Lifetime Measurements using the Jefferson Lab JLAB LoadLock Gun J. Grames, P. Adderley, M. Baylac, J

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Lifetime Measurements using theJefferson Lab
(JLAB) Load-Lock GunJ. Grames, P. Adderley, M.
Baylac, J. Brittian, D. Charles,J. Clark, J.
Hansknecht, M. Poelker, M. Stutzman, K.
Surles-LawOctober 7-9, 2004PESP 2004Mainz,
Germany
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Lifetime Studies
Our job at JLAB is to provide up to 200 mA CW of
polarized electrons (20 C/day). Future
photoinjectors will require 10s of milliAmps of
polarized electrons.
Photocathode quantum efficiency (QE) degrades
with gun operation, correlated with the charge
evolved. If QE were initially exceptionally high
or laser power were unlimited then degradation of
the QE might be tolerable, but neither are
practically so. Additionally, non-uniform QE
across the illuminated surface may be intolerable
for some parity violation experiments.
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Presentation Outline
A 100 keV load lock gun and beam line have been
installed for the studies of QE degradation as a
function of gun operation.

• Gun/Beamline Commissioning
• QE Degradation Trends
• vs. vacuum level
• vs. laser spot location
• vs. active area
• Lifetime Rates
• Mask vs. Anodization
• Improved vacuum monitoring
• Higher current studies
• Model construction

• Part 1 (Summer-Fall 03)
• Part 2 (Spring-Summer 04)

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100 keV Load-Lock Electron Gun
High Voltage Chamber
Load-Lock Chamber
Preparation Chamber
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100 keV Beam line for Lifetime Studies
Summer-Fall 03
Spring-Summer 04
780 nm diode, 50 mW, 500 mm Commissioned to 350 mA
780 nm diode, 50 mW, 500 mm 532 nm NdYVO4, 11 W,
1000 mm Commissioned to 8 mA
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Versus Vacuum, Anodization, Beam Location
Unanodized (11 mm)
Anodized (5mm)
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QE Degradation vs. Gun Pressure (Extractor Gauge)
2.5E-11 Torr
5.0E-11 Torr
gt15.0E-11 Torr
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QE Degradation Trend vs. Laser Spot Location
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QE Degradation Trend vs. Anodization Size
5mm (red) anodized
11 mm (green) unanodized
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Lifetime vs. Time (fast slow process ?)
Data
Lifetime(s) QE falls by 1/e
Slow Lifetime Fit
Fast Lifetime Fit
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Part 2 Mask Activation Vacuum Monitoring
Improved UHV on-line vacuum monitoring using nAmp
holding ion pump power supplies.
A mask to limit activation surface (as
demonstrated at Mainz) was installed to the prep
chamber (5, 7, 9 mm).
Manipulator mis-alignment has resulted in dropped
pucks and detours
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Lifetime vs. Time (9 mm mask)
Goal Develop self-consistent model.
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Lifetime vs. Intensity (7 mm 9 mm mask)
Threshold ?
7 mm 9 mm
Beam Off
7 mm 9 mm
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Lifetime vs. Laser Spot Location (7 mm 9 mm
mask)
Mask Radius
Lifetime dependence on spot location can be
sensitive to optics.
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QE 300 V vs. 100 kV
QE at 300 V
4.15 mA
QE at 100 kV (2500,3500)
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Transient Steady State Details

59 C
0.1 C
0.01 C
4 C
8 C
19 C
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Summary
We are using a 100 keV load-lock gun and beamline
to study photocathode lifetime vs. gun operating
parameters. Our experimental setup evolves
often. Measurements consistently indicate that
improved vacuum and limited active area enhance
photocathode lifetime. Measurements indicate
that photocathode lifetime as a function of laser
spot location may depend upon a greater set of
parameters, such as beam line optics and laser
spot size. In the coming months emphasis will be
placed to develop a quantitative lifetime model
to better describe collected data and design
future measurements.