schottky enabled photoemission & dark current measurements john power, eric wisniewski, wei gai...

Schottky Enabled Photoemission

& Dark Current Measurements

John Power, Eric Wisniewski, Wei Gai

Argonne Wakefield Accelerator GroupArgonne National Laboratory

U.S. High Gradient WorkshopSLAC, Feb 9, 2011

at the S-band RF Gun Facility at Tsinghua

CERN, CLIC StudiesTsinghua EP department

John Power, SLAC 2011

Tsinghua U has an rf gun facility available to study copper surfaces under high fields

Laser• 400 nm, • 1 mJ• 0.1 – 3 ps

S-band RF gun57 – 73 MV/m

cathode

Laseralignment

Faraday Cup

ICT

Dark CurrentMeasurements photocathode

Schottky Enabled Photoemission Measurements

2 configurations


Tsinghua S-band rf gun Facility Features

• Dark current measurement• The cathode is a solid copper plate (no gap)

• Schottky photoemission measurement • RF field level and laser parameters are suitable• = 400 nm laser (h= 3.1 eV)• E = 50-73 MV/m

• The research facility is operational


Schottky Enabled Photoemission Measurements

Experimental parameters– work function of copper = 0 = 4.65 eV

– energy of =400nm photon = h= 3.1 eV – Laser pulse length

• Long = 3 ps• Short = 0.1 ps

– Laser energy ~1 mJ (measured before laser input window) – Field (50 – 73 MV/m)

ICT

e-First

results

from TsinghuaData 2010-10-04

Should not get photoemission


Long Laser Pulse (~ 3ps) E=55 MV/m@ injection phase=80 55sin(80)=54

Q(p

C)

laser energy (mJ)photocathode input window

First resultsfrom TsinghuaData 2010-10-04

Q Isingle photon emission

y = 125. 82x - 10. 065

R2 = 0. 907

0

10

20

30

40

50

60

0 0. 1 0. 2 0. 3 0. 4 0. 5

i ct Li near ( i ct )


Short Laser Pulse (~ 0.1ps) E=55 MV/m@ injection phase=80 55sin(80)=54

First resultsfrom TsinghuaData 2010-10-04y = 133. 91x - 2. 5869

0102030405060708090

100

0 0. 2 0. 4 0. 6 0. 8

i ctLi near ( i ct )

Q(p

C)


Q Isingle photon emission


Short Laser Pulse (~ 0.1ps) E=50 MV/m@ injection phase=30 50sin(30)=25


0

50

100

150

200

250

300

0 200 400 600 800

Q(p

C)


Q aI + bI2

multiphoton emission


Dark Current Measurements

Experimental parameters– work function of copper = 0 = 4.65 eV

– Field (57– 73 MV/m)– Note: field could be lowered more but Faraday Cup signal was

too weak to measure current

First results

from Tsinghua

S-band RF gun57 – 73 MV/m

cathodeFaraday

Cup


Copper work function Φ0=4.65 eV

Fit: β ~ 130

Fowler Nordheim plot of dark current data


Field (57– 73 MV/m)


Summary of the first measurements

• Schottky Enabled Photoemission Measurements • Schottky enhanced emission observed at all the field

levels measured. • h=eV,0=4.6 eV =1.5eV (Schottky effect required)

• The lowest field 25 MV/m• (Schottky effect)• implies >=60

• note: also observed emission at lower fields, but data was noisy. This implies even larger exists.

• Dark current measurements • is 130. (this is consistent with typical SLAC data E~10 GV/m)

0πεEe= 4ΔΦ 3

What questions about the surface can be investigated with an s-band

gun?

Some possibilities/speculation …Alternative interpretation of Fowler-Nordhiem plots

Measurement of the field enhancement and work function


Image potentiale2/160z

Electrostatic potential-eEz

Effective potential

z0

eff

EF

0πεEe= 4ΔΦ 3eff = -

The Schottky Effect:applied field lowers the effective potential

eEzz

eeze

0

2

0 16

metal

e- field emission

Field emission


Electron emission

0

0e

βE

φ

φ

βE)Aφ(=I

1.50

9

1.750

2.50.50

12 6.53x10exp

9.35exp105.79

Copper surface

typical picture geometric perturbations ()

Fowler Nordheim Law (RF fields):

1. High field enhancements () can field emission.

peaksgrainboundaries

cracks

(suggested by Wuensch and colleagues)

(, Ae, E0)IFN

oxides

inclusions

alternate picture material perturbations ()

2. Low work function () in small areas can cause field emission.

E0

E0


Field emission enhancement factor

=130 seems unphysical– h/ ~ 100– fresh surfaces machines

to ~10nm roughness– h=10 nm, =0.1 nm

“a tower of single atoms”


β from Fowler-Nordheim plot

Raw Data– Field emitted current– E-field on surface

Fit– Different combinations of and can fit the same raw data

– Can we find a way to measure what role each effect plays?

Slope

φ=β

1.50

9102.84

(β=5, Φ0 =0.5 eV)

(β=130, Φ0 =4.66 eV)


Ae from Fowler-Nordheim plot

Raw Data– Field emitted current– E-field on surface

00

e

βEφ

βE)φ(

φI=A

1.50

92.50.5

012

1.750

6.53x10-exp9.35exp105.79

Fit– Typical fits give areas so small

that they are difficult/impossible to measure.Does this give us a way to probe

whether or 0 dominates?



z0

eff

EF


metal

Photoemission

hh

h photoemissionh No photoemission

h




Effective potential

z0

eff

EF

0πεEe= 4ΔΦ 3eff = -


eEzz

eeze

0

2

0 16

metal

h

e- photoemission

excess energy

Eexcess, metal = ħ-eff

Normal Photoemission in an rf gun1,2

Photoemission

1D.H.Dowell,J.F.Schmerge,Phys.Rev.Spec.Top.Accel.Beams 12 074201 (2009)2K.L. Jensen et al., J. Appl. Phys. 104, 044907 (2008)




Effective potential

z0

eff

EF

0πεEe= 4ΔΦ 3

eff = -


eEzz

eeze

0

2

0 16

metal

h

e- photoemissionexcess energy

Schottky Enabled Photoemissionvia (external field)

Photoemission



z0

eff

EF


metal

h

e- photoemissionexcess energy

Schottky Enabled Photoemissionvia (work function lowering)

Photoemission

e- photoemission


z0

eff

EF


I

II

III-eEz-eEz (bulk of cathode)

(high )


Photoemission

h

eff

Q

heff

Q (heff)2

Ideas to measure the effective work function??

– sweep the laser energy (OPO)

– sweep the RF phase, which changes field (Schottky effect)


z0

eff


h

I

II

-eEz


Photoemission

eff III

-eEz


Can we measure the relative strength of and 0

Q

heff

Q (h0)2

E (MV/m)

eff (

eV

)


summary

An S-band facility at Tsinghua University is available to study surface emission– Schottky Enabled Photoemission– Dark Current Emission

Facility Parameters– laser: 400 nm laser (pulse length: 0.1 ps, 3 ps)– rf field: <73 MV/m

First measurements have been madeAlternative interpretation of FN plots being investigated

– and o

Developing techniques to measure the effects

schottky enabled photoemission & dark current measurements john power, eric wisniewski, wei gai...

Documents

mvm slide

photoemission slide

tsinghua data

laser parameters

ps laser energy

ev field

laser input window field

typical slac data e