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Helical Collaboration. I.R. Bailey, J.B. Dainton, L.J. Jenner, L.I. Malysheva (University of Liverpool / Cockcroft Institute) D.P. Barber (DESY / Cockcroft Institute) G.A. Moortgat-Pick (IPPP, University of Durham / CERN / Cockcroft Institute)

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ilc positron source spin tracking

Helical Collaboration

I.R. Bailey, J.B. Dainton, L.J. Jenner, L.I. Malysheva

(University of Liverpool / Cockcroft Institute)

D.P. Barber

(DESY / Cockcroft Institute)

G.A. Moortgat-Pick

(IPPP, University of Durham / CERN / Cockcroft Institute)

A. Birch, J.A. Clarke, O.B. Malyshev, D.J. Scott

(CCLRC ASTeC Daresbury Laboratory / Cockcroft Institute)

E. Baynham, T. Bradshaw, A. Brummit, S. Carr, Y. Ivanyushenkov, J. Rochford

(CCLRC Rutherford Appleton Laboratory)

P. Cooke

(University of Liverpool)

ILC Positron Source & Spin Tracking

Ian Bailey

University of Liverpool / Cockcroft Institute

ilc positron source spin tracking1

ILC Positron Source & Spin Tracking

Ian Bailey

University of Liverpool / Cockcroft Institute

EUROTeV: WP4 (polarised positron source) PTCD task

I. Bailey, J. Dainton, L. Zang (Cockcroft Institute / University of Liverpool)

D. Clarke, N. Krumpa, J. Strachan (CCLRC Daresbury Laboratory)

C. Densham, M. Woodward, B. Smith, (CCLRC Rutherford Appleton Laboratory)

J.L. Fernandez-Hernando, D.J. Scott (CCLRC ASTeC Daresbury Laboratory / Cockcroft Institute)

P. Cooke, P. Sutcliffe (University of Liverpool)

In collaboration with

Jeff Gronberg, David Mayhall, Tom Piggott, Werner Stein (LLNL)

Vinod Bharadwaj, John Sheppard (SLAC)


Photons(≈ 10 MeV )

Helical Undulator

(≈ 100 m)


(150 GeV)



(≈ 5 MeV)


Target (0.4X0 Ti)

Photon Collimator

Undulator-Based Positron Source for ILC

  • The ILC requires of order 1014 positrons / s to meet its luminosity requirements.
  • A factor ~60 greater than the ‘conventional’ SLC positron source.
  • Undulator based source  lower stresses in the production target(s) and less activation of the target station(s).
  • Collimating the circularly-polarised SR from the undulator leads to production of longitudinally-polarised positrons.

ILC Positron Source Layout

Original baseline layout of ILC with undulator at 150GeV position in main linac.


125 MeV e+

400 MeV e+

ILC Positron Source Beamlines

EUND - undulator

TAPA - target station A

TAPB - target station B

Not to scale in any dimension!


Baseline Positron Source R&D

Area Systems Group R&D topics

  • Undulator - CI
    • Topic Leader: Jim Clarke
  • Target station - CI
    • Topic Leader: Ian Bailey / Tom Piggott
  • OMD (capture optics)
  • Target hall (eg layout, remote-handling) - CI
  • Capture rf cavity
  • Accelerator physics (eg production/ capture) - CI
    • Topic Leader: Gudi Moortgat-Pick
  • Polarisation (collimators, spin transport) - CI

Collaborating Institutes









CI - denotes a significant Cockcroft Institute contribution


Superconducting undulator prototypes for ILC

Parameters of first prototype

Superconducting bifilar helix

First (20 period) prototype

Hall probe measurements of first prototype

Cut-away showing winding geometry


Superconducting undulator prototypes for ILC

Superconducting bifilar helix

First (20 period) prototype

Section of second prototype, showing NbTi wires in Al former.


Prototype Parameters

All completed prototypes have reached design field

Peak field specification of < +/- 1% demonstrated

Demonstrated predicted enhancement of field by ~0.44T using iron former


ILC Undulator Simulations

Undulator simulations showing winding bore and period of device needed for ILC parameters.

Simulations of photon desorption of absorbed gases from undulator beam pipe. Collimation required to maintain vacuum of 10-8 Torr.

Energy spread increase in ILC electron beam due to resistive wall impedance in undulator vacuum vessel. Red is room temperature, blue is at 77K


ILC Undulator Simulations

Simulations of photon desorption of absorbed gases from undulator beam pipe. Collimation required to maintain vacuum of 10-8 Torr.

Energy spread increase in ILC electron beam due to resistive wall impedance in undulator vacuum vessel. Red is room temperature, blue is at 77K


5% rms

2 Simple dipole steerers added to model

Trajectory Modelling

  • Simple simulation of 2 x 2m undulators per module
  • RMS of Peak Field 5% (5 times worse than measured)

No correction

With correction

  • The trajectory can be corrected to within a few microns over 4m
  • It may be possible to operate without corrections

He bath vessel

Thermal shield

Two sections of the undulator magnet

Long Prototype

Long prototype (4m) now under detailed design and will be manufactured by Summer ‘07.

Turret region

Cryostat wall – Thermal shield - He vessel - Magnet



Future Undulator Activities

  • Finalise design and construct long undulator prototype (4m)
  • Prototype beam tests
    • ERLP at Daresbury Laboratory
    • 70 MeV electron beam  visible light emitted
  • Pre-production prototype
    • Construct with UK industry
    • Module alignment issues
    • Module instrumentation
    • Collimation
    • Integrated vacuum system
  • Extend simulations
    • e.g. Geometric Wakefields

Target Systems

The CI plays a key role in the EUROTeV-funded task to carry out design studies of the conversion target and photon collimator for the polarised positron source.

Capture Optics

Positron beam pipe/

NC rf cavity

Target wheel


beam pipe


Vacuum feedthrough

LLNL - draft design

  • Working in collaboration with SLAC and LLNL.
  • Developing water-cooled rotating wheel design.
  • 0.4 radiation length titanium alloy rim.
  • Radius approximately 0.5 m.
  • Rotates at approximately 2000 rpm.

Target Wheel Design

LLNL - draft design

  • Iterative design evolution between LLNL and DL
  • Constraints:
  • Wheel rim speed fixed by thermal load and cooling rate
  • Wheel diameter fixed by radiation damage and capture optics
  • Materials fixed by thermal and mechanical properties and pair-production cross-section (Ti6%Al4%V)
  • Wheel geometry constrained by eddy currents.

DL - draft design


Power Deposition & Cooling

LLNL - draft design

LLNL - draft design

Approximately 10% of photon beam

power is deposited in target wheel


Water fed into wheel via rotating water union on drive shaft.


Eddy Current Simulations

LLNL - preliminary

LLNL - preliminary

  • Initial “Maxwell 3D” simulations by W. Stein and D. Mayhall at LLNL indicated:
  • ~2MW eddy current power loss for 1m radius solid Ti disc in 6T field of AMD.
  • <20kW power loss for current 1m radius Ti rim design.
  • However - Simulations do not yet agree with SLAC rotating disc experiment.
    • 8” diameter Cu disc rotating in field of permanent magnet.
  • OPERA-3D simulations are starting at RAL.

Future Target Activities

  • Prototyping centred at Daresbury
  • Proposing 3 staged prototypes over 3 years (LC-ABD funding bid)
  • Measure eddy current effects
    • top priority
    • major impact on design
  • Test reliability of drive mechanism and vacuum seals.
  • Test reliability of water-cooling system for required thermal load
  • Develop engineering techniques for manufacture of water-cooling channels.
  • Develop techniques for balancing wheel.
  • CI staff to work on design, operation and data analysis.
  • Timeline integrated with our international collaborators.
  • Remote-handling design centred at RAL
  • Essential that remote-handling design evolves in parallel with target design.
  • Determines target hall layout and cost.
  • Related CI activities
  • Activation simulations (in collaboration with DESY and ANL)
  • Positron production and capture simulations (see spin tracking activities)
  • Photon collimator design and simulation (CI PhD student + ASTeC expertise).

Robust Spin Transport

  • Developing reliable software tools that allow the machine to be optimised for spin polarisation as well as luminosity. Aiming to carry out full cradle-to-grave simulations.
  • Currently carrying out simulations of depolarisation effects in damping rings, beam delivery system and during bunch-bunch interactions.
  • Developing simulations of spin transport through the positron source.
  • Will soon extend simulations to main linac, etc.

Energy spectrum and circular polarisation of photons from helical undulator.

Trajectories of electrons through helical undulator.

Example of SLICKTRACK simulation showing depolariation of electrons in a ring.

  • Collaborating with
    • T. Hartin (Oxford)
    • P. Bambade, C. Rimbault (LAL)
    • J. Smith (Cornell)
    • S. Riemann, A. Ushakov (DESY)

Depolarisation Processes

Both stochastic spin diffusion through photon emission and classical spin precession in inhomogeneous magnetic fields can lead to depolarisation.

1 mrad orbital deflection  30° spin precession at 250GeV.

Largest depolarisation effects are expected at the Interaction Points.

Photon emission

Spin precession


Software Tools

e+ source

Packages in parentheses will be evaluated at a later date.


Positron Source Simulations

  • Polarisation of photon beam
    • Ongoing SPECTRA simulations (from SPRING-8)
    • Benchmarked against URGENT
  • Depolarisation of e- beam
    • So far, review of analytical studies only
    • eg Perevedentsev etal “Spin behavior in Helical Undulator.” (1992)
  • Target spin transfer
    • GEANT4 with polarised cross-sections provided by E166 experiment.
    • Installed and commissioned at University of Liverpool.
  • Capture Optics
    • Adding Runge-Kutta and Boris-like T-BMT integration routine to ASTRA

10MeV photons


Bunch-Bunch Simulations

During Interaction

Before Interaction

After Interaction

Spread in Polarisation

Large Y

Before Interaction

During Interaction

After Interaction

Low Q

  • Opposing bunches depolarise one another at the IP(s).
  • Studies of different possible ILC beam parameters (see table on right).
  • Much work ongoing into theoretical uncertainties.

SLICKTRACK Simulations

  • Damping Rings
    • OCS and TESLA lattices analysed for ILC DR group.
    • Depolarisation shown to be negligible.
    • Ongoing rolling study.
  • Beam Delivery System
    • First (linear spin motion) simulations presented at EPAC ’06 conference.
    • Ongoing rolling study.

Future Spin Transport Activities

  • MERLIN development as a cross-check of main results
    • Andy Wolski training CI RA’s and PhD students
  • Non-linear orbital maps interfaced to SLICKTRACK
    • Modelling sextupoles, octupoles, undulator, etc
  • Integrated positron source simulations
    • Rolling study
  • Beam-beam theoretical uncertainties
    • Incoherent pair production and EPA, T-BMT validity, etc…
    • Comparison with GUINEA-PIG
  • Novel polarisation flipping in positron source
    • Flipping polarity of source without spin rotators (cost saving)
  • Polarimetry and polarisation optimisation (University of Lancaster)
    • Developing techniques to optimise polarisation at the IP
  • Optimising use of available computing resources at DL, Liverpool and on the GRID

Summary and Outlook

  • CI has leadership role in many areas of ILC positron source R&D
  • Recent results presented at PAC ’05, HEP ’05, EPAC ’06, ICHEP ’06, SPIN ’06, …
  • Proposed new prototyping programme to be based at Daresbury
    • Addressing key issues for undulator + target
  • Establishing base of spin tracking expertise at CI
  • CI paradigm is working