Lifetime Measurements using the JLab LoadLock Gun Joe Grames September 2226, 2003 Polarized Sources

1
Lifetime Measurements using theJLab Load-Lock
GunJoe GramesSeptember 22-26, 2003Polarized
Sources and Targets WorkshopNovosibirsk, Russia
2
Preface
The good news is that Isabel let me come here,
but
the bad news is that she kept most of my talk.
3
Outline

• Lifetime studies purpose and parameters
• Load-Lock Gun Testbed
• Lifetime vs. vacuum level
• vs. anodization
• vs. laser spot location
• 4. Trends from Data (preliminary)
• 5. Planned Improvements
• 6. Conclusions

4
Lifetime Studies
Our job at JLab is to provide the nuclear physics
program with 200 mA CW of polarized electrons
(this amounts to 5-20 C/day).
The electron gun photocathode quantum efficiency
degrades with the amount of charge extracted. If
the cathode QE were very high to begin with, or
laser power were unlimited the degradation would
be tolerable, but neither are so. Strong
evidence indicates the QE degradation results due
to ion back-bombardment. The purpose of these
studies is to quantify (and qualify) the manner
by which the QE degrades while operating the gun.
5
Experimental Test Bed100 kV Load-Lock Electron
Gun
6
Load-Lock Diagnostic Beamline
Cathode Bulk GaAs w/ Cs-NF3 Laser 780 nm
diode, DC 500 mm spot size Beam 150-180 mA _at_
100keV Optics Copy of CEBAF injector
7
Versus Vacuum, Anodization, Beam Location
Unanodized (11 mm)
Anodized (5mm)
8
Versus Vacuum
2.5E-11 Torr
5.0E-11 Torr
gt15.0E-11 Torr
9
Versus Spot Location
2.4 mm
1.6 mm
0.8 mm
Center
10
Versus Anodization
5mm (red) anodized
11 mm (green) unanodized
11
Versus Something Not Understood
Data
Slow Lifetime Fit
Fast Lifetime Fit
12
Analysis Plans
Include QE surface and ion-pump nA-meter data to
improve interpretation.
13
Improvements to Load Lock Gun
Cathode Handling Improve transfer between
chambers (dropped, mis-aligned holder) Provide
capability to store multiple cathodes (one at a
time) Load Chamber Increase ion pump capacity to
20 L/s (11 L/s capacity too small) Replace H
cleaner (inefficient SS resonator, large H
plumbing volume) Add N2 vent port toTurbo pump
(bleeds to atmosphere) Prep Chamber Add
heating/cooling (now done in less clean load
chamber) Ground cathode holder (cathode can float
during activation) High Voltage Chamber Improve
handling and between chambers (dropped,
mis-aligned) Provide capability to store multiple
cathodes (one at a time)
14
Conclusions
We are performing a series of measurements to
study photocathode lifetime vs. operating
parameters in our 100 kV load-lock gun and
beamline. We have learned and added the
diagnostics to now make a good series of
measurements at typical HV chamber vacuum
level. Results of the measurements will be
summarized this Fall and prepared for
publication. Improvements to the load-lock gun
will begin shortly after returning to JLab. The
product of that work is planned to be installed
in the accelerator in summer, 2004.