1. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Strained Superlattice GaAs photocathodes at JLab M. Baylac Qweak collaboration meeting August 17, 2004

2. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Polarized Electron Guns at JLab photocathode NF 3 Laser Cs anode e - -100 kV HV insulator NEG pump s Strained GaAs in Gun2 (“old” material) Strained-superlattice GaAs in Gun3 (“new” material) NEG-coated Beamline Photoemission from GaAs semiconductor

3. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Strained layer GaAs photocathodes From 1998 through 2003, we have used strained layer GaAs photocathodes at JLab (Bandwidth Semiconductor, Inc.). Reliable, well understood material. Stained-layer GaAs provides; Good polarization: P ~ 75% at 840 nm Moderate quantum efficiency: QE ~ 0.2% at 840 nm Limitations that keep polarization < 80%: limited band splitting relaxation of the strain for thickness > critical thickness (~10 nm) e

4. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Strained GaAs/GaAsP superlattice Very thin quantum well layers alternating with lattice- mismatched barrier layers Each superlattice layer is < critical thickness Natural splitting of valence band adds to the strain-splitting Developed by SLAC with SVT Associates, Inc. SLAC-PUB-10331 (2004), submitted to Appl.Phys.Lett First samples received at JLab October 2003, characterized at the injector test cave

5. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Superlattice structure Be doping (cm ) -3 GaAs P 1-x x, 0<x<0.36 (2.5 μm) p-type GaAs substrate GaAs (5 nm) GaAs (4 nm) GaAsP (3 nm) GaAs P 0.64 0.36 (2.5 μm) 14 pairs 5.10 19 5.10 17 5.10 18 SVT associates, per SLAC specs.

6. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Quantum Efficiency QE ~ 1% versus 0.2% from strained layer material Wavelength (nm) QE (%) we operate here

7. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Beam polarization Wavelength (nm) Polarization (%) Highest polarization ever measured at the Test Cave Wavelength for Good QE and Polarization

8. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Analyzing power (aka QE anisotropy) Analyzing power smaller by factor of 3 compared with strained-layer GaAs: 4% versus 12% This means smaller inherent intensity & position asymmetries on beam. Wavelength (nm) Analyzing power (%) Wavelength for good QE and polarization

9. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility QE vs hydrogen cleaning Hydrogen exposure time (min) QE (%) Drawback: Delicate material Can’t clean with atomic hydrogen Makes it tough to anodize edge of cathode Typical H-dose to clean anodized samples

10. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Superlattice Photocathodes at CEBAF Several failed attempts to load superlattice photocathodes inside tunnel guns Successful installation of un-anodized superlattice photocathode in Gun 3 (March, 2004) Activation gave QE ~ 0.4% at 780 nm (vs 1% in test cave) Used during HAPPEx-He and portion of HAPPEx-H (June, 2004)

11. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Poor lifetime Frequent spot moves were required to maintain 40  A beam current at Hall A every week at start of run, every day as we approached July 4 shutdown! HAPPEx-He OK. HAPPEx-H not so good. Injector conditions changing too often. HC asymmetries were not stable. Poor gun lifetime atypical of CEBAF photoinjector. QE profile after 3 weeks of running 14 mm

12. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Polarimetry in hall A Compton (D. Lhuillier) 5 MeV Mott (J. Grames) Preliminary photon electron P ~ 86  3 % e P ~ 85.2  3.2 % e

13. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Parity quality beam? Short run + numerous spot moves => Jury is still out. Poor gun lifetime made it difficult to assess performance of superlattice photocathode from a parity violation experiment perspective. HAPPEX reports; Charge asymmetry OK for both photocathodes Position asymmetries were smaller using gun2 strained layer photocathode (no active position feedback) Gun2 strained layer GaAsGun3 superlattice GaAs From HAPPEx-H

14. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility QE drops as laser power increases: photoelectrons build up in band bending region create opposing E field that reduces NEA G.A. Mulhollan et al, Phys. Lett. A 282, 309 (2001) Reduces maximum available beam current. Lose laser headroom. Makes for shorter operating lifetime of gun. Surface Charge Limit QE is not constant

15. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Our new commercial Ti-Sapphire lasers provide more laser power (~ 300 mW) compared to our “old” diode lasers (~ 50 mW). They are wavelength tunable. Now we can tune to peak polarization. Successful and reliable running since G0. Ti-Sapp laser + superlattice photocathode a good match for high current Qweak experiment. 300 mW laser power + QE of 1% can provide 1800 uA beam current. Max current only 360 uA with strained layer cathode. Not as much headroom. Lasers http://www.tbwp.com

16. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Summary Highest polarization ever measured at JLab: P = 86% Measurements of many samples at test stand indicates this is no fluke. 5 times higher QE than strained layer material. Smaller analyzing power should provide smaller inherent charge and position asymmetry. (Recent HAPPEx results do not support this claim.) Delicate material, more difficult to handle. Cannot be H-cleaned. Can’t recover QE from a dirty superlattice, unlike strained layer We suffered surface charge limit. QE drops with increasing laser power. A concern for high current experiments like Qweak. e

17. Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy Thomas Jefferson National Accelerator Facility Outlook Poor lifetime due to supperlattice? Doubt it: Gun 3 has a bad lifetime in 2003 using strained layer Un-anodized wafer increases damage on the wafer Reworked Gun 3 over the shutdown, hoping to boost lifetime QE lower in the tunnel than in test cave: Hopefully due to the gun itself, not the wafer Received arsenic capped samples: easier to handle and anodize (to be tested in lab) Smaller inherent HC asymmetries? Surface charge limit? Need more operating experience.