Transcript Lifetime Measurements using the JLab Load-Lock Gun Joe Grames September 22-26, 2003 Polarized Sources and Targets Workshop Novosibirsk, Russia Thomas Jefferson National Accelerator Facility Operated by the.

Lifetime Measurements using the
JLab Load-Lock Gun
Joe Grames
September 22-26, 2003
Polarized Sources and Targets Workshop
Novosibirsk, Russia
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Preface
The good news is that Isabel let me come here, but…
the bad news is that she kept most of my talk.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Outline
1. Lifetime studies: purpose and parameters
2. Load-Lock Gun Testbed
3. Lifetime vs. vacuum level
vs. anodization
vs. laser spot location
4. Trends from Data (preliminary)
5. Planned Improvements
6. Conclusions
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
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.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Experimental Test Bed:
100 kV Load-Lock Electron Gun
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Load-Lock Diagnostic Beamline
Cathode:
Bulk GaAs w/ Cs-NF3
Laser:
780 nm diode, DC
500 mm spot size
Beam:
150-180 mA @ 100keV
Optics:
Copy of CEBAF injector
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Versus: Vacuum, Anodization, Beam Location
Unanodized (11 mm)
Leak
Fix/Bake
>15.0
4.9
Anodized (5mm)
NEG’s w/ IP
NEG’s w/ Turbo
2.3
(x10-11 Torr)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
1.9
Versus Vacuum
~2.5E-11 Torr
~5.0E-11 Torr
>15.0E-11 Torr
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Versus Spot Location
2.4 mm
1.6 mm
0.8 mm
Center
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Versus Anodization
Location #1
5mm (red)
anodized
11 mm (green)
unanodized
Location #2
Location #3
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Versus Something Not Understood…
Data
Slow Lifetime Fit
Fast Lifetime Fit
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Analysis Plans
Include QE surface and ion-pump nA-meter data to improve interpretation.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
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)
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
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.
Thomas Jefferson National Accelerator Facility
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy