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

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2000

2500

3000

2400V

2050V

1900V

Series7

Time [Hour]

Cu

rren

t [A

]

Bia

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V]