Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5...
Transcript of Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5...
Secondary-electron emission from hydrogen-terminated diamond
Erdong WangBrookhaven National Laboratory
30-May-12 1
Outline
• Motivation-High average current electron source requirement-Diamond amplifier concept
• Experiments-Hydrogenation optimization-High gain emission test-Lifetime and uniformity
• Emission models development-Effective NEA surface-Energy Spread-Emission gain depend on pulse width
• Summary
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Motivation
•High average current, high-brightness, low emittance injector requirement:
-ERL light source-E-cooling
•Desired cathode:-High peak current-High average current-Low transverse emittance-Fast temporal response-Small energy spread-Simple installation expose to air, no load-lock mechanism requirement-Long life time-Should not contaminate gun
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Diamond amplifier concept
Laser
Photocathode
Metal coating
RF cavity
Hydrogenated surface
Primary beam
-10kV
Diamond
Secondary beam
Gap
Focusing
Metal photocathode: QE 0.1%,long life time (months)Semiconductor photocathode: QE 1%~10%, short life time (weeks)Diamond amplifier, high current, long lifetime
Diagram courtesy of D.A.Dimitrov (Tech-X Crop.)30-May-12 4
Why we choose diamond?
• Wide band gap– Diamond has a band gap of 5.47 eV;
indirect band gap– Material with larger band gap will have the
ability of tolerant stronger electric fields.Prevent field emission and dark current.
– Could have negative electron affinity(NEA) with hydrogen termination on thesurface, due to the large band gap
• Best rigidity– Diamond window will protect the gun from
cathode material.• High thermal conductivity
-- Easily handle heat power from primarycurrent deposited
• Very high Mobility and Saturation Velocity– Saturation velocity is 2.7×105 m/s. It have
high frequency application
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Diamond amplifier fabrication
•Single crystal CVD diamond (Element 6); X ray topography.
•Surface cleanThe diamond sample was cleaned ultrasonically in acetone and then in alcohol for 15 minutes.
•Metal Pt SputteringSputter 30nm Pt layer
•HydrogenationGenerate NEA surface,determine the high gain of diamond
(110) X ray topography
primary metal
e h
E EG
E −
−=
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Hydrogenation
1E-06
1E-05
0.0001
0.001
0.01
0.1190 200 210 220 230 240 250 260 270 280
QE
Wavelength(nm)
1E-06
1E-05
0.0001
0.001
0.01
0.1
190 200 210 220 230 240 250 260 270 280
QE
wavelength(nm)
1. Heat to 800 ⁰C for 30 minute.2. Cool down and measure background
emission spectrum, then heat to 800 ⁰C.3. Expose to hydrogen atoms with10-6 torr.4. Monitor the photocurrent increase once
turn off the heater.5. After the photocurrent reached its peak,
Turned off the hydrogen atoms6. Measured the emission spectrum after
sample cool down.
After hydrogenation
Before hydrogenation
Good sample
Contaminated sample
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Hydrogenation optimization
High temperature hydrogenation decay
RM temperature hydrogenation decay
•Slow decay after high temperature Hydrogenation (4.8 hrs)•Twin decay after RM temperature Hydrogenation (0.25hr,4.8hrs)•Slow decay be recovered by baking ,but not in quick decay
RM temperature hydrogenation
0
5
10
15
20
25
0 10 20 30 40 50
Phot
ocur
rent
(nA)
Minutes
High temperature hydrogenationHydrogenation stop
0
5
10
15
20
200 300 400 500 600 700 800
Phot
ocur
rent
(nA)
Temperature(⁰C)
Optimized temperature
~200
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Lifetime of diamond amplifier
12.5
13
13.5
14
14.5
15
15.5
16
0 2 4 6 8 10 12 14
Phot
ocur
rent
(nA)
Hours
Decay current
Lamp off/HV on
Lamp on/HV off
Off axis lamp on/HV on
Lamp off/HV off
Total 18%
Residual gas 1.9%
UV light 9.8%
Bias 0.7%
Ion back bombardment(emission)
4.2%
Ion back bombardment(UV)
1.4%•Hydrogenated diamond exposed to atmosphere for 30 minute, then gain drop 5%•New diamond amplifier gain is 135•One month later, gain still above 90•Exposed to atmosphere four months, gain 130 after re-hydrogenation
Hydrogen-termination is robust!
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Emission gain test
Hydrogenated surface Diamond
0- to 10-keV Electron beam
A
CCD camera
Phosphor ScreenFocusing Channel Pt metal coating
Anode with holes
H.V. pulse generator
Primary current
HV pulse
Secondary current
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Secondary electron beam
Frequency:1000Hz Pulse voltage:3000VPulse width:10usCurrent:169nA
After focusing <500um
The highest bunch charge: 500 pC @1 mm2
Current density:70 μA/mm2
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Gain measurement
0
20
40
60
80
100
120
140
160
0 0.5 1 1.5 2 2.5 3
Gain
Gradient(MV/m)
0.2us
0.6us
1us
2us
4us
6us
10us
20us
50us
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50
Gain
Pulse width(us)
500V
1000V
1500V
0.0 0.5 1.0 1.5 2.0 2.5MV�m50
100
150
200
250
300Gain
•Diamond biased to negative voltage, thermionic gun is -10 kV. Primary electrons energy drop when the voltage of diamond increase, so gain drop•The gain drop due to trapping electron accumulate on the surface when the pulse width increase•Without the surface trapping,ideally gain is 280
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Secondary emission uniformity
•Emission diameter 2.8 mm•Maximum gain178•Center gain143•Edge gain increase due to thin metal layer
A
A’
B
B’
A-A’
B-B’
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Reproducible
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Emission probability
Diamond Vacuum
Metal coating Hydrogenated surface Anode
Ei
Ld
Ee
Lg
Ip Ii(t) Ie(t)Is it
p
IGI
=
Unmeasureable:P=Ie/Ii Measureable when pulse width tend to infinitely small:P=Ia/(Ii ∙w∙ f ) =Ge/Gt
0( )
1/
w
ea
I t dtI
f= Ge =
Ia
I p
1f ⋅ w
Defined a parameter independent on primary beam, metal coating:Emission probability
Average emission gain:
Transmission gain:
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Effective negative electron affinity
No barrier on the surface, totally emission
Barrier on the surface, some electrons be trapped
The hydrogenated diamond (100) surface is effective NEA
Effective NEA True NEA
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Schottky effect on diamond surface
Vc
Vv
φ1φmφs
Diamond Vacuum
_ 00.0318 ( / )[ ]sch dia E MV m eVφ =
2
2( )
2
2( )
2
m
ef x
ϕσ
πσ
−−
=
11 ( )2 2
m sP Erfc ϕ ϕσ
+= −
Schottky effect:
Distribution:
Emission probability:
Energy spread
Gauss peak to vacuum level
•Prepared four nearly identical samples and measured the emission probability• Obtain σ and φm by data fitting•Energy spread estimated 0.12±0.01eV
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Gain depend on the pulse width
0
1( ) (1 ) ( , ( ) )te
i p t p ir
EE t P I G E t d tSε ε
= − − × ×× E
3000V
5000V
3000V
5000V
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Summary & prospective
1. Hydrogen terminated diamond was demonstrated toamplify the electron beam from traditional cathode.
2. The standard procedure was developed after systemically hydrogenation study. The high quality diamond amplifier is reproducible now.
3. The highest measured gain was 200.4. Developed an emission model on the effective NEA
surface and the energy spread was estimated.5. The emission gain decrease when the HV pulse
width increase. This phenomena is explained well by the surface trapping model.
6. The 112MHz SRF gun for diamond amplifier test is under development.
7. Thermal emittance measurement is under going.
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Acknowledgements
Special thanks to the diamond amplifier group:
Ilan Ben-zviTriveni Rao
Sergey BelomestnykhXiangyun Chang
Dimitre .A. DimitrovQiong Wu
John smedleyErik MullerTianmu Xin
Mengjia Gaowei
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Thanks for your attention!