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Progress in Superconducting Undulator Development at LBNL

Soren Prestemon Lawrence Berkeley National Laboratory

D. Arbelaez, E. Rochepault, H. Pan, T. Ki, S. Myers, T. Seyler, M. Morsch, R. Oort, C. Swenson, R. Schlueter

Soren Prestemon, Workshop on Superconducting Undulators, April 28, 2014

Outline

• Motivation • Main areas of development • Progress and status • Future outlook

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Motivation• SCU’s can provide the best performance characteristics for X-ray facilities

• High-field, short-period devices provide spectral range with shortest FEL footprint, lowest beam energy • Fine trajectory and phase-shake correction provides requisite field quality and access to harmonics

• SCU design is focused on performance and cost • Anticipate lower fabrication costs versus competing technologies • Anticipate faster commissioning time / undulator section

• Need to clearly demonstrate technology for applications to storage rings and FEL’s

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LCLS, most storage rings!SACLA, many storage rings!a few storage rings!New capability!!


Main areas of R&D focus at LBNL

• Superconducting undulator technology using Nb3Sn ✓ Choice and specification of superconductor ✓ Materials compatibility ✓ Fabrication: processes, tooling

• Field trajectory correction and phase tuning ✓ End design and correction ✓ Internal trajectory wander reduction and phase error tuning

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Part of a collaboration between ANL, LBNL and SLAC


Conductor options• Using 0.48mm diameter OST MJR • Working with OST to investigate RRP 217 strand

✓ Dia: 0.6mm, Deff: 30 microns, RRR: ~50-100?, ✓ Jc: ~2000-2400?, Cu:SC: 1.1:1

• Working with SupraMagnetics, Inc on PIT

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Nb3Sn


Undulator components

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Single'Wire'Winding'


Number of layers

Num

ber o

f turns per layers

Design optimization

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Load Line Margin

8 turns per layer

Load Lines

3.65 3.7 3.75 3.8 3.85 3.9 3.95700

750

800

850

900

Peak Magnetic Field on Conductor [T]

Ope

ratin

g Cu

rrent

[A]

Operating current for B0 = 1.86 T

8 by 7

8 by 9Design Point Peak Conductor Field

On-‐Axis Field

Cur

rent

[A]


Motivations to consider Nb3Sn

• Nb3Sn prototype example design: ✓ λ=20mm, gm=7.5mm

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Beam steering considerations• Ideal condition consists of…

✓ Beam arrival on axis ➡ parallel to nominal path (NP), and with no offset

✓ Undulator entry results in electron transverse oscillation about NP ✓ Periodic section results in identical transverse oscillations ✓ Beam exit results in beam on NP (parallel, no offset)

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End Design options and selection• Want zero net displacement and steering due to the ends • Even or Odd number of poles

✓ Even – zero net steering, non-zero net displacement ✓ Odd – zero net displacement, non-zero net steering

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2δ

Even number of poles

δ+K

-‐K

Odd number of poles

+K +K

+δ -‐δ

Steering + Displacement

Displacement Only

Ideal


End design optimization• Odd poles/even coils • Binomial expansion pattern

✓ Poles: 0, +1/4, -3/4, +1, -1,… ✓ Coils: +1/8, -4/8, +7/8, -1, +1,…

• 7 x 8 turns/pocket: ✓ Turns/coil: 7, 28, 49, 56, 56,…

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+1/8 -‐1/2 +7/8 -‐1

Yoke

Poles Coils

• Example requirements: • I1 (end) < 40 μT•m, I2 (end) < 50 μT•m2


Permeability effects• Non-ideal effects due to finite permeability and differential saturation of end poles

✓ End kick is dependent on the undulator field ✓ Dipole field is generated by unbalanced yoke field

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x x x

xx x

x x x

xx x

1 2

Second Field Integral

End Kick

Curvature due to dipole field

End Kick


End correctors for compensation: Correction of distributed dipole

• Wound on top of the main coil in the remaining pocket on each end • Adds both a dipole and end kicks

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End correctors for compensation: Correction of end kicks

• Wound in a separate yoke on each end • Decoupled from the main yoke

• => adds only end kicks

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Tuning for internal trajectory and phase errors

• Concept of in-situ tuning of undulators ✓ Selectable correction locations ✓ Corrections at all locations have the same strength ✓ Strength can be varied with a single power supply as a

function of the undulator field strength

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Once correction locations and current calibration are known, hardwire with final system


Scaling of Trajectory and Phase Errors (random)

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RMS value of second integral error

[µm]

[µT•

m2 ]

LCLS-II requirement

RMS value of phase shake

[o ]

[µm]

σ I2=

23λu

Lu3/2I1(σ err ) σ I2

=2λuLu1/2I 2 (σ err )

Pole Errors Coil Errors

Trajectory errors scale with the undulator length to the power of 3/2

Trajectory and phase error scaling with respect to fabrication tolerances


Switch-Based Tuning Concept

• One superconducting path - with heater • One resistive path (low resistance) • When heater is on the superconducting path becomes resistive (high resistance)

174/21/14

Superconducting path

Heaters

CurrentResistive path (high resistance)

Resistive solder joint (low resistance)

Heaters ON


Current path via lithography on YBCO Tapes• Commercial tape from SuperPower Inc. • Masks designed for photolithography process • Chemical etching used to remove Copper, Silver, and YBCO layers where desired

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Possible tuning layouts

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Example: • Icorr = 50 A • Max operating field


Strength of single-loop corrections• Corrector strength approximately varies linearly with corrector current • For a given current the corrector strength varies with the undulator field

strength due to saturation of the poles

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Sensitivity to longitudinal position• Misalignment from

✓ fabrication ✓ differential thermal contraction

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∆z#

Signature#(not#to#scale)#shi4ed#longitudinally#

By I1 I2 Phase advance


Full Length Layout Concept• Correctors are placed on both sides of the vacuum chamber • Top and bottom correctors are used together • Drive current on each side of the vacuum chamber

✓ Allows for loops with positive and negative orientation ✓ Return current line is directly below the drive current

224/21/14


Correction Configurations• Various configurations allow for:

✓ Increase and decrease in the phase error without introducing a net kick ✓ Positive and negative net kicks without net changes in the phase ✓ Individual correctors give both a kick and phase change

234/21/14

Net phase increase No net kick

Net phase decrease No net kick

Net kick No net phase change


Testing capabilities• Test cryostat for superconducting undulators • Being extended to allow for testing of 1.5m tuning system

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PT415%Cryocooler%

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Pulsed wire measurements complement Hall-probe systems• Dispersion correction algorithm provides high-accuracy • End-damping allows averaging with wire in vacuum • System has been successfully used on PM EPU’s

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Looking forward

• Our plans for the next year include: ✓ Fabrication of a 1.5m Nb3Sn prototype meeting FEL requirements ✓ Develop tuning concepts and test off-line

➡ Critical for FEL and higher-harmonic applications ✓ Demonstrate tuning of NbTi and Nb3Sn prototypes

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