High Brightness and High Polarization Electron Source for Electron Microscope

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High Brightness and High Polarization Electron
Source for Electron Microscope
PESP-2008 Workshop (3. October, 2008 _at_J-Lab)

• Tsutomu NAKANISHI
• (Department of Physics, Graduate School of
  Science,
• Nagoya University)

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Beam requirements for electron sources
fromthree kinds of electron accelerators
High peak current (10A)
F10mm
ILC
High average current (10mA)
ERL
F1mm
High current density ( 1A/mm2 )
F1µm
SPLEEM
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Contents of this talkhave already explained
partially in1) Talk by Toru Ujihara, R/D of
transmission PC 2) Poster by Naoto Yamamoto
(small Y) Proto-type SPLEEM gunperformances

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LEEM / PEEM type Electron Microscope (developed
by E. Bauer)
Energy analyzer ( 90or 180bend)
LaB6 emitter
Focus lens
Image lens
LEEM
Objective lens
Beam separator
Contrast aperture
Screen
Objective lens
hn
PEEM
Specimen
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SPELEEM (??????OECU)
Koshikawa Yasue Group
CCD???
????????
LaB6 Gun
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?????? ??????
????????????
???? ??????
????????? (???????? ??????)
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?????
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Made by ELMITEC co. (Germany)
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LEEM images
Dynamic observation of Cu thin film growth on
W(110) at 100 ?
FOV10mmf 0-3.2MLbcc
layer-by-layer growth
(Room temp. - 150?)
The third layer does not start just after
2.13MLbcc and it start at around 2.47 MLbcc.
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Imaging of magnetic domains
I I ?
LEEM Image
I? I ? ?
?
?
Mag. Domain Image
A Magnetic contrast (Asymmetry)
?


P
Polarization of incident beam

• Pure Spin effects can be obtained in Magnetic
  domain images

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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)
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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
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Our first trial toward higher brightness (2003?)
HV-20kV?gap-width5.34mm
Needle-tip (?20nm? 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)
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This work toward much higher brightness (2005?)
? Conventional Type Laser spot size ? 50?m
? New transmission type Laser spot size
diffraction limit ? a few ?m
Advantage Electron Laser beam lines do not
interfere
Laser spot size (exp.) 1.3µm(FWHM) _at_?777nm)?
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Lens stage to make the minimum laser spot
Optical Fiber
Ti-Sapphire Laser
Fiber Collimater
Polarizing Beam splitter
Quarter Waveplate
Imaging Lens
Positioner
XHV
Focusing Lens
Focusing Lens
Photocathode
Photo- cathode
Electron Beam
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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
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A 20keV test-guns Compositions
NEA activation chamber
Laser optics equipment
Gun chamber
100keV-Mott Analyzer
Beam size monitor
Spherical condenser
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Beam simulation
20keV
adopted
? Laser spotf3µm ? Electrode gap4mm ?
Voltage20kV Field gradient5MV/m ?
Electrode Mo (cathode) material Ti
(anode) ? Photocathode exchanged by a
load-lock system
Beam simulation
4keV
Dark current could be suppressed below 10nA under
25kV
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20keV proto-type-gun designed for SPLEEM (JPES-1)
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Mott detector system
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Apparatus performance of JPES-1
Gun assembly
Activation chamber
Load-lock transfer-rod
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Beam size (Brightness) measurement?
Conditions Beam energy (U) 20keV,Beam current
(I)5.3µA
Current (I)
Current density (dI/dx)
L531mm
Source size (S) estimation Laser spot size
Electron diffusion length 0.65µm(HWHM) ?1µm
1.50.3µm
Beam size R1.000.02mm (HWHM)?
1.00.4107 A m-2 sr-1 V-1
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Performance of GaAs-GaAsP superlattice
(Reflection PC by Nagoya group)
_at_778nm
? GaAs-GaAsP superlattice shows the
best performance !
Polarization 92 Q.E. 0.5
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Transmission PC
90 Polarization achieved (2007/10/26)
Position dependence of Polarization
Uniformity of Polarization assured
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Polarization improvement by change of strain
property of GaAsP buffer-layer
Pol.? 65
Pol.? 90
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Summary of JPES-1Performances
Performances of 20keV polarized electron gun with
transmission type photocathode (PC)

• Beam size at PC ? 1.3?m? (780nm laser)
• Polarization 90
• Quantum efficiency 0.1
• Average Current 15?A
• Brightness 2?107A/cm2/str (_at_20keV)
• Brightness (reduced) 1?107A/m2/str/V
• NEA lifetime 200h (without beam)
• NEA lifetime 30h (with 5 ?A)
• Vacuum at PC ? 9.0 ?10-10 Pa

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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?????????? ? ???????????????????????
??????? (Nagoya University?2007??)

• Patents
• T. Nakanishi ????????????? 2006-084303
• T. Ujihara?T. Nakanishi ?5?????????????????????
  ?2008-079292 (2008/3/25??)

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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
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JPES-2 (gun, spin-manipulator beam SW line)
for SPLEEM
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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
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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. ..

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Possible applications of the new-type PES
We start to contact with various fields
researchers (Looking for the academic users of
our PES)
Biology ? Chirality studies
SPLEEM Surface magnetic domain ? Magnetic
memories
TEM Bulk magnetic properties ? Electron
holography
HE Accelerators ? High current low emittance
electron source
Inverse Photo-emission Spectroscopy ? Spin IPES
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SPLEEM collaboration

• 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)?
• T. Saka
• (Daido Institute of Technology)
• T. Kato
• (Daido Steel Co. Ltd.)
• T. Koshikawa, T. Yasue, M. Suzuki
• (Osaka Electro-Communication University)?
• T. Ohshima?T. Kohashi
• (Central Research Laboratory, Hitachi Ltd.)?

High Energy Physics
Semiconductor Physics
LEEM Physics
Electron Microscope Physics
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• Thanks
• for your attentions !

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Examples of SPLEEM image
?
Electron injection energy Ei0.7 eV?50
sec/image
FOV30 ?m
FOV10 ?m
FOV6 ?m
f
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Mechanisms of spin-flip depolariztion
Crystal defects of buffer-layer carried onto
SL-layer
GaAs-substrate
GaP-substrate
Spin-flip occurs
Spin-flip does not occur
SL-layers
GaAsP buffer-layer
GaP-substrate
Cracks do not meet with electrons
Dislocations meet with electrons
Crack-like defects are favorable than
dislocation-like defects