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Polarized Electron Guns for Qweak: Superlattice Photocathodes and Load Locked Gun

This article discusses the use of superlattice photocathodes and load locked guns for polarized electron guns in the Qweak experiment. It explores the challenges and successes of using these technologies, as well as the impact on polarization and analyzing power. The article also highlights the need for limiting active area and proper management of extracted beam for high current operation.

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Polarized Electron Guns for Qweak: Superlattice Photocathodes and Load Locked Gun

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  1. Polarized Source Issues for Qweak • 180 uA and polarization > 80% • Parity quality • Long experiment • Success requires: • Superlattice photocathodes • High power, reliable modelocked Ti-Sapphire laser • Load locked gun (new!!) • 300 Hz pockels cell flipping

  2. photocathode anode Laser -100 kV - e Cs NF 3 Polarized Electron Guns at JLab HV insulator Photoemission from GaAs semiconductor NEG pumps NEG-coated Beamline Strained GaAs in Gun2 (“old” material) Strained-superlattice GaAs in Gun3 (“new” material)

  3. Superlattice Photocathode from SVT here here here Wavelength (nm) Wavelength (nm) Wavelength (nm) QE (%) Polarization Analyzing Power From Hall A Compton Polarimeter • The highest polarization yet measured at CEBAF; ~ 85% • QE 0.8% versus 0.15% • Analyzing power 4 % versus 12% photon electron

  4. Superlattice Photocathodes at CEBAF • Good results at EEL and Test Cave. Reliable 85% polarization and QE ~ 1%. • First attempts to load superlattice photocathodes inside tunnel guns failed; can’t anodize and hydrogen clean. • Un-anodized superlattice photocathode in Gun 3 (March, 2004). QE ~ 0.4%. Used during HAPPEx-He and portion of HAPPEx-H (June, 2004). Gun lifetime poor at 80 uA. • Another sample loaded into gun3 Sept. 2004. Tantalum mask used to limit active area. Good polarization and QE but gun performance poor.

  5. 14 mm From HAPPEx-H Gun3 superlattice GaAs Gun2 strained layer GaAs First Experience with Superlattice at CEBAF QE profile after 3 weeks of running • No anodization. • Operate with large photocathode active area. • Poor gun lifetime. • Spot moves daily while running ~ 100 uA from gun.

  6. High current operation requires that we properly manage the extracted beam (both “good” and “bad” beam). e beam Why? To preserve good vacuum and limit QE degradation associated with ion-backbombardment • Limit photocathode active area • Eliminate stray light • Large diameter beampipes • NEG coated chambers to limit ESD • Proper electrode geometry • Proper lens configuration Cathode Anode Hits anode Hits gun chamber Hits Wien faceplate Hits beampipe

  7. Anodized photocathode Photocathode “out of box” Typical H-dose to clean anodized samples QE (%) Atomic Hydrogen Exposure (min) “The Effects of Atomic Hydrogen Exposure On High Polarization GaAs Photocathodes”, M. Baylac, in press. Limiting Active Area via Anodization We have not successfully anodized superlattice material – it cannot be hydrogen cleaned.

  8. Ion Pump Locations Pumps detect bad orbit and beamloss Ta-mask Gun chamber pump Wien filter Laser chamber Second Experience with Superlattice at CEBAF • Limit active area using tantalum mask • Poor lifetime again • Mask disturbs gradient at surface of photocathode • “halo” electrons strike vacuum chamber walls

  9. As-capped samples – Can they be anodized? • No! Recent test at EEL building unsuccessful. • Installing yet another superlattice sample April 14, 2005 for low current Hall B program (lifetime not such an issue). • Exploring other options for limiting active area (sputter coat the edges, ozone etch, use an electrode with small hole) • Load lock gun solves problem….

  10. Load lock gun will solve anodization problems Mask limits application of cesium. • Other benefits for Qweak; • Better gun vacuum (gun chamber never vented). • Samples can be ~ quickly replaced without bakeout • Multiple samples in gun enhance operating lifetime • Aggressive sample selection for best polarization

  11. Modelocked Ti-Sapphire Lasers from TimeBandwidth SESAM: passive modelocking for high rep rates

  12. strained superlattice • Passive modelocking for high rep rates: CEBAF model = 499 MHz • Selectable wavelength ranges near 770 nm or 850 nm • Phase-locked pulse train is stable for days, weeks, months however… • “laser jock” required.

  13. Summary e • 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. We have not yet observed good lifetime from gun using superlattice. Work in progress.

  14. Outlook • Must find a way to limit active area of superlattice photocathodes by June 2005 (HAPPEx want 100 uA). • Load locked gun very important for Qweak • No anodization required • Samples can be quickly swapped, enhanced operating lifetime at high current. • Installation September, 2005 • Three Ti-sapp lasers on site. Ti-sapp lasers are finicky. More experience required to minimize tunnel accesses. • 300 Hz flipping awaits testing.

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