A Helical Undulator for the Production of

Polarised Positrons

Duncan ScottASTeC

Daresbury Laboratory

01/07/03 Duncan Scott: Super-Conducting Workshop 2

Contents

TESLA Positron ProductionPolarised Positron ProductionHelical Undulators DesignsOptimising the GeometryEffects of IronPower Dissipated & VacuumConclusions & Further Work

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TESLA

T eVEnergy Super-conducting Linear Accelerator250 GeV (per beam) e+ e- Linear Collider

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TESLA Layout

Injector systemParticles created initial acceleration

Damping RingsBeam size is reduced by emitting of synchrotron radiation

Very long LINAC to accelerate particles to 250 GeVInteraction Point

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TESLA Layout

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TESLA Positron Source

Efficient positron production is a limiting factor on the performance of a LCThe present system involves

An undulator to produce radiationA target where radiation creates electron positron pairsPositrons “captured” and accelerated

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Positron Production Schematic

250 GeV e- Beam

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Polarised Positrons

A ‘Helical Undulator’ would provide Polarised γ rays, These could produce Polarised Positrons

Helical Field -Rotating dipole field in the transverse planes

e- beam follows a helical path

Circularly Polarised Photons

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Required Magnetic Field

Required B Field to Produce 20 MeV Photons

0

1

2

3

4

5

6

7

0.4 0.9 1.4 1.9 2.4 2.9

Undulator Period (cm)

On

Axi

s B

Fie

ld (T

)

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Permanent Magnet Undulator Design

“Halbach” undulator (Klaus Halbach NIM Vol. 187, No1)

PPM blocks create Dipole Field

Rotate many rings to create Helical Field

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Super-Conducting Design

Super-Conducting Wires 2 wires wrapped around vacuum vessel

HelixDiameter

Undulator Period, u

Current, -i

Current, i

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3D-Modelling

Based on meeting the required on-axis field looked atDifferent Period LengthsDifferent Helix BoresDifferent Wire Dimensions

Calculated conductor margins for each caseThe assumptions for calculating the conductor margins were

Wire packing factor of 0.74Normal:SC ratio of 1:1Super-Conductor is NbTiTemperature is nominal 4.2K

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3D - Model Used

Period

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3D-Modelling Example Result

Picture shows peak field plots in the windings for the FEA model

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Results - Period

A 12mm period would work, howeverFormer has to be made & wires wrapped around itSpace is needed for the e- beam

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Results - Changing Bore

A 14mm period is recommendedWires will wrap around it4mm beam stay clear for the electron beam

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Qualitative Effects of Iron

Iron can be added in places to improve the field strength Electron Beam

Former

Iron Sleeve Increases on-axis Field strength and is fairly easy to do

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Qualitative Effects of Iron

Vacuum VesselElectron Beam

Iron sleeve and iron between windings creates the best resultAttaching iron to the vacuum vessel would be difficult

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Qualitative Effects of Iron

For a completely iron former and iron sleeve the field is less than the other two cases - but still better than no iron

Electron Beam

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Power Dissipated - SRPower dissipated as a function of the distance through the undulator

far field approximation used4mm beam stay clear assumed

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Vacuum System

TESLA Requirements are 10-8 mbar (CO Equivalent)Single-pass Machine

This is equivalent to photon flux of ~ 1017 photons s-1 m-1

Super-Conducting MagnetPower levels indicate flux is less than this amount

Pure Permanent MagnetNEG Coating will be requiredNEG coating of a 4mm Diameter Pipe has never been achieved

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Making the Former

Prototype former made from cutting grooves into cylinder

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Magnet Design Conclusions

A super-conducting double helix design is recommended with the following properties

14mm period6mm helix diameter4mm electron beam stay clearWires made up of 16 filaments bonded into former

(A similar result is reached for the PPM “Halbach” design)

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Further Work

Complete magnet designLooking at having iron to help reduce the required current density

Analyse effects of magnetic field errorsBuild a Prototype

Looking to build a short (~20 Period) device - hopefully of both technologiesUse a custom probe to measure magnetic field

Use PrototypeTest emitted radiation by placing undulator in a test beam, e.g. 4GLS, TTF2

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Acknowledgements

Super-Conducting Group at RALElwyn BaynhamJames RochfordTom BradshawIouri Ivaniouchenkov

Insertion Devices Group at DLJim Clarke

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