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.