Further Measurements of Photocathode Operational Lifetime at Beam Intensity > 1mA with the NEW 100 kV DC GaAs Photogun J. Grames, M. Poelker, P. Adderley, J. Brittian, J. Clark, J. Hansknecht, E. Pozdeyev, M. Stutzman, K. Surles-Law 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. Will undoubtedly allow us to improve existing polarized guns operating at lower average current and unpolarized guns at milliAmp beam currents (e.g., Lightsources).

CEBAF Polarized Source CEBAF is doing very well. Busy, productive Nuclear Physics program. 35 weeks of beam delivery per year 100 mA at 85% polarization is fairly routine One photocathode operates for year(s), and three or four activations 2-3 Users simultaneously; one is always Parity Violation experiment Photocathode Lifetime limited by ion back-bombardment. Before heat/activation After heat/activation

CEBAF Gun Charge Lifetime (2001-2004) Data compiled by M. Baylac NEG replacement Summer 2003 improves lifetime Charge Lifetime Steadily Decreasing

Can increasing the laser spot size improve charge lifetime? (Best Solution – Improve Vacuum, but this is not easy) Bigger laser spot, same # electrons, same # ions electron beam OUT residual gas cathode Ionized residual gas strikes photocathode anode laser light IN Ion damage distributed over larger area

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 Anode e beam Hits anode Hits gun chamber Hits Wien faceplate Hits beampipe

NEG-coated large aperture beam pipe Experimental Setup Bulk GaAs NEW 100 kV load locked gun Faraday Cup Baked to 450C NEG-coated large aperture beam pipe Differential Pumps w/ NEG’s 1W green laser, DC, 532 nm Focusing lens on x/y stage Spot size diagnostic Insertable mirror

High Voltage Chamber Top View Activation Chamber Beam Activation Chamber Manipulators 150 C bake New & Used puck storage Suitcase & Load Lock 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

Side View High Voltage Chamber Activation 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

Side View High Voltage Chamber Activation 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

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” 304SS without (blue) and with (red) electroplishing and vacuum firing NEG coating (Ti/Zr/V) 100 hrs @ 70 C 200 L/sec

Experimental Setup Faraday Cup Laser (1 W @ 532 nm) High Voltage (100 kV) NEG pipe Activation (Cs/NF3, 5 mm) Spot Size Adjustment 350 mm 1500 mm Load lock (GaAs on puck)

Example Run (5 mA) 1/e Charge Lifetime = Charge Extracted ln (QE /QE ) Set beam current (1-10 milliAmps) at Faraday Cup 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 1/e Charge Lifetime = Charge Extracted ln (QE /QE ) i f

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

Dependence of Gun Pressure on Beam Intensity Gun Ion Production ~ Beam Intensity x Gun Pressure ~ (a∙I + b∙I2) Leakage Current Pgun = P0 + 4 pA/mA July Sept

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

Is Ionized Gas from the Beamline Limiting Charge Lifetime? Ionization cross section for H2 Wtot= σ(E)∙dE ~ 4∙10-18 cm2 ∫ Lgap ~ 5 cm Pgun ~ 5∙10-12 Torr Ygun ~ 1.5∙107 ions/C W(100keV) = 4∙10-19 cm2 Lbeamline ~ 100∙Lgap Pbeamline ~ 20∙Pgun Ybeamline ~ 200∙Ygun

Repelling Beamline Ions with Biased Anode Contributed by E. Pozdeyev Beamline Ions Design Shield Anode Design = Trap!!

Biased Anode: Null Result 5.1 mA (40 C) 2.9 mA (120 C) 3.2 mA (140 C) +1 +2 +2.5 -2.5 BIAS Conclusion: Not observing improvement.

Preparation Chamber: Hydrogen Degradation of QE 10-9 10-10 10-11 Preparation chamber dominated by hydrogen Pressure (Torr) 30 min ~2 L

Tantalyzing Discovery: Hydrogen Barrier Enhances Lifetime Wafer QE improves as the hydrogen barrier is removed. All three spots ~25 C before QE starts to fall. Once the barrier is removed QE falls as usual. 1 3 2

QE Largest at Beam Spot Location 2 weeks 1000x reduction (12% to 0.012%) ~5 Coulombs extracted 10x improvement at spot 4x improvement on surface ~103 L Trenching?

Experience with Superlattice @ 1 mA Superlattice QE @ 532 nm: ~8% at bias (300V) ~4% at HV (100 kV) Lifetime ~200 C at 1 mA (532 nm)

Laser Position Stability What’s Next ? Modifications to Load Lock Gun Fix a short in the heater assembly Add puck/heater cooler (8 hr => 2 hr) Increase HV gate valve ID 20 mm steps, ~ few hrs Prepare running with polarized material Load bulk, strained layer & superlattice Add 780 nm laser (2 Watt and RF) Improve laser intensity/position monitoring Run ~1000 C at 1-10 mA for accurate lifetimes Fully commission Load Lock gun Demonstrate 1 mA from superlattice >500 C lifetime Install in CEBAF June 2007 Complete 2nd Load Lock gun Laser Position Stability

Conclusions NEW gun charge lifetime better by 2-3 times; likely due to vacuum improvements. Large spot provides better Charge Lifetime, consistent with previous experiments. Exceptionally good Charge Lifetime >1000 C at high currents >1mA In fact, difficult to measure when using large laser spot. Anode biasing to +/- 2.5kV yields no measurable improvement; ions created downstream of anode not a problem, at least not in test stand with good vacuum. First demonstration of surface barrier that enhances operating lifetime, albeit at expense of initial QE. Look for other coating material that preserves QE, but does not reduce QE. Polarized beam studies at >1 mA and 80% polarization during Fall 2006. Install load lock in tunnel in Spring 2007.

New Load Lock Gun Assembled & Running Spring ‘06

Reality is more complicated… Ions accelerated & focused to electrostatic center We don’t run beam from electrostatic center residual gas cathode electron beam OUT anode Which ions more problematic? laser light IN QE trough to electrostatic center