Depolarization Effects and
Other Aspects
Ian Bailey
University of Liverpool / Cockcroft Institute
heLiCal collaboration
L.I. Malysheva 1,2, I.R. Bailey 1,2, D.P. Barber 3,2,1,
E. Baynham 6, A. Birch 1,5, T. Bradshaw 6, A. Brummitt 6,
S. Carr 6, J.A. Clarke 1,2,5, P. Cooke 1,2, J.B. Dainton 1,2,
Y. Ivanyushenkov 6, L.J. Jenner 1,2, A. Lintern 6,
O.B. Malyshev 1,5, G.A. Moortgat-Pick 1,4, J. Rochford 6,
P. Schmid 3 and D.J. Scott 1,2,5
1Cockcroft
Institute,
2Department of Physics, University of Liverpool,
3DESY, Deutsches Electronen Synchrotron,
4Institute of Particle Physics Phenomenology, University of Durham,
5CCLRC ASTeC Daresbury Laboratory.
6CCLRC Rutherford Appleton Laboratory
Talk Overview
• Accelerator Physics Issues (GigaZ) on
behalf of Gudi Moortgat-Pick
• Status of Robust Spin Transport
simulations by HeLiCal collaboration and
colleagues.
mZ= 91.1876+/- 0.0021 GeV
Z=2.4952 +/-0.0023 GeV
=> undulator at 147.5 GeV
position?
Robust Spin Transport
3E+14
• Currently carrying out simulations of
depolarisation effects in damping rings,
beam delivery system and during bunchbunch interactions.
•Currently extending simulations to main
linac, etc.
Collaborating with
T. Hartin (Oxford)
P. Bambade, C. Rimbault (LAL)
J. Smith (Cornell)
S. Riemann, A. Ushakov (DESY)
3E+14
0.6
0.4
2E+14
0.2
20 x 20 urad flux
2 x 2 urad flux
20 x 20 urad polarisation
2 x 2 urad polarisation
2E+14
1E+14
0.0
-0.2
-0.4
-0.6
5E+13
-0.8
0E+00
0.0
20.0
40.0
60.0
Photon Energy (MeV)
80.0
-1.0
100.0
Circular Polarisation Rate
0.8
Flux (photons/s/mA/0.1%)
• 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.
1.0
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.
Depolarisation Processes
Both stochastic spin diffusion through
photon emission and classical spin
precession in inhomogeneous magnetic
fields can lead to depolarisation.
 spin
( g  2)

 orbit
2
Photon emission
1 mrad orbital deflection  30° spin
precession at 250GeV.
Largest depolarisation effects are
expected at the Interaction Points.
Spin precession
Software Tools
Undulator
Capture
Optics
Standard Model
T-BMT
(spin spread)
GEANT4, FLUKA
ASTRA
URGENT
Damping ring
T-BMT
Physics
Process (spin diffusion)
Packages SLICKTRACK,
(Merlin)
Main Linac /
BDS
Interaction
Region
T-BMT
Bunch-Bunch
SLICKTRACK
(Merlin)
CAIN2.35
(Guinea-Pig)
Packages in parentheses will be evaluated at a later date.
e+ source
Electrodynamics
Physics
Process
Packages SPECTRA,
Collimator / Target
Positron Source Simulations
Polarisation of photon beam
•Ongoing SPECTRA simulations (new version from SPRING-8)
•Benchmarked against URGENT (F77 code)
•Depolarisation of e- beam
•Analytic studies
•eg Perevedentsev etal “Spin behavior in Helical Undulator.” (1992)
•c.f. trajectory simulations
•Target spin transfer
• GEANT4 (v 8.2) with polarised cross-sections provided by Andreas Schaelicke,
DESY (E166 experiment)
• Installed and commissioned at University of Liverpool
•Capture Optics
•Adding Runge-Kutta and Boris-like T-BMT integration routine to ASTRA
Bunch-Bunch Simulations
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.
CAIN simulatons
Before
Interaction
During
Interaction
After
Interaction
Spread in
Polarisation
Large Y
Before
Interaction
Low Q
During
Interaction
After
Interaction
Bunch-Bunch Simulations (2)

Theoretical work ongoing into
 validity of T-BMT equation in strong fields (checked by Gudi)
 higher-order QED processes
 spin correlations in pair-production processes
 validity of equivalent photon approximation (EPA) for incoherent pair
production processes
Dominant at
ILC energies
Tony Hartin, Oxford
Tony Hartin, Oxford
Tony Hartin, Oxford
Damping rings
• In ideal Damping Ring depolarising effects are expected
to be negligible
• Enhancement of synchrotron radiation (wigglers) might
lead to the depolarisation effects
• Two out of seven reference lattices were selected: OCS
6km (circle) and TESLA 17 km (dogbone)
• Two energies: 5.066GeV and 4.8 GeV (close to
resonance)
• SLICKTRACK: Monte-Carlo simulation of the effects of
synchrotron radiation, i.e. evolution of the spin distribution
over a few damping times including full 3-D spin motion
OCS Spin Diffusion at 5.066GeV for spins
initially at 100 mrad from n0
P S
bunch
S  1   2   2 nˆ0 ( s)   mˆ ( s )   lˆ( s )
of the
of spins
dP
1 Spread
d
1 d projections
2
2
2
2

  
(     )
0
reaches
dt
2 plane
dt
2 dt equilibrium (25 ) :
on a horizontal
Longitudinal
polarisation can survive DR!!! Direction of
polarisation vector depends on time.
OCS Spin Diffusion at 4.8 GeV
and 5.066 GeV for all spins
parallel to n0
The loss of polarisation is negligible
SLICKTRACK Simulation
Summary
• Loss of the vertical component of polarisation in DR is insignificant.
• Contrary to common belief there is little decoherence of the
horizontal components of spin, thus the direction of the horizontal
component polarisation vector depend on time at which the kickers
are fired
• Our results are in excellent agreement with simple analytical model
( see http://www.desy.de/~mpybar/mypapers.html and arXiv:physics/9709025)
• Spin rotators before DR required
• Loss of polarisation in BDS is negligible confirming earlier work
(J.Smith, Cornell)
Future plans
We will maintain a rolling study to include extra effects as necessary
Include non-linear optics (Collaboration with E. Forest)
Linac simulations (started)
Further Spin Transport Activities
• MERLIN development as a cross-check of main results
• 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