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SLIDE 1
THE CALCULATED EMITTANCE OF A PHOTOCATHODE
THE CALCULATED EMITTANCE OF A PHOTOCATHODE
Ninth Annual Directed Energy Symposium 2006Albuquerque, NM 30 October - 2 November 2006
K. L. Jensen, N. A. Moody, D. W. Feldman, J. Yater, J. Shaw, P. G. O’Shea
Code 6841, ESTD, Naval Research Laboratory
Washington, DC 20375-5347 USA
IREAP, University of MarylandCollege Park, Maryland, D 20742-3511 USA
Funding provided by Joint Technology Office and Office of Naval Research
Acknowledgements (alph): S. Beidron, C. Bohn, C. Brau, M. Cahay, D. Dimitrov, D. Dowell, Y.Y. Lau, J. Lewellen, E. Nelson, J. Petillo, J. Smedley, M. Virgo
SLIDE 2
INTRODUCTION
High brightness beams for high rep rate MW-class FELs require robust, long-lived, high duty factor photocathode with needs at odds w/ drive laser.
Cs-based controlled porosity dispenser cathode is basis of UMD/NRL exp. & theoretical effort to make rugged, self-rejuvenating cathode with high QE
Characterization & prediction of QE from Cs-covered surfaces lead to a material- and laser-parameter dependent model from which the emission distribution can be used to calculate Emittance and Brightness.
Accuracy is confirmed by comparison to exp. measurements of QE from cesiated W and Ag, and bare metals
From this model, we derive an asymptotic limit for expressions for Emittance and Brightness and compare it to numerical calculations using the full transport model, with specific accommodations for:
Scattering
Surface Work function
Quantum Effects
SLIDE 3
BACKGROUNDPhotoemitter Capable of in situ Rejuvenation With a High Quantum Efficiency (QE) Needed for High Power FELs & Linear Accelerators. Coatings Such As Cesium Reduce Work Function = Affects QE
GOALS: Custom Engineered Controlled
Porosity Photocathodes Photoemission Models Validated By
Experiment and Adapted to Needs of Beam Simulation Codes
STATUS Development of Advanced Photo-Electron Emission Microscope System for
the characterization of metals, semiconductors, cesiated surfaces Prototype Dispenser Cell; QE vs. Coverage Diagnostic Tool (UMD) From Integrated Simulation Model, We have Developed Photoemission
Modules Appropriate for Beam Simulation Code Parallel development of theoretical models with experimental effort
Bare Metals
Coated, Solid Metals
Coated Porous Metals
Surface Diffusion
Dispenser Cathode
SLIDE 4
EMISSION “MOMENTS” CALCULATIONS
“Moments” of the Emission Distribution: Electron E augmented by photon, but direction of propagation distributed over sphere Photon absorbed by an electron at depth x: Probability of escape depends upon
occupation of initial state, probability electron final state is empty electron path to surface & probability of collision, energy component directed at barrier & probability of escape
Mn= 2π( )
−3 2mh2
⎛
⎝⎜⎞
⎠⎟
3/ 2
E1/ 2dE sinθdθ2mh2
Esin2(θ)cos2(θ)
⎧⎨⎩⎪
⎫⎬⎭⎪
⎛
⎝⎜
⎞
⎠⎟
n/ 2
×0
π / 2
∫0
∞
∫T (E +hω)cos2θ{ }
transmission probability1 24 4 4 34 4 4
fλ (cosθ,E +hω)scattering factor
1 24 4 34 4fFD(E) 1− fFD(E +hω){ }
Energy distribution & occupation1 24 4 4 4 34 4 4 4
Longitudinal (n = 1) - CURRENT DENSITY
Transverse (n = 0 & 2) - EMITTANCE
n =1⇒ kz = 2mE / h2( )cosθ
n=0,2 ⇒ kρ2 = 2mE / h2( )sin2θ
SLIDE 5
Emittance and Beam Brightness Quality of Electron Source Used to Generate Bunches; Beams with higher current and smaller emittance…
Enable Shorter Wavelength / More Powerful Felselectron beam must be focused inside laser beam for interaction
Emittance Related to Gain of FELits magnitude is a critical and oft-used measure of beam quality, as is brightness.
Intrinsic emittance – what originates at photocathode – important: cannot be compensated for by subsequent beam optics
EMITTANCE DOMINATED BEAMS
O ≡drrd
rkO
rr,
rk( ) f
rr,
rk( )∫
drrd
rkf
rr,
rk( )∫
kx2 =
kρ2
2≈
exp −βTh2kρ2 / 2m{ }
0
∞
∫ kρ3dkρ
2 exp −βTh2kρ2 / 2m{ }
0
∞
∫ kρdkρ
Expectation Value
x
kx
εn,rms(z) =
hmc
x2 kx2
εn,rms(z) =
hmc
x2 kx2
Emittance
Bn =2Ie / πεn( )2
Bn =2Ie / πεn( )2
Brightness
for an axisymmetric, flat, circular, uniformly emitting surface:
SLIDE 6
MODEL CALCULATION: THERMAL EMITTANCE
Apply “Moments” of the Emission Distribution to Thermionic emitters
No photon
Uniform Emission: distribution function independent of x:
T(E): Richardson Approximation
No Scattering factor (e- at barrier)
Incident Distribution is Maxwell-Boltzmann
No “final state” occupation issue
x2 = 1
2ρ2 = 1
2ρc
2
x2 = 1
2ρ2 = 1
2ρc
2
hω =0 hω =0
T E⊥( ) =Θ E⊥ −μ −Φ+ 4QF( ) T E⊥( ) =Θ E⊥ −μ −Φ+ 4QF( )
fλ =1 fλ =1
f
FDE( ) ∝ exp −βT E −μ( ){ } ; βT =1/ kBT
fFD
E( ) ∝ exp −βT E −μ( ){ } ; βT =1/ kBT
1− fFD E( ) =1 1− fFD E( ) =1
F is q x FieldT is temperatureμ is Fermi level
Φ is Work function
F is q x FieldT is temperatureμ is Fermi level
Φ is Work function
εn,rms (z) =h
mc
ρ c2
2
⎛
⎝⎜⎞
⎠⎟
1/2 k⊥3
0
∞
∫ exp −βT h2k⊥2 / 2m{ }dk⊥
2 k⊥0
∞
∫ exp −βT h2k⊥2 / 2m{ }dk⊥
⎧
⎨⎪
⎩⎪
⎫
⎬⎪
⎭⎪
1/2
=hρ c
2mc
M 2
2M 0
⎛
⎝⎜⎞
⎠⎟
1/2
=ρ c
4βT mc2
εn,rms (z) =h
mc
ρ c2
2
⎛
⎝⎜⎞
⎠⎟
1/2 k⊥3
0
∞
∫ exp −βT h2k⊥2 / 2m{ }dk⊥
2 k⊥0
∞
∫ exp −βT h2k⊥2 / 2m{ }dk⊥
⎧
⎨⎪
⎩⎪
⎫
⎬⎪
⎭⎪
1/2
=hρ c
2mc
M 2
2M 0
⎛
⎝⎜⎞
⎠⎟
1/2
=ρ c
4βT mc2
Common representation of the emittance of a thermionic cathode
εn,rms =ρ c
2
kBT
mc2
⎛⎝⎜
⎞⎠⎟
1/2
SLIDE 7
101
102
103
104
105
106
107
-3 -2 -1 0 1 2 3 4 5
Emission from a Tungsten Needle heated to 1570 K
400500600800100012001600
E-EF
Voltage
J. W. Gadzuk, E. W. Plummer,
Phys. Rev. B3, 2125 (1971)Figure 2: Experimental Data
QUANTUM (FIELD) EFFECTSThe General Equation of Electron Emission (M1)
Transmission Probability based on WKB form Widely used D(E) = exp(-2θ(E)) not good near barrier max
WKB Factors Determined From “Area Under Curve”
Quadratic Form (for Near Barrier Max)
Fowler Nordheim Form (for Near Fermi Level μ)
T kx( ) ≈1 / 1+ exp βF (Eo −Ex( )⎡⎣ ⎤⎦T kx( ) ≈1 / 1+ exp βF (Eo −Ex( )⎡⎣ ⎤⎦
J(F,T ) =
e2π( )3
hkx
mT(kx)
0
∞
∫ fFD E(rk)( )d3k
J is the current density F = field in eV/nm
βT = 1/kBT in 1/eV μ = Fermi level in eV
θ E( ) =
1
2π
2m
h2
⎛⎝⎜
⎞⎠⎟
1/2
Q1/4F−3/4 μ + φ − E( ) θ E( ) =
1
2π
2m
h2
⎛⎝⎜
⎞⎠⎟
1/2
Q1/4F−3/4 μ + φ − E( )
βF = −∂Eθ E = Em( ); Eo = Em + θ Em( ) / βF( )
θ E < μ + φ( ) = 22m
h2V (x) − E( )dx
x−
x+
∫
θ E( ) =
4v(y)
3hF2mΦ3 1+
3t(y)
2Φv(y)E − μ( )
⎧⎨⎩
⎫⎬⎭
F-likeT-like
-10
-5.0
0.0
5.0
10
15
20
7 8 9 10 11 12
Num.FN-likeQuadμμ+φ
Energy
Φ = 4.8 eV μ = 8 eV
= 1570 T K = 4 /F eV nm
SLIDE 8
1st & 2nd GEN. MODELS FOR BEAM CODES
QE Algorithm used in distributed code:
Revised Fowler Dubridge Model Parts:
Scattering (Fλ,where ) Scattering factor is proportion of e- to get from
excitation site to surface
Reflectivity (R) & Penetration depth () Calculated from exp. dielectric n & k data
Emission Probability (U(x)) Probability that photo-excited e- will surmount or
tunnel through surface barrier Ratio of incident to transmitted J for allowed e- Depends on Temp, Photon E, Barrier Height
Emission Models For Beam Codes developed with increasing complexity and inclusion of material-dependent factors
Next Gen Code: “Moments” based analysis
Fλ ≈dθ
0
π /2
∫ exp −x−
xcos(θ)v(E)τ (E,T )
⎛⎝⎜
⎞⎠⎟0
∞
∫ dx
dθ0
π
∫ exp −x / ( )0
∞
∫ dx
10
100
0.1 1 10
delta(W)%R(W)delta(Cu)%R(Cu)delta(Au)%R(Au)
[ ]Wavelength micron
10
100
0.1 1 10
delta(W)%R(W)delta(Cu)%R(Cu)delta(Au)%R(Au)
[ ]Wavelength micron
QE =
qhω
Fλm
hk(E)τ (E)⎛⎝⎜
⎞⎠⎟1−R(ω)( )
U hω −φ( ) / kBT⎡⎣ ⎤⎦U μ / kBT[ ]
QE =q
hωFλ
mhk(E)τ (E)
⎛⎝⎜
⎞⎠⎟1−R(ω)( )
U hω −φ( ) / kBT⎡⎣ ⎤⎦U μ / kBT[ ]
E =hω + μ
4
8
12
-4 -2 0 2 4
U(x)
Argument
4
8
12
-4 -2 0 2 4
U(x)
Argument
U x( )≈12
x2 +π 2
6
SLIDE 9
1
2
3
4
5
0 0.2 0.4 0.6 0.8 1
Ba Dispenser (Theory)LongoHaasCs on W (Theory)WangTaylor
Coverage (fraction of monolayer)
Gyftopolous-Levine Theory for Coverage-Dependent Work function: Determination of Φθ depends on
Covalent Radii rx and their sums R
Work function of bulk f and monolayer m
Factors f and w = “Atoms Per Cell” Values Depend on Crystal Face: General Surface = “Bumpy [B]” f : w = 1:4 (Cs on W, Mo, Ta)
= 1:2 (Ba on Sr, Th, W, etc)
n factor: alkali (n = 1); alkaline-earth (n = 1.65)
COVERAGE DEPENDENT WORK FUNCTION
x =x / 2rx( )
2
β R
Hard Sphere Model of Surface Dipole
W
C
Modified Gyftopolous-Levine Theory Φ θ( ) =φf − φf −φm( )θ 2 3−2θ( ) 1−G θ( ){ }
G θ( ) =
rorC
⎛
⎝⎜⎞
⎠⎟
2
1−2w
rWR
⎛
⎝⎜⎞
⎠⎟
2⎛
⎝⎜⎜
⎞
⎠⎟⎟
1+ nrCR
⎛
⎝⎜⎞
⎠⎟
3⎛
⎝⎜⎜
⎞
⎠⎟⎟
1+9n8
fθ( )3/ 2⎛
⎝⎜⎞
⎠⎟
fθ
Dipole modification factor
C-S Wang, JAP44, 1477 (1977)J. B. Taylor, I. Langmuir, PR44, 423 (1933).R. T. Longo, E. A. Adler, L. R. Falce, Tech. Dig. of IEDM 1984, 12.2 (1984).G. A. Haas, A. Shih, C. R. K. Marrian, ASS16, 139 (1983)
SLIDE 10
THEORETICAL EVALUATION OF SCATTERINGScattering in metals due to acoustic phonons and e-e (electron). If mechanisms independent, then:
Values of & Ks from Monte Carlo & Thermal Conductivity data
[1] A. V. Lugovskoy, I. Bray, JPD:AP31, L78 (1998)
Mathiessen’s Rule τ −1 =τee
−1 +τ ac−1 +τ imp
−1
-2
0
2
4
6
8
10
12
14
0 1 2 3 4 5 6 7
ln(Tau [fs])ln(Tac)ln(Tee)ln(TOTL)Liq. NitrogenRoom Temp
ln(Temperature [Kelvin])
Cu
0
20
40
60
80
100
0 0.5 1 1.5 2 2.5 3 3.5 4
TauLugovskoy Ks =1Lugovskoy Ks=5.2tau-ee [fs]tau-ac [fs]
E-EF [eV]
Cu
τ ac =πρh3vs
2 TD / T( )5
mkBTkF2
s5ds
es −1( ) 1−e−s( )0
TD /T
∫⎧⎨⎪
⎩⎪
⎫⎬⎪
⎭⎪
−1
Phonons:• TD = Debye Temp = Deformation potential• vs = Sound velocity
Electron-Electron: (Lugovskoy & Bray [1]) E = Electron energy above Fermi level• qo = Thomas Fermi Screening wave
number• Ks = Dielectric constant
τee =4hKs
2
α 2πmc2(kBT)2
1+E −μπkBT
⎛
⎝⎜⎞
⎠⎟
2kF
qo
⎛
⎝⎜⎞
⎠⎟⎡
⎣⎢⎢
⎤
⎦⎥⎥
−1
(x) =x3
4tan−1 x+
x1+ x2
−tan−1 x 2 + x2
( )
2 + x2
⎛
⎝
⎜⎜⎜
⎞
⎠
⎟⎟⎟
Empirical thermal conductivity data
Monte Carlo simulation of e-e
scattering
SLIDE 11
COMPARISON TO EXPERIMENT: Metals
Bulk Metal Comparisons for Low Laser Intensity & Field - Methodology:
Thermal Photoemission Moments Approach (Next Gen Model) using τee(E=μ+h) for each incident laser intensity or wavelength
• Pulses were Gaussian in time: Total E and Q evaluated via integration over pulse (Laser) and Emitted charge profile (electron)
• Parameters = standard values obtained from Literature or Source (no adjustable parameters)
10-6
10-5
10-4
10-3
200 220 240 260 280 300
Exp (BNL)Theory (Φ = 4.0 )eV
( )Wavelength nm
Lead (Pb)β = 3.0F
o = 1.0 MV/m
T = 300 KData: J. Smedley
200 220 240 260 280 300
Exp (SLAC)4.31 eV5.1 eV60:40 Mix
Wavelength (nm)
Copper (Cu)β = 1.0F
o = 0.01 /MV m
Φ = 4.31 eV = 300 T K
: . Data D Dowell
SLAC:Two grain model
of Cu Surface
60% 4.31 eV(Dowell)
40% 5.1 eV[100] face
0.0
0.5
1.0
1.5
2.0
0 20 40 60 80 100
Charge [nC] 2% AnyCharge 70˚ pCharge 70˚ sTime Dep. Theory
Laser [µJ]
Field = 39 MV/mBeta = 1t = 16 psA = π(0.39 mm)^2λ = 266 nm
, ., 341,379 (1994)Rosenzweig et al NIMA
9.28.06
SLIDE 12
COMPARISON TO EXPERIMENT: Cs on Metals
Notes1. Standard Library Parameters for
W,Ag,Cs: No Adj. Parameters2. Cs-Ag Data taken by A. Balter:
Low θ : Difficulty removing Cs from prior runs
0
0.02
0.04
0.06
0.08
0.1
0 20 40 60 80 100Theta [%]
808 nm655 nm532 nm405 nm375 nm
0.01
0.1808 nm655 nm532 nm405 nm375 nm
Cesium on Argon-Cleaned Tungsten Surface
0.00
0.04
0.08
0.12
0 20 40 60 80 100
Theo
Theo x 0.85
ExpA
ExpB
θ [%]
Cs on Agλ = 405 nm = 0.0174 /F MV m
Φmin
= 1.6 eV
α = 5.3 Angstroms
Cesium on Silver
QU
AN
TU
M E
FF
ICIE
NC
Y [
%]
SLIDE 13
ASYMPTOTIC EMITTANCE FORMULA
Take the weak-field, low temperature limit: Transmission probability and Fermi-Dirac distributions replaced by Step Functions
M n = 2π( )−3
2mh2 hω −φ( )⎡
⎣⎢⎤⎦⎥
(n+3)/2 μhω −φ( )
+ x−1⎡
⎣⎢
⎤
⎦⎥0
1
∫(n+1)/2
G p (μ +φ)(x+1)( ),1
x+1,n2
⎡⎣⎢
⎤⎦⎥dx
fλ cosθ,E( ) =exp −
z−
zl E( )cosθ
⎛
⎝⎜⎞
⎠⎟dz
0
∞
∫
exp −z
⎛⎝⎜
⎞⎠⎟dz
0
∞
∫
=cosθ
cosθ + p E( )scattering
factor
εn,rms =
ρ c
2
M 2
2M 0
⇒ρ c
3hc
6μ hω − φ( )
hω + μ( ) εn,rms =
ρ c
2
M 2
2M 0
⇒ρ c
3hc
6μ hω − φ( )
hω + μ( )
M n ≈1
2π( )22mh2 hω −φ( )⎡
⎣⎢⎤⎦⎥
(n+3)/2 μ1/2 hω −φ4 μ +φ( ) p hω + μ( ) +1⎡⎣ ⎤⎦
μ3 hω + μ( )
n=2( )
1 n=0( )
⎧
⎨⎪
⎩⎪
Generic Causes of Theory-Exp. Differences: • Non-linear Field Components in Cavity; • Wakefields; • Non-uniformity of the Laser Illumination Source; • Thermal Effects; • Quantum Efficiency Non-uniformity Due to
Contamination or Cathode Structure;• Space-charge Effects for Sufficiently High
Bunch Charge
μ barrier heightω Laser freq.E Electron energyθ angle wrt normal to surface
laser penetration depthτ relaxation timeR reflectivityμ Fermi Level
p E( ) =
mhk(E)τ (E)
; G a,b,y( ) =x 1−x2( )
y
x+ ab
1
∫ dx; =hω −φμ +φ
LengthRatio
Angularintegral
EnergyRatio
SLIDE 14
0.0
0.2
0.4
0.6
0.8
1.0
0
50
100
150
200
0.05 0.1 0.15 0.2 0.25 0.3Wavelength [µm]
Copper @ 110 MV/m
EMITTANCE & BRIGHTNESS (analytic)
Consider conditions from:
D. H. Dowell, F. K. King, R. E. Kirby, J. F. Schmerge, J. M. Smedley, PRST-AB 9, 063502 (2006).
Copper Cathode
Work function 4.31 eV(surface cleaned with hydrogen)
Illumination area 4 mm2
Field 110 MV/m
Exp. Value (Dowell, et al.):εrms(233 nm) 0.60 mm-mrad
Theory Value εrms(233 nm) 0.42 mm-mrad
Brightness:
Relaxation time approximation for Copper Parameters (based on fit)(time in fs, energy in eV)
τ ee(E) = 42.9 E − μ( )−1.90
BN =2Ie
4πεrms( )2
For BN, an optimal wavelength exists
εn,rms =
ρ c
3hc
6μ hω − φ( )
μ + φ εn,rms =
ρ c
3hc
6μ hω − φ( )
μ + φ
Emittance
Bn
1−R ω( )( ) I λA≈
3qmc2
2π 2μ 3hωhω + μ( ) hω −φ( )1+ p μ +hω( )( )
Bn
1−R ω( )( ) I λA≈
3qmc2
2π 2μ 3hωhω + μ( ) hω −φ( )1+ p μ +hω( )( )
Ratio of Brightness to Absorbed Laser Power
SLIDE 15Energy [eV]
co
s(θ
)c
os
(θ)
Energy [eV]
co
s(θ
)c
os
(θ)
NUMERICAL STUDY: Cu (LHS) & Cs on Cu (RHS)
F = 110 MV/mμ = 7.0 eVT = 300.0 KΦ = 4.5 eVλ = 266 nmρc = 0.113 mm
M(2)
M(0)
F = 110 MV/mμ = 7.0 eVT = 300.0 KΦ= 1.6 eVλ= 266 nmρc = 0.113 mm
FermiLevel
M(2)
M(0)
SLIDE 16
100 101 102 103
FIELD [MV/m]
270 nm
240 nm
210 nm
180 nm
1600K0.1
0.2
0.3
0.4
0.5
0.6
100 101 102 103
[ / ]FIELD MV m
270 nm
240 nm
210 nm
180 nm
300K
NUMERICAL VS ANALYTIC: Cu
PERFORMANCE
Analytic model works best when F & T not large, and photon energy significantly higher than barrier
Error @ 1 MV/m, 300K: -5% to 10%
NUMERICAL EVALUATION
ASYMPTOTIC FORMULA (photo)
ASYMPTOTIC FORMULA (thermal)
Bare copper metal
Work function 4.5 eV
Illumination radius 1.13 mm
SLIDE 17
100 101 102 103
FIELD [MV/m]
270 nm
240 nm
210 nm
180 nm
1600K
0.6
0.7
0.8
100 101 102 103
[ / ]FIELD MV m
270 nm
240 nm
210 nm
180 nm
300K
NUMERICAL VS ANALYTIC: Cs on Cu
PERFORMANCE
Analytic model works best when F & T not large, and photon energy significantly higher than barrier
Error @ 1 MV/m, 300K: -6% to -13%
NUMERICAL EVALUATION
ASYMPTOTIC FORMULA (photo)
ASYMPTOTIC FORMULA (thermal) (not visible)
Copper W/ Cs coating
Work function 1.6 eV
Illumination radius 1.13 mm
SLIDE 18
IDENTIFY factors that affect QE (e.g., laser, environment, photocathode material)DEVELOP a custom-engineered controlled porosity photo-dispenser cathode
DISPENSER PHOTOCATHODES
Metal
TopView
CsO
SideView
Interpore ≈ 6 µm; Grain Size≈ 4.5 µm; Pore Diam. ≈ 3 µm
Conventional Dispenser
Controlled Porosity
0
2
4
6
8
1 2 3 4 5 6 7 8
Grain Index
Grain SizeAve Diam = 4.8 μm
0
1
2
3
4
5
6
7
0 16 32 48 64 80 96 112 128 144 160
SEPARATION (pixel)
LogNorm(x)
bin size = 8 pixelsLog-Normal Parametersμ = 35.3 pixelsσ = 0.786
- - :Mean pore to pore35.3 (10 x μ / 143 ) = 2.47 m pixel μm
Cs Dispenser Cathode
Dispenser Cathode Surface showing pores & grains
SLIDE 19
WORK FUNCTION MODEL FOR BEAM CODE
Analysis of dispenser cathode surface shows grains
Different faces have different f factors in GL Theory Work function variation may impact beam: perform
modeling of emission using MICHELLE
421 Pixels
Scale: 421 Pixels = 67 µm
0
1000
2000
3000
4000
0 20 40 60 80 100 120
yCutoffs
GRAY SCALE (RGB index)
Value
1417A1
52.062B1
3198.8A2
62.608B2
3440.7A3
82.955B3
y(n) = Aj
n−Bj
8⎛⎝⎜
⎞⎠⎟
2
+1⎧⎨⎪
⎩⎪
⎫⎬⎪
⎭⎪
−1
j=1
3
∑
Grain A Grain B Grain C
SLIDE 20
GENERATION OF MICHELLE MODEL FOR GRAIN
Use actual image and its behavior under processing to motivate method for creating “artificial” grain surface
Method: Generate random matrix of RGB Pixels
Smooth (iterate 3 - 5 times)
Rescale & Truncate at Cut-off values
Pi, jn =
1Ro + 8
Ro −1( )Pi, jn−1 + Pi+k, j+l
n−1l=−1
1∑k=−1
1∑( )
ModelExp
SLIDE 21
CONCLUSION
NEED FOR PHOTOCATHODE
Rugged & Long-lived Photocathodes Critical for MW-class FELsDemands Placed on Photocathode Reflect Needs of Drive Laser & Visa Versa.
EXPERIMENTAL - THEORETICAL PROGRAM
QE of Bare Metals & with (sub)-monolayer coatings of Cs: GOOD AGREEMENT
Development of Custom Controlled Porosity Photocathodes
Validated Photoemission Models for Beam Simulation Codes
HIGHLIGHTS
Next Generation (Moments based) Models for PIC & Beam Simulation
Analytical Models for Emittance, Brightness
Analytical Scattering Operator based on Model of Lugovsky & Bray Reflectivity model of surface based on experimental grain distribution
Surface non-uniformity distribution and analysis