1. Field Emission Measurements with CERN DC-Spark System Tomoko MURANAKA Hebrew University / CERN

2. Outline 28/01/20152  Experimental setup  Previous measurements  Improvements  Ongoing work – Emission fluctuation measurement  Summary & Outlook T.Muranaka, CLIC Workshop

3. Experimental setup at CERN – DC Spark System-II 28/01/20153T.Muranaka, CLIC Workshop Anode tip Cathode sample HV supply Current cable Step motor 3-axis table Cathode: Cu plate, treated as CLIC ACS Anode: Cu tip, Φ 2mm Current measurement: pA – μA Voltage supply: 0V – 12kV, ΔV min ~ 10V Gap setting: 10-60μ m, ±1 μ m accuracy Position setting: By eye, through a viewport

4. Previous measurements with the DC system in Uppsala 28/01/20154  Measured emission did not always follow Fowler–Nordheim.  Local activities?  Surface condition? T.Muranaka, CLIC Workshop Current vs Time Current vs Voltage

5. Improvements at CERN DC system 28/01/2015T.Muranaka, CLIC Workshop5  Reliable field emission measurement  Preparing surface with breakdowns  High resolution current vs time measurement  Add external circuit with an oscilloscope

6. Surface Stabilization 28/01/20156  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 1 2 3

7. 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/mDischarge at 140 MV/m First stable EFE at 240 MV/m o 14 emission sites (including discharge) at E act = 300 MV/m →Emitter number density : 56 cm -2 o EFE free region in the scanned area at E = 300 MV/m o Activation field E act > onset field E on E act = 260 MV/m, E on =128 MV/m Slide from S. Lagotzky, Wuppertal Univ.

8. Stable field emission measurements 28/01/2015T.Muranaka, CLIC Workshop8 Current vs E-Field F-N Plot

9. Improvements at CERN DC system 28/01/2015T.Muranaka, CLIC Workshop9  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.

10. 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

11. What are we looking for Model predicts strong fluctuations in observed current as the critical point is approached Current as measured in an uncontrolled DC gap setup Simulated 2 nd and 3 rd moment protrusion size and distribution vs driving force Proof of concept? Uncontrolled gap, low I resolution, Slide from Y. Ashkenazy

12. Current vs Time measurements 28/01/201512  Current vs time measurement  Additional external circuit with an oscilloscope. (based on M. Barnes and I. Profatilova’s work) T.Muranaka, CLIC Workshop

13. 750V (Preliminary) Current fluctuation measurements 28/01/201513 800V 850V T.Muranaka, CLIC Workshop  At higher field, higher average current & higher fluctuation were observed.  Still need to improve current resolution.

14. Summary 28/01/201514  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 Workshop

15. Outlook 28/01/2015T.Muranaka, CLIC Workshop15  Higher resolution current vs time measurements.  Microscopy on pre-breakdown surface (after certain emission without a breakdown).

16. 28/01/2015T.Muranaka, CLIC Workshop16