Download Field Emission Measurements From Cesiated Titanium and Stainless

Field Emission Measurements
From Cesiated Titanium and
Stainless Steel Electrodes
K.Surles-law, P.Adderley, J.Brittian,
D.Charles, J.Clark, J.Grames, J.Hansknecht,
M.Poelker, M.Stutzman
Field Emission from 25deg Electrodes
POISSON model of 25deg
electrode
E-fields shown are for
V = -100kV
Field Emission
Electrons are confined in a
metal by a potential well
Energy of electron
insufficient to escape from
metal
Electron must be given extra
energy to escape (thermal,
photoemission)
QM demonstrates the
electron wavefunction
attenuates rapidly outside
potential barrier
Field Emission
Application of external
fields lowers and thins
the potential barrier
Fowler- Nordheim used
quantum mechanics to
demonstrate that some
electrons can tunnel
through the potential
barrier and escape into
the vacuum
Field Emission: Fowler-Nordheim Equation
6
I (E) 
1.54 x10 A (  E )

2
(
Exp 
3 3/ 2
6.83 x10 
 E
)
This is the Fowler-Nordheim expression for field
emission
E is the electric field (MV/m)
 is the work function of the material (eV)
 is the field enhancement factor
A is the effective emitting area
Field Emission: Fowler-Nordheim Equation
F-N with Cesium
Work Function = 2.12eV
2
Field Emission Current Density (Amps/m )
1600
1400
1200
1000
800
600
400
200
0
-200
0
1
2
3
4
5
6
E (MV/m)
Plot of Fowler-Nordheim equation with b = 200. The electric field gradients
were generated from POISSON model of the 25 degree electrode. The
potential varied from 10 to 125kV
Field Emission: Fowler-Nordheim Equation
One can use exponential form of FN to
compare field emission from Ti and SS
Field Emission: Fowler-Nordheim Plot
Ti and SS 7th Application of Cs
8000
7000
FE Current (pA)
6000
5000
SS (7th Cs)
4000
Ti (7th)
3000
2000
1000
0
0
20
40
60
80
100
120
HV (kV)
210min of Cs for Ti and SS
140
Field Emission: Fowler-Nordheim Plot
More interesting to plot ln(I/V^2) vs.
1/V
The slope of the fitted data is
proportional to the work function to the
3/2 power
If work function changes, the slope of
FN plot changes
Field Emission: Fowler-Nordheim Plot
2
d ln( J / E )
3/ 2
Slope  
  a
1
d( )
E
Field Emission: Fowler-Nordheim Plot
Now let’s observe FN plots for Ti and SS
with no Cs and 210min of Cs
Field Emission: Fowler-Nordheim Plot
SS No Cs
Linear Fit of Data10_B
-7.5
SS 210min Cs
Linear Fit of Data10_I
-3.5
-4.0
-8.0
-4.5
-8.5
ln ( I / V )
-9.0
2
2
ln ( I / V )
-5.0
-9.5
-6.0
slope = -975
slope = - 934
-10.0
-5.5
-6.5
-7.0
-10.5
-7.5
0.0080
0.0085
0.0090
0.0095
1/V
0.0100
0.0105
0.0110
0.0115
0.0080
0.0085
0.0090
0.0095
1/V
0.0100
0.0105
0.0110
0.0115
Field Emission: Fowler-Nordheim Plot
Ti no Cs
Linear Fit of Data14_B
Ti with 210min Cs
Linear Fit of Data12_I
-0.5
-7.9
-8.0
-1.0
-8.1
ln ( I / V )
-8.2
2
2
ln ( I / V )
-1.5
-8.3
-8.4
-2.0
-2.5
slope = - 816
slope = - 352
-8.5
-3.0
-8.6
-3.5
-8.7
0.0080
0.0085
0.0090
1/V
0.0095
0.0100
0.0080
0.0085
0.0090
0.0095
1/V
0.0100
0.0105
0.0110
0.0115
Field Emission: Fowler-Nordheim Plot
SS no Cs
SS 210min Cs
Ti no Cs
Ti 210min Cs
0
-2
2
ln ( I / V )
-4
-6
-8
-10
0.0080
0.0085
0.0090
0.0095
1/V
0.0100
0.0105
0.0110
0.0115
Field Emission: Fowler-Nordheim Equation
Next, decrease cathode-anode spacing
Take FE data for Ti and SS with no Cs
Compare FE vs. spacing: Is Ti or SS
better, or same?