Polarized Positrons at Jefferson LabJonathan Dumas (JLab/LPSC), Joe Grames (JLab), Eric Voutier (LPSC)
Outlook
2008: Develop simulation tools and design experiment 2009: Build and install experiment beam line, demonstrate e+ production 2010: Characterize e+ (polarization, yield, energy and angular distributions)
Goal = Deliver simulation tools, experimental data, baseline design for e+ source
A polarized positron source is intended for the ILC. E166 has demonstrated feasibility of a polarized positron source by pair creation:
- high energy e- beam (≈50 GeV)
- synchrotron photons- thin low power target
We investigate the idea of generating polarized positrons using the CEBAF polarized electron photoinjector.
- low energy (e- beam energy ≤ 8.5 MeV)
- bremsstrahlung photons- low radiation level- thin LOW/HIGH power target
Transfer polarization from e- to e+
The design goal for the electron driver is to produce a high average current (1 mA), high polarization (85%) with energy up to ~ 10 MeV electron beam. The CEBAF photoinjector provides the necessary characteristics:
Electron Gun
• GaAs/GaAsP superlattice photocathode• High QE ~1% at 780 nm (~6 mA/mW)ÞTest surface charge limit
• High Polarization ~85% at 780 nm
=> Test polarization at high current
• 1.5 GHz fiber laser (500 mW @ 780 nm)ÞTest new 1.5GHz rf control module
• 100kV DC Load Lock Electron Gun• demonstrated lifetime >250 C at 1 mA
(J. Grames et al., in Proc. of the 2007 Particle Accelerator Conference, THPMS064, p. 3130.)
=> Field emission is limiting lifetime
Photoinjector
“G0” high bunch charge demonstrated
• 1.3 pC @ 31 MHz ~2 mA at 1.5 GHzÞ Restore G0 setupÞ Test pulsed mode DF~0.1%Þ Consider 200 kV gun option?
• Normal & SRF cavities => ~8.5 MeV
• Target spot size set by quadrupoles
In a material, e- radiate gs (Bremsstrahlung). gs
(>1.022MeV) can create a e+/e- pair.
Electron Driver
Polarization transfer depends on:
-e- energy Ee-
- g energy Eg
-Target atomic number: Z
-g scattering angle
Bremsstrahlunge- longitudinal g
circular
Polarization transfer calculation for both
interactions. Olsen & Maximon, Phys. Rev. 114
(1957)
Geant4 simulation: polarization distribution at
the creation vertex
Pair creationg circular e+
longitudinal
Electromagnetic shower :-Bremsstrahlung- Pair creation
Electron Beam EnergyThe goal is to measure
electron momentum with precision better than 0.5%. Magnetic field map and operational range increased to 8.5 MeV to support this goal.Electron
PolarimetryHigh precision (~1%) Mott measures electron polarization (at higher current) and calibrates positron polarimeter.
Positron Polarimetry- Transmission Compton
- Well suited for low momentum- Rapid relative monitor- Successfully used, e.g., E166
(P.Schuler et al., in Proc. of the 17th international spin physics symposium, p. 1095.)
The target: a Tungsten foil
Electron distribution
e- momentum ~ 4.7 MeV/cScattering angle < 30o
Photon distribution
Large amount of photonsLow energy
Positron distribution
Low momentum (0.5->2 MeV/c)
Particle Distributions
Particle distributions (scattering angle vs energy) after the foil for:
Tungsten foil:- Thickness: 0.25 mm
Electron beam:- 1 bunch (0.67 pC-1 mA at
1.5 GHz)
- energy of 5 MeV- transverse size of 1
mm
The Experiment
Use tungsten because:• High Z => greater EM shower• Low power target => Low cost • Thin target => better beam quality• Good thermal properties => extends deposited power limit
Considerations:• Positron yield (foil thickness optimization) • Power deposition (melting foil)
Fig.7: Positron yield after the foil and power deposit in the target.
e
e PAPP
poweranalyzingA
polarmagnetP
asymmetryphoton
e
:
..:
:
Polarization transfer depends on:
- g energy Eg
-e+ energy Ee+
-Target atomic number: Z
-e+ scattering angle
Eg= 5 MeV g circular= 100%Z=74 (Tungsten)
e+ scattering angle=all
Eg= 5 MeV g circular= 100%Z=74 (Tungsten)
e+ scattering angle=all
Target thickness=0.25mm
at the exitof the foil
Ee-= 5 MeV e- longitudinal =
100%Z=74 (Tungsten)
g scattering angle=all
Ee-= 5 MeV e- longitudinal =
100%Z=74 (Tungsten)
scattering angle=allTarget
thickness=0.25mm
~58% at 2 MeV
Ee-= 5 MeV e- longitudinal = 85%
Z=74 (Tungsten)e+ scattering angle=all
Target thickness=0.25mm
Recommended
