C-AD electron-cooling

BNL Photocathode R&DBNL Photocathode R&DAn overview of research on high An overview of research on high

quantum efficiency photocathodes quantum efficiency photocathodes and associated laser systemsand associated laser systems

Andrew BurrillHPHB WorkshopNov 9, 2004

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High Quantum Efficiency High Quantum Efficiency 0.5 A at 351 MHz (1/2 RF frequency) = 1.3nC/bunch0.5 A at 351 MHz (1/2 RF frequency) = 1.3nC/bunch

Uniform emission surfaceUniform emission surface Long lifetimeLong lifetime RobustRobust Reproducible preparation techniqueReproducible preparation technique Photoemission at convenient laser Photoemission at convenient laser

wavelengthwavelength

Photocathode CriteriaPhotocathode Criteria

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Research GoalsResearch Goals

Establish deposition system with a reproducible Establish deposition system with a reproducible optimized recipe optimized recipe

Study Lifetime related issues: Dependence on Study Lifetime related issues: Dependence on laser intensity and wavelength laser intensity and wavelength current density current density ContaminantsContaminants Vacuum conditions Vacuum conditions

Integration into load lock or diamond capsuleIntegration into load lock or diamond capsule

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Cathode OptionsCathode OptionsPhotocathodePhotocathode CsKCsK22SbSb CsCs22TeTe GaAsGaAs MgMg

Laser wavelengthLaser wavelength 532/355532/355 266 nm266 nm 800 nm800 nm 266 nm266 nm

QEQE 3%/10%3%/10% 3%3% 5-10%5-10% .1%.1%

Laser power to Laser power to achieve .5Aachieve .5A

38W/17W38W/17W 77 W77 W 15 W15 W 2300 W2300 W

Prompt emissionPrompt emission YesYes YesYes NoNo YesYes

Commercial Laser Commercial Laser availableavailable

YesYes NoNo MaybeMaybe NoNo

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Photocathode Deposition SystemPhotocathode Deposition System

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Fabrication ProcedureFabrication Procedure

Chemical Deposition Chemical Deposition systemsystem

Polished 1” Molybdenum Polished 1” Molybdenum substratesubstrate

Multi-stage deposition Multi-stage deposition processprocess

200 Å Sb, 150 Å K, ~200 Å 200 Å Sb, 150 Å K, ~200 Å Cs Cs

Current is monitored as a Current is monitored as a function of Cs depositionfunction of Cs deposition

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Current vs. Cs Deposition timeCurrent vs. Cs Deposition time

0

2

4

6

8

10

12

14

16

18

20

0 50 100 150 200 250 300 350 400

time (sec)

Cu

rre

nt (

uA

)

0

0.5

1

1.5

2

2.5

3

QE

(%

)

Current

QE %

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Surface UniformitySurface Uniformity

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

0 5 10 15 20 25 30

Position

curr

en

t (u

A)

545 nm

365 nm

Position for Cs/Kdeposition

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ReproducibilityReproducibility

0

5

10

15

20

25

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

curr

en

t in

uA

2.6%

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Lifetime StudiesLifetime Studies

0

5

10

15

20

25

0 10 20 30 40 50 60

Days

Cur

rent

(uA

)

normal laser lifetime

focused laser

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Cathode Gun InterfaceCathode Gun Interface

Load-lock to attach deposition system to Load-lock to attach deposition system to SCRF gunSCRF gun

Capsule design with diamond window Capsule design with diamond window sealed onto photocathodesealed onto photocathode

Outcome of SEY program will determine Outcome of SEY program will determine final course takenfinal course taken

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Laser System RequirementsLaser System Requirements

351.875 MHz351.875 MHz 532 nm, 355 nm532 nm, 355 nm 10 ps pulse length10 ps pulse length Synchronized to Synchronized to

master RF clockmaster RF clock Adjustable output Adjustable output

powerpower Variety of amplifier Variety of amplifier

systemssystems

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Laser RequirementsLaser Requirements

Laser Laser WavelengthWavelength

CsKCsK22Sb QESb QE SEYSEY Desired Desired CurrentCurrent

Laser Laser Power to Power to CathodeCathode

532 nm532 nm 3%3% 00 0.5 A0.5 A 38 W38 W

532 nm532 nm 3%3% 5050 0.5 A0.5 A 0.7 W0.7 W

355 nm355 nm 9%9% 00 0.5 A0.5 A 17 W17 W

355 nm355 nm 9%9% 5050 0.5 A0.5 A 0.35 W0.35 W

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Laser Layout optionsLaser Layout options

Oscillator multi-pass Amplifier Chain Harmonic Conversion

CsK2Sb cathode in SCRF gun

CsK2Sb cathode with diamond secondary emitter

Oscillator Amplifier Harmonic Conversion

1064 nm351 MHzFew watts

Multi-passMulti-stageAdjustable output power to 80 W

2nd or 3rd Harmonic40 W green20 W UV

1064 nm351 MHzFew watts

Single pass(Optional)

2nd or 3rd Harmonic1 W green0.5 W UV

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ConclusionsConclusions

Cathode research Cathode research Moderate QE obtainedModerate QE obtained Good surface Good surface

uniformityuniformity Lifetime and current Lifetime and current

density studies are density studies are promisingpromising

Reproducibility needs Reproducibility needs to be betterto be better

Different Substrate Different Substrate materials will be materials will be studiedstudied

Laser SystemLaser System Commercial oscillators Commercial oscillators

are availableare available Outcome of SEY Outcome of SEY

experiments will experiments will determine amplifier determine amplifier needsneeds

Cathode-Gun Cathode-Gun interface being interface being addressedaddressed