1. 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).

2. 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

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

4. 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

5. 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

6. 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

7. 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 250 C
No H-Cleaning

8. 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

9. 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

10. Improvements to the High Voltage Chamber
304 SS: Electropolished & Vacuum Fired
(AVS: x10-6 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) C
200 L/sec

11. 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)

12. 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

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

14. 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

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

16. 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

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

18. 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.

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

20. 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

21. 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?

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

23. 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

24. 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.

25. New Load Lock Gun Assembled & Running Spring ‘06

26. 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