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Field Emission Measurements with CERN DC-Spark System
Tomoko MURANAKA
Hebrew University / CERN
T.Muranaka, CLIC Workshop2
Outline
28/01/2015
Experimental setup Previous measurements Improvements Ongoing work – Emission fluctuation
measurement Summary & Outlook
T.Muranaka, CLIC Workshop3
Experimental setup at CERN– DC Spark System-II
28/01/2015
Anode tip
Cathode sample
HV supply
Currentcable
Step motor
3-axis table
Cathode: Cu plate, treated as CLIC ACSAnode: Cu tip, Φ2mm Current measurement: pA – μAVoltage supply: 0V – 12kV, ΔVmin ~ 10VGap setting: 10-60μm, ±1 μm accuracyPosition setting: By eye, through a viewport
T.Muranaka, CLIC Workshop4
Previous measurementswith the DC system in Uppsala
28/01/2015
Measured emission did not always follow Fowler–Nordheim. Local activities? Surface condition?
Current vs TimeCurrent vs Voltage
T.Muranaka, CLIC Workshop5
Improvements at CERN DC system
28/01/2015
Reliable field emission measurement Preparing surface with breakdowns
High resolution current vs time measurement Add external circuit with an oscilloscope
6
Surface Stabilization
28/01/2015
Stabilized surface => Breakdown field and Electron emission behavior are reproducible.
Stabilization was completed by over 20 breakdowns.
Probably due to oxidation layer removal. Consistent with “surface activation”Reported by S. Lagotzky
T.Muranaka, CLIC Workshop
0 500 1000 1500 2000 2500 3000 3500 40000
2E-10
4E-10
6E-10
8E-10
0.000000001
0.00000000121 23
Bias Voltage [V]
Cu
rre
nt
[A]
6 8 10 12 14 16 18 20 22 240
500
1000
1500
2000
2500
3000
# BD
Bia
s V
olt
age [
V]
1
23
EFE RESULTS AFTER DIC ON DT + SLAC SAMPLE (17E)o Field maps between 120 - 300 MV/m, 20 (10) MV/m steps for E < (>) 200 MV/m
o Scanned area: 5x5 mm², truncated cone anode (W, Ø= 300 µm), step size = 150 µm, Δz = 25 µm (E ≥ 240 MV/m), 40 µm (200 – 240 MV/m) or 50 µm (E < 200 MV/m)
No EFE at 120 MV/m Discharge at 140 MV/m First stable EFE at 240 MV/m
o 14 emission sites (including discharge) at Eact = 300 MV/m→ Emitter number density : 56 cm-2
o EFE free region in the scanned area at E = 300 MV/mo Activation field Eact > onset field Eon
• Eact = 260 MV/m, Eon =128 MV/m
Slide from S. Lagotzky, Wuppertal Univ.
T.Muranaka, CLIC Workshop8
Stable field emission measurements
28/01/2015
30 40 50 60 70 80 900.0E+00
2.0E-07
4.0E-07
6.0E-07
8.0E-07
1.0E-06
1.2E-06
Run1Run2Run3
Electric Field [MV/m]
Curr
ent
[A]
0.01 0.015 0.02 0.025 0.03 0.035-36
-34
-32
-30
-28
-26
-24
-22Run1
Run2
1/E
ln(I
/E**
2)
Current vs E-Field F-N Plot
T.Muranaka, CLIC Workshop9
Improvements at CERN DC system
28/01/2015
Stable field emission measurement Preparing surface with breakdowns <-Done!
High resolution current vs time measurement Add external circuit with an oscilloscope
Validate & Calibrate “Stochastic plastic model for breakdown formation” suggested by Y. Ashkenazy.
Model
• Stochastic plastic model for breakdown formation:o BD caused by localized protrusions. These are formed due to
dislocation activity within the sample resulting in protrusion growth.o The stochastic model, describes dislocation evolution leading to critical
protrusion formation.o The sub breakdown population can be characterized through dark
currents.o As it approaches the critical point – protrusion population increases
leading to larger fluctuation in dark currents
Surface protrusion as observed in the Field emission area of the DC sample.
Slide from Y. Ashkenazy
What are we looking for• Model predicts strong fluctuations in observed
current as the critical point is approachedCurrent as measured in an uncontrolled DC gap setup
Simulated 2nd and 3rd moment protrusion size and distribution vs driving force
Proof of concept?Uncontrolled gap, low I resolution,
Slide from Y. Ashkenazy
T.Muranaka, CLIC Workshop12
Current vs Time measurements
28/01/2015
Current vs time measurement Additional external circuit with an oscilloscope.
(based on M. Barnes and I. Profatilova’s work)
0 100 200 300 400 500 600 700 800 900 10001.0E-12
1.0E-11
1.0E-10
1.0E-09
1.0E-08
1.0E-07
1.0E-06
1.0E-050807_R10807_R20807_R30807_R40807_R5P2S40815_P2S5_up
Voltage on the gap [V]
Curr
ent
[A]
T.Muranaka, CLIC Workshop13
750V
(Preliminary)Current fluctuation measurements
28/01/2015
800V
850V
At higher field, higher average current & higher fluctuation were observed.
Still need to improve current resolution.
T.Muranaka, CLIC Workshop14
Summary
28/01/2015
Technical improvements to measure reliable electric field emission were produced in the CERN DC-Spark system. Over 20 breakdowns on a spot stabilized the
surface. Reliable current vs field measurements were performed.
Current vs time measurement circuit was developed. Overall current levels were consistent with I-V measurements. Improvements on current accuracy are ongoing to validate the stochastic plastic model for breakdown formation.
T.Muranaka, CLIC Workshop15
Outlook
28/01/2015
Higher resolution current vs time measurements.
Microscopy on pre-breakdown surface (after certain emission without a breakdown).
T.Muranaka, CLIC Workshop16 28/01/2015
0 10 20 30 40 50 601.00E-08
4.10E-07
8.10E-07
1.21E-06
0
500
1000
1500
2000
2500
3000
2400V
2050V
1900V
Series7
Time [Hour]
Cu
rren
t [A
]
Bia
s v
olt
ag
e [
V]