Ian Bailey

University of Liverpool / Cockcroft Institute

Depolarization Effects and Other Aspects

heLiCal collaborationL.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

• 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.

•Currently extending simulations to main linac, etc.

0E+00

5E+13

1E+14

2E+14

2E+14

3E+14

3E+14

0.0 20.0 40.0 60.0 80.0 100.0

Photon Energy (MeV)

Flu

x (p

ho

ton

s/s/

mA

/0.1

%)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

Cir

cula

r P

ola

risa

tio

n R

ate

20 x 20 urad flux2 x 2 urad flux20 x 20 urad polarisation2 x 2 urad polarisation

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)

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.

Depolarisation Processes

Photon emission

Spin precession

( 2)

2spin orbit

g

Undulator Collimator / Target Capture Optics

PhysicsProcess

Electrodynamics Standard Model T-BMT

(spin spread)

Packages

SPECTRA, URGENT

GEANT4, FLUKA ASTRA

Damping ring Main Linac / BDS

Interaction

Region

PhysicsProcess

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.

e+ source

Software Tools

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 SimulationsOpposing 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.

Large Y

During Interaction

Before Interaction

After Interaction

Spread in Polarisation

Low Q

Before Interaction

During Interaction

After Interaction

CA

IN s

imul

aton

s

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

Bunch-Bunch Simulations (2)

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

bunchP S

2 20

ˆˆ ˆ1 ( ) ( ) ( )S n s m s l s

2 2 2 21 1( )

2 2

dP d d

dt dt dt

Spread of the projections of spins on a horizontal plane reaches equilibrium (250) : 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)

• 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

Further Spin Transport Activities