1. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 An EIC-like Polarized Electron Injector: Source, Beam & Polarimeter(s) Joe Grames Jefferson Lab August 23, 2007

2. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Challenging e- Source Requirements High Electron Polarization > 80 % High Current ~ 1 mA for JLab ELIC ~ 25 (250) mA for BNL eRHIC ~ Surface Charge Limit Sustainable Photocathode (QE ~1%) 2.5 mA => 210 C/day (~ state of the art lifetime) 25 mA => 2.1 kC/day (~ represents significant effort) 250 mA => 21 kC/day (~ ?) Laser Heating (6 mA/Watt/%) photocathode cooling becomes necessary I > ~few mA? photocathode lifetime => laser “headroom” necessary (~10x)

3. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 GaAs Photocathodes: last 30 years… High QE ~ 8% Pol ~ 35% @ 780 nm Superlattice GaAs: Layers of GaAs on GaAsP Bulk GaAs “conventional” material QE ~ 0.2% Pol ~ 75% @ 850 nm Strained GaAs: GaAs on GaAsP 100 nm No strain relaxation QE ~ 0.8% Pol ~ 85% @ 780 nm 100 nm 14 pairs FOM  I P 2

4. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 P I 2 2 sup. str. = 1.38 Experiment Figure of Merit Routinely Achieve Stable Polarization ~ 85% Polarization 85% No depolarization over time!  QE reduced by 2x, but polarization constant  One sample used from Sept ‘05 through Apr ‘07  7 activations, ~ 1000 C extracted, I ~ 200uA  Max QE: 0.7 to 0.4%, P ~ constant at 85%

5. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 New Fiber Technology Based Drive Laser  Gain-switching better than modelocking; no phase lock problems  Very high power  Telecom industry spurs growth, ensures availability  Useful because of superlattice photocathode (requires 780nm) J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, 063501 (2006) Ti-Sap Diode

6. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Compact, Off-The Shelf, Rack Mountable… RF locked low-power 1560 nm fiber diode High power 1560 nm fiber amplifier Non-linear frequency doubling converter for 1560 nm to 780 nm

7. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 New CEBAF “load-lock” gun “suitcase” Loading chamber Preparation/activation chamber HV chamber Vent/bake gun x4 Goal: 8 hours swap photocathode Present: ~12 hours

8. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 304SS without (blue) and with (red) electroplishing and vacuum firing Improvements to Gun Vacuum 304 SS: Electropolished & Vacuum Fired (AVS: 3 hrs @ 900 C @ 3x10 -6 T) NEG coating (Ti/Zr/V) 100 hrs @ 70 C 200 L/sec

9. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Comparing NEW & OLD load locked guns OLD NEW Bulk GaAs, Green light and DC beam “Further Measurements of Photocathode Operational Lifetime at Beam Current > 1mA using an Improved 100 kV DC High Voltage GaAs Photogun,” J. Grames, et al., Proceedings Polarized Electron Source Workshop, SPIN06, Tokyo, Japan Vacuum gauges indicated same pressure in both guns, Suggesting gauges don’t work below 1.5x10 -11 Torr

10. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 1 mA from a High Polarization Photocathode* ParameterValue Laser Rep Rate499 MHz Laser Pulselength30 ps Wavelength780 nm Laser Spot Size450 mm Current1 mA Duration8.25 hr Charge30.3 C Lifetime210 C Charge Lifetime160 kC/cm 2 High Initial QE Vacuum signals Laser Power Beam Current * Electron polarization not measured

11. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 An EIC-like Polarized e - Source 100 nm Superlattice GaAs: Layers of GaAs on GaAsP No strain relaxation QE ~ 0.8% Pol ~ 85% @ 780 nm chekc 14 pairs Fiber-based Laser Good gunGood PhotocathodeGood Laser

12. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 What are desirable (necessary) features of electron polarimeters? Large total analyzing power High luminosity to rapidly achieve small statistical uncertainty Non-invasive continuous measurement does not disrupt experiment Designs with reduced sensitivity to major systematics An EIC-like Injector Polarimeter  = P beam · P target · A total analyzing power N= ·(P b ·P t ·A) 2 PbPb PbPb 2

13. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Mott Scattering Spin-orbit coupling of the beam electron and the target nucleus.  =    1+ S(  ) P b · k  k´ | k  k´| Sherman Phys.Rev. 103(6) 1956, p1601-7 (-) A tot Target thickness effects Dilution by multiple/plural scattering Sherman function sets scale Target thickness extrapolation necessary MeV double-scattering is important

14. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 High cross-section of low energy (<1 MeV) Mott polarimeters is problematic: Significant plural and multiple scattering => reduces effective analyzing power Beam current limited to nanoamps High energy Mott scattering (MAMI, 1994) J. Sromicki demonstrated Mott scattering experiments from lead at 14 MeV J. Sromicki, Phys.Rev.Let. 81(1), 1999, p.57-60 Reduced cross-section =>  A currents are tolerated and dilution of the analyzing power is suppressed => sensitivity to target thickness is similarly reduced Uncertainty of Sherman function Coulomb screening at lower energy Ross et.al Phys.Rev A 38(12) 1988, p6055-8 Finite nuclear size at higher energy Ugincius et. al Nucl.Phys. A158 1970, p418-32 20% effect at 14 MeV 1.5% effect at 5 MeV Mott at Higher (MeV) Energy

15. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Jlab 5 MeV Polarimeter Jlab built a 5 MeV Mott polarimeter (typ. 1  m Au foil and 2  A beam current) J.S. Price et al., Pol. Gas Targets and Pol. Beams 7 th Int’l. Workshop, Urbana, IL 1997 Inelastic background discrimination was the largest problem HAPPEX used injector Mott results with ~5% uncertainity JLab 5 MeV Mott Polarimeter P b ~ 70% A tot ~ 40% Target: 1  m Au  = A tot · P b = N + + N - N + - N -

16. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Late 90’s M. Steigerwald joined the source group from MAMI Dramatic improvement eliminating background signal by means of collimation, shielding, time of flight, and coincidence methods Mott studied over range of 2-8 MeV with Au, Ag, Cu foils. Results presented at Spin 2000; M. Steigerwald, 14 th International Spin Physics Symposium 500 Å Gold Reducing Mott Systematics

17. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Effective Analyzing Power Collaboration with Horowitz at Indiana Univ. for Sherman function calculations (dominant contribution of total uncertainty about 1%) Applied double-scattering model to describe dilution of AP in targets of finite thickness. PRL describing analysis, model of double scattering, and results of 1.1% total measurement was drafted, but not published.

18. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Mott @ 20 keV, 100 keV, 500 keV & 5 MeV 20 kV “mini”-Mott will mate to new Load Lock Gun 5 MeV 100 keV or 500 keV New, July 2007

19. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 GaAs Photoguns @ Higher Average Current Year Ave. Beam Current (mA) First polarized beam from GaAs photogun eRHIC First low polarization, then high polarization at CEBAF JLab FEL program with unpolarized beam Cornell/JLab (construction) Qweak (2009) ELIC (LL demo) JLab Load Lock

20. Workshop on Precision Electron Beam Polarimetry for the Electron Ion Collider, University of Michigan, Aug 23-24, 2007 Conclusions EIC-like Electron Source Great progress, but work still to be done… 10-100x increase in superlattice lifetime laser cooling for 1 … 10 …100 mA sources gun technology for Vacuum or HV (e.g., 200 kV gun) surface charge limit (working with vendor) EIC-like Injector Polarimeter ~MeV Mott can be a “1%”-ish polarimeter limiting uncertainty is Sherman function invasive, yet rapid useful for the e- source scientist useful for polarimeter cross-comparisons