June 28, 2012

Triveni Rao

Experience with GaAs Cathodes at BNL

Triveni Rao

Gatling Gun Review

June 28, 2012

June 28, 2012

Triveni Rao

Outline

• Objective• Description of system 1• Results from system 1• Description of system 2• Results from system 2• Summary and Future Plans

June 28, 2012

Triveni Rao

Objective

• Establish 10-11-10-12 Torr vacuum in fabrication chamber• Fabricate GaAs cathodes• Optimize fabrication process

– Cs source– Oxygen source– yo-yo vs co-deposition– Cleaning temperature

QE, surface roughness, elemental properties• Basic Physics studies

– Effect of high intensity, high charge, high current on QE and polarization

System 1- Erdong Wang System 2- Riehn and Rahman

June 28, 2012

Triveni Rao

System 1

• Built to test GaAs in SRF gun– Load-lock transfer system– GaAs cleaned mostly by

radiant heat

10” diameter sphere, baked to 400 CPumped by 270 l/s ion pump and 1000 l/s TSPRGA, Gauge, Cs, O2, anodeBase pressure of 2x 10-11 TorrStalk heater temp 720 C, cathode temp 630 C Max

Cathode: P doped (~ 3x 1018 Zn) GaAsLaser: 543 nm, ~ 300 µm diameter on cathodeAnode Bias: 50 VDetector: PicoammeterSource:

Alvatec-pure Cs, too high evaporation, can not be exposed to air once opened, possible indium contaminationSAES Getter-Cs2CrO4 with ZrAl, activate to release Cs, typical activation current 4A, temp. 550 C

Activation: yo-yo and saturation

June 28, 2012

Triveni Rao

yo-yo: Relatively simple Longer time Lower Life time

Saturation:

Shorter activation time Longer Life time Sensitive to oxygen flow rate

Long life time at 2 A/cm2 current density

June 28, 2012

Triveni Rao

System 2

Stalk heater similar to Jlab, Substrate temperature up to 640 CIon pump and TSPBase pressure < 6x10-12 TorrQuick turn around time (~1 wk/sample)

June 28, 2012

Triveni Rao

620 625 630 635 640 645 6500.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

Bottle Source Boat Source

Anneal Temp (C)

QE

Comparison of Oxygen Sources- High Purity Oxygen Bottle, Manganate in Ag Boat

Bottle chosen due to availability

550 560 570 580 590 600 610 6200

0.5

1

1.5

2

2.5

650 nm780 nm

Temperature [C]

norm

. QE

[%]

Max QE: 650 nm-~ 10.6%, 780 nm.~ 5.5% Sample to sample

variation in QE QE(T) Normalized to QE

(560 C) does not depend on T

Variation in QE for 780 nm is indicative of spot to spot variation of QE

0 500 1000 1500 2000 2500 30000

2

4

6

8

10

12

Time [s]

QE

at 6

50 n

m [%

]

Typical cathode activation

June 28, 2012

Triveni Rao

SEM pictures: Sample heated to 610 C

At the Boundary Inside the boundary

June 28, 2012

Triveni Rao

Outside the boundary

Area inside the boundary is affected differently from that outside the boundaryElemental analysis of the features and surface profile with AFM to be done soonSamples heated from 560 C to 600 C to be analyzed soon

June 28, 2012

Triveni Rao

Summary and Future Plans

Summary– Built two XHV systems– Third under

construction– Fabricated bulk GaAs

cathodes– Optimized the

activation process– Started Basic science

experiments

Future Plans Build prep chamber

compatible with Gatling Gun

Fabricate cathodes for polarized electrons

Conduct systematic basic science studies: Changes in surface morphology, QE, Polarization using polarimeter, SEM, AFM, x-ray beams

Study performance in Gatling Gun