Polarized Electron Beams for the Jefferson Lab Nuclear Physics Program

1. (www.jlab.org/accel/inj_group) 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.) Polarized Electron Beams for theJefferson Lab Nuclear Physics Program SES Meeting of the American Physical Society Williamsburg, VA November 9-11, 2006

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 mA) => target or acceptance • energy selection (multiples of linac energy) => flexibility

3. Gallium Arsenide Photocathodes 3” wafer cleaved into square photocathodes (15.5 mm) & mounted on a “stalk” using In and a Ta cup.

4. Optical pumping Gallium Arsenide between P and S 3/2 1/2 - +   =0.34 eV m J 3-1 P == 50% S P P 3+1 e E < E <E 3/2 1/2 1/2  gap+ gap = 1.42 eV hc/l

5. 100 nm 100 nm Strained GaAs 14 pairs Bulk GaAs GaAsP GaAsP Bulk GaAs Bulk GaAs Strained Superlattice GaAs Unstrained GaAs Strained GaAs Superlattice GaAs P ~ 35 - 40% P ~ 70 – 75 % P ~ 80 - 90 % Degeneracy Broken degeneracy, but relaxation No relaxation, quantum well structure

6. Beam Polarization at CEBAF 2 2 P I P I sup. = 1.38 str. Experiment Figure of Merit

7. A B C Continuous Electron Beam Accelerator Facility Two SRF 600 MeV linacs (1497 MHz) 67 MeV injector (1497 MHz) RF Lasers (499 MHz) RF deflectors A B C Double-sided septum Pockels cell Wien filter P 100 keV DC Electron Gun

8. A B C Continuous Electron Beam Accelerator Facility Two SRF 600 MeV linacs (1497 MHz) 67 MeV injector (1497 MHz) RF Lasers (499 MHz) RF deflectors A B C Double-sided septum Pockels cell Wien filter Spin Precession Degrees of Freedom P 100 keV DC Electron Gun

9. Right-handed electron beam Left-handed electron beam Helicity Reversal & Parity Violation Experiments Pockels cell 30 Hz 10 to 1 to 0.1 ppm Ideally only the electron helicity reverses, but… • Intensity • Position and Angle on target • Energy Detector L R Target

10. Origins of Helicity Correlated False Asymmetries • Residual linear polarization in the laser beam • Anisotropy in GaAs crystal results in asymmetric electron yield • Induces helicity-correlatedintensity asymmetry • Lensing of Pockels cell at High Voltage • Pockels cells lensing may steer off-centered laser beam • Induces helicity-correlatedposition difference High Voltage • Intrinsic birefringence gradient in the Pockels cell • Spatial variation of laser polarization shifts beam centroid • Induces helicity-correlated position difference • Beam loading in rf cavities • Correlation between cavity gradient and beam intensity • Induces helicity-correlatedenergy difference (courtesy G. Cates)

11. 1999 2008 Parity Violation Experiments at CEBAF

12. Future High Current & High Polarization Projects Proposed (>1 mA) facilities ELIC, eRHIC Qweak to test standard model 180 mA at 85% polarization Solutions: Fiber-based laser + Next Generation e-Gun