カソードワークショップ報告
第48回ERL検討会
2010年11月9日
山本将博

1st P3 workshop @ BNL

期間：10月12日～14日の3日間
参加者：総勢40人程度

Topics・カソードの加速器利用の現状
カソ ドの加速器利用の現状
・カソードの作成手法
・カソードの解析の最新情報
・光電子のスペクトル解析
光電子のスペクトル解析
・カソード解析のための諸施設の利用
・光電子放出の理論
・将来のカソード開発案
将来のカソ ド開発案
・カソード開発、評価の協力関係について

USA groups
BNL, ANL, Cornell, SLAC, JLAB,
UCLA,, LBNL,, PNNL,, IBM,,U. Maryland
Europep ggroupspDaresbury, PSI, HZBpAsia ggroups
KEK/Nagoya （山本・桑原の2名）

各所の電子銃・フォトカソード
RF電子銃
常伝導型：BNL(CsK2Sb diamond) ANL(Cs2Te,CsK2Sb),
常伝導型：BNL(CsK2Sb,diamond),
ANL(Cs2Te CsK2Sb)
SLAC(Cu), LBNL(CsK2Sb), UCLA(Cu,Nb,MgF2)
U. Maryland (dispenser W cathode)
超伝導型：HZB(Nb,Pb,CsK2Sb..)
DC電子銃
C電子銃
パルス：PSI (Cu, Mg, Ti, SUS, Bronze, Y, Al, Mo, Nb, FEA)CW： Cornell(GaAs,
Cornell(GaAs AlGaAs) , JLAB(2‐photon GaAs),
GaAs)
SLAC(GaAs (Li)), Daresbury(GaAs),
KEK/Nagoya (Superlattice)
RF電子銃関連のグループからの報告が7割程度。
発表スライドは下記よりダウンロ ド可能。
発表スライドは下記よりダウンロード可能
https://indico.bnl.gov/conferenceDisplay.py?confId=290

H. Padmore (LBNL)LBNL Photocathodes

INFN – FLASH cathode load lock / transfer system

Frequency

187
MHz

Operation modeCW

Gap voltage

750 kV

Field at thecathode

19.47
MV/m

Q0

30887

Shunt
impedance

6.5 M

RF Power

87.5 kWStored energy

2.3 J

Peak surface
field

24.1
MV/m

Peak wall powerddensity
it

25.0
W/ 2
W/cm

Accelerating
gap

4 cm

Diameter69 4 cm
69.4

Total length

35.0 cm

H. Padmore (LBNL)H. Padmore (LBNL)H. Padmore (LBNL)H. Padmore (LBNL)X‐ray photoelectron spectroscopy (XPS)O 1s

AlK 1487 eV

e‐

Evac

Evac
h

S

A

Ti 3p & Sr 4s
O 2s & Sr 4p
VB

Sr 3p

Ti 2p
p

Ek

Sr 3s

E k'

Sr 3d

analyzer
coun
nts

semiconductor
sampleSrTiO 3 (001)

600

500

400

300
200
binding energy ( eV )

100

EF

Eb

h = Eb + Ek + A

Scott A. Chambers (PNNL)0UV photoelectron spectroscopy (UPS)He I 21
21.2
2 eV

0

kinetic energy, Ek (eV)
5
10
15
E

e‐

Countssemiconductor
samplesecondaryelectrontailanalyzer

15
E k'

Ek

Evac
h

S

Evac

VB

10
5
binding energy, Eb (eV)0

E = h ‐ S

A

Ek = 0
EF

E k'

Evac
h

Eb

S

Evac

A

EF
Eb
h = Eb + Ek + A

Scott A. Chambers (PNNL)High energy resolution XPS/UPS spectrometer
High‐energy

Scott A. Chambers (PNNL)

n‐Si(001)Si(001)‐‐(2x1)
XPS VB & GGA
normal emission
VB
monochromatic AlK
(0.46 eV resolution)
DFT GGA DOS ‐‐ (E)∞∫(E´)g(E‐E´)dE´0 = 0.46 eV

0.56 eV

0.58 eV
2.0

12

10

8
6
4
2
binding energy (eV)

0

0.0

‐2
2
Scott A. Chambers (PNNL)Diamond Science at BNL

J. Smedley (BNL)

Imaging
SEM

Scanning Electron Microscopy

Surface morphology

LEEM

Low Energy Electron Microscopy

Imaging of hydrogenated surface, spatially localized LEED, work function mapping

AFM

Atomic Force Microscopy

Surface morphology

X‐ray diffraction, time resolved

Characterization of metal contacts, including temperature of formation and crystalline texture

X‐ray diffraction

Diamond crystal quality; evaluation of stress caused by laser shaping

Diffraction
XRD
XRD
Topography
LEED

Diamond crystal quality, localization and identification of defects
Low Energy Electron Diffraction

Surface crystal analysis, evaluation of hydrogenated surface

Spectroscopy
Ultraviolet Photoemission Spectroscopy
Electron affinity, energy & angular distribution of emitted electrons, lifetime of NEA surface
UPS/ARPES
Total Yield Spectroscopy
Evaluation of hydrogenated surface, lifetime
TYS
Near Edge X‐ray Absorption Fine Structure Surface elemental analysis, characterization of surface bonding, carbon formation
NEXAFS
X‐ray absorption fine structure
Titanium/diamond surface chemistry
XAFS
Energy Dispersive X‐ray Spectroscopy
Surface elemental analysis
EDS
Fourier Transform Infrared Spectroscopy
Impurities in diamond
FTIR
Impurity analysis, identification of carbon chemistry, mapping
Photoluminescence & Raman Spectroscopy

Carrier Transport and Emission
Electron Generated
Photo‐electron Generated
Soft X‐ray, Monochromatic
Hard X‐ray, MonochromaticHigh Flux White beam
Micro‐beam Mapping

Carrier Transport vs Field, Emission, Gain, Thermal EmittanceGain, Timing
Charge collection distance, Charge trapping/detrapping effects
Measurement of mean ionization energy (gain)
Current Limits, Contact requirements, Heat management
Localization of electrically active sites

EQUATIONS OF ELECTRON EMISSION
K. L. Jensen (NRL)

J(F,T ) =1 ∞∫ 0 D ( E ) f ( E ) dE
2π 

THERMAL
Low Field, High Temperature

FIELD
High Field, Low Temperature

PHOTO
Low Field & Temp, Photon

Richardson-Laue-Dushman Eq.

Fowler Nordheim Eq.

Fowler-Dubridge Relation

D ( E ) = Θ ⎡⎣E − (µ + Φ)⎤⎦

D ( E ) ≈ exp ⎡⎣− ( B / F ) − C (µ − E )⎤⎦

D ( E ) = Θ ⎡⎣E + ω − (µ + Φ)⎤⎦

mkBT
exp ⎡⎣(µ − E ) / kBT ⎤⎦2π
⎛ mkB2 ⎞ 2
J RLD (T ) = ⎜ 2 3 ⎟ T exp [−Φ / kBT ]⎝ 2 π  ⎠m(µ − E ) Θ (µ − E )π 2F2J FN ( F ) =exp (−B / F )16 π 2 Φf (E) =

f (E) =

f (E) =

B=

T = 1273 K
Φ = 2.1 eV
JRLD≈ 1 A/cm2

Thermionic cathodes run at lower
T levels than possible topreserve lifetime

More? See K.L. Jensen. J. Appl. Phys. 102, 024911 (2007).

4
2mΦ3 ;
3

C=

2
2mΦ
F

F = 7 GV/m
Φ = 4.5 eV
JRLD ≈ 1.5x105 A/cm2
Field emission occurs over
(5 nm)2 emission areasm(µ − E ) Θ (µ − E )π 2m2
J FD ( ω ) =ω − Φ)2 3 (4π 

ω = 4.66 eV
Φ = 4.5 eV, µ = 7 eVJ( ω − Φ )QEFD ≈ over =J allµ2= 0.1%

2

Losses due to scattering
lowers estimate
Photocathode Physics for Photoinjectors Workshop

5 of 32

EMITTANCE EVALUATION BY MOMENTS APPROACH
Transverse Moments Evaluation for Metals & Semiconductors

K. L. Jensen (NRL)

Moments Approach can be used to calculate the weighted averages used in calculation of emittance
3/2

( )

M n = 2π−3⎛ 2m ⎞⎜ 2 ⎟⎝  ⎠∞

1/2∫ E dE ∫
0

π /2
0

⎧
n/2
⎧ 2m
⎫
⎪ f FD ( E) 1− f FD ( E + ω )
2
2
⎡
⎤
sin θ dθ ⎨ 2 E + ω sin θ ⎬ D ( E + ω )cos θ f λ ⎣cosθ , p ω ⎦ ⎨⎩ ⎭Θ ω + E − Eg
⎪
⎩

(

Important: Cons. of k across surface forkx & ky of wave function demands appending
photon energy to E in moment term
Dowell, Schmerge, PRSTAB 12, 074201 (2009).

ε n,rms =


mc

x2

kx2 − xkx

2)

This is transverse
momentum (sin would
be cos for J calc)

{

}

( )

This is transmission
probability forsurface barrierThis is scattering
loss factor forbulk transport<...> in ε are weighted over f(x,k) of emitted electrons, andtherefore already in language of Moments

THERMAL EMISSION)

This is initial & final state
occupation factorO ( x, kx ) ≡ ∫ dr dkO ( x, kx ) f ( r, k )Photons ω > 02
2
2
Uniform emission 2 x = ρ = ρ cUniform emission 2 x 2 = ρ 2 = ρc2Richardson Approx. D ( k ) = Θ ( E(k) − µ − φ )No Scatteringfλ ( x, p ) = 1

Maxwell-Boltzmann f(x,k) D(k) f (k) ∝ exp {− ( E(k) − µ ) / kBT }No “Final state” issues 1− fFD ( E ) ⇒ 1


mc

(

)

PHOTO-EMISSION

No photons ω = 0

ε n,rms (thermal) =(x2

JWKB Approx. D ( k ) = DJWKB ( E, F )Scattering fλ ( x, p ) = x / ( x + p ( E ))2
Schottky Lowering φ = Φ − q F / 4 πε 0leading order (metal)

1/2

kx2
1/2

 ⎛ ρ c ⎞ ⎛ M 2 ⎞=⎜ ⎟ ⎜⎟
mc ⎝ 2 ⎠ ⎝ 2M 0 ⎠

1/2ρ ⎛ k T ⎞
= c ⎜ B 2 ⎟
2 ⎝ mc ⎠ ⎛ ρ c ⎞ ⎛ M 2 ⎞
ε n,rms ( photo) =⎜ ⎟ ⎜⎟
mc ⎝ 2 ⎠ ⎝ 2M 0 ⎠

1/2ρ ⎡ ( ω − φ ) ⎤
≈ c ⎢⎥
2 ⎣ 3mc 2 ⎦

Metals p large & fλ ≈ cosθ/p: therefore, emittance indep. of p.
Semiconductors larger ε due to p small, but D also has impact
Photocathode Physics for Photoinjectors Workshop 17 of 32

• Start with 4000 electrons• Laser penetration depth = 12 nm, incident frombottom of frame;• λ = 266 nm,,
F = 50 MV/m, T = 300 K• Time Step = 0.2 fs / frame, 100 frames
Sim Region 20 nm x 20 nm• When electron energy drops below barrier“Fatal Scattering” Approx K. L. Jensen (NRL)• Left:• Right:• Acoustic changes electron direction• e-e bleeds away energy by creating 2nd
electron & sharing KE, final direction isalong one of 6 axes wrt incident direction• Red electrons travel towards surface (down)Blue electrons travel away from surface (up)height E < μ + Φ - (4QF)1/2, electron isremoved from simulation visualization

caveat: pseudo-Monte Carlo

If initial velocities of twocolliding e- are pz & qz, where +z
is direction of incident electron pz
and qz is velocity of any allowed
second electron, then:• p' = (pz qz)1/2
q' = (pz2 + qz2 - pzqz)1/2
and θ = 0 (π reverses role)with 33% Probability• p' = pz
q' = qzand θ = π/2
with 67% Probability
Approx: Scattering is fatal

Approx: Scattering redirects & shares energy
Photocathode Physics for Photoinjectors Workshop 28 of 32

Current Status at HZB – sc-cavity (HoBiCaT) 2

At the moment the laser (I. Will, MBI) and electron-beam diagnostics are arranged

0.26 µm, 30 kHz, 2…3 ps, 0.15 µJ pulses from
Yb:YAG oscillator + regen. amp. + 4th harm conv.
BNL, Oct. 2010

Susanne Schubert

4Current Status at HZB – Cathode Project 1

Design of UHV - chambers for CsK2Sb - preparation:
p= 10-11 mbar range
plug-system from FZ-Dresden for possible sample exchange
still in search of a nice and suitable methode to heat the substrate (laser, boralectric…)
and measure its temperature (pyrometer, thermocouple …)

BNL, Oct. 2010

Susanne Schubert

5Current Status at HZB – Cathode Project 2

Preparation
Substrate (Mo):-polishing-pre-treatment in UHV by ion-sputtering and heating cycles
Film growth:-elevated substrate temperature-vaporvapor deposition (Cs(Cs, Sb
Sb, K)

Cathode-holder from FZD

BNL, Oct. 2010

Manipulate the film appearance by:a)substrate ( element, surface roughness,
pre-treatment)b)substrate temparturec)deposition)d
iti ratet andd sequenced)presence of gases (O2, Ar)

Susanne Schubert

6Photoemission emittance measured in a Diode – RF Gun

166 mmDiode – RF gun Combination:
Diode: 500 kV; 5 mm; < 3 Hz; Mg,
Cu, …
RF: 2 Cell – L Band
Band, 1.499
1 499 GHz
Beam: 6 MeV; 200 pC
CathodeAnode

PulsedSolenoid

RF Cavity

Intrinsic Emittance versus Laser Spot Sizepepperpot
solenoid scan
Theory: 0.54 mm.mrad / mm (Cu=4.35 eV)

Normalize
ed Emittanc
ce (mm.mra
ad)

0.3
02
0.2

Q < 1 pC; 5.2 MeV
272 nm0.1

Copperpp ((Cu)
Cu)
0.0
0.3

Theory: 0.62 mm.mrad / mm (Nb=4.13 eV)02
0.2

Pepperpot
Solenoid Scan

0.1

 Intrinsic   x

Niobium (Nb)
Nb

0.0
0.0

0.1

0.2
0.3
0.4
Laser spot size rms (mm)Feff1/ 2(4 0 )1/ 2

3mc 2
3

Laser spot size (rms)0.5Good Agreement between Pepperpot and Solenoid Scan.
G d AgreementGoodAt withith Theory
Th(Linear(Li
B
Behavior)h i )h   0  e

3/ 2

Norrmalized E
Emittancee (mm.mraad)Intrinsic Emittance versus Laser Wavelength

0.35

282 nm272 nm
261 nmTheory 0.41 mm.mrad /mm
Theory 0.54 mm.mrad /mm
Theory 00.68
68 mm
mm.mradmrad /mm

0.30
0.25

261 nm: 0.68 mm.mrad / mm

272 nm: 0.54 mm.mrad / mm

0.20

282 nm: 0.41 mm.mrad / mm

0.15
0.10
0 05
0.05
0.00
07 12 09

 Intrinsic   x

0.0

Exp. Cond.:
Q < 1 pC; 5.2 MeV
σt,laser= 4 ps rms; 25 MV/m
Copper Cu
Solenoid Scan

0.1

0.2
0.3
0.4
L
Laser
rms sizei ((mm))h   0  e

Feff1 / 2

3/ 2(4 0 )1 / 2

3mc 2

0.5

ΦCu= 4.3
4 3 eVWith Feff = 25 MV/m

Intrinsic Emittance versus Laser Wavelength
;

1.0

285

y

Photon Wavelength (nm)
280
275
270
265

260

255

n/x (mm.mrad / mm)

0.9
0.8
Cu
C
Mo
Nb
Al
BronzeYttrium
Mg 'dry'
Mg 'wet'Th Formula (0= 4.4 eV)

0.7
06
0.6
0.5

Th Formula (0= 4.3 eV)
Th Formula (0= 4.1
4 1 eV)

0.4

Th Formula (0= 3.9 eV)
Th Formula (0= 3.66 eV)

0.3
4.3

4.4

4.5

4.6

4.7

4.8

4.9

Photon Energy (eV)Q < 1 pC; 5 MeV; Fcathode= 25 MV/m

 Intrinsic   xh   0  e3 / 2
3mc 2

Feff1ff/ 2(4 0 )1 / 2

Q
Quantumm Efficie
ency

Quantum Efficiency of different materials

-5
5

10

-6

10

25

30
Surface Gradient (MV/m)

Duettino ;6 ps rms; 4-6 muJ on Cathode; 5.2 MeV; gap 6mm
DLC Hollow Cathode + Flat Insert;

Cu
St. St.
Ti
Mo
Nb
AlBronzeTiVAl
Mg 1Yttrium
Mg 2

Evidence of Ion Back Bombardment
The figures show the QE
degradation resulting fromillumination of three
separate spots over a
period of many weeks. The
damage appears only along
a line joining theilluminated spot and theelectrostatic center of thegun.
The damage ischaracterized (imperfectly)
in terms of the
Coulombs/cm2 delivered
f
from
the
h illuminatedill id spot.
11/9/2010

BNL PPP Workshop 10/10

C. Sinclair (Cornell)

9

Jlab results up to 10 mA in 100 kV Gun
Must eliminate large radius electrons
to achieve these results
Clearly factors other than the charge
delivered per unit area are involved.
Whether these are fundamental or not
isn’tt clear
isn
clear.
From Grames et al., in Proceedings ofthe 17th International Spin Symposium,
Kyoto, Japan, 2006

11/9/2010

BNL PPP Workshop 10/10

C. Sinclair (Cornell)

10Constant Current QE Lifetime

Q0 A  Pmax 
0 

T
l 
ln
I0
 1.24 I 0 
Where Qo is the charge density lifetime, A is theill i t d area,  isilluminatedi th
the wavelength
l th ini , andd P iis ththelaser power in W.
For a Qo of 106 C/cm2 you can deliver 100 mA for 100
h
hours
with
ith 10 W off llaser power att 527 nm on a 10%
initial QE cathode, from a 1.8 mm diameter spot.
11/9/2010

BNL PPP Workshop 10/10

C. Sinclair (Cornell)

11Complete Energy Distribution at 90K
From
Orlov et al.
APL 78,2721 (2001)

11/9/2010

BNL PPP Workshop 10/10

C. Sinclair (Cornell)

12Angular Distribution Measurement

From Liu et alal., JVST B 23,
23 2758 (2005)
11/9/2010

BNL PPP Workshop 10/10

C. Sinclair (Cornell)

13Compact ERL (test facility)

PF-AR south area
(DC gun test facility)

M.Yamamoto (KEK)cERL test area

Commissioning will be started end of 2012 FY
FY.
PPPW2010 BNL

4Policy of cathode R&D for ERL

M.Yamamoto (KEK) Superlattice structure for ultra low emittance- narrow conduction mini
mini-band
band width- higher conduction mini-band level
 High QE cathode for reducing laser power- DBR structure- control density of state (SL)- wide band gap material
 Transmission type cathode for suppression
laser heating- transmitting substrate- high thermal conductivity substrate

PPPW2010 BNL

9Laser system for injector commissioning at KEK• Drive laser for AR-south injector commisioning testarea (started since 2009)• Requirement for 10mA operation of cERL:••

1.3GHz(repetition), 530nm(wavelength), 20ps(pulse
duration), 1.5W(power)
System has been built based on commercial units(1.3GHz oscillator, fiber amplifier, SHG, etc.)
500mW (2ω) output has been achieved. Enough forfirst commisioning of the injector upto 3mA
3mA.
Development of higher power amplifier is on going.

PPPW2010 BNL

13

1 3GHz oscillator
1.3GHzM.Yamamoto (KEK)

pulse compressor
pre amplifier
pre-amplifier
10W-amplifier(30W-amplifier)pulse train shaping
Second Harmonics
transport , shaping

200 kV DC Gun

M.Yamamoto (KEK)

Ti anode & Mo cathode, Ecath: 3.0 MV/m
Segmented insulators, Ultimate P: 2E-9 Pa
200 kV polarized electron gun (NPES3)
has been developed at Nagoya Univ.
Details was presented in PESP2008
PESP2008.
PPPW2010 BNL

14

The
e gu
gun wasas ttransferreda s e ed fromo Nagoyaagoya
to KEK in 2009.

Layout of Beam Diagnostic Line

200 kV
DC gung& FC

Laser injection port

PPPW2010 BNL

15

M.Yamamoto (KEK)Emittance measurement result (SL)	

M. Kuwahara (Nagoya)5	

Photon energy dependence (SL and Bulk-GaAs)	
In SL photocathode, the increase ofemittance is lower than that of bulkGaAs

This effect is explained by thedispersion of the JDOS.

The DOS in C.B. is quantized and shrinks.
(Miniband is formed by the micro-structure.)	
⇒ An emittance growth is suppressed in shorter wavelength
SL structure has a low emittance with high QE comparing with a bulk-GaAs	Superlattice photocathode	

M. Kuwahara (Nagoya)7	

Toward a low emittance with a high QE

1. Layer-structure -> SL (suppressing the emittane growth withincreasing the excitation energy)
2. Using excitation both of HH and LH
3. Expand the active layer with suppressing the expansion of pulse width	

The JDOS of C.B. is quantized and shrinks.
(Miniband is formed by the superlattice)	

Schematic of an extraction process in a SL
photocathode	Superlattice photocathode	

M. Kuwahara (Nagoya)8	

Excitation energy toward the high QE	
Joint Density of State	

The Brillouin zone
depends the SL repetition	

The dispersion of JDOS is dominantly determined by the C.B. because theeffective mass is smaller than that of V.B.
Separation between 1st and 2nd mini-band in C.B. must be larger than that of
HH and LH. 	

Temporal response in SL (not in Bulk)	

M. Kuwahara (Nagoya)11	

Average emission time
In Bulk-GaAs, the pulse width is mainly determined by the absorption length.
However, if the active-layer thickness is limited, the tail can be cut.
GaAs /GaAsP Supperlattice
d2
d2
t ≈
=
= 2 ps2
τ re = 55 ps,τ s = 140 ps, α = 7 × 10 3 cm −1
12D 12⋅ 27cm /s
d ~ 254nm
D = µe k B T /e = 1000cm 2 /Vs⋅ 27mV = 27cm 2 /s
Ref.: K. Aulenbacher, et al., J. Appl. Phys. 92 (2002) 7536.	

Using the QE of 0.9% @751nm in active-layer width of 96nm,
In the case of 254-nm width,€QE is expect to be 2.1%. (Without recombination effect)	

€

Simple estimation using Pol. , relaxation time and recombination time
α v +1/τ r 1 − exp( −(α v +1/τ s +1/τ r )⋅ d / v )PCW = P0
= 0.92
α v +1/τ s +1/τ r 1 − exp( −(α v +1/τ r )⋅ d / v )v ≈ 9 × 10 4 m /s

Corresponding with the temporal response time in short pulse	
t = 96nm / v ≈ 1ps
The difference ~ additional spin-relaxation time orscattering in drifting process or BBR ?	

Recent R&D	

M. Kuwahara (Nagoya)12	

A transmission-type spin-polarized photocathode has beendeveloped for application to several types of electron microscopy,
e.g., spin-polarized LEEM, spin-polarized TEM and so on.
Spin-polarized
electrons

Transmission-type photocathode with
GaAs/GaAsP strained superlattice

GaAs/GaAsP
superlatticeHigh spin-polarization：
90％ 	

GaP substrate	

Super-highbrightness：1.3×107
A⋅cm-2⋅sr-1
Xiuguang JIN et al., Appl. Phys. Express 1 (2008) #045002

Laser light（@20 kV）

まとめ・現状のカソード材料について、様々な手法で表面分析が進められている。（BNLではNSLSを利用した表面・構造分析
が実際に行われている。（見学コース））・アメリカ各所では、カソード開発・分析（特にその場観測）を各所で協力して行う体制を模索している。
（カソードPuck、真空スーツケースの規格化）
・カソード開発のためのシミュレーションコードの開発が進んでいる。
・マルチアルカリカソード、酸化膜カソードなどの使用を検討している所が多い。（高QE, カソ
カソード寿命の点で。）
ド寿命の点で。）
・カソードとペアで考えるべきレーザーに関する議論はほとん
ど無かった。
ど無かった・2年後にコーネル大で2回目を開催予定。