High brightness and high polarization PES for SPLEEM and/or High

PESP-2008 Workshop (3. October, 2008 @J-Lab)
“High Brightness and High Polarization
Electron Source
for Electron Microscope”
Tsutomu NAKANISHI
(Department of Physics, Graduate School of Science,
Nagoya University)
Beam requirements
for electron sources from
three kinds of electron accelerators
ILC
ERL
SPLEEM
Φ10mm
Φ1mm
Φ1μm
High peak current
（≥10A）
High average current
（≥10mA）
High current density
（≥ 1A/mm2 ）
Contents of this talk
have already explained
partially in
1) Talk by Toru Ujihara,
[ R/D of transmission PC ]
2) Poster by Naoto Yamamoto (“small” Y)
[ Proto-type SPLEEM gun
performances ]
LEEM / PEEM type Electron Microscope
(developed by E. Bauer)
Energy analyzer
（ 90°or 180°bend）
LaB6 emitter
Image lens
Focus lens
LEEM
Objective lens
Screen
Beam separator
Contrast aperture
Objective lens
hn
PEEM
Specimen
Koshikawa & Yasue Group
エネルギー分析器
CCDカメラ
LaB6 Gun
電子銃
コントラスト
アパーチャー
高輝度水銀
ランプシステム
制限視野
アパーチャー
入射側アパーチャー
（イルミネーション
アパーチャー）
測定室
試料準備室
マニピュレーター
Made by ELMITEC co. (Germany)
Dynamic observation
of Cu thin film growth
on W(110) at 100 ℃
FOV10mmf
0-3.2MLbcc
layer-by-layer growth
(Room temp. - ~ 150℃)
0
1
2
Coverage[MLbcc] 2.13
2.47
3
The third layer
does not start just after
2.13MLbcc
and it start at around
2.47 MLbcc.
Imaging of magnetic domains
LEEM contrast : I

P
I+ = I + 

P
+ Mag. Domain contrast : 
I = I  
LEEM Image
I

I  I
2

Mag. Domain Image
A : Magnetic contrast (Asymmetry)

A
PM


P : Polarization of incident beam

M :Magnetization of surface

A
I  I
I  I
P

M
Pure Spin effects can be obtained in Magnetic domain images
Proposal of this work (April 2008)
Real-time observation of
magnetic domain formation process
Approved at September 2005 by Japan Science and Technology Agency (JST), as
Technology Development Program for Advanced Measurement and Analysis (Program-T)
talk contents
1. Procedure for Higher Brightness
Transmission Type Photocathode
2. A 20keV Test-Gun
Apparatus Performances
Beam Performances
3. A 20keV gun for SPLEEM
Assembling finished
Beam test in Progress
Our first trial toward higher brightness （2003）
HV=－20kV、gap-width=5.34mm
Needle-tip (20nmf radius)
NEA-GaAs emitter
M.Kuwahara et al. JJAP 45 (2006) 6245
● Field emitter polarized electron source
▲ Serious Problem : Current limit
( Tip melt-down by self-heating）
（current / tip  30 nA）
This work toward much higher brightness (2005)
▲ Conventional Type
● New transmission type
Laser spot size  50mm
Laser spot size
≈ diffraction limit  a few mm
Advantage : Electron & Laser beam lines do not interfere
Photocathode
Lens
Laser f >20cm
Anode
Laser
Mirror
Electron
Photocathode
Electron
Anode
Spherical Condenser
Lens
f=a few mm
Laser
Lens
Electron f >20cm
Laser spot size (exp.)
1.3μm(FWHM) @λ：777nm)
Lens stage to make the minimum laser spot
Optical
Fiber
Ti-Sapphire
Laser
Fiber Collimater
CCD
camer
a
Polarizing Beam splitter
Quarter Waveplate
Laser Spot Profile
Imaging Lens
1
Focusing
Lens
Photocathode
XHV
Focusing Lens
Intensity [A.U.]
Positioner
Y
X
0.75
FMHW
Y : 1.4 mm
X : 1.3 mm
0.5
0.25
Photocathode
Electron Beam
0
-3
-2
-1
0
1
Posision [mm]
2
3
talk contents
1. Procedure for Higher Brightness
Transmission Type Photocathode
2. A 20keV Test-Gun
Apparatus Performances
Beam Performances
3. A 20keV gun for SPLEEM
Assembling finished
Beam test in Progress
A 20keV test-gun’s Compositions
NEA activation chamber
Laser optics equipment
Gun chamber
100keV-Mott Analyzer
Beam size monitor
Spherical condenser
Electrode
Beam simulation
SL-PC
Mo
Ti
Ceramic
20keV
○ Laser spot=φ3μm
○ Electrode gap=4mm
○ Voltage=20kV
Field gradient=5MV/m
○ Electrode: Mo (cathode)
material Ti (anode)
○ Photocathode exchanged
by a load-lock system
Dark current could be suppressed
below 10nA under 25kV
adopted
Beam simulation
4keV
20keV proto-type-gun designed for SPLEEM
（JPES－1）
Mott detector system
Apparatus performance
of JPES-1
Gun
assembly
Activation
chamber
Load-lock
transfer-rod
Apparatus
Laser system
HV
UHV system
Specification
 10mm spot
Achieved
 2μm
Dark current
10nA
 6nA (25kV)
10-10Pa
at NEA surface
 9×10-10 Pa
Beam size (Brightness) measurement
Conditions： Beam energy (U) = 20kｅV，Beam current (I)＝5.3μA
Gun
Current
電流値(I)
Current density
電流密度(dI/dx)
Current [mA]
knife-edge
L
Farady cup
dI/dX [mA/mm]
6
4
2
0
-3
L=531mm
0
Position [mm]
Source size (S) estimation
= Laser spot size +Electron diffusion length
= 0.65μm(HWHM) + 1μm ～1.5±0.3μm
Reduced
Brightness
Br I
1
π S2 π R
2
L
3
2
1
0
-3
0
Position [mm]
Beam size
R=1.00±0.02mm (HWHM)
1
S
2
U
=1.0±0.4×107 A m-2 sr-1 V-1
3
Performance of GaAs-GaAsP superlattice
(Reflection PC by Nagoya group)
@778nm
Polarization ~ 92%
Q.E.
~ 0.5%
☆ GaAs-GaAsP superlattice
shows
the best performance !
Transmission PC
90% Polarization achieved
（2007/10/26）
Position dependence
of Polarization
Uniformity of
Polarization assured
Polarization improvement
by change of
strain property of GaAsP buffer-layer
Pol. 65%
Pol. 90%
Summary of JPES-1Performances
Performances of 20keV polarized electron gun
with transmission type photocathode (PC)
 1.3mmf (780nm laser)
•
Beam size at PC
•
•
•
Polarization
≥ 90％
Quantum efficiency ≥ 0.1％
Average Current
≥ 15mA
•
•
Brightness
≥ 2107A/cm2/str (@20keV)
Brightness (reduced) ≥ 1107A/m2/str/V
•
•
•
NEA lifetime
NEA lifetime
Vacuum at PC
≥ 200h (without beam)
≥ 30h (with 5 mA)
 9.0 10-10 Pa
Documents on a transmission PC PES
［Published Papers］
（1）“High brightness and high polarization electron source using
transmission photocathode with GaAs-GaAsP superlattice layers“
N. Yamamoto et al. Journal of Applied Physics vol.103, (2008), 064905
（2） “Super-high brightness and high spin-polarization photocathode”
X. Jin et al. Applied Physics Express Vol. 1 (2008), Article No.: 045002
［Doctor Thesis］ Naoto Yamamoto： “NEA-GaAs型超格子薄膜結晶を用
いた高輝度・高スピン偏極度・大電流密度ビームを生成する電子源の開発”
（Nagoya University、2007年度）
［Patents］
1)
2)
T. Nakanishi： “スピン偏極電子源装置”、特願 2006-084303
T. Ujihara、T. Nakanishi 他5名：“透過光吸収フォトカソード型偏極電子源”、
特願2008-079292 （2008/3/25出願）
talk contents
1. Procedure for Higher Brightness
Transmission Type Photocathode
2. A 20keV Test-Gun
Apparatus Performances
Beam Performances
3. A 20keV gun for SPLEEM
Assembling finished and
final beam test in progress
JPES－2 (gun, spin-manipulator & beam SW line)
for SPLEEM
High Brightness & High Polarization
Electron Source for LEEM
LEEM (Osaka)
PES (Nagoya)
Within one month,
this PES system will be transferred to Osaka and jointed with LEEM
Additional remarks (1)
○ Advantages of transmission-PC PES
Freedom to design both of laser & electron beam
Lines independently.  Laser beam line can be
optimized to satisfy various requirements.
•
•
•
•
•
Minimum laser spot size obtained (this work)
Symmetrical beam distribution to beam axis
Relax the laser heating problem for ERL-PC
Two photon excitation becomes easily.
Others, etc. etc. …..
Possible applications of the new-type PES
We start to contact with various fields researchers
(Looking for the academic users of our PES)
［SPLEEM］
Surface magnetic
domain

Magnetic memories
［Inverse Photo-emission
Spectroscopy］

Spin IPES
[TEM］
Bulk magnetic
properties

Electron holography
［Biology］

Chirality
studies
［HE Accelerators］

High current + low emittance
electron source
SPLEEM collaboration
High Energy
Physics
T. Nakanishi, S. Okumi, M. Yamamoto, [M. Kuwahara],
[N. Yamamoto], [A. Mano], Y. Nakagawa
(Faculty of Science, Nagoya University)
Y. Takeda, T. Ujihara, X. J. Kim
(Faculty of Engineer, Nagoya University)
Semiconductor
Physics
T. Saka
(Daido Institute of Technology)
T. Kato
(Daido Steel Co. Ltd.)
LEEM
Physics
Electron Microscope
Physics
T. Koshikawa, T. Yasue, M. Suzuki
(Osaka Electro-Communication University)
T. Ohshima、T. Kohashi
(Central Research Laboratory, Hitachi Ltd.)
Thanks
for your attentions !
θ
Examples of


PM


A  P M
SPLEEM image
Electron injection energy
Ei=0.7 [eV]、50 [sec/image]

P

Co : 4 ML
M
W(110)
φ
FOV=30 [mm]
FOV=10 [mm]
FOV=6 [mm]
θ
 
P // M
φ
 
P // M


PM
2 mm
Mechanisms of spin-flip depolariztion
Crystal defects of buffer-layer carried onto SL-layer
GaP-substrate
GaAs-substrate
Spin-flip occurs
Spin-flip does not occur
SL-layers
GaAsP
buffer-layer
GaP-substrate
GaAs-substrate
Dislocations meet
with electrons
Cracks do not meet
with electrons
Crack-like defects are favorable than dislocation-like defects

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