Transcript CEBAF Polarized Electron Guns Marcy L. Stutzman for the Jefferson Lab Polarized Source Group.

CEBAF Polarized Electron Guns
Marcy L. Stutzman
for the
Jefferson Lab
Polarized Source Group
Status of the Gun 2001
• Horizontal polarized guns
• Strained layer GaAs
photocathode material
(70% polarization)
• Homemade Ti:Sap lasers or
diodes
• Vacuum not well understood
– Leaks in load-locked gun
prototype
Photocathode Improvements
• Strained layer
– relaxation limits polarization to 70%
• Superlattice material
– SLAC Small Business Innovation
Research (SBIR) collaboration
– 85% polarization routinely
– Higher QE, 780 nm lasers
– Delicate: no hydrogen cleaning
– New anodization procedure
developed
– Beam from superlattice
photocathodes since summer 2005
Laser Improvements
2000: High maintenance, high power
Ti:Sap or low power diode lasers
2002: Transition to commercial Ti:Sap
lasers starting with G0 experiment for
32 MHz repetition rate, same vendor for
499 MHz regular running
2004: Clean room installed
2006: Gain-switched fiber-coupled diode
laser and ErYb-doped fiber amplifier
– More power
– Better reliability
– Compatible with superlattice
– Final solution for CEBAF lasers?
Vacuum Improvements
• Better vacuum leads to longer lifetime
• Real time UHV monitoring
• Outgassing rate studies showed
benefit of EP, vacuum fire
• Pump speed measurements quantified
improvement with activation
• NEG coating characterization
• XHV pressure measurement?
Discharge
event in
beamline
Outgassing Rates vs. Bakes
6 E-12
5 E-12
Outgassing Rate
New load-locked gun July 2007
– Best vacuum practices
– No chemicals in HV chamber
– No Cs on electrodes
– Multiple photocathodes
– No anodization required
~10-10 Torr
Full Scale
4 E-12
3 E-12
2 E-12
1 E-12
0 E+00
0
2
4
6
8
Bake number
10
12
14
Future
• Always looking for next “100%” polarization cathode
– Developing a micro-Mott polarimeter to characterize
photocathodes (collaboration with Tim Gay, U. Nebraska)
– 100/500 kV Mott polarimeter for injector segmented mode
– CrO2 photocathode? “half metal” developed for spintronics
– Need to continue GaAs photocathode research - SBIR
• Load locked gun utilizing vacuum improvements will be
installed in tunnel – no more need for anodization
• We have the final “last” perfect laser?