An EIC-like Polarized Electron Injector: Source, Beam & Polarimeter(s) Joe Grames Jefferson Lab

1. An EIC-like Polarized Electron Injector: Source, Beam & Polarimeter(s) Joe Grames Jefferson Lab August 23, 2007

2. 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. GaAs Photocathodes: last 30 years… 100 nm 100 nm 2 FOM  I P Superlattice GaAs: Layers of GaAs on GaAsP Strained GaAs: GaAs on GaAsP Bulk GaAs 14 pairs “conventional” material QE ~ 0.2% Pol ~ 75% @ 850 nm No strain relaxation QE ~ 0.8% Pol ~ 85% @ 780 nm High QE ~ 8% Pol ~ 35% @ 780 nm

4. 2 2 P I P I sup. = 1.38 str. 85% Polarization No depolarization over time! Routinely Achieve Stable Polarization ~ 85% Experiment Figure of Merit • 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. New Fiber Technology Based Drive Laser Ti-Sap Diode J. Hansknecht and M. Poelker, Phys. Rev. ST Accel. Beams 9, 063501 (2006) • 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)

6. 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. New CEBAF “load-lock” gun Preparation/activation chamber Goal: 8 hours swap photocathode Present: ~12 hours HV chamber Loading chamber x4 “suitcase” Vent/bake gun

8. 304SS without (blue) and with (red) electroplishing and vacuum firing Improvements to Gun Vacuum NEG coating (Ti/Zr/V) 100 hrs @ 70 C 200 L/sec 304 SS: Electropolished & Vacuum Fired (AVS: 3 hrs @ 900 C @ 3x10-6 T)

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

10. 1 mA from a High Polarization Photocathode* * Electron polarization not measured High Initial QE • Vacuum signals • Laser Power • Beam Current

11. Superlattice GaAs: Layers of GaAs on GaAsP Fiber-based Laser 100 nm 14 pairs chekc No strain relaxation QE ~ 0.8% Pol ~ 85% @ 780 nm An EIC-like Polarized e- Source Good gun Good Photocathode Good Laser

12. -1 N= ·(Pb·Pt·A) 2 2 dPb Pb An EIC-like Injector Polarimeter • 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 e = Pbeam· Ptarget· Atotal analyzing power

13. k k´ 1+ S(q)Pb· s(q,f) = s0(q) | k k´| Sherman Phys.Rev. 103(6) 1956, p1601-7 Mott Scattering Spin-orbit coupling of the beam electron and the target nucleus. • Target thickness effects • Dilution by multiple/plural scattering • Sherman function sets scale • Target thickness extrapolation necessary • MeV double-scattering is important (-) Atot

14. Mott at Higher (MeV) Energy • 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 => mA 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

15. Pb ~ 70% Atot ~ 40% Target: 1mm Au N+ - N- e = Atot· Pb = N+ + N- JLab 5 MeV Mott Polarimeter • Jlab 5 MeV Polarimeter • Jlab built a 5 MeV Mott polarimeter (typ. 1mm Au foil and 2 mA beam current) • J.S. Price et al., Pol. Gas Targets and Pol. Beams 7th Int’l. Workshop, Urbana, IL 1997 • Inelastic background discrimination was the largest problem • HAPPEX used injector Mott results with ~5% uncertainity

16. Reducing Mott Systematics • 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, 14th International Spin Physics Symposium 500Å Gold

17. 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. 5 MeV 100 keV or 500 keV New, July 2007 Mott @ 20 keV, 100 keV, 500 keV & 5 MeV 20 kV “mini”-Mott will mate to new Load Lock Gun

19. First polarized beam from GaAs photogun GaAs Photoguns @ Higher Average Current JLab Load Lock Cornell/JLab (construction) JLab FEL program with unpolarized beam eRHIC ELIC (LL demo) Ave. Beam Current (mA) Qweak (2009) Year First low polarization, then high polarization at CEBAF

20. 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