Date post: | 21-Dec-2015 |
Category: |
Documents |
View: | 216 times |
Download: | 1 times |
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
John Power, SLAC 2011
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
John Power, SLAC 2011
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
John Power, SLAC 2011
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 )
John Power, SLAC 2011
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)
laser energy (mJ)photocathode input window
Q Isingle photon emission
John Power, SLAC 2011
Short Laser Pulse (~ 0.1ps) E=50 MV/m@ injection phase=30 50sin(30)=25
First resultsfrom TsinghuaData 2010-10-04
0
50
100
150
200
250
300
0 200 400 600 800
Q(p
C)
laser energy (mJ)photocathode input window
Q aI + bI2
multiphoton emission
John Power, SLAC 2011
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
John Power, SLAC 2011
Copper work function Φ0=4.65 eV
Fit: β ~ 130
Fowler Nordheim plot of dark current data
First resultsfrom TsinghuaData 2010-10-04
Field (57– 73 MV/m)
John Power, SLAC 2011
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
John Power, SLAC 2011
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
John Power, SLAC 2011
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
John Power, SLAC 2011
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”
John Power, SLAC 2011
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)
John Power, SLAC 2011
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?
John Power, SLAC 2011
Image potentiale2/160z
z0
eff
EF
The Schottky Effect:applied field lowers the effective potential
metal
Photoemission
hh
h photoemissionh No photoemission
h
John Power, SLAC 2011
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
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)
John Power, SLAC 2011
Image potentiale2/160z
Electrostatic potential-eEz
Effective potential
z0
eff
EF
0πεEe= 4ΔΦ 3
eff = -
The Schottky Effect:applied field lowers the effective potential
eEzz
eeze
0
2
0 16
metal
h
e- photoemissionexcess energy
Schottky Enabled Photoemissionvia (external field)
Photoemission
John Power, SLAC 2011
Image potentiale2/160z
z0
eff
EF
The Schottky Effect:applied field lowers the effective potential
metal
h
e- photoemissionexcess energy
Schottky Enabled Photoemissionvia (work function lowering)
Photoemission
e- photoemission
John Power, SLAC 2011
z0
eff
EF
The Schottky Effect:applied field lowers the effective potential
I
II
III-eEz-eEz (bulk of cathode)
(high )
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)
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)
John Power, SLAC 2011
z0
eff
The Schottky Effect:applied field lowers the effective potential
h
I
II
-eEz
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)
Photoemission
eff III
-eEz
John Power, SLAC 2011
Can we measure the relative strength of and 0
Q
heff
Q (h0)2
E (MV/m)
eff (
eV
)
John Power, SLAC 2011
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