Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Lifetime Measurements using theJLab Load-Lock Gun
Joe Grames
September 22-26, 2003
Polarized Sources and Targets WorkshopNovosibirsk, Russia
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
The good news is that Isabel let me come here, but…
the bad news is that she kept most of my talk.
Preface
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
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
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Lifetime Studies
Our job at JLab is to provide the nuclear physics programwith 200 A 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.
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Experimental Test Bed:100 kV Load-Lock Electron Gun
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Load-Lock Diagnostic Beamline
Cathode:Bulk GaAs w/ Cs-NF3
Laser:780 nm diode, DC500 m spot size
Beam:150-180 A @ 100keV
Optics:Copy of CEBAF injector
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Versus: Vacuum, Anodization, Beam Location
Leak Fix/Bake
4.9
NEG’s w/ TurboNEG’s w/ IP
(x10-11 Torr)
2.3 1.9>15.0
Unanodized (11 mm) Anodized (5mm)
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Versus Vacuum
>15.0E-11 Torr
~5.0E-11 Torr
~2.5E-11 Torr
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Versus Spot Location
Center
0.8 mm
1.6 mm
2.4 mm
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Versus Anodization
Location #1
Location #2
Location #3
5mm (red)anodized
11 mm (green)unanodized
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Versus Something Not Understood…
Fast Lifetime Fit
Slow Lifetime Fit
Data
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Analysis Plans
Include QE surface and ion-pump nA-meter data to improve interpretation.
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
Improvements to Load Lock Gun
Cathode HandlingImprove transfer between chambers (dropped, mis-aligned holder)Provide capability to store multiple cathodes (one at a time)
Load ChamberIncrease 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 ChamberAdd heating/cooling (now done in less clean load chamber)Ground cathode holder (cathode can float during activation)
High Voltage ChamberImprove handling and between chambers (dropped, mis-aligned)Provide capability to store multiple cathodes (one at a time)
Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy
Thomas Jefferson National Accelerator Facility
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.