Electron Polarization
In MEIC
Pavel Chevtsov
Review 09/2010
Page 1
Outline
• MEIC Electron Polarization Requirements
• MEIC Electron Polarization Design
• Universal Spin Rotator
• Spin dynamics in MEIC Electron Ring
• Summary
Review 09/2010
Page 2
Electron Polarization Requirements
• High (>80%) polarization of stored high-current
electron beam
• Sufficiently long polarization lifetime (>10 min) in 3 to
11 GeV energy range
• Longitudinal polarization available at all interaction
points
• Alternating bunch-to-bunch polarization
• Comparably high polarization of a stored positron
beam through S-T self-polarization effect
• Sufficiently fast self-polarization at low positron beam
energies (~3 GeV)
Review 09/2010
Page 3
Electron Polarization Design Choices
• High electron polarization through full-energy injection
from CEBAF
• Over 80% electron beam polarization from CEBAF’s state-ofthe-art polarized source/injector
• High polarization well preserved during acceleration in the
multi-pass recirculated SRF Linac of CEBAF
• Polarization direction coming out of CEBAF easily controlled
by two Wien filters in the CEBAF injector
• Longitudinally-polarized electron beam injected from CEBAF
into a long straight section of the MEIC electron collider ring
• After polarization degradation, beam replaced by new bunches
from CEBAF.
• Possibility of continuous top-off injection from CEBAF
Review 09/2010
Page 4
Electron Polarization Design Choices
• Maintaining high electron beam polarization
• S-T self-polarization by making the spin anti-parallel to bending
magnetic field in the arcs
• Spin-tune superconducting solenoid(s) in straight(s) to adjust the
spin tune for polarization stability
• Longitudinal electron polarization at IP’s using spin rotators
•
•
•
•
Four 90º spin rotators at the ends of the arcs
Upstream spin rotator rotates spin from vertical to longitudinal
Downstream spin rotator rotates spin back to vertical
Spin rotators work over entire energy range
Review 09/2010
Page 5
Electron Polarization In Figure-8 Collider Ring
Review 09/2010
Page 6
Impact of Figure-8 Shape
• Figure-8 shape chosen exclusively due to its unique advantages for
ion polarization
• Electron collider ring matches ion figure-8 footprint to share same
tunnel and accommodate interaction points
• No complications due to figure-8 shape for electrons, potentially
advantageous due to energy-independent zero spin tune
• SC solenoids in the straights (where spin is longitudinal) to improve
polarization stability
aˆ  2n  
aˆ  (2n   )
Review 09/2010
x

P
s
2n    (2n   )  0
Page 7
Positron Beam
• Positrons can be accelerated in CEBAF
as effective as electrons with similar
beam quality
• Both polarized and unpolarized positron
sources being developed for fixed target
program
• Only unpolarized positron source can
provide sufficiently high beam current
• Positron beam polarized by S-T selfpolarization
• Polarization time too long at low energies
(<6 GeV). Shortening polarization time by
• Using small damping ring to enhance S-T effect
• Accelerating to high (~7 GeV) energy for quick
self-polarization and subsequent deceleration
Review 09/2010
Self-Polarization Time
GeV
Hours
3
14.6
4
3.5
5
1.1
7
0.21
9
0.06
Page 8
Spin Flip and Polarization Lifetime
From CEBAF and at IP’s
spin
spin
spin
spin
In arcs
spin
spin
Polarization
spin
spin
depolarization
• Bunch trains with alternating polarization from CEBAF by changing polarization
of photo-injector’s driving laser
• S-T effect could cause significant depolarization of wrong-direction polarization
• Depolarized beam can be replaced periodically by injection from CEBAF,
provided polarization lifetime is not too short (>10 min)
• Possibility of multiple spin flips by spin resonance crossing
Review 09/2010
Page 9
Universal Spin Rotator
• Spin rotator rotates spin from vertical to
longitudinal
• works in whole 3 to 11 GeV energy range
• same orbit geometry for different energies
• Universal spin rotator (USR) composed
of two solenoid interspersed with arc
bending dipoles
• Fixed dipole bending angles  no orbit
change, solenoid strengths adjusted for
different energies
• x-y coupling compensated for each
solenoid individually
E (GeV)
1
BL1 (Tm)
1
2
BL2 (Tm)
2
3
/2
15.7
/3
0
0
/6
4.5
/4
11.8
/2
/2
23.6
/4
6
0.62
12.3
2/3
1.91
38.2
/3
9
/6
15.7

2/3
62.8
/2
12
0.62
24.6
4/3
1.91
76.4
2/3
Review 09/2010
Page 10
Solenoid Coupling Compensation
• Transport matrix of a solenoid
I 22 sin  

I 22 cos  
Tue Jul 13 23:59:50 2010
OptiM - MAIN: - C:\Working\ELIC\MEIC\Optics\5GeV Electe. Ring\sol_rot_2.opt
1
0   I 22 cos 

AL    I 22 sin 
15
A
M sol   L
0
(H. Sayed)
BdL = 28.7 T m
B
B
L
Decoupling
insertion
L/2
B
-1
e-
0
v
DISP_X&Y[m]
(V. Livinenko & A. Zholents, 1980)
BETA_X&Y[m]
• X-Y decoupling scheme
0
Review 09/2010
BETA_Y
solenoid
4.16 m
L/2
 A TA
T 0 
  M sol / 2   L / 2 L / 2
M sol / 2  
0
 0 T 

BETA_X
0


 AL / 2TAL / 2 
DISP_X
DISP_Y
17.9032
decoupling quad insert
M=
T
0
0 T
Page 11
solenoid
4.16 m
Spin Dynamics
• Analytic estimate of equilibrium electron polarization
gives ~85%
• Preliminary tracking results (D. Barber) consistent
with ~85%
• More detailed depolarization/polarization studies
including spin matching underway (in collaboration
with Profs. D. Barber and A. Kondratenko).
Review 09/2010
Page 12
Summary
• Electron polarization requirements
•
•
•
•
High polarization (>80%)
Sufficient polarization lifetime
Longitudinal polarization at IP’s
Alternating bunch-to-bunch polarization / spin-flipping
• High electron polarization by full-energy injection from CEBAF
• High degree of polarization maintained by S-T effect and by spin
tune control with spin-tune solenoids
• Longitudinal polarization at IP’s provided by four spin rotators in
the entire 3 to 11 GeV/c energy range without affecting orbit
• No complications due to figure-8 shape, potential advantage
• Possibility of polarized positron beam with unpolarized positron
source and S-T self-polarization
• Alternating polarization by flipping polarization at the source
• Detailed spin tracking and simulations underway in collaboration
with Profs. D. Barber and A. Kondratenko
Review 09/2010
Page 13