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 ParametersILCCLIC Electrons/microbunch~3E106E9 Number of microbunches2625312 Width of Microbunch1 ns~100 ps Time between microbunches~360 ns500.2 ps Width of Macropulse1 ms156 ns Macropulse repetition rate5 Hz50 Hz Charge per macropulse~12600 nC300 nC Average current from gun 63  A15  A Peak current of microbunch4.8 A9.6 A Current density (1 cm radius)1.5 A/cm3.0 A/cm Polarization>80% F. Zhou/SLAC 22 PESP workshop at JLAB, Oct. 1-3, 2008

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 F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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. Capabilities at Cathode Test System 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). F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

8. SBIR graded Al x Ga (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: –1 st sample InAlGaAs/AlGaAs (measurements done) –2 nd sample InAlGaAs/AlGaAs installed –Internal graded sample Al x Ga (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. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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

11. InAlGaAs/AlGaAs: QE uniformity 1 1 0.84 0.86 0.78 0.790.83 0.81 0.750.84 0.82 0.86 0.72 0.75 20 mm F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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

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

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

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. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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

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

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

20. Charge limit vs beam size Space charge limit (Child law): Hqgf 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. F. Zhou/SLAC 7x10 /cm 18 3 PESP workshop at JLAB, Oct. 1-3, 2008 J =(2.33x10 )V /d 0 -63/2 2 J =10 A/cm 0 2 J =0.06 A/cm 0 2

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

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

23. What’s the possible indications from the measurements for ILC and CLIC: surface charge & space charge? F. Zhou/SLAC ILCCLIC Microbunch 300 ns  1 ns Surface charge better Space charge worse 300 ns  100 ps Surface charge better Space charge much worse Macropulse1 ms (360 ns spacing) Accumulated surface charge may be much worse? 156 ns (0.5 ns spacing) Surface charge may accumulate Current intensity (r=1cm) Surface charge and space charge combined; Surface charge may be serious in macropulse? Space charge serious; Surface charge may accumulate in macropulse 1.5 A/cm 2 3.0 A/cm 2 PESP workshop at JLAB, Oct. 1-3, 2008

24. 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. F. Zhou/SLAC 7x10 /cm 18 3 0.06 A/cm 2 PESP workshop at JLAB, Oct. 1-3, 2008

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. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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 F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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. F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

28. 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. 50 Hz – 20 ms 126 ns CLIC 100 ps/bunch, 312 bunches, bunch spacing: 0.5 ns  5 Hz – 200 ms 1 ms ILC 1 ns/bunch, ~2800 bunches bunch spacing: ~357 ns  F. Zhou/SLAC PESP workshop at JLAB, Oct. 1-3, 2008

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