1. Polarized Positrons at the Jefferson Laboratory Idaho State University, Idaho Accelerator Center, Jefferson Lab, LPC Clermont-Ferrand, LPSC Grenoble, Old Dominion University o A group of physicists is developping the possible concepts and technics for a high intensity positron source at JLab. o In this effort, the LPSC team is studying the production of polarized positrons from polarized electrons. J. Dumas is a Grenoble PhD student co-funded by JLab & IN2P3. Jonathan Dumas (JLab/LPSC), Joe Grames (JLab), Arne Freyberger (JLab), Serkan Golge (ODU), Charles Hyde (LPC), Eric Voutier (LPSC)

2. Physics Motivations DVCS Beam Charge Dependent Cross Section DVCS Beam Spin Dependent Cross Section  The beam charge asymmetry (BCA) in deeply virtual Compton scattering (DVCS) isolates the real part of the DVCS amplitude. It is however 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 Re,Im[DVCS*BH] terms from separate e + and e - data. Eric Voutier

3. Principe of Operation Unique Challenges for CW Positron Beams 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. Photon circular polarization Positron longitudinal polarization Simulations for a 5 MeV longitudinally polarized (100 %) e - beam on 250 µm tungsten foil Simulations for a 5 MeV circularly polarized (100 %)  beam on 250 µm tungsten foil Eric Voutier

4. GEANT4 Simulations Positron longitudinal polarization  At 5 MeV/c, 85% of polarization and a 250 µm tungsten foil, the integrated production rate in the forward direction is ~7 x 10 -5 e + /e -, in absence of a magnetic collection system. The longitudinal polarization of positrons can be as high as 70%, depending on the positron energy, and is ~ 40% on the average. Eric Voutier

5. QE ~ 1% & P ~ 85% @ 780nm 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 ~ 1W @ 780 nm &  t ~ 50 ps @ 499 Hz Eric Voutier A beam current of 1 mA was demonstrated for 10 h. Beam polarization not measured.

6. Test Experiment  A test experiment is being designed in order to validate this concept.  The angular, energy and polarization distributions of positrons will be measured. A possible detection system would constitute of a selecting dipole magnet followed by a Compton transmission polarimeter.  2009 : design completion and construction.  2010 : data taking. Eric Voutier

7. JPos09 International Workshop on Positrons at Jefferson Lab Jefferson Lab, March 25-27, 2009 Contact: grames@jlab.org Organizing Committee: L. Ceraul (JLab) L. Ceraul (JLab) L. Elouadrhiri (JLab) L. Elouadrhiri (JLab) T. Forest (Idaho State University) T. Forest (Idaho State University) J. Grames (JLab) J. Grames (JLab) W. Melnitchouk (JLab) W. Melnitchouk (JLab) E. Voutier (LPSC Grenoble) E. Voutier (LPSC Grenoble) Internationnal Advisory Committee: X. Artru (IPN Lyon) X. Artru (IPN Lyon) L. Cardman (JLab) L. Cardman (JLab) P. Cole (Idaho State University) P. Cole (Idaho State University) A. Freyberger (JLab) A. Freyberger (JLab) P. Guichon (CEA Saclay) P. Guichon (CEA Saclay) R. Holt (Argonne National Laboratory) R. Holt (Argonne National Laboratory) A. Hunt (Idaho Accelerator Center) A. Hunt (Idaho Accelerator Center) C. Hyde (LPC Clermont Ferrand) C. Hyde (LPC Clermont Ferrand) M. Klein (University of Liverpool) M. Klein (University of Liverpool) K. Kumar (University of Massachusetts) K. Kumar (University of Massachusetts) M. Poelker (JLab) M. Poelker (JLab) J. Sheppard (SLAC) J. Sheppard (SLAC) A. Variola (LAL Orsay) A. Variola (LAL Orsay) Selected Topics: Technology of positron sources Technology of positron sources Polarized positrons Polarized positrons Electron/photon drivers Electron/photon drivers Positron & electron polarimetry Positron & electron polarimetry Generalized parton distributions Generalized parton distributions Electromagnetic form factors Electromagnetic form factors Positron annihilation spectroscopy Positron annihilation spectroscopy