Transcript Fowler Nordheim Eq. (RF fields):

Schottky Enabled Photoemission
&
Dark Current Measurements
at the S-band RF Gun Facility at Tsinghua
CERN, CLIC Studies
Tsinghua EP department
John Power, Eric Wisniewski, Wei Gai
Argonne Wakefield Accelerator Group
Argonne National Laboratory
U.S. High Gradient Workshop
SLAC, Feb 9, 2011
Tsinghua U has an rf gun facility available to
study copper surfaces under high fields
2 configurations
Dark Current
Measurements
Schottky Enabled
Photoemission
Measurements Laser
photocathode
alignment
ICT
S-band RF gun
57 – 73 MV/m
cathode
Faraday
Cup
Laser
• 400 nm,
• 1 mJ
• 0.1 – 3 ps
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
• l = 400 nm laser (hn= 3.1 eV)
• E = 50-73 MV/m
• The research facility is operational
John Power, SLAC 2011
Schottky Enabled Photoemission
Measurements
ICT
eg
 Experimental parameters
Should not get
photoemission
– work function of copper = f0 = 4.65 eV
– energy of l=400nm photon = hn= 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)
John Power, SLAC 2011
Long Laser Pulse (~ 3ps)
E=55 MV/m@ injection phase=80  55sin(80)=54
First results
from Tsinghua
Data 2010-10-04
60
ict
50
y = 125.82x - 10.065
R 2 = 0.907
40
Q(pC)
Linear (ict)
Q I
single photon emission
30
20
10
0
0
0.1
0.2
0.3
0.4
laser energy (mJ)
photocathode input window
John Power, SLAC 2011
0.5
Q(pC)
Short Laser Pulse (~ 0.1ps)
E=55 MV/m@ injection phase=80  55sin(80)=54
First results
from Tsinghua
100
90
80
70
60
50
40
30
20
10
0
Data 2010-10-04
y = 133.91x - 2.5869
ict
Linear (ict)
Q I
single photon emission
0
0.2
0.4
0.6
laser energy (mJ)
photocathode input window
John Power, SLAC 2011
0.8
Short Laser Pulse (~ 0.1ps)
E=50 MV/m@ injection phase=30  50sin(30)=25
300
First results
from Tsinghua
250
Data 2010-10-04
Q(pC)
200
Q aI + bI2
multiphoton emission
150
100
50
0
0
200
400
600
laser energy (mJ)
photocathode input window
John Power, SLAC 2011
800
Dark Current Measurements
S-band RF gun
57 – 73 MV/m
cathode
Faraday
Cup
 Experimental parameters
– work function of copper = f0 = 4.65 eV
– Field (57– 73 MV/m)
– Note: field could be lowered more but Faraday Cup signal was
too weak to measure current
John Power, SLAC 2011
Fowler Nordheim plot of dark
current data
First results
from Tsinghua
Data 2010-10-04
 Copper work function Φ0=4.65 eV
 Fit: β ~ 130
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.
• hn =3.1 eV, f0=4.6 eV  DF=1.5eV (Schottky effect
required)
• The lowest field 25 MV/m
•  ΔΦ = e 3 b E 4 πε 0 (Schottky effect)
•  implies b>=60
• note: also observed emission at lower fields, but data was noisy. This
implies even larger b exists.
• Dark current measurements
• b is 130. (this is consistent with typical SLAC data
bE~10 GV/m)
John Power, SLAC 2011
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
Field emission
The Schottky Effect:
applied field lowers the effective potential
0
z
Image potential
e2/160z
DF
Electrostatic potential
-eEz
F0
Feff
EF
e- field emission
eF  z   eF 0 
e
2
16  0 z
 eEz
Effective potential
Feff = F0 - DF
metal
John Power, SLAC 2011
ΔΦ =
3
e E 4 πε 0
Electron emission
IFN (b, f0, Ae, E0)
Fowler Nordheim Law (RF fields):
I =
5.79  10
 12
 0.5
exp ( 9.35 φ 0
)A e  βE 0 
1.75
φ0
2.5
  6.53x10
exp 
βE 0

9
1.5
φ0 



1. High field enhancements (b) can
field emission.
2. Low work function (f0) in small
areas can cause field emission.
typical picture 
geometric perturbations (b)
E0
bE0
alternate picture 
material perturbations (f0)
grain
peaks
boundaries
cracks
Copper surface
John Power, SLAC 2011
oxides
inclusions
(suggested by Wuensch
and colleagues)
Field emission enhancement factor
 b=130 seems unphysical
– h/r ~ 100
– fresh surfaces machines
to ~10nm roughness
– h=10 nm, r=0.1 nm
“a tower of single atoms”
John Power, SLAC 2011
β from Fowler-Nordheim plot
(β=5, Φ0 =0.5 eV)
 2.84  10 φ 0
9
β=
1.5
Slope
 Raw Data
– Field emitted current
– E-field on surface
John Power, SLAC 2011
(β=130, Φ0 =4.66 eV)
 Fit
– Different combinations of b and
f0 can fit the same raw data
– Can we find a way to measure
what role each effect plays?
Ae from Fowler-Nordheim plot
1.75
I φ0
Ae =
5.79  10
 12
 0.5
exp ( 9.35 φ 0
 - 6.53x10 9 φ 01.5
2.5
) βE 0  exp 
βE 0

 Raw Data
– Field emitted current
– E-field on surface
John Power, SLAC 2011




 Fit
– Typical fits give areas so small
that they are difficult/impossible
to measure.
Does this give us a way to probe
whether b or f0 dominates?
Photoemission
The Schottky Effect:
applied field lowers the effective potential
hw<
hw> F
F00
No
photoemission
photoemission
0
z
Image potential
e2/160z
hw F0
hw
Feff
EF
metal
John Power, SLAC 2011
Photoemission
Eexcess, metal =
ħw-feff
The Schottky Effect:
applied field lowers the effective potential
Normal Photoemission in an rf gun1,2
e- photoemission
0
excess
energy
z
Image potential
e2/160z
DF
Electrostatic potential
-eEz
hw F0
Feff
Effective potential
eF  z   eF 0 
EF
e
2
16  0 z
 eEz
Feff = F0 - DF
metal
John Power, SLAC 2011
ΔΦ =
3
e E 4 πε 0
1D.H.Dowell,J.F.Schmerge,Phys.Rev.Spec.Top.Accel.Beams
2K.L.
Jensen et al., J. Appl. Phys. 104, 044907 (2008)
12 074201 (2009)
Photoemission
The Schottky Effect:
applied field lowers the effective potential
Schottky Enabled Photoemission
via (external field)
0
z
Image potential
e2/160z
DF
excess
energy
Electrostatic potential
-eEz
3
F0
hw
ΔΦ =
Feff
Effective potential
eF  z   eF 0 
EF
e- photoemission
metal
e
2
16  0 z
 eEz
Feff = F0 - DF
John Power, SLAC 2011
e E 4 πε 0
Photoemission
The Schottky Effect:
applied field lowers the effective potential
Schottky Enabled Photoemission
via f0 (work function lowering)
excess
energy
e- photoemission
0
z
Image potential
e2/160z
hw
F0
Feff
EF
metal
John Power, SLAC 2011
Photoemission
The Schottky Effect:
0 applied field lowers the effective potential
z
DF
I
DF
hw
II
F0
Feff
III
Feff
Q
EF
(hn  feff)2
Q
 Ideas to measure the effective work
function??
– sweep the laser energy (OPO)
– sweep the RF phase, which
changes field (Schottky effect)
hn  feff
John Power, SLAC 2011
1D.H.Dowell,J.F.Schmerge,Phys.Rev.Spec.Top.Accel.Beams
2K.L.
Jensen et al., J. Appl. Phys. 104, 044907 (2008)
12 074201 (2009)
Photoemission
0
The Schottky Effect:
applied field lowers the effective potential
z
I
hw
F0
Feff
II
F0
III
Feff
John Power, SLAC 2011
1D.H.Dowell,J.F.Schmerge,Phys.Rev.Spec.Top.Accel.Beams
2K.L.
Jensen et al., J. Appl. Phys. 104, 044907 (2008)
12 074201 (2009)
Can we measure the relative strength of
b and f0
(hn  f0)2
feff (eV)
Q
Q
E (MV/m)
hn  feff
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 made
 Alternative interpretation of FN plots being investigated
– b and fo
 Developing techniques to measure the effects
John Power, SLAC 2011