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Measurements of Photocathode Operational Lifetime at Beam Currents up to 10 mA

Measurements of Photocathode Operational Lifetime at Beam Currents up to 10 mA using an Improved DC High Voltage GaAs Photogun. J. Grames , M. Poelker , P. Adderley, J. Brittian, J. Clark, J. Hansknecht, M. Stutzman, K. Surles-Law BNL C-A Department February 9, 2007. Purpose & Overview.

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Measurements of Photocathode Operational Lifetime at Beam Currents up to 10 mA

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  1. Measurements of Photocathode Operational Lifetime at Beam Currents up to 10 mA using an Improved DC High Voltage GaAs Photogun J. Grames, M. Poelker, P. Adderley, J. Brittian, J. Clark, J. Hansknecht, M. Stutzman, K. Surles-Law BNL C-A Department February 9, 2007

  2. Purpose & Overview Goal: Deliver high average current (> 1mA) and high polarization (> 80%) with long photocathode operational lifetime in support of new accelerator initiatives. Enhance our understanding of photocathode decay mechanism. This will undoubtedly allow us to improve existing polarized guns operating at lower average current and unpolarized guns at milliAmp beam currents (e.g., Lightsources). • Background • R&D Program • New DC HV Load Lock Gun • Low-P GaAs Studies • High-P GaAs Studies

  3. CEBAF Polarized e- Source • CEBAF’s first polarized e-beam experiment 1997 • Now polarized beam experiments comprise ~ 80% of our physics program, in fact, we only deliver polarized electrons • All beam originates via photoemission from a Gallium Arsenside crystal inside a 100 kV photogun • 35 weeks of beam delivery per year • 100 mA at 85% polarization is fairly routine • Three experimental areas may simultaneously receive: • high polarization (~85%) => large asymmetry/ figure of merit • continuous wave (499 MHz) => high statistics/ low couting rates • independent intensity (50 pA to 200 mA) => target / acceptance • energy selection (multiples of linac energy) => flexibility

  4. A B C ContinuousElectronBeamAcceleratorFacility Two SRF 600 MeV linacs (1497 MHz) 67 MeV injector (1497 MHz) RF Lasers (499 MHz) RF deflectors A B C Double-sided septum Pockels cell Wien filter Spin Precession Degrees of Freedom P 100 keV DC Electron Gun

  5. Ceramic Insulator Cathode (GaAs) Anode -100 kV Laser - e Cs NF 3 NEG coated beampipe Non evaporable getter pumps (NEG) 4,000 liter/s pump speed  5E-12 Torr Present JLab “Vent/Bake” Polarized Electron Gun

  6. GaAs Photocathodes 100 nm 100 nm Strained GaAs 14 pairs Bulk GaAs GaAsP GaAsP Bulk GaAs Bulk GaAs Unstrained GaAs Strained GaAs Superlattice GaAs P ~ 35 - 40% P ~ 70 – 75 % P ~ 80 - 90 % Degeneracy Broken degeneracy, but relaxation No relaxation, quantum well structure

  7. Beam Polarization at CEBAF 2 2 P I P I sup. = 1.38 str. Experiment Figure of Merit

  8. Future High Current/ High Polarization Projects Qweak to test standard model >200 mA at 85% polarization Proposed (>1 mA) facilities ELIC, eRHIC Ring <1 C/day Linac >100 C/day ~20 C/day

  9. CEBAF Gun Charge Lifetime (2001-2004) Data compiled by M. Baylac QE(q)=QE0∙e(-q/CL) NEG replacement Summer 2003 improves lifetime Charge Lifetime Steadily Decreasing

  10. CEBAF Polarized Source Photocathode “QE” Lifetime limited by ion back-bombardment. Before heat/activation After heat/activation laser beam One photocathode operates for year(s), and multiple activations, usually limited by field emission from the cesiated electrode.

  11. Ions accelerated & focused to electrostatic center We don’t run beam from electrostatic center laser light IN electron beam OUT anode residual gas cathode Which ions more problematic? QE trough to electrostatic center Ion Back-Bombardment

  12. e beam Experiment Requires Managing Electron Beam • Limit photocathode active area • Eliminate stray light • Large diameter beampipes • NEG coated chambers to limit ESD • Proper electrode geometry • Proper lens configuration Cathode (-100 kV) Anode (ground) Strikes anode Strikes gun chamber Strikes Wien Faceplate (aperture) Strikes beampipe

  13. GaAs wafer…becomes a photocathode Wafer from vendor Stalk Mounted

  14. Paradigm Shift (Peggy Style => Load Lock Gun) Wafer from vendor Stalk Mounted Puck Mounted

  15. BTLLPEG Test Stand (2003-2006) • 3 Chambers • Load/Hydrogen/Heat • Prepare NEA surface • High Voltage, Good Vacuum • Photocathode Lifetime Test Bed • Low-P bulk GaAs • High QE (15-20%) => mA’s • 200 C/day vs. 10 C/day

  16. Improvements limiting the active area No more hydrogen cleaning Study one sample without removal

  17. Ion Pump Locations Improvements to monitor gun & beamline pressure

  18. Ion Pump Locations Improvements to monitor gun & beamline pressure

  19. Photocathode Lifetime Studies & Operation (2003-2006) • We’ve learned about photocathode lifetime… • vs. gun & beamline pressure (leaks, pumping, gauging) • vs. laser (spot size, position, reflections, power levels) • vs. GaAs preparation (active area, cleaning) • vs. beam handling (optics, orbits, beam losses) • We’ve learned about functionality of a Load Lock gun… • Round pucks + gravity = rolling • Manipulator alignment + bake-outs • Activation, heating, cooling • Sensitivity of manipulators to bake temperature • Multiple photocathodes > 1 photocathode Work mainly presented at workshops & recorded in proceedings…

  20. NEW Load Lock PhotoGun for CEBAF • What’s next (really, now!)… • Improve gun vacuum, photocathode lifetime • Load multiple photocathodes with the “suitcase” • Evolve the technology, i.e., design-out “features” • Transfer the technology to the CEBAF program

  21. High Voltage Chamber Top View Beam • Activation Chamber • Manipulators 150 C bake • New & Used puck storage • Suitcase & Load Chamber • Mount wafer on puck in lab • Holds 4 pucks (e.g., bulk, SL, SSL) • Load Lock: 8 hour bake @ 250 C • No H-Cleaning

  22. Docking Chamber & “Suitcase”

  23. Side View • High Voltage Chamber • “Side ceramic” design • load chamber at ground potential • No moving parts at HV • Activation Chamber • Mini-stalk heater • Mask selects active area • UHV IP supplies gauge activation • Keyed & eared pucks

  24. Side View • High Voltage Chamber • “Side ceramic” design • load chamber at ground potential • No moving parts at HV • Activation Chamber • Mini-stalk heater • Mask selects active area • UHV IP supplies gauge activation • Keyed & eared pucks

  25. 304SS without (blue) and with (red) electroplishing and vacuum firing Improvements to the High Voltage Chamber 304 SS: Electropolished & Vacuum Fired (AVS: 3 hrs @ 900 C @ 3x10-6 T) • Careful electrode alignment • Lipped to flatten field profile • Bias anode or support • Rear windows view “tee” NEG coating (Ti/Zr/V) 100 hrs @ 70 C 200 L/sec

  26. New Load Lock Gun Assembled & Running Spring ‘06 Heat/activation chamber Small bake Load region NEG-coated HV chamber Suitcase

  27. laser light IN electron beam OUT anode residual gas cathode Ionized residual gas strikes photocathode Ion damage distributed over larger area Benchmarking Photogun with Operational Lifetime (Best Solution – Improve Vacuum, but this is not easy) Bigger laser spot, same # electrons, same # ions

  28. Experimental Setup Faraday Cup (450 C bake) Laser (1 W @ 532 nm) & attenuators High Voltage (-100 kV) NEG pipe Activation (Cs/NF3, Mask=5 mm) Solenoid Centering Spot Size Adjustment 7 Precision Ion Pump Supplies 350 mm 1500 mm Load lock port (GaAs on puck)

  29. 1/e Charge Lifetime = Charge Extracted ln (QE /QE ) i f Example Run (5 mA) • Run laser power (<1 Watt) PID to fix beam current • Record ion pump current at 7 beam line locations • Record laser power/setpoint via “pickoff” detector

  30. Measurements Limited by HV Power Supply 13 mA!

  31. NEW vs. OLD Load Lock Design (small laser spot) Damage ~ (a∙I + b∙I2) NEW OLD

  32. Pgun = P0 + 4 pA/mA July Sept Leakage Current HV Chamber Pressure vs. Beam Intensity Gun Ion Production ~ Beam Intensity x Gun Pressure ~ (a∙I + b∙I2) New UHV

  33. Tough to measure >1000 C lifetimes with 100-200 C runs! 2 1500 Expectation: ≈ 18 350 5 15 SMALL vs. LARGE Laser Spot (BP vs. LL)

  34. Side-by-Side Comparison of Original/Improved Guns

  35. Photocathode Aging Superlattice Charge Lifetime The “100 mA” 85% Photocathode • We have no operational experience operating with superlattice at > 100 mA. • Surface charge limit. QE droops at higher laser power. Old wafers get tired, must be replaced.

  36. High Surface Charge Density Superlattice Photocathodes (M. Yamamoto, Nagoya University) • Superlattice photocathode: • Surface <100 nm is GaAs • Similar doping, e.g., Zinc • Concern: heat => diffuses dopant

  37. Superlattice Test June ‘05: 1 mA @ 532 Brief opportunity to test superlattice photocathode with 532 nm DC laser in the original load lock gun Lifetime ~200 C at 1 mA (532 nm)

  38. Fiber-based Laser Superlattice GaAs: Layers of GaAs on GaAsP chekc 100 nm 14 pairs No strain relaxation QE ~ 0.6% Pol ~ 85% @ 780 nm Now: High Current & High Polarization Ingredients: Good gun, good photocathode, powerful laser NEW Load Lock Gun (Was: Ti:Sap)

  39. Superlattice in LL Gun Successful activation QE ~ 10% @ 532 nm (low-P) QE ~ 0.6% @ 780 nm (high-P) We have, so far, only measured poor photocathode lifetime (10’s of C) at low average current (100 mA).

  40. We have just begun… …to measure how our experience with bulk translates to superlattice: Bulk (robust) Band-deep light (532 nm) DC (peak=ave, no SC) Superlattice (fragile) Band-gap light (780 nm) RF (ps & MHz) ? Cause & effect is not always obvious, so we will replace the sample, repeat the measurement, verify the baseline and ... enhance our understanding of photocathode decay mechanisms.

  41. Conclusions => NEW gun charge lifetime 2-3x better; likely vacuum, electrode improvements. => Larger laser spot improves charge lifetime, but not simple model prediction. => Exceptionally good Charge Lifetime >1000 C at high currents >1mA; in fact, difficult to measure when using large laser spot. => Photocathode lifetime measurements at higher (>1 mA) currents using GaAs/GaAsP superlattice, but so far poor lifetime. => Install load lock in tunnel in July 2007.

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