Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Strained Superlattice GaAs

photocathodes at JLab

M. Baylac

Qweak collaboration meeting

August 17, 2004

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Polarized Electron Guns at JLab

photocathode

NF3

Laser

Cs

anode

e -

-100 kV

HV insulator

NEG pumps

Strained GaAs in Gun2 (“old” material)

Strained-superlattice GaAs in Gun3 (“new” material)

NEG-coated Beamline

Photoemission from GaAs semiconductor

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Strained layer GaAs photocathodes

• From 1998 through 2003, we have used strained layer GaAs photocathodes at JLab (Bandwidth Semiconductor, Inc.).

• Reliable, well understood material.

• Stained-layer GaAs provides;

• Good polarization: P ~ 75% at 840 nm

• Moderate quantum efficiency: QE ~ 0.2% at 840 nm

• Limitations that keep polarization < 80%:

• limited band splitting

• relaxation of the strain for thickness > critical thickness (~10 nm)

e

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Strained GaAs/GaAsP superlattice

• Very thin quantum well layers alternating with lattice-mismatched barrier layers

• Each superlattice layer is < critical thickness

• Natural splitting of valence band adds to the strain-splitting

• Developed by SLAC with SVT Associates, Inc.

SLAC-PUB-10331 (2004), submitted to Appl.Phys.Lett

• First samples received at JLab October 2003, characterized at the injector test cave

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Superlattice structure

Be doping (cm )-3

GaAs P1-x x, 0<x<0.36 (2.5 μm)

p-type GaAs substrate

GaAs (5 nm)

GaAs (4 nm)

GaAsP (3 nm)

GaAs P0.64 0.36 (2.5 μm)

14 pairs

5.1019

5.1017

5.1018

SVT associates, per SLAC specs.

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Quantum Efficiency

QE ~ 1% versus 0.2% from strained layer material

Wavelength (nm)

QE

(%

)

we operatehere

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Beam polarization

Wavelength (nm)

Pol

ariz

atio

n (

%)

Highest polarization ever measured at the Test Cave

Wavelength for Good QE and Polarization

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Analyzing power (aka QE anisotropy)

Analyzing power smaller by factor of 3 compared with strained-layer GaAs:4% versus 12%

This means smaller inherent intensity & position asymmetries on beam.

Wavelength (nm)

An

alyz

ing

pow

er (

%)

Wavelength for good QE and polarization

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

QE vs hydrogen cleaning

Hydrogen exposure time (min)

QE

(%

)

Drawback:Delicate material

Can’t clean with atomic hydrogen

Makes it tough to anodize edge of cathode

Typical H-dose to clean anodized

samples

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Superlattice Photocathodes at CEBAF

• Several failed attempts to load superlattice photocathodes inside tunnel guns

• Successful installation of un-anodized superlattice photocathode in Gun 3 (March, 2004)

• Activation gave QE ~ 0.4% at 780 nm (vs 1% in test cave)

• Used during HAPPEx-He and portion of HAPPEx-H (June, 2004)

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Poor lifetime

• Frequent spot moves were required to maintain 40 A beam current at Hall A

every week at start of run, every day as we approached July 4 shutdown!

• HAPPEx-He OK. HAPPEx-H not so good. Injector conditions changing too often. HC asymmetries were not stable.

• Poor gun lifetime atypical of CEBAF photoinjector.

QE profile after 3 weeks of running

14 mm

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Polarimetry in hall A

• Compton (D. Lhuillier)

• 5 MeV Mott (J. Grames)

Preliminary

photonelectron

P ~ 86 3 %e

P ~ 85.2 3.2 %e

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Parity quality beam?

• Short run + numerous spot moves

=> Jury is still out. Poor gun lifetime made it difficult to assess performance of superlattice photocathode from a parity violation experiment perspective.

• HAPPEX reports;

• Charge asymmetry OK for both photocathodes

• Position asymmetries were smaller using gun2strained layer photocathode(no active position feedback)

Gun2 strained layer GaAsGun3 superlattice GaAs

From HAPPEx-H

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

• QE drops as laser power increases: photoelectrons build up in band bending region create opposing E field that reduces NEA G.A. Mulhollan et al, Phys. Lett. A 282, 309 (2001)

• Reduces maximum available beam current. Lose laser headroom. Makes for shorter operating lifetime of gun.

Surface Charge Limit

QE is not constant

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

• Our new commercial Ti-Sapphire lasers provide more laser power (~ 300 mW) compared to our “old” diode lasers (~ 50 mW).

• They are wavelength tunable. Now we can tune to peak polarization.

• Successful and reliable running since G0.

• Ti-Sapp laser + superlattice photocathode a good match for high current Qweak experiment. 300 mW laser power + QE of 1% can provide 1800 uA beam current.

• Max current only 360 uA with strained layer cathode. Not as much headroom.

Lasers

http://www.tbwp.com

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

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. Cannot be H-cleaned.

Can’t recover QE from a dirty superlattice, unlike strained layer

• We suffered surface charge limit. QE drops with increasing laser power. A concern for high current experiments like Qweak.

e

Operated by the Southeastern Universities Research Association for the U.S. Depart. Of Energy

Thomas Jefferson National Accelerator Facility

Outlook

• Poor lifetime due to supperlattice? Doubt it:

• Gun 3 has a bad lifetime in 2003 using strained layer

• Un-anodized wafer increases damage on the wafer

Reworked Gun 3 over the shutdown, hoping to boost lifetime

• QE lower in the tunnel than in test cave:

• Hopefully due to the gun itself, not the wafer

• Received arsenic capped samples: easier to handle and anodize (to be tested in lab)

• Smaller inherent HC asymmetries? Surface charge limit? Need more operating experience.