History of CEBAF Polarized Source: From Bulk GaAs to Superlattice Cathode

1. A bit of CEBAF polarized source history… • Our original polarized source used bulk GaAs. • QE ~ 6% @ 780 nm • Power ~ 100 mW from a diode • Polarization ~ 35% • In 1999 we began running from strained GaAs. • QE ~ 0.2% @ 840 nm • Power ~ 100 mW from a diode • Polarization ~ 75% • Since then we’ve seen improvements in laser power at 840 nm. • Power ~ 350 mW from a Ti:Sapphire laser • In June, 2003 we heard about a really SUPER cathode at the JLAB • Polarimetry workshop. This is the superlattice cathode. • QE ~ 1% @ 780 nm • Power ~ 500 mW from a Ti:Sapphire laser • Polarization ~ 85%

2. 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. Unfortunately, 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.

3. Superlattice Photocathode from SVT here here Preliminary here Wavelength (nm) Wavelength (nm) Wavelength (nm) QE (%) Polarization Analyzing Power photon electron From Hall A Compton Polarimeter • The highest polarization yet measured at CEBAF; ~ 85% • QE 0.8% versus 0.15% • Analyzing power 4 % versus 12%

4. 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 NEG’s. • We installed a superlattice cathode that has it’s “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…

5. Gun3 SVT4722#2 Hall A 776 nm Hall B 655 nm QE ~ 0.6% QE ~ 6% Hall C 782 nm

6. ELOG#1202899 SVT4722#3VGUN (Reblochon)

7. Demonstration of “Qweak” high polarization beam

8. What does this mean to the program…?

9. 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