1. Polarized Electrons for Polarized Positrons
A Proof of Principle Experiment
Alexandre Camsonne*, Jonathan Dumasǂ*, Arne Freyberger*, Joe Grames*, Matt Poelker*, Jean-Sébastien Réalǂ, Eric Voutierǂ…
ǂ Laboratoire de Physique Subatomique et de Cosmologie
Grenoble, France
* Thomas Jefferson National Accelerator Facility
Newport-News, Virginia, USA
Physics motivations
Polarized positron production
Technical challenges
GEANT4 simulations
PEPPo experiment
Conclusions
GDR Nucléon
February 11, 2010, Orsay

2. Physics Motivations
Eric Voutier
Unpolarized cross section
Beam charge difference
Beam spin difference
The combination of polarized electrons and polarized positrons offers the unique opportunity to access a pure interference signal.
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3. The photon polarization is transferred to positrons via pair creation.
Polarized Positron Production
Eric Voutier
Compton Scattering
1.28 GeV
≤ 56 MeV
~36 MeV
A circular photon beam is produced by Compton scattering of a laser beam off electrons.
The photon polarization is transferred to positrons via pair creation.
Polarized e+ convert into circularly polarized photons into a lead target.
The photon polarization is analyzed via Compton scattering off a magnetized iron target.
P(e+) = 73 ± 15 ± 19 %
M. Fukuda et al, PRL 91 (2003) T. Omori et al, PRL 96 (2006)
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4. iron e+ Undulator Scheme Polarized Positron Production reconversion
Eric Voutier
Undulator Scheme
e+ Diag. γ
Diag.
e- Dump
46.6 GeV e-
W Target
10 MeV γ
Undulator
Spectrometer
e- to Dump
Bending magnets
reconversion
target
magnetized
iron
detector e+
spectrometer
G. Alexander et al, PRL 100 (2008) G. Alexander et al, NIM A610 (2009) 451
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5. polarized electron source
Polarized Positron Production
Eric Voutier
Polarized Bremsstrahlung
E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395
Within a high Z target, longitudinally polarized e-’s radiate circularly polarized g’s.
Within the same/different target, circularly polarized g’s create longitudinally polarized e+’s.
bulk GaAs
Strained GaAs
Superlattice GaAs
Pe-
35%
35%
75%
75%
85%
85%
85%
1995
1998
1999
2000
2004
2007
2010
Ie-
30 mA
100 mA
50 mA
100 mA
150 mA
1 mA
180 mA
All operating with suitable photocathode lifetime to sustain weeks of operation
Evolution of CEBAF
polarized electron source
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6. e+ Beam Concept Technical Challenges
Eric Voutier
e+ Beam Concept
Geometric Emittance < 5 mm-mrad Absolute Energy Spread < 1 MeV
Beam Current > 50 nA Bunch Length < 2 ps
Duty Factor = 100 % Frequency = 1497 MHz
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7. Technical Challenges
Eric Voutier
S. Golge et al., Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009
A possible concept involves the construction of a dedicated e+ tunnel at the end of the injector and parallel to the north linac.
Positrons would be produced with 120 MeV e- (JLab 12 GeV) incident on a tungsten target.
e+’s are selected with a quadrupole triplet and transported to the accelerator section.
G4beamline simulations indicate a global efficiency of 10-5 e+/e- for 120 MeV e- off a 3 mm W target.
10 mA e- → 100 nA e+
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8. Technical Challenges g
Eric Voutier
S. Golge et al., Proc. of XXIInd Particle Accelerator Conference, Albuquerque (NM, USA), June 25-29, 2007
Previous studies of the collection system (10 MeV e- beam) indicate that a large fraction of the beam power deposits in the conversion target. g e+ e-
Possible solutions are a rotating target or a liquid metal target but have not yet been investigated in the JLab context.
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9. Technical Challenges Accelerator magnets Beam diagnostics Beam modes
Eric Voutier
A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009
Accelerator magnets
Most magnet power supplies are reversible except the arc dipoles which requires a manual action.
The e- to e+ switching time will limit the precision on a charge asymmetry measurement.
Beam diagnostics
Beam position monitors and viewers will work as long as the e+ current is ≥ 50 nA.
Beam modes
The tune mode based on a pulsed beam about tens of µA and 250 µs long and used for beam steering will need to be redefined because of the small e+ current.
RF system
Each pass in the linac are adjusted in phase with each other via the adjustement of their pathlength with the arc dogleg sections. The diagnostic that measures the phase difference between passes require tune mode beam of sufficient current (µA).
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10. R. Dollan, K. Laihem. A. Schälicke, NIM A559 (2006) 185
GEANT4 Simulations
Eric Voutier
e+ Source Polarization
Simulation are performed within the GEANT4 framework, taking advantage of the polarization capabilities developed by the E166 Collaboration.
R. Dollan, K. Laihem. A. Schälicke, NIM A559 (2006) 185
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11. the incident flux of particles and its polarisation.
GEANT4 Simulations
Eric Voutier
e+ Figure of Merit
The Figure of Merit is the quantity of interest for the accuracy of a measurement which combines
the incident flux of particles and its polarisation.
Optimum
FoM
Optimum energy
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12. GEANT4 Simulations
Eric Voutier
Simplistic cuts are applied to mimic a capture system and the accelerator acceptance.
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13. The positron yield and polarization distributions will be measured.
PEPPo Experiment
Eric Voutier
A Proof of Principle
An experiment to test the production of polarized positrons from polarized bremsstralhung is currently designed.
The positron yield and polarization distributions will be measured.
The experiment will be performed at the CEBAF injector (T ≤ 10 MeV) on a new dedicated e+ line, designed to sustain ~30 µA electron current, and is expected to run during the 6 months shutdown of 2011.
The e+ line will be equiped with g and e+ production production targets, and the magnetic collection & selection system & Compton transmission polarimeter used in the E166 experiment at SLAC.
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14. PEPPo Experiment
Eric Voutier
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15. Experimental Strategy
PEPPo Experiment
Eric Voutier
Experimental Strategy
The electron beam will be characterized in energy and polarization with the standard measurement devices.
The transmission polarimeter will be cross-calibrated in electron with respect to the Mott polarimeter.
Systematic misalignement and transverse polarization effects will be evaluated.
The energy distribution of the positron polarization and yield will be measured.
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16. Compton Transmission Polarimetry
PEPPo Experiment
Eric Voutier
Compton Transmission Polarimetry
Polarized e± convert into circularly polarized photons into a tungsten target.
The photon polarization is analyzed via Compton scattering off a magnetized iron target.
T = 7.5 MeV
P = 85%
tw = 1 mm
I = 1 pA
Dt = 100 s
The analyzing power Ae is obtained from electron beam calibration data and simulations.
A data acquisition system with high rate capabilities (~1 MHz) is forseen (250 MHz flash ADC).
DPe = ±0.03
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17. via bremsstralhung and pair creation.
Conclusions
Eric Voutier
Summary
The possibilities of developing a CW « low cost » polarized positron source at JLab look promising.
Technical challenges concern the electron beam driver (10 85%) and the high power conversion target
(optimum positron capture, accelerator acceptance, damping/accumulator ring ?…)
PEPPo,
a proof-of-principle experiment is designed to quantify the performances of the polarization transfert from polarized electrons to positrons
via bremsstralhung and pair creation.
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