Transcript E-166 Undulator-Based Production of Polarized Positrons A proposal for the 50 GeV Beam in the FFTB Thursday, June 12, 2003 K-P.

E-166
Undulator-Based Production
of Polarized Positrons
A proposal for the 50 GeV Beam in the FFTB
Thursday, June 12, 2003
K-P. Schüler and J. C. Sheppard
EPAC June 2003
Undulator-Based Production of Polarized Positrons
E-166 Collaboration
(45 Collaborators)
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EPAC June 2003
Undulator-Based Production of Polarized Positrons
E-166 Collaborating Institutions
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(15 Institutions)
EPAC June 2003
E-166 Experiment
E-166 is a demonstration of undulator-based
polarized positron production for linear colliders
- E-166 uses the 50 GeV SLAC beam in conjunction with 1 m-long, helical
undulator to make polarized photons in the FFTB.
- These photons are converted in a ~0.5 rad. len. thick target into polarized
positrons (and electrons).
- The polarization of the positrons and photons will be measured.
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The Need for a Demonstration Experiment
Production of polarized positrons depends on the
fundamental process of polarization transfer in an
electromagnetic cascade.
While the basic cross sections for the QED processes
of polarization transfer were derived in the 1950’s,
experimental verification is still missing
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The Need for a Demonstration Experiment
Each approximation in the modeling is well justified in
itself.
However,the complexity of the polarization transfer
makes the comparison with experiment important so
that the decision to build a linear collider w/ or w/o a
polarized positron source is based on solid ground.
Polarimetry precision of 5% is sufficient to prove the
principle of undulator based polarized positron
production for linear colliders.
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Physics Motivation for Polarized Positrons
Polarized e+ in addition to polarized e- is recognized as a
highly desirable option by the WW LC community (studies in
Asia, Europe, and the US)
Having polarized e+ offers:
– Higher effective polarization -> enhancement of effective
luminosity for many SM and non-SM processes,
– Ability to selectively enhance (reduce) contribution from
SM processes (better sensitivity to non-SM processes,
– Access to many non-SM couplings (larger reach for non-SM
physics searches),
– Access to physics using transversely polarized beams (only
works if both beams are polarized),
– Improved accuracy in measuring polarization.
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Physics Motivation: An Example
 
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Separation of the selectron pair eL eL in ee  eL,ReL,R
with longitudinally polarized beams to test association of
chiral quantum numbers to scalar fermions in SUSY
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transformations
NLC/USLCSG Polarized Positron System Layout
2 Target assembles for redundancy
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TESLA, NLC/USLCSG, and E-166 Positron Production
Table 1: TESLA, NLC/USLCSG, E-166 Polarized Positron Parameters
Parameter
Units
TESLA*
NLC
E-166
GeV
150-250
150
50
Beam Energy, Ee
10
9
3x10
8x10
1x1010
Ne/bunch
2820
190
1
Nbunch/pulse
Hz
5
120
30
Pulses/s
planar
helical
helical
Undulator Type
1
1
0.17
Undulator Parameter, K
cm
1.4
1.0
0.24
Undulator Period u
st
MeV
9-25
11
9.6
1 Harmonic Cutoff, Ec10
photons/m/e
1
2.6
0.37
dN/dL
m
135
132
1
Undulator Length, L
Ti-alloy Ti-alloy Ti-alloy, W
Target Material
r.l.
0.4
0.5
0.5
Target Thickness
%
1-5
1.8†
0.5
Yield
%
25
20
Capture Efficiency
12
12
8.5x10
1.5x10
2x107
N+/pulse
3x1010
8x109
2x107
N+/bunch
%
40-70
40-70
Positron Polarization
*TESLA baseline design; TESLA polarized e+ parameters (undulator and
polarization) are the same as for the NLC/USLCSG
† Including the effect of photon collimation at  = 1.414.
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E-166 Vis-à-vis a Linear Collider Source
E-166 is a demonstration of undulator-based
production of polarized positrons for linear colliders:
- Photons are produced in the same energy range and
polarization characteristics as for a linear collider;
-The same target thickness and material are used as in
the linear collider;
-The polarization of the produced positrons is expected
to be in the same range as in a linear collider.
-The simulation tools are the same as those being used
to design the polarized positron system for a linear
collider.
- However, the intensity per pulse is low by a factor of
2000.
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EPAC June 2003
E-166 Beamline Schematic
50 GeV, low emittance electron beam
2.4 mm period, K=0.17 helical undulator
0-10 MeV polarized photons
0.5 rad. len. converter target
51%-54% positron polarization
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E-166 Helical Undulator Design, =2.4 mm, K=0.17
PULSED HELICAL UNDULATOR FOR TEST AT
SLAC THE POLARIZED POSITRON PRODUCTION
SCHEME. BASIC DESCRIPTION.
Alexander A. Mikhailichenko
CBN 02-10, LCC-106
Table 3: FFTB Helical Undulator System Parameters
Parameter
Number of Undulators
Length
Inner Diameter
Period
Field
Undulator Parameter, K
Current
Peak Voltage
Pulse Width
Inductance
Wire Type
Wire Diameter
Resistance
Repetition Rate
Power Dissipation
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T/pulse
Units
m
mm
mm
kG
Amps
Volts
s
H
mm
ohms
Hz
W
0
C
Value
1
1.0
0.89
2.4
7.6
0.17
2300
540
30
0.9x10-6
Cu
0.6
0.110
30
260
2.7
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Helical Undulator Radiation
Circularly Polarized Photons
30.6
K2



photons / m / e  0.37 photons / e
2
dL u  mm 1  K
dN
Ec10  24  MeV 
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E
e
50 GeV 
u  mm 1  K
2

2
 9.6 MeV
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Photon Intensity, Angular Dist., Number, Polarization
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Polarized Positrons from Polarized ’s
Circular polarization of
photon transfers to the
longitudinal polarization
of the positron.
Positron polarization
varies with the energy
transferred to the
positron.
(Olsen & Maximon, 1959)
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Polarized Positron Production in the FFTB
Polarized photons pair produce
polarized positrons in a 0.5 r.l.
thick target of Ti-alloy with a
yield of about 0.5%.
Longitudinal polarization of the
positrons is 54%, averaged over
the full spectrum
Note: for 0.5 r.l. W converter,
the yield is about 1% and the
average polarization is 51%.
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Polarimetry
K-Peter Schüler Presentation
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Polarimeter Overview
4 x 109   4 x 107
1 x 1010 e 4 x 109 
4 x 109 
 2 x 107 e+
4 x 105 e+  1 x 103 
2 x 107 e+
 4 x 105 e+
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Transmission Polarimetry of
(monochromatic) Photons
M. Goldhaber et al. Phys. Rev. 106 (1957) 826.


 comp 
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 phot   comp   pair
 0  P Pe  P
all unpolarized contributions cancel
in the transmission asymmetry 
(monochromatic case)
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Transmission Polarimetry of
Photons
Monochromatic Case
Analyzing Power:
But, undulator photons are not monochromatic:
 Must use number or energy weighted integrals 
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Transmission Polarimetry
of Positrons
2-step Process:
•
•
re-convert e+   via brems/annihilation process
– polarization transfer from e+ to  proceeds
in well-known manner
measure polarization of re-converted photons
with the photon transmission methods
– infer the polarization of the parent positrons
from the measured photon polarization
Experimental Challenges:
•
•
large angular distribution of the positrons
Fronsdahl & Überall;
Olson & Maximon;
at the production target:
Page; McMaster
– e+ spectrometer collection & transport efficiency
– background rejection issues
angular distribution of the re-converted photons
– detected signal includes large fraction of Compton scattered photons
– requires simulations to determine the effective Analyzing Power
Formal Procedure:
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Spin-Dependent Compton
Scattering
Simulation with modified GEANT3
(implemented by V. Gharibyan)
• standard GEANT is unpolarized
• ad-hoc solution:
- substitute unpolarized Compton subroutines
with two spin-dependent versions (+1 and -1)
and run these in sequence for the same
same beam statistics
- then determine analyzing power from this data
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Analyzer Magnets
g‘ = 1.919  0.002
for pure iron,
Scott (1962)
Error in e- polarization is dominated by knowledge
in effective magnetization M along the photon
trajectory:
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Photon Analyzer Magnet:
Positron Analyzer Magnet:
Pe  0.07
Pe / Pe  0.05
active volume
50 mm dia. x 150 mm long
50 mm dia. x 75 mm long
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Photon Polarimeter Detectors
E-144 Designs:
Si-W Calorimeter
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Threshold Cerenkov (Aerogel)
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Positron Polarimeter Layout
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Positron Transport System
e+
transmission
(%) through
spectrometer
photon
background
fraction
reaching
CsI-detector
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CsI Calorimeter Detector
Crystals:
Number of crystals:
Typical front face of one crystal:
Typical backface of one crystal:
Typical length:
Density:
Rad. Length
Mean free path (5 MeV):
No. of interaction lengths (5 MeV):
Long. Leakage (5 MeV):
from BaBar Experiment
4 x 4 = 16
4.7 cm x 4.7 cm
6 cm x 6 cm
30 cm
4.53 g/cm³
8.39 g/cm² = 1.85 cm
27.6 g/cm² = 6.1 cm
4.92
0.73 %
Photodiode Readout (2 per crystal):
Hamamatsu S2744-08
with preamps
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Expected Photon
Polarimeter Performance
Si-W Calorimeter
Expected measured energy asymmetry δ = (E+-E-)/(E++E-)
and energy-weighted analyzing power
AE
determined through analytic integration and, with good agreement,
through special polarized GEANT simulation
  0.0266
Pe  0.07
AE  0.62
Energy-weighted Mean
Aerogel Cerenkov
will measure P for E > 5 MeV
(see Table 12)
1% stat. measurements very fast (~ minutes),
main syst. error of ΔP /P ~ 0.05 from Pe
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Expected Positron
Polarimeter Performance I
Simulation based on modified GEANT code, which correctly
describes the spin-dependence of the Compton process
Photon Spectrum & Angular Distr.
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Number- & Energy-Weighted
Analyzing Power vs. Energy
10 Million simulated e+ per point & polarity
on the re-conversion target
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Expected Positron
Polarimeter Performance II
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Analyzing Power
vs. Target Thickness
Analyzing Power
vs. Energy Spread
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Expected Positron
Polarimeter Performance III
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Table 13
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Polarimetry Summary
• Transmission polarimetry is well-suited
for photon and positron beam measurements
in E166
• Analyzing power determined from simulations
is sufficiently large and robust
• Measurements will be very fast
with negligible statistical errors
• Expect systematic errors of ΔP/P ~ 0.05
from magnetization of iron
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EPAC June 2003
Beam Request
J. C. Sheppard Presentation II
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E-166 Beam Request
E-166 Beam Parameters
Ee
GeV
50
frep
Hz
30
Ne
e1x1010
x=y
m-rad
3x10-5
xy
m
5.2, 5.2
x,y
m

E/E


6 weeks of activity in the SLAC FFTB:
•2 weeks of installation and check-out
•1 week of check-out with beam
•3 weeks of data taking:
roughly 1/3 of time on photon measurements,
2/3 of time on positron measurements.
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E-166 Beam Measurements
•Photon flux and polarization as a function of K
(P ~ 75% for E > 5 MeV).
•Positron flux and polarization for K=0.17, 0.5 r.l. of Ti vs.
energy. (Pe+ ~ 50%).
•Positron flux and polarization for 0.1 r.l. and 0.25 r.l. Ti and
0.1, 0.25, and 0.5 r.l. W targets.
•Each measurement is expected to take about 20 minutes.
•A relative polarization measurement of 5% is sufficient to
validate the polarized positron production processes.
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E-166 Institutional Responsibilities
Electron Beamline
Undulator
Positron Beamline
Photon Beamline
Polarimetry:
Overall
Magnetized Fe Absorbers
Cerenkov Detectors
Si-W Calorimeter
CsI Calorimeter
DAQ
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SLAC
Cornell
Princeton/SLAC
SLAC
DESY
DESY
Princeton
Tenn./ S. Carolina
DESY/Humboldt
Humboldt/Tenn./S. Car.
EPAC June 2003
E-166 as Linear Collider R&D
– E-166 is a proof-of-principle demonstration of
undulator based production of polarized positrons
for a linear collider.
– The hardware and software expertise developed
for E-166 form a basis for the implementation of
polarized positrons at a linear collider.
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E-166 Costs
Experiment E-166_attach1-052703.xls
(J.
Weisend, E-166 Impact Report)
Sub-system
EFD
Labor
SLAC
Labor
SLAC
M&S
Coll.
Contr.
Exist.
FFTB
Elec. BT
50
63
49
99
Gam. BT
54
18
58
Posi. BT
60
47
Gen/Infrstr
10
Grand Total
172
39
Total
67
Exist.
nonFFTB
55
50
29
35
244
75
60
41
33
315
15
25
50
20
10
130
143
207
259
157
133
1071
382
(All entries in k$)
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E-166 Institutional Responsibilities
Experiment E-166_attach1-052703.xls
(J. Weisend, E-166 Impact Report)
SLAC
Cornell
Princeton
DESY
U. Tenn/U. S. Carolina
Humboldt U.
Simulations
Erroneous
Beamline, infrastructure
Undulator, pulser
Spctr. Magnets, Aerogel cntr
Fe absorber magnets
Si-W cal., DAQ
CsI cal, DAQ
All
HSB PS
207
85
60
20
15
15
50
14
(All entries in k$)
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(polarimetry extra)
Undulator Photon Beam I
Undulator basics (1st harmonic shown only)
E166 undulator parameters
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(polarimetry extra)
Undulator Photon Beam II
photon spectrum, angular distribution and polarization
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(polarimetry extra)
Positron Beam Simulation
distributions behind the converter target (0.5 r.l. Ti)
based on polarized EGS shower simulations by K. Flöttmann
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(polarimetry extra)
Low-Energy Polarimetry
Candidate Processes
• Photons: Compton Scattering on polarized electrons
– forward scattering
(e.g. Schopper et al.)
– backward scattering
– transmission method
(e.g. Goldhaber et al.)
• Positrons: all on ferromagnetic = polarized e- targets
– Annihilation polarimetry (e+e-  )
(e.g.
Corriveau et al.)
– Bhabha scattering (e+e-  e+e-)
(e.g.
Ullmann et al.)
– brems/annihilation (e+  ) plus -transmission (Compton)
polarimetry
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(polarimetry extra)
Trade-offs
Principal difficulties of e+ polarimetry:
– huge multiple-scattering at low energies even in thin targets
– cannot employ double-arm coincidence techniques
or single-event counting due to poor machine duty cycle
– low energies below 10 MeV, vulnerable to backgrounds
All of the candidate processes have been explored by us:
 the transmission method is the most suitable
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(polarimetry extra)
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Compton Cross Section
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(polarimetry extra)
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e+ polarimeter: typical
GEANT output (example) I
EPAC June 2003
(polarimetry extra)
e+ polarimeter: typical
GEANT output (example) II
*
*
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Assuming 2 x 105 e+ per pulse
(1% e+ spectrometer transmission)
EPAC June 2003