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Recent Polarized Cathodes R&D at SLAC and Future Plans

Recent Polarized Cathodes R&D at SLAC and Future Plans. Feng Zhou Axel Brachmann, Jym Clendenin (ret.), Takashi Maruyama, and John Sheppard SLAC Workshop on polarized e- sources, JLAB, Oct. 1 – Oct. 3, 2008. Outline.

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Recent Polarized Cathodes R&D at SLAC and Future Plans

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  1. Recent Polarized Cathodes R&D at SLAC and Future Plans Feng Zhou Axel Brachmann, Jym Clendenin (ret.), Takashi Maruyama, and John Sheppard SLAC Workshop on polarized e- sources, JLAB, Oct. 1 – Oct. 3, 2008

  2. Outline • Critical R&D goals: demonstrate full charge productions (highly polarized) for future linear colliders • Recent measurements on InAlGaAs/AlGaAs: • Measurements at Cathode Test System (CTS) • Measurements at Gun Test Lab (GTL) • Summary for the measurements • Future plans on polarized cathode developments F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

  3. Major parameters of ILC and CLIC e- sources 2 2 PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  4. Challenges • Full charge production limited by space charge and surface charge: • Lasers to demonstrate production of electron beam with ILC and CLIC time structures • Good polarized cathodes to overcome surface charges with high QE and polarization • H.V. gun to overcome space charge, and high vacuum to overcome contamination. • Cathode candidates for linear colliders: • Less charge limit (surface charge and space charge) • High polarization (>85%) • High QE and QE lifetime PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  5. InAlGaAs/AlGaAs cathode • Strained-well InAlGaAs/AlGaAs structures designed and grown by St. Petersburg in Russia: • Large valence band splitting (~60 meV) due to combination of deformation and quantum confinement effects in quantum well. • Good BBR engineering • Thick working layer without strain relaxation • 88%-93% of polarization obtained at Russia F. Zhou/SLAC

  6. Cathode preparation and cleaning processes QE and polarization measured at 20 KV QE and polarization of Cathode can be quickly characterized in few days. More fast and convenient compared with GTL Drawback: is unable to characterize surface charge limit (time evolution of bunch). Capabilities at Cathode Test System PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  7. InAlGaAs/AlGaAs @ CTS • Polarized cathode measurements: • Cathode preparation • Chemical cleaning • Load lock system to change cathode in UHV • Heat cleaning • Atomic hydrogen cleaning • 4 samples measured at CTS: 0.1%-0.3% QE and 82%-87% of polarization. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  8. SBIR graded AlxGa(1-x)As/GaAs(Grown by SVT) • The graded bandgap active region provides an internal accelerating field for the photo-generated electrons in the conduction band. QE is increased by the field. • But, the polarization is decreased; need to tune the structure parameters in SBIR phase II. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

  9. Capabilities at Gun Test Lab • Re-established all measurements at GTL after three-year down time: • Charge limit (time evolution of bunch) • QE and QE lifetime • Polarization • Take full measurements at GTL: • 1st sample InAlGaAs/AlGaAs (measurements done) • 2nd sample InAlGaAs/AlGaAs installed • Internal graded sample AlxGa(1-x)As/GaAs • To demonstrate full charge production once it is mated to ILC and CLIC lasers • Planned programs on cathode developments • System is also available for other R&D projects, such as test different electrodes and guns. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  10. GTL layout 60-300 ns Flash Ti:Saphhire @60 Hz 120 kV DC-gun Nanoammeter to measure QE Fast Faraday cup to measure time evolution of electron bunch Mott polarimeter Spin rotator PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  11. InAlGaAs/AlGaAs: QE uniformity 0.75 0.75 0.75 0.86 0.82 0.72 20 mm 1 1 0.75 0.84 0.81 0.84 0.86 0.83 0.79 0.78 PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  12. InAlGaAs/AlGaAs: QE lifetime PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  13. QE vs wavelength @ certain laser energy =10 mm bandgap PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  14. InAlGaAs/AlGaAs: polarization measurements PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  15. InAlGaAs/AlGaAs: polarization vs surface charge limit • The cathode is driven into saturation, electrons photoexcited into conduction band can still escape if they diffuse to a non-saturated region. • But, these electrons spend long time inside structure so it is likely that they suffer spin relaxation. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  16. Surface photovoltaic effect – surface charge limit • Photon absorption excites electrons to conduction band • Electrons can be trapped near the surface • Electrostatic potential from trapped electrons raised affinity. • Increased affinity decreases emission probability. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  17. Surface charge vs laser energy 10 10 0.73x10 e- @ 1x laser energy 10 mm full size 2.5x10 e- @ 8x laser energy 10 mm full size PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  18. Surface charge vs laser location Same laser energy 10 10 2.5x10 e- production 10 mm full size @ good location 1.4x10 e- production 10 mm full size @ bad location PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  19. Charge limit vs laser energy 10 mm laser full size PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  20. J =(2.33x10 )V /d -6 3/2 2 0 2 2 7x10 /cm 18 J =0.06 A/cm J =10 A/cm 3 0 0 Charge limit vs beam size • Space charge limit (Child law): • H qgf • Wqjh • WJH • Space charge (Child’s law) @ GTL gun • Take beam parameters d=5mm, Q=3.75 nC, 300 ns, • Space charge negligible at current conditions; thus surface charge limit dominates at smaller size with of doping at the surface layer. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  21. 60 ns Surface charge vs pulse length (good location) 300 ns And what about at 1 ns? same laser energy 10 mm laser full size PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  22. Surface charge vs pulse length (bad location) 60 ns 300 ns And what about at 1 ns? same laser energy 10 mm laser full size PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  23. 3.0 A/cm 2 1.5 A/cm 2 What’s the possible indications from the measurements for ILC and CLIC: surface charge & space charge? PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  24. 7x10 /cm 18 3 Summary • Combination of CTS & GTL at SLAC is an unique diagnostic to characterize polarized photo-cathodes. • Recent systematic measurements for one InAlGaAs/AlGaAs sample (sample # 7-632) at both CTS & GTL: • 0.3% QE at CTS against 0.7% QE of Russian data; QE lifetime measured at GTL is 120-150 hrs. • 82% (CTS) and 84% (GTL) of polarization against 88% polarization of Russian data. • Surface charge limit is observed, current intensity with @ of doping in surface. • First observation of polarization dependence on surface charge limit. • Need optimize parameters of InAlGaAs/AlGaAs to meet cathode critical requirements for linear collider sources. 0.06 A/cm 2 PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  25. Future cathode R&D at SLAC • Measure another sample of InAlGaAs/AlGaAs and graded AlGaAs/GaAs cathode at GTL. • Planned programs: • Study doping level in the structure of GaAs/GaAsP • Gradient doping in the active layer • Apply both techniques into GaAs/GaAsP • Optimize InAlGaAs/AlGaAs cathode parameters • Demonstrate charge production (surface charge and space charge) for the ILC once its laser ready. The ILC laser expected ready in the early of next year. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  26. Doping level in GaAs/GaAsP • Doping level at least affects: • Smearing band edge and broadening hole spectrum • Spin relaxation in transport stage; BAP process - one of major mechanisms -, exchange interaction between electrons and holes: • Spin relaxation in BBR • Surface charge limit • Plan to study doping level in surface and active layer of GaAs/GaAsP PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  27. Gradient doping in the active layer • Electrons are accelerated when getting through band-bending regions • High QE expected • Much interest in gradient doping in the active layer of SL structure. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  28. 100 ps/bunch, 312 bunches, bunch spacing: 0.5 ns   CLIC 126 ns 50 Hz – 20 ms 1 ns/bunch, ~2800 bunches bunch spacing: ~357 ns   ILC 1 ms 5 Hz – 200 ms Future cathode R&D (con’t) • Demonstrate CLIC-like beam production by simply modifying ILC laser: • ILC laser: existing 76 MHz ML oscillator/25 times=3MHz • CLIC-like laser: directly use existing 76 MHz ML oscillator to generate CLIC-like beam (~13 ns spacing). • Real CLIC laser (need extra funds) • The gun lab at SLAC is an ideal diagnostic to characterize critical parameters of a photocathode for both ILC and CLIC: surface charge limit, polarization, QE, and QE lifetime. PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

  29. Thank colleagues at St. Petersburg and SVT Associates for the collaboration PESP workshop at JLAB, Oct. 1-3, 2008 F. Zhou/SLAC

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