Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility Polarized Electron Beams for the Jefferson Lab Nuclear Physics Program Joe Grames, P. Adderley, J. Brittian, J. Hansknecht, J. McCarter, M. Poelker, M. Stutzman, R. Suleiman *, K. Surles-Law (JLab Electron Gun Group, * Virginia Polytechnic Inst. & State U.) SES Meeting of the American Physical Society Williamsburg, VA November 9-11, 2006 (

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 2 Everyone Gets Polarized Electrons! CEBAF’s first polarized e-beam experiment 1997 Now polarized beam experiments comprise ~ 80% of our physics program, in fact, we only deliver polarized electrons All beam originates via photoemission from a Gallium Arsenside crystal inside a 100kV photogun Three experimental areas may simultaneously receive: –high polarization (~85%) => increases asymmetry, figure of merit –continuous wave (499 MHz) => high statistics, low counting rates –independent intensity (50 pA to 200  A) => target or acceptance –energy selection (multiples of linac energy) => flexibility

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 3 Gallium Arsenide Photocathodes 3” wafer cleaved into square photocathodes (15.5 mm) & mounted on a “stalk” using In and a Ta cup.

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 4 P = = 50% e E < E < E ga p  gap+   +  - m J S 1/2 P 3/2 P 1/2 = 0.34 eV = 1.42 eV hc/ Optical pumping Gallium Arsenide between P and S 3/ 2 1/2

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 5 P ~ %P ~ 70 – 75 %P ~ % Strained Superlattice GaAs Bulk GaAs Strained GaAs 100 nm 14 pairs Bulk GaAs GaAsP Bulk GaAs GaAsP Unstrained GaAsStrained GaAsSuperlattice GaAs DegeneracyBroken degeneracy, but relaxation No relaxation, quantum well structure

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 6 Beam Polarization at CEBAF P I 2 2 sup. str. = 1.38 Experiment Figure of Merit

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 7 Continuous Electron Beam Accelerator Facility A B C ABC Pockels cell 100 keV DC Electron Gun Two SRF 600 MeV linacs (1497 MHz) 67 MeV injector (1497 MHz) RF deflectors Double-sided septum Wien filter RF Lasers (499 MHz) P

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 8 Continuous Electron Beam Accelerator Facility A B C ABC Pockels cell 100 keV DC Electron Gun Two SRF 600 MeV linacs (1497 MHz) 67 MeV injector (1497 MHz) RF deflectors Double-sided septum Wien filter RF Lasers (499 MHz) P Spin Precession Degrees of Freedom

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 9 Helicity Reversal & Parity Violation Experiments Right-handed electron beam Left-handed electron beam Ideally only the electron helicity reverses, but… Intensity Position and Angle on target Energy Target L R Detector Pockels cell 30 Hz 10 to 1 to 0.1 ppm

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 10 Origins of Helicity Correlated False Asymmetries Lensing of Pockels cell at High Voltage –Pockels cells lensing may steer off-centered laser beam –Induces helicity-correlated position difference Residual linear polarization in the laser beam –Anisotropy in GaAs crystal results in asymmetric electron yield –Induces helicity-correlated intensity asymmetry Beam loading in rf cavities –Correlation between cavity gradient and beam intensity –Induces helicity-correlated energy difference Intrinsic birefringence gradient in the Pockels cell –Spatial variation of laser polarization shifts beam centroid –Induces helicity-correlated position difference High Voltage (courtesy G. Cates)

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 11 Experiment Physics Asymmetry (ppm) Position Difference (nm) Intensity Asymmetry (ppm) SpecAchievedSpecAchieved HAPPEx-I [1999] G 0 Forward [2001] HAPPEx-He [2004] HAPPEx-He [2005] HAPPEx-II-H [2004] HAPPEx-II-H [2005] G 0 Backward [2006] PREX Q weak Parity Violation Experiments at CEBAF

Operated by Jefferson Science Associates, LLC for the U.S. Department of Energy Thomas Jefferson National Accelerator Facility 12 Future High Current & High Polarization Projects Q weak to test standard model 180  A at 85% polarization Solutions: Fiber-based laser + Next Generation e-Gun Proposed (>1 mA) facilities ELIC, eRHIC