Journées Instrumentation du GDR Nucléon 8-9 Avril 2008, CEA Saclay

PPolarized olarized PPositrons at ositrons at the the

JJeffersonefferson Lab Laboratory oratory

(i) Physics motivations(ii) Principe of operation(iii) Technical challenges(iv) GEANT4 simulations(v) Road map

Jonathan Dumasǂ*, Joe Grames*, Eric Voutierǂ ǂ Laboratoire de Physique Subatomique et de Cosmologie

Grenoble, France* Thomas Jefferson National Accelerator Facility

Newport-News, Virginia, USA

2/10Journées Instrumentation du GDR Nucléon

Eric VoutierPhysics Motivations

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DVCS Beam Charge Asymmetry

DVCS Beam Spin Asymmetry

The beam charge asymmetry (BCA) in deeply virtual Compton scattering (DVCS) is a golden case for a positron beam at JLab, but it is very unlikely that e+ and e- will be delivered and measured simultaneously in the experimental halls.

A polarized positron beam having the quality features of the CEBAF polarized electron beam

will provide, via the beam spin asymmetry, a unique way to master systematics and insure a better

determination of the BCA from separate e+ and e- data.

Principe of Operation

3/10Journées Instrumentation du GDR Nucléon

Eric Voutier

Undulator Source Scheme

A high energy e- beam travelling through an helical undulator emits circularly polarized ’s.

The polarized ’s create longitudinally polarized e+’s by interaction in a Ti-alloy target.

A. Mikhailichenko et al., AIP CP 915 (2007) 1095

Simulations for a 50 GeV beam

E166 @ SLAC scheme

4/10Journées Instrumentation du GDR Nucléon

Eric VoutierPrincipe of Operation

W.H. McMaster, RMP 33 (1961) 8

Proposed JLab Scheme

The e- source would consist of a 5-10 MeV high intensity longitudinally polarized e- beam.

The polarized e-’s create circularly polarized ’s by Bremsstrhalung radiation in a W target.

Within the same target, the polarized ’s create longitudinally polarized e+’s.

H. Olsen, L.C. Maximon, PR 114 (1959) 887

In general, ’s are elliptically polarized; the linear composant does not transfer to positrons but introduces an asymmetry with respect to the polarization plane. In the considered energy range, the transverse polarization of positrons remains small compared to the longitudinal polarization.

5/10Journées Instrumentation du GDR Nucléon

Eric VoutierTechnical Challenges

State of the Art @ JLab

Strained single-layer and multi-layer (superlattice) GaAs are used to achieve polarization greater than 50%.

Strained superlattice photocathodes provide the largest polarization (P~85%).

They operate at JLab since 09/2005 delivering a constant polarization over time, even when the quantum efficiency drops.

QE ~ 1% & P ~ 85% @ 780nm

6/10Journées Instrumentation du GDR Nucléon

Eric VoutierTechnical Challenges

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Load-lock gun

Strained Superlattice GaAs

Fiber-based laser system

The determinant elements for the achievement of a high intensity high polarization e- beam are:

a performant electron gun, a very good photocathode, and a powerful laser.

Improved electron gun with ultra-high vaccuum

J. Grames et al., AIP CP 915 (2007) 1037

Pw ~ 1W @ 780 nm & t ~ 50 ps @ 499 Hz

7/10Journées Instrumentation du GDR Nucléon

Eric VoutierTechnical Challenges

RF Fiber Laser100kV Load Lock

Polarized e- Photogun

Load & PrepareSuperlattice Photocathode

1 mA e- beam

J. Grames et al., Proc. of XXIInd Particle Accelerator Conference, Albuquerque (NM, USA), June 25-29, 2007

A beam current of 1 mA was demonstrated for 10 h with the superlattice photocathode; the beam polarization was however not measured.

8/10Journées Instrumentation du GDR Nucléon

Eric VoutierGEANT4 Simulations

GEANT4 simulations are performed for a 10 MeV/c longitudinally polarized (100%) e- beam in perpendicular incidence on a 1.2 mm thick tungsten foil.

Photon longitudinal polarization

The longitudinal polarization of the photon at the creation vertex exhibits a typical behaviour displaying a small effect originating from the emission angle of the photon.

Electrons can interact several times in the foil leading to longitudinal polarization larger than expected for 10 MeV/c e-.

9/10Journées Instrumentation du GDR Nucléon

Eric VoutierGEANT4 Simulations

Positron longitudinal polarization

At 10 MeV/c and a 1.2 mm tungsten foil, the integrated production rate in the forward direction is ~1.6 x 10-3 e+/e-, in absence of a magnetic collection system.

In the positron energy range considered for beam production, the expected average longitudinal polarization is ~60%.

Promising results motivating further investigations…

10/10Journées Instrumentation du GDR Nucléon

Eric VoutierRoad Map

Simulations

Implementing the tracking of the spin of the particles in « GEANT4 Beam Line »Study & optimization of the polarization of positrons

up to the entrance of the acceleration stage Feasability of high intensity 5-10 MeV e- (> 1 mA) at the CEBAF injector

Design

Beam line diagnostics for electron and positronPositron polarimetry

Positron acceptance and capture...

Experimental Demonstration