Polarized Source Issues for Qweak

1
Polarized Source Issues for Qweak

• 180 uA and polarization gt 80
• Parity quality
• Long experiment

• Success requires
• Superlattice photocathodes
• High power, reliable modelocked Ti-Sapphire
  laser
• Load locked gun (new!!)
• 300 Hz pockels cell flipping

2
Polarized Electron Guns at JLab
HV insulator
Photoemission from GaAs semiconductor
NEG pumps
NEG-coated Beamline
Strained GaAs in Gun2 (old material) Strained-s
uperlattice GaAs in Gun3 (new material)
3
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

photon
electron
4
Superlattice Photocathodes at CEBAF

• Good results at EEL and Test Cave. Reliable 85
  polarization and QE 1.
• First attempts to load superlattice
  photocathodes inside tunnel guns failed cant
  anodize and hydrogen clean.
• Un-anodized superlattice photocathode in Gun 3
  (March, 2004). QE 0.4. Used during HAPPEx-He
  and portion of HAPPEx-H (June, 2004). Gun
  lifetime poor at 80 uA.
• Another sample loaded into gun3 Sept. 2004.
  Tantalum mask used to limit active area. Good
  polarization and QE but gun performance poor.

5
First Experience with Superlattice at CEBAF
QE profile after 3 weeks of running

• No anodization.
• Operate with large photocathode active area.
• Poor gun lifetime.
• Spot moves daily while running 100 uA from gun.

6
High current operation requires that we properly
manage the extracted beam (both good and bad
beam).
Why? To preserve good vacuum and limit QE
degradation associated with ion-backbombardment

• Limit photocathode active area
• Eliminate stray light
• Large diameter beampipes
• NEG coated chambers to limit ESD
• Proper electrode geometry
• Proper lens configuration

Cathode
Anode
Hits anode
Hits gun chamber
Hits Wien faceplate
Hits beampipe
7
Limiting Active Area via Anodization
We have not successfully anodized superlattice
material it cannot be hydrogen cleaned.
8
Second Experience with Superlattice at CEBAF

• Limit active area using tantalum mask
• Poor lifetime again
• Mask disturbs gradient at surface of
  photocathode
• halo electrons strike vacuum chamber walls

9
As-capped samples Can they be anodized?

• No! Recent test at EEL building unsuccessful.
• Installing yet another superlattice sample
  April 14, 2005 for low current Hall B program
  (lifetime not such an issue).
• Exploring other options for limiting active
  area (sputter coat the edges, ozone etch, use an
  electrode with small hole)
• Load lock gun solves problem.

10
Load lock gun will solve anodization problems
Mask limits application of cesium.

• Other benefits for Qweak
• Better gun vacuum (gun chamber never vented).
• Samples can be quickly replaced without
  bakeout
• Multiple samples in gun enhance operating
  lifetime
• Aggressive sample selection for best polarization

11
Modelocked Ti-Sapphire Lasers from TimeBandwidth
SESAM passive modelocking for high rep rates
12
superlattice

• Passive modelocking for high rep rates CEBAF
  model 499 MHz
• Selectable wavelength ranges near 770 nm or 850
  nm
• Phase-locked pulse train is stable for days,
  weeks, months however
• laser jock required.

13
Summary

• Highest polarization ever measured at JLab P
  86
• Measurements of many samples at test stand
  indicates this is no fluke.
• 5 times higher QE than strained layer material.
• Smaller analyzing power should provide smaller
  inherent charge and position asymmetry. (Recent
  HAPPEx results do not support this claim.)
• Delicate material, more difficult to handle. We
  have not yet observed good lifetime from gun
  using superlattice. Work in progress.

e
14
Outlook

• Must find a way to limit active area of
  superlattice photocathodes by June 2005 (HAPPEx
  want 100 uA).
• Load locked gun very important for Qweak
• No anodization required
• Samples can be quickly swapped, enhanced
  operating lifetime at high current.
• Installation September, 2005
• Three Ti-sapp lasers on site. Ti-sapp lasers are
  finicky. More experience required to minimize
  tunnel accesses.
• 300 Hz flipping awaits testing.