation overlap error.

J. Welch, SLAC

Assembly concept exploded
Assembly Concept exploded item

J. Welch, SLAC