Polarized Electrons for Polarized Positrons A Proof of Principle Experiment Alexandre Camsonne*, Jonathan Dumasǂ*, Arne Freyberger*, Joe Grames*, Matt Poelker*, Jean-Sébastien Réalǂ, Eric Voutierǂ… ǂ Laboratoire de Physique Subatomique et de Cosmologie Grenoble, France * Thomas Jefferson National Accelerator Facility Newport-News, Virginia, USA Physics motivations Polarized positron production Technical challenges GEANT4 simulations PEPPo experiment Conclusions GDR Nucléon February 11, 2010, Orsay

Physics Motivations Eric Voutier Unpolarized cross section Beam charge difference Beam spin difference The combination of polarized electrons and polarized positrons offers the unique opportunity to access a pure interference signal. Orsay, Febryary 11, 2010 2/17

The photon polarization is transferred to positrons via pair creation. Polarized Positron Production Eric Voutier Compton Scattering 1.28 GeV ≤ 56 MeV ~36 MeV A circular photon beam is produced by Compton scattering of a laser beam off electrons. The photon polarization is transferred to positrons via pair creation. Polarized e+ convert into circularly polarized photons into a lead target. The photon polarization is analyzed via Compton scattering off a magnetized iron target. P(e+) = 73 ± 15 ± 19 % M. Fukuda et al, PRL 91 (2003) 164801 T. Omori et al, PRL 96 (2006) 114801 Orsay, Febryary 11, 2010 3/17

iron e+ Undulator Scheme Polarized Positron Production reconversion Eric Voutier Undulator Scheme e+ Diag. γ Diag. e- Dump 46.6 GeV e- W Target 10 MeV γ Undulator Spectrometer e- to Dump Bending magnets reconversion target magnetized iron detector e+ spectrometer G. Alexander et al, PRL 100 (2008) 210801 G. Alexander et al, NIM A610 (2009) 451 Orsay, Febryary 11, 2010 4/17

polarized electron source Polarized Positron Production Eric Voutier Polarized Bremsstrahlung E.G. Bessonov, A.A. Mikhailichenko, EPAC (1996) A.P. Potylitsin, NIM A398 (1997) 395 Within a high Z target, longitudinally polarized e-’s radiate circularly polarized g’s. Within the same/different target, circularly polarized g’s create longitudinally polarized e+’s. bulk GaAs Strained GaAs Superlattice GaAs Pe- 35% 35% 75% 75% 85% 85% 85% 1995 1998 1999 2000 2004 2007 2010 Ie- 30 mA 100 mA 50 mA 100 mA 150 mA 1 mA 180 mA All operating with suitable photocathode lifetime to sustain weeks of operation Evolution of CEBAF polarized electron source Orsay, Febryary 11, 2010 5/17

e+ Beam Concept Technical Challenges Eric Voutier e+ Beam Concept Geometric Emittance < 5 mm-mrad Absolute Energy Spread < 1 MeV Beam Current > 50 nA Bunch Length < 2 ps Duty Factor = 100 % Frequency = 1497 MHz Orsay, Febryary 11, 2010 6/17

Technical Challenges Eric Voutier S. Golge et al., Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009 A possible concept involves the construction of a dedicated e+ tunnel at the end of the injector and parallel to the north linac. Positrons would be produced with 120 MeV e- (JLab 12 GeV) incident on a tungsten target. e+’s are selected with a quadrupole triplet and transported to the accelerator section. G4beamline simulations indicate a global efficiency of 10-5 e+/e- for 120 MeV e- off a 3 mm W target. 10 mA e- → 100 nA e+ Orsay, Febryary 11, 2010 7/17

Technical Challenges g Eric Voutier S. Golge et al., Proc. of XXIInd Particle Accelerator Conference, Albuquerque (NM, USA), June 25-29, 2007 Previous studies of the collection system (10 MeV e- beam) indicate that a large fraction of the beam power deposits in the conversion target. g e+ e- Possible solutions are a rotating target or a liquid metal target but have not yet been investigated in the JLab context. Orsay, Febryary 11, 2010 8/17

Technical Challenges Accelerator magnets Beam diagnostics Beam modes Eric Voutier A. Freyberger, Proc. of the International Workshop on Positrons at Jefferson Lab, Newport News (VA, USA), March 25-27, 2009 Accelerator magnets Most magnet power supplies are reversible except the arc dipoles which requires a manual action. The e- to e+ switching time will limit the precision on a charge asymmetry measurement. Beam diagnostics Beam position monitors and viewers will work as long as the e+ current is ≥ 50 nA. Beam modes The tune mode based on a pulsed beam about tens of µA and 250 µs long and used for beam steering will need to be redefined because of the small e+ current. RF system Each pass in the linac are adjusted in phase with each other via the adjustement of their pathlength with the arc dogleg sections. The diagnostic that measures the phase difference between passes require tune mode beam of sufficient current (µA). Orsay, Febryary 11, 2010 9/17

R. Dollan, K. Laihem. A. Schälicke, NIM A559 (2006) 185 GEANT4 Simulations Eric Voutier e+ Source Polarization Simulation are performed within the GEANT4 framework, taking advantage of the polarization capabilities developed by the E166 Collaboration. R. Dollan, K. Laihem. A. Schälicke, NIM A559 (2006) 185 Orsay, Febryary 11, 2010 10/17

the incident flux of particles and its polarisation. GEANT4 Simulations Eric Voutier e+ Figure of Merit The Figure of Merit is the quantity of interest for the accuracy of a measurement which combines the incident flux of particles and its polarisation. Optimum FoM Optimum energy Orsay, Febryary 11, 2010 11/17

GEANT4 Simulations Eric Voutier Simplistic cuts are applied to mimic a capture system and the accelerator acceptance. Orsay, Febryary 11, 2010 12/17

The positron yield and polarization distributions will be measured. PEPPo Experiment Eric Voutier A Proof of Principle An experiment to test the production of polarized positrons from polarized bremsstralhung is currently designed. The positron yield and polarization distributions will be measured. The experiment will be performed at the CEBAF injector (T ≤ 10 MeV) on a new dedicated e+ line, designed to sustain ~30 µA electron current, and is expected to run during the 6 months shutdown of 2011. The e+ line will be equiped with g and e+ production production targets, and the magnetic collection & selection system & Compton transmission polarimeter used in the E166 experiment at SLAC. Orsay, Febryary 11, 2010 13/17

PEPPo Experiment Eric Voutier Orsay, Febryary 11, 2010 14/17

Experimental Strategy PEPPo Experiment Eric Voutier Experimental Strategy The electron beam will be characterized in energy and polarization with the standard measurement devices. The transmission polarimeter will be cross-calibrated in electron with respect to the Mott polarimeter. Systematic misalignement and transverse polarization effects will be evaluated. The energy distribution of the positron polarization and yield will be measured. Orsay, Febryary 11, 2010 15/17

Compton Transmission Polarimetry PEPPo Experiment Eric Voutier Compton Transmission Polarimetry Polarized e± convert into circularly polarized photons into a tungsten target. The photon polarization is analyzed via Compton scattering off a magnetized iron target. T = 7.5 MeV P = 85% tw = 1 mm I = 1 pA Dt = 100 s The analyzing power Ae is obtained from electron beam calibration data and simulations. A data acquisition system with high rate capabilities (~1 MHz) is forseen (250 MHz flash ADC). DPe = ±0.03 Orsay, Febryary 11, 2010 16/17

via bremsstralhung and pair creation. Conclusions Eric Voutier Summary The possibilities of developing a CW « low cost » polarized positron source at JLab look promising. Technical challenges concern the electron beam driver (10 mA @ 85%) and the high power conversion target (optimum positron capture, accelerator acceptance, damping/accumulator ring ?…) PEPPo, a proof-of-principle experiment is designed to quantify the performances of the polarization transfert from polarized electrons to positrons via bremsstralhung and pair creation. Orsay, Febryary 11, 2010 17/17