Transcript High Average Current Photocathodes for Accelerators

Response time of Alkali Antimonides
John Smedley
Brookhaven National Laboratory
Overview
• The Three-Step Model and Response Time
– Metallic Photocathodes
– Semiconductor Photocathodes
• Positive Electron Affinity (Alkali Antimonide)
• Negative Electron Affinity (Cs: GaAs)
• Diamond Electron Amplifier
Modern Theory and Applications of Photocathodes
W.E. Spicer & A. Herrera-Gómez
SAC-PUB-6306 (1993)
Three Step Model of Photoemission - Metals
1) Excitation of e- in metal
Reflection
Absorption of light
Energy distribution of excited ee--e- scattering
mfp ~50 angstroms
Direction of travel
Φ
Φ’
Φ
3) Escape surface
Overcome Workfunction
Reduction of  due to applied
field (Schottky Effect)
Filled States
Energy
Empty States
h
Vacuum level 2) Transit to the Surface
Integrate product of probabilities over
all electron energies capable of
escape to obtain Quantum Efficiency
Medium
Vacuum
M. Cardona and L. Ley: Photoemission in Solids 1,
(Springer-Verlag, 1978)
“Prompt”
Metals have very low quantum efficiency, but they are prompt emitters,
with fs response times for near-threshold photons:
To escape, an electron must be excited with a momentum vector
directed toward the surface, as it must have
 2 k2

2m
The “escape” length verses electron-electron scattering is typically
under 10 nm in the near threshold case. Assuming a typical hot
electron velocity of 106 m/s, the escape time is 10 fs.
(this is why the LCLS has a Cu photocathode)
W.F. Krolikowski and W.E. Spicer, Phys. Rev. 185, 882 (1969)
D. H. Dowell et al., Phys. Rev. ST Accel. Beams 9, 063502 (2006)
T. Srinivasan-Rao et al., PAC97, 2790
Three Step Model - Semiconductors
1) Excitation of eEmpty States
Reflection, Transmission,
Interference
Energy distribution of excited e-
h
2) Transit to the Surface
Eg
Filled States
Energy
No States
Φ
Ea
e--phonon scattering
mfp ~100 angstroms
many events possible
e--e- scattering (if hν>2Eg)
Spicer’s Magic Window
Random Walk
Monte Carlo
Response Time (sub-ps)
3) Escape surface
Overcome Electron Affinity
Medium
Vacuum
K2CsSb DOS
0.9
0.8
Filled States
0.7
0.6
States/eV
Empty States
0.5
0.4
0.3
0.2
0.1
Band Gap
PHYSICAL REVIEW B 66, 115102
0.0
-3
-1
1
3
5
7
9
11
eV
A.R.H.F. Ettema and R.A. de Groot, Phys. Rev. B 66, 115102 (2002)
Spectral Response – Bi-alkali
In “magic window”
Unproductive absorption
Onset of e-e
scattering
Cs3Sb (Alkali Antimonides)
Work function 2.05 eV, Eg= 1.6 eV
Electron-phonon scattering length
~5 nm
Loss per collision ~0.1 eV
Photon absorption depth
~20-100 nm
Thus for 1 eV above threshold, total
path length can be ~500 nm
(pessimistic, as many electrons will
escape before 100 collisions)
This yields a response time of
~0.6 ps
Alkali Antimonide cathodes have been
used in RF guns to produce
electron bunches of 10’s of ps
without difficulty
D. H. Dowell et al., Appl. Phys. Lett., 63, 2035 (1993)
W.E. Spicer, Phys. Rev., 112, 114 (1958)
Diamond Amplifier Concept
(first strike solution?)
Thin Metal
Layer
(10-30 nm)
Transparent
Conductor
Photon
Photocathode
3-10 kV
Primary
Electron
Diamond
(NEA)
Secondary
Electrons
MCP
Diamond Amplifier Setup
Phosphor
Screen
CCD camera
Focusing
Channel
Hydrogenated
surface
Diamond
Pt metal
coating
0- to 10-keV
Electron beam
Anode with
holes
A
H.V. pulse
generator
Diamond Amplifier Results
With focusing
Demonstrated
emission and gain
of >100 for 7 keV
primaries
Would need large
area polycrystalline
diamonds, probably
still too expensive
Maybe NEA GaAs
amplifier?
X. Chang et al., Phys. Rev. Lett. 105, 164801 (2010).
Closing Thoughts
While not strictly “prompt” in the manner of metals,
the alkali atimonides have sub-ps response time
Could be improved to some extent (at the cost of QE)
by making the cathode very thin
Electron stimulated desorption/Ion backbombardment?
Thanks!
D. Dowell (SLAC/LCLS), Henry & Klaus for the invitation; V. Radeka, I.
Ben-Zvi, and my colleagues at BNL
Three Step Model – NEA Semiconductors
1) Excitation of eEmpty States
Reflection, Transmission,
Interference
h
2) Transit to the Surface
Eg
3) Escape surface
Filled States
Energy
No States
Ea
e--lattice scattering
thermalization to CBM
diffusion length can be 1µm
recombination
Random Walk
Monte Carlo
Response Time (10-100 ps)
Medium
Vacuum
Step 1 – Absorption and Excitation
Fraction of light absorbed:
Iab/I = (1-R)
Probability of absorption and electron excitation:
P( E , h ) 
N ( E ) N ( E  h )
E f  h
 N ( E ' ) N ( E 'h )dE'
Ef
•Medium thick enough to absorb all transmitted light
•Only energy conservation invoked, conservation of k
vector is not an important selection rule
Step 2 – Probability of reaching the surface w/o e--e- scattering
e ( E )  ph ( )
T ( E, ) 
1  e ( E )  ph ( )
•Energy loss dominated by e-e scattering
•Only unscattered electrons can escape
EDC and QE
At this point, we have N(E,h) - the Energy Distribution Curve of
the emitted electrons
Yield:
h  E f
Y ( )  I ( )(1  R( ))  P( E )T ( E , ) D( E )dE
 E f
Quantum efficiency:
h  E f
QE( )  (1  R( ))  P( E )T ( E , ) D( E )dE
 E f
Step 3 - Escape Probability
•
•
•
•
•
Criteria for escape:
 2 k 2
 ET  E f  
2m

Requires electron trajectory to fall
within a cone defined by angle:
k
E 1
cos  min  ( T ) 2
E
k
Fraction of electrons of energy E
falling with the cone is given by:
2
1 
1
1
ET 12
D( E ) 
sin  ' d '  d  (1  cos )  (1  ( ) )

4 0
2
2
E
0
For small values of E-ET, this is the
dominant factor in determining the
emission. For these cases:
This gives:
QE( ) 
h  E f
( h  )  ET
E f
ET
D( E )dE 


QE( )  (h   )2
 D( E )dE
Cathode Parameters
K2CsSb
5%-12% QE @ 527nm
Peak Current 45-132A
Average Current 35 mA
(140 mA @ 25% DC)
Lifetime 1-10 hrs
Gun Parameters
433 MHz
26 MV/m peak field
0.6 MW RF Power
D. H. Dowell et al., Appl. Phys. Lett., 63, 2035 (1993)
Laser Propagation and Interference
Laser energy in media
0.8
Calculate the amplitude of the
Poynting vector in each media
0.6
Not exponential decay
0.4
543 nm
0.2
2 10
Vacuum
K2CsSb
200nm
-7
4 10
Copper
-7
6 10
-7
8 10
-7
1 10
-6
Thickness dependence @ 543 nm
0.1
0.6
Ref
0.09
trans
0.08
Total QE
0.5
QE w/o R&T
0.4
0.07
0.06
0.05
0.3
0.04
0.03
0.2
0.02
0.1
0.01
0
0
50
100
150
Thickness (nm)
200
0
250
QE
Transmission/Reflection
0.7
QE
QE vs Cathode Thickness
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
50 nm
200 nm
20 nm
20 nm
10 nm
2
2.2
2.4
2.6
2.8
photon energy [eV]
3
3.2
3.4
Spatial Variation of QE for a Thin K2CsSb Cathode
QE in reflection mode
QE %
1.4
1.2
1
0.8
0.6
0.4
0.2
0
465
470
475
480
Position in mm
485
490
495
Energy
Electron Transport in Diamond
Empty States
Ea
Eg
Surface Trap
Bulk Trap
Filled States
- penetrate
Primary
eSome
1μm into
diamond
- are<trapped
evia
--phonon
Hydrogen
Secondary
termination
e lose energy
lowers
electron
e--e-- and
affinity
e
(achieve
scattering
NEA)
- scattering
Lose
energy
via
e
-e
Most
drift
side
(hopefully)
- trapped
Eventually,
e- reaches
Sometoevacuum
the
bottom
at of
surface
the conduction band
Excite
e
into
conduction
band
e modify
field
in ediamond
- into diamond
HolesTrapped
drift
Mosttoward
will
bemetal
emitted
layer,
(hopefully)
Some e- and holes will diffuse to metal (probability based on drift velocity)
Challenges
Watanabe
et al, J.
of Applied Physics, 95 4866 (2004)
• Electrons
must escape
diamond
– Diamond must <30 μm for 700 MHz RF
– Negative Electron Affinity (NEA) surface for emission
– Field in the diamond is a critical parameter
• Field should be high enough for ve to saturate
• Field should be low enough to minimize e- energy
• Modeling suggests 3 MV/m – good for SRF injector
• Diamond must not accumulate charge
– Material must have a minimum of bulk/surface traps
– Stimulated detrapping
– Metal layer required to neutralize holes
• Minimize energy loss in metal (low Z, low ρ)
• Practical aspects
– Electron stimulated desorption
– Heat load and thermal stresses (1100K to 77K)
– Effect of ion/electron back-bombardment on H-terminated surface
Diamond Measurements in Transmission Mode
x-rays/e-
Diamond is metallized on
both sides
Contact is made by annular
pressure
Electrodes are used to bias
diamond and measure
current
Outer electrodes biased to
prevent photoemission
Gain in Transmission Mode
300
4keV 330nA
5keV 340nA
6keV 250nA
7keV 270nA
8keV 260nA
250
Gain
200
150
100
50
0
0
0.5
1
1.5
Field in diamond [MV/m]
2
2.5
Diamond X-ray Response
Responsivity (A/W)
0.1
0.01
1 keV photon
0.001
0.0001
0
0.1
0.2
0.3
Field (MV/m)
0.4
0.5
Diamond X-ray Response
NSLS U3C/X8A
0.09
0.08
Responsivity (A/W)
0.07
Pt edge
0.06
0.05
0.04
measured
0.03
modeled
0.02
Ti edge
0.01
1/W (13.3 eV)
C edge
0.00
0
1000
2000
3000
4000
Photon energy (eV)
5000
6000
Diamond Timing – Hard X-rays
Diamond Timing – Soft X-rays