Journées Instrumentation du GDR Nucléon 8-9 Avril 2008, CEA Saclay Polarized Positrons at the Jefferson Laboratory (i) Physics motivations (ii) Principe of operation (iii) Technical challenges (iv) GEANT4 simulations (v) Road map Jonathan Dumas ǂ *, Joe Grames *, Eric Voutier ǂ ǂ Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France * Thomas Jefferson National Accelerator Facility Newport-News, Virginia, USA

2/10 Journées Instrumentation du GDR Nucléon Eric Voutier Physics Motivations 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 3/10 Journées 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 SLAC scheme

4/10 Journées Instrumentation du GDR Nucléon Eric Voutier Principe 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.

5/10 Journées Instrumentation du GDR Nucléon Eric Voutier Technical Challenges State of the 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 ~ 780nm 6/10 Journées Instrumentation du GDR Nucléon Eric Voutier Technical Challenges 100 nm chekc 14 pairs 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 ~ 780 nm & t ~ Hz

7/10 Journées Instrumentation du GDR Nucléon Eric Voutier Technical Challenges RF Fiber Laser 100kV Load Lock Polarized e - Photogun Load & Prepare Superlattice Photocathode 1 mA e - beam J. Grames et al., Proc. of XXII nd 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.

8/10 Journées Instrumentation du GDR Nucléon Eric Voutier GEANT4 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 -.

9/10 Journées Instrumentation du GDR Nucléon Eric Voutier GEANT4 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 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…

10/10 Journées Instrumentation du GDR Nucléon Eric Voutier Road 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 positron Positron polarimetry Positron acceptance and capture... Experimental Demonstration