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Undulator Prototype Status and Plans. Marion M. White APS-ASD. Outline – Prototype Undulator Status. Design Challenges Mechanical Design Features Performance Improvements Canted Pole Undulator Measurements Plans Summary. Undulator Design Parameters. LCLS – Familiar Design Challenges.

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outline prototype undulator status
Outline – Prototype Undulator Status
  • Design Challenges
  • Mechanical Design Features
  • Performance
  • Improvements
  • Canted Pole Undulator Measurements
  • Plans
  • Summary
lcls familiar design challenges
LCLS – Familiar Design Challenges

Between APS insertion devices and the LEUTL FEL, the APS team has a lot of undulator experience with:

  • High-quality undulator magnetic fields
  • Magnetic tuning for phase errors and trajectory straightness
  • Variable and fixed gaps
  • Phasing undulator ends
  • Magnetic design
    • NdFeB magnets
    • Vanadium permendur poles
    • 30-mm period
    • K=3.63 so Beff=1.296 T
magnetic design
Magnetic Design

Standard undulator design considerations:

  • Maximize the field
  • Don’t demagnetize the magnets
  • Don’t oversaturate the poles
magnetic design 2
Magnetic Design (2)

Prevent Radiation Damage

  • Chose a new grade of magnet with higher coercivity (N39SH) for the prototype
  • Attention to minimizing the demagnetizing field
  • Design goal to be as restrictive as usual on the demagnetizing field, maybe even at the cost of higher pole saturation, then use the high Hc magnets
new challenges uniformity and stability
New Challenges – Uniformity and Stability

Achieving a field-strength uniformity of 1.5 x 10-4 along the undulator line is a challenge

  • Gap change of 1.4 microns
  • Vertical shift ~ 50 microns
  • Temperature coefficient of the magnet is 0.1%/°C
  • Thermal expansion

And there may be a desire to taper in the future

outline prototype undulator status1
Outline – Prototype Undulator Status
  • Design Challenges
  • Mechanical Design Features
  • Performance
  • Design Improvements
  • Canted Pole Measurements
  • Plans
  • Summary
complete undulator module
Complete Undulator Module



Magnet Assembly

Rails CAM Movers Cradle

mechanical design features
Mechanical Design Features
  • Housing is made from a forged Ti bar
  • Ti was preferred over other materials because:
    • Nonmagnetic
    • Low thermal expansion
    • Long-term stability
    • Rigidity to density ratio for minimal deflection
  • Al baseplate provides partial thermal compensation
  • Open on one side for magnetic measurement access and shimming
mechanical design features cont
Mechanical design features, cont.

Magnetic side shims. Steel bars approach side of pole. Correction up to ~3% in field.

Shims and push-pull screws adjust the gap.

Magnets are clamped from only one side.

pole clamping
Pole clamping

Poles have titanium wings, and are clamped on both sides

pole simplification now under consideration
Pole simplification now under consideration

Eliminate the wings and screw the pole in from the bottom.

Still being refined; will be used in a segment of the prototype and reviewed.

end phase adjusters in the prototype
End-phase adjusters in the prototype
  • Piezo translators on end sections allowed gap & field strength adjustment
  • Over the last seven periods only
  • Adjusted phasing between undulators
  • Can relax the requirement for constant Beff between undulators to 7x10-4
  • Travel range 100 micron each jaw (200 microns in total gap). 100 microns corresponds to 29°.
eccentric cam movers
Eccentric cam movers

Each cam is driven by a separate motor

Adjustable in both transverse directions & in roll, pitch, & yaw

outline prototype undulator status2
Outline – Prototype Undulator Status
  • Design Challenges
  • Mechanical Design Features
  • Performance
  • Design Improvements
  • Canted Pole Measurements
  • Plans
  • Summary
assembly magnet sorting
Assembly - Magnet Sorting

Single Magnets

Matched Pairs

Magnets were sorted by strength (Total Moment), then the strongest and the weakest were matched together.

Very important – saved lots of time since we found we could use this vendor’s measurements “as is” for sorting; not all vendors routinely make these measurements.

assembly pole sorting
Assembly - pole sorting

After magnet sorting, the main contributor to field errors was pole height variation.

Tall and short poles were paired, and RMS deviation in gap was reduced from 6.3 to 2.4 microns

After sorting:

But this pairing neglected the contribution of the Al base plate thickness, and variation due to the attachment to the Ti. Final gap variation was ±50 microns.

Note – Put tighter tolerances on the Al baseplate for production

magnetic tuning
Magnetic Tuning

Nonetheless, the device met the trajectory straightness requirement (±2 micron) without tuning.

After tuning, the wiggle-averaged trajectory was within a range of about 0.5 microns.

phase error tuning
Phase Error Tuning

The calculated spontaneous emission amplitude needed tuning to raise it from 93% to over 99% of ideal.

(The rms phase error decreased from 11.2° to 6.5°.)

temperature dependence
Temperature Dependence

Care must be taken in the measurements to allow the undulator sufficient thermal equilibration time

Also need to correct for temperature dependence of the Hall probe: (DBeff/Beff)/DT = -5.5 x 10-4 /°C

outline prototype undulator status3
Outline – Prototype Undulator Status
  • Design Challenges
  • Mechanical Design Features
  • Performance
  • Post-Prototype Design Improvements
  • Canted Pole Measurements
  • Plans
  • Summary
post prototype considerations
Post-prototype Considerations
  • End phase adjustments
    • Piezos long-term stability for this application is untested
  • Adjustment to final gap has not yet been done, but can do this with the cant anyway.
  • Assume temperature dependence is handled by the conventional facilities specifications.
radiation damage post prototype
Radiation Damage – Post-prototype
  • Had considered using SmCo magnets
    • Better radiation resistance
    • Smaller decrease in strength with temperature rise
    • But overall weaker strength and more brittle
    • Ruled out based on schedule – no time for R&D.
  • Instead, take advantage of APS radiation exposure and damage experience at the APS.
  • Provide dose limit guidance and information to SLAC to be used as input into the undulator protection system.
  • Do not operate LCLS under conditions likely to result in damage to the undulators.
post prototype cont
Post-prototype, cont.
  • A comb shunt for adjusting the field strength was proposed
  • Initial tests look promising, but added design complexity; (remote capability - considerable added design complexity)
  • Also a possibility for end phase correction only
outline prototype undulator status4
Outline – Prototype Undulator Status
  • Design Challenges
  • Mechanical Design Features
  • Performance
  • Post-Prototype Design Improvements
  • Canted Pole Measurements
  • Plans
  • Summary
canting the gap
Canting The Gap
  • A scheme ( thanks to J. Pflueger) of canting the poles so that field strength varies with lateral (horizontal) position was very promising.
  • A test section was “canted” and measured with excellent results. Canting was adopted into the baseline.
canted cross section exaggerated
Canted Cross-section (exaggerated)




Section with




Effective Magnetic Field

  • Measured slope of 6.6 Gauss/mm agrees with calculations (~ 5.7 Gauss/mm for 3 mrad cant).
  • Alignment accuracy needed for DB/B ~ 1.5x10-4 ~ 2 Gauss -> 0.3 mm
rms phase error
RMS Phase Error
  • No significant dependency on X
  • An RMS phase error of ~ 6.5 degree is an upper limit for near-perfect (~100%) performance.
horizontal trajectory averaged over period length at 14 1 gev
Horizontal Trajectory (averaged over period length) at 14.1 GeV
  • Trajectory vs. X well behaved and well within the tolerance requirement of 2 mm maximum walk-off from a straight line.
  • Operational range is ±1.2 mm for ±1.0°C temperature compensation.
fringe fields at x 65 and 100 mm
Fringe Fields at X=65 and 100 mm
  • Fringe fields with new shims are close to earth field for X=100 mm. (Earth field contribution to trajectory shift has to be corrected.)
fine adjustment of effective magnetic field isaac s field tuning procedure
Fine adjustment of effective magnetic field(Isaac’s field-tuning procedure )
  • Select spacers with thickness step ~ 15 µm to set the effective field in the range of ±30 Gauss (1 µm in gap corresponds to ~ 2 Gauss in field).
  • Set spacer horizontal position to adjust the effective field to ~ ±6 Gauss (spacers are wedged with 3 mm/mm cant)
  • Set horizontal position of the undulator as a whole so the effective field is in the range ±2 Gauss (DB/B ~ ±1.5x10-4) (This step saves time and provides better accuracy)
  • The undulator horizontal position could be remotely controlled during operation to compensate for in-tunnel temperature variations (motion of ±1.2 mm for ±1°C needed). Such option is available, if quadrupoles are separated from undulator sections.
magnetic needles for alignment
Magnetic needles for alignment
  • Only one needle is required for alignment in the X direction
  • One more needle has to be added at Y=0 for alignment in the Y direction
outline prototype undulator status5
Outline – Prototype Undulator Status
  • Design Challenges
  • Mechanical Design Features
  • Performance
  • Post-Prototype Design Improvements
  • Canted Pole Measurements
  • Plans
  • Summary
scope and plans undulator systems
Scope and Plans – Undulator Systems
  • 33 Precision magnetic arrays with canted poles
  • 33 Support/alignment systems including:
    • Cradle that supports the undulator, BPM, and quadrupole magnet.
    • Precision CAM movers and motors enabling positioning, alignment, and adjustment of the cradle.
    • Rail system to move the undulator, facilitating manual retraction of an undulator out of the beamline and precision reproducible re-insertion.
  • 7 Spare Undulator Modules
  • 1 Undulator Transport Device for Installation
plan undulators 1
Plan – Undulators (1)
  • To meet schedule and funding profiles, and to ensure that the Undulator Systems are complete by July 2007, we plan to procure the following long-lead items as early as possible in FY05:
      • Precision-machined titanium strongbacks
      • NdFeB Magnet blocks
      • Vanadium Permendur Magnet poles
  • The same APS undulator experts, who were relied upon for design, construction, and assessment of the prototype, will finalize procurement packages for the LL items, in accordance with our Advance Procurement Plans [APP].
scope quadrupole magnet systems
Scope – Quadrupole Magnet Systems
  • 33 Quadrupole Magnet Systems - installed
    • Permanent Magnet Quadrupole
    • Support with Precision Translator [settable to 5 um; readout to 1um]
  • 5 Spare Magnet Systems
  • Separate steering is not included
  • A full-scale prototype undulator was constructed and tested at APS, and met LCLS performance goals.
  • A subsequent design improvement, that of introducing a 3-mrad cant in the pole gap, was implemented using wedged spacers between the aluminum base plates and the titanium core. It was successfully tested and the concept was adopted in the baseline.
  • A disadvantage of the canted-pole design is the necessity to provide a separate support for vacuum chamber
  • Magnetic measurements show good agreement with calculated change of the effective magnetic field versus X (horizontal motion).
  • No significant change of the RMS phase error versus X was measured.
current status
Current Status
  • Significant effort has been devoted to planning, resulting in a detailed undulator construction schedule that is integrated with the BPM, quadrupole and vacuum chamber construction and testing. The undulator schedule and the magnet measurement schedule are mostly integrated, and are consistent with completion of undulator systems in July 2007.
  • A skeleton installation schedule exists; details are being added and integration with the rest of the schedule is ongoing.
  • Schedule refinement is ongoing.
  • Costs were estimated by in-house experts with relevant experience and were based on vendor quotes and previous experience. Cost scrubbing will continue.
  • The greatest schedule risks come from:
    • Design changes
    • Delayed funding