Field Emission

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Field Emission Electron Sources - FEL
Application
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PSI FEL Context
PSI FEL Project 0.1nm lt ? lt 10nm
10-100fs Pulses 10Hz Compact Machine lt
800 m Expected Commissioning 2016
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Low Emittance Gun
Anode
1.5 GHz 4.5GHz RF Cavity
Electron Source
Pulsed Voltage 1MV in 200ns 4mm Gap 10 Hz
250 MV/m
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Thermal Emittance
Ultimate limit in Accelerators Thermal
emittance of the Electron Source
Size of the produced Electron Beam
Thermal Agitation of produced electrons
Goal Emittance lt 5.10-8 m.rad
Uniform Beam over r Maxwell Distribution
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F.E.A. or Single Tip for LEG
Field Emission requires High Field
FSurface gt 3 GV / m
Field Enhancement with Tip Effect
Needle Cathode
Field Emitters Arrays (FEAs)
1mm
50µm
1µm
1mm
Etched Wire
Microelectronic Techniques Thin Film Deposition
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Field Emitters Arrays (FEA) Electron Sources
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Field Emitters Arrays (FEAs)
Principle of FEAs
FEAs Characteristics
10000 Tips / FEA Rtip 20 100nm øFEA lt
1mm Typical IFEA lt 1mA
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Emittance of FEA Beam
Optics Analogy
Thermal Emittance / Tip r15nm
Ekin300meV en/Tip 5.10-12 m.rad
After Focusing en lt 10-7 m.rad
Initially en 2.10-6 m.rad
r
r
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Tests on Commercial Field Emitter Arrays
120 mA (Pulses of 30ns at 10HZ)
(Commercial Device)
Projected Emittance Single Gated FEA en 2.10-6
m.rad at 40 keV Source S.C. Leemann
50000 Tips Ø 1 mm
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Field Emitter Arrays Fabrication at PSI
Pyramid Shape PSI FEA
2 µm
2 µm
Source E. Kirk

• Mo Tips (Field Enhancement Factor ltßgt 90)
• Metallic wafer

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Needle Cathode Electron Source
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Single Tip Electron Source
ZrC robust, FZrC 4 eV
Possibility of Faceting the Tip
rapex 1 to 5 µm
10 µm
1 mm
Required Operating Voltage VTip gt 10 kV
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Single Tip ZrC performances
1GHz Feedthrough
Experimental Setup
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High Current Pulses from a ZrC Tip
Tip Voltage waveform
Peak Current By Field Emission 500 mA
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Fowler Nordheim Analysis
F 5 GV/m J 1012 A/m2
Emitting Area S 12000nm2 (Ø 120nm)
ZrC Tip, Gate 2mm Voltage Pulses 1ns at
10Hz FZrC 3.5eV, 10-9 Torr
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Rough estimation of the Emittance from a ZrC Tip
? 45º 0.78 rad
1mm
1mm
Measure 1 S 12000nm2 ØBeam 125nm ß 192000
m-1 rapex 1/(5ß) 1µm
en rbeam.? 62 nm0.78 rad 5.10-8 m.rad
1014 A.m-2.rad-2
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Needle Cathode Beam Attractive but too long
bunches (ns)
How to modulate the emission in the ps range ?
Laser assisted field emission (Photo-Field
Emission)
(ref. M. Boussoukaya et. al. , NIM A 264,
131-134, 1988)
Combination of Field Emission and Photoemission
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Photo-Field Emission Single Tip
NdVAN Laser 11µJ, 16ps (rms) 266nm, 30Hz Spot
Radius sr 100 µm (rms)
Tip Radius lt 100µm
electrons
Ø4mm
16ps
High Voltage Pulse - 40 kV, 1ns, 30Hz
Laser Spot (off axis) ø 100µm
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Photo-Field Emission Principle
Larger Electron Reservoir for Photoemission
Higher Quantum Efficiency
Schottky Effect
E (eV)
Vacuum
E (eV)
Metal
E0
IPhotoemission
266nm
FZrC 3.6eV
E109 V.m-1
h?4.6 eV
EF
IField Emission
2 nm
0
Energy range from where electrons can be directly
photo-emitted !
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Photo-Field Emission Current
2µJ at 266nm 49pC Q.E. gt 10-4
Limited by Scope Bandwidth (lt1GHz)
Laser 2µJ / sz16ps / sr75µm / 30Hz / 266nm ZrC
tip / VTip-51kV
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Emitting Area (Photo-Field Emission)
How many photons intercepted by Tip ?
rtip 30 µm QE10-4 rtip 10 µm QE10-3
sr,laser
rtip
Laser Pulse 266 nm 2µJ
Tip
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Exponential Increase of the Emitted Charge
Combination of Fowler-Nordheim and Schottky
Dependence
5 µ J / 30Hz / 266nm sr,laser91µm
(rms) sz,laser16ps (rms)
Photo-emission term
Field Emission term
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Delay Voltage Pulse / Laser Pulse
At Synchronisation Photo-field Emission
Field Emission
Photo-Emission
Laser 11µJ / 30Hz / 266nm / 10-9 Torr Scope
1GHz Setup ZrC tip 2mm aperture Focus
Elect VDC0V Vsolenoid14V VTip-40kV
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Electron Beam Transverse Properties
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Photo-Field Emission Beam Spot size
Electron Beam Spot (Phosphor Screen)
Laser 11µJ / 30Hz / 266nm / 10-9 Torr Scope
1GHz Setup ZrC tip 2mm aperture Focus
Elect VDC0V Vsolenoid14V VTip-40kV
11µJ at 266nm Q12pC 40 keV FWHM 740 µm
Electron Beam Profile
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Photo-Field Emission Emittance
Weak Signal after Pinhole Array
e (mm.mrad) 1.3 (/- 0.2)
Normalized Emittance (27 keV 20pC) en ß?e en
0.5 0.1 mm.mrad
Laser 2.3µJ / sz16ps / sr75µm / 30Hz /
266nm ZrC tip VTip-27kV
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Threshold for Plasma initiation
High EField easier to trigger Plasma
Explosive Regime
EEE numbers J 108 A/cm2 Losses 10-8 g/C EEEE gt
104 J/g P 109 Pa
Photo-Field Emission
Litterature IAblation 20GW/cm2 for Copper when
No Gradient
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Laser Induced Plasma Emission
Explosive Electron Emission Stationary Process
Laser Pulse 532nm 20µJ 16ps 10Hz PLaser gt1 G
W/cm2
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Conclusion
FEA Technology Great Potential No laser
needed PSI Status All metal single gated FEA
Issues Double gate fabrication Uniformity
Needle Cathode Laser Approaching PSI FEL
requirements (1A 0.5mm.mrad) Issues
Synchronization and Pointing stability
Laser induced Plasma Emission Very Intense
Stable Issue Control of bunch length