Polarized Positrons at Jefferson Lab

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Polarized Positrons at Jefferson Lab Jonathan 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) Þ Test surface charge limit • High Polarization ~85% at 780 nm => Test polarization at high current • 1.5 GHz fiber laser (500 mW @ 780 nm) Þ Test 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 In a material, e - radiate gs (Bremsstrahlung). gs (>1.022MeV) can create a e + /e - pair. Electron Driver Polarization transfer depends on: -e - energy E e- -g energy E g -Target atomic number: Z -g scattering angle Bremsstrahlung e - longitudinal g circular Polarization transfer calculation for both interactions. Olsen & Maximon, Phys. Rev. 114 (1957) Geant4 simulation: polarization distribution at the creation vertex Pair creation g circular e + longitudinal Electromagnetic shower : -Bremsstrahlung - Pair creation Electron Beam Energy The goal is to measure electron momentum with precision better than 0.5%. Magnetic field map and operational range increased to 8.5 this Electron Polarimetry High 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/c Scattering angle < 30 o Photon distribution Large amount of photons Low 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 P A P P g g power analyzing A polar magnet P asymmetry photon e : . . : : g - Polarization transfer depends on: - g energy E g -e + energy E e+ -Target atomic number: Z -e + scattering angle E g = 5 MeV g circular= 100% Z=74 (Tungsten) e + scattering angle=all E g = 5 MeV g circular= 100% Z=74 (Tungsten) e + scattering angle=all Target thickness=0.25mm at the exitof the foil E e- = 5 MeV e - longitudinal = 100% Z=74 (Tungsten) g scattering angle=all E e- = 5 MeV e - longitudinal = 100% Z=74 (Tungsten) scattering angle=all Target thickness=0.25mm ~58% at 2 MeV E e- = 5 MeV e - longitudinal = 85% Z=74 (Tungsten) e + scattering angle=all Target thickness=0.25mm

description

Polarized Positrons at Jefferson Lab Jonathan Dumas ( JLab /LPSC ), Joe Grames ( JLab ), Eric V outier (LPSC). Outlook. The Experiment. We investigate the idea of generating polarized positrons using the CEBAF polarized electron photoinjector . - low energy (e - beam energy ≤ 8.5 MeV ) - PowerPoint PPT Presentation

Transcript of Polarized Positrons at Jefferson Lab

Page 1: Polarized Positrons at Jefferson Lab

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