A bit of CEBAF polarized source history

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A bit of CEBAF polarized source history Our
original polarized source used bulk GaAs. QE 6
_at_ 780 nm Power 100 mW from a diode Polarization
35 In 1999 we began running from strained
GaAs. QE 0.2 _at_ 840 nm Power 100 mW from a
diode Polarization 75 Since then weve seen
improvements in laser power at 840 nm. Power
350 mW from a TiSapphire laser In June, 2003 we
heard about a really SUPER cathode at the
JLAB Polarimetry workshop. This is the
superlattice cathode. QE 1 _at_ 780 nm Power
500 mW from a TiSapphire laser Polarization 85
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We ordered a wafer immediately and prepared for
greatness!!! However, our first two attempts in
the tunnel failed. This was a frustrating
period, particularly with the G0 and HAPPEX-2
experiments on the horizon. We spent much effort
in 2003 and 2004 studying the problem, finally
learning that an essential step in our process,
hydrogen cleaning, results in poor quantum
efficiency and degraded polarization. Unfortunate
ly, hydrogen cleaning is an essential step in our
process for chemically anodizing the cathode. We
anodize the outer edge of the cathode (kill QE
there) to prevent bad electron emission, which
results in cathode damage. In June 04, prior to
HAPPEX-2, we chose to skip the anodization (and
hydrogen cleaning) steps, and install a bare
cathode.
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Superlattice Photocathode from SVT
From Hall A Compton Polarimeter

• The highest polarization yet measured at CEBAF
  85
• QE 0.8 versus 0.15
• Analyzing power 4 versus 12

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However, the lifetime was poor and we went from
running to limping, moving the laser spot often,
finally every day. Was it the exposed cathode
area or was Gun3 ready for maintenance? In Aug.
04 during the summer shutdown we gave Gun3 an
overhaul, notably installing fresh gas pumping
NEGs. We installed a superlattice cathode that
has its bad area covered by a metal mask. We
spent great care to polish and prepare this item,
as it becomes part of the high voltage
electrode. We planned for success, hoping to
run from the superlattice cathode during the Hall
A DVCS experiment, however, a laser problem
prevented us from doing so. Finally, we are here
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Gun3 SVT47222
Hall A 776 nm
Hall B 655 nm
QE 0.6
Hall C 782 nm
QE 6
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ELOG1202899 SVT47223VGUN (Reblochon)
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Demonstration of Qweak high polarization beam
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What does this mean to the program?
Bulk Strained Superlattice
Maximum Polarization 35 75 85
QE _at_ Max. Pol. 6 0.2 0.6
Polarized Experiment Efficiency 1 month 1 week 5 days
Advanced Concepts (1 mA) 1 month 2 weeks 6 days
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SUMMARY

• Status
• SVT superlattice high polarization 85 cathode
  in use
• Delivering 150 mA (soon 200 mA) of high
  polarization beam
• Rebuilt Gun3 with metallic mask operating well
• For Jan 05
• Evaluation of QE lifetime at 200 mA gun
  current
• Interval Mott and on-line Compton polarization
  measurements
• Parity quality beam tests occur parasitically
  and during BS periods