ILC Positron Source

1
ILC Positron Source Spin Tracking
Ian Bailey University of Liverpool / Cockcroft
Institute
2
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)
3
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.

4
ILC Positron Source Layout
Original baseline layout of ILC with undulator at
150GeV position in main linac.
5
ILC Positron Source Beamlines
EUND - undulator TAPA - target station A TAPB -
target station B
Not to scale in any dimension!
6
Baseline Positron Source RD

• Area Systems Group RD 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 DL RAL Cornell SLAC LLNL
ANL DESY BINP
CI - denotes a significant Cockcroft Institute
contribution
7
Superconducting undulator prototypes for ILC
Parameters of first prototype
Superconducting bifilar helix
Design field 0.8 T
Period 14 mm
Magnet bore 4 mm
Winding bore 6 mm
Winding section 4 ? 4 mm2
Overall current density 1000 A/mm2
Peak field 1.8 T
First (20 period) prototype
Hall probe measurements of first prototype
Cut-away showing winding geometry
8
Superconducting undulator prototypes for ILC
Superconducting bifilar helix
First (20 period) prototype
Section of second prototype, showing NbTi wires
in Al former.
9
Prototype Parameters
I II III IV V
Former material Al Al Al Iron Iron
Pitch, mm 14 14 12 12 11.5
Groove shape rectangular trapezoidal trapezoidal trapezoidal rectangular
Winding bore, mm 6 6 6.35 6.35 6.35
Vac bore, mm 4 4 4 4.5 (St Steel tube) 5.23 (Cu tube)
Winding 8-wire ribbon, 8 layers 9-wire ribbon, 8 layers 7-wire ribbon, 8 layers 7-wire ribbon, 8 layers 7-wire ribbon, 8 layers
Sc wire CuSc 1.351 CuSc 1.351 CuSc 1.351 CuSc 1.351 CuSc 0.91
Status Completed and tested Completed, tested and sectioned Completed and tested Completed and tested Manufacture in progress
All completed prototypes have reached design
field Peak field specification of lt /- 1
demonstrated Demonstrated predicted enhancement
of field by 0.44T using iron former
10
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
11
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
12
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

13
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 connection
14
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

15
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
Photon beam pipe
Motor
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.

16

• 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
  (Ti6Al4V)
• Wheel geometry constrained by eddy currents.

DL - draft design
17
Approximately 10 of photon beam power is
deposited in target wheel (30kW)
Water fed into wheel via rotating water union on
drive shaft.
18

• 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.
• lt20kW 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.

19

• 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).

20

• 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)
21
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
22
Software Tools
Undulator Collimator / Target Capture Optics
Physics Process Electrodynamics Standard Model T-BMT (spin spread)
Packages SPECTRA, URGENT GEANT4, FLUKA ASTRA
e source
Damping ring Main Linac / BDS Interaction Region
Physics Process T-BMT (spin diffusion) T-BMT Bunch-Bunch
Packages SLICKTRACK, (Merlin) SLICKTRACK (Merlin) CAIN2.35 (Guinea-Pig)
Packages in parentheses will be evaluated at a
later date.
23
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
24

• 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.

25

• 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.

26

• MERLIN development as a cross-check of main
  results
• Andy Wolski training CI RAs 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

27

• CI has leadership role in many areas of ILC
  positron source RD
• 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