1. DC Spark Test System for CLIC
Trond Ramsvik
TS / MME
10 June 05

2. Outline Introduction Experimental Setup Field Emission Breakdown Field
Motivation, Origin of Breakdown, Materials
Experimental Setup
Field Emission
Breakdown Field
Local Field
Automatic Spark Conditioning
Effect of Residual Gas
Further work
Conclusions
Introduction
Motivation, Origin of Breakdown, Materials
Experimental Setup
Field Emission
Breakdown Field
Local Field
Automatic Spark Conditioning
Effect of Residual Gas
Further work
Conclusions
10 June 05

3. Introduction: Why DC spark experiments?
With a DC spark-test system, materials can be subjected to high electric fields, and their properties and responds to different treatments can be examined relatively easy and quickly.
Goal: To find materials that withstand the highest field without breakdown or have low level of deterioration even when breakdown events occur.
Need to understand the details of the breakdown phenomena
10 June 05

4. Introduction: Origin of Breakdown
The physics of breakdown is still not perfectly understood
Commonly accepted that breakdown at small gaps (d ≤ 0.5 mm) is initiated by an electron field emission based mechanism from one or a few microprotrusions*
Process towards breakdown. Suggested Models:
“Anode-initiated”
electron bombardment of anode
release of gas and/or anode material through intense localized heating or electron stimulated desorption (ESD)
Avalanche ionization of the released species
“Cathode-initiated”
The microprotrusion on the cathode becomes unstable
ohmic heating from high field emission current density
fracture of the surface due to the tensile stress produced by the electric field
Exchange
Avalanche of mutual secondary emission of ions, electrons between the electrodes
10 June 05
*High Voltage Vacuum Insulation, edited by R. V. Latham (Academic, New York, 1995).

5. Introduction: Choice of materials
Criteria:
low vapor pressure
high tensile strength
high melting point
high thermal conductivity
high electrical conductivity
Molybdenum & Tungsten
10 June 05

6. Breakdown Measurements
Experimental Setup
Sphere / Plane geometry
HV supply
0 to + 12 kV
UHV
Sample
Tip
Q-meter
Scope C
Switch
Breakdown Measurements
HV supply
0 to + 12 kV
UHV
Sample
Tip
A-meter
Field Emission Measurements
10 June 05

7. Field Emission Fowler – Nordheim plot: 2r r h Ex ∆
V(x) ~ 3 GV/m
M ~ 4.5 eV F B
Metal
Vacuum e-
e-
0 Å
10 Å
5 Å  2r e h E r
local
Fowler – Nordheim plot:
R. A. Millikan and C. C. Lauritsen, Proc. Nat. Acad. Sci. (US), 14, (1928)
R. H. Fowler and L. Nordheim, Proc. Roy. Soc. A 119 (1928) 173.
10 June 05

8. Molybdenum Tip and Sample
Fowler – Nordheim plot
Enhancement factor:
Molybdenum Tip and Sample
Emission Radius:
10 June 05

9. Enhancement Factor (β)
Results from Fowler – Nordheim plots
Molybdenum Tip and Sample
Enhancement Factor (β)
Emission Radius (re) [nm]
Before bake-out: β ~ (40 ± 10) re ~ (2 → 11) nm
After bake-out: β ~ (60 ± 10) re ~ (5 → 40) nm
High uncertainty
10 June 05

10. Molybdenum Anode and Cathode
Spark Cycle
The voltage over the gap is increased until breakdown occurs
Molybdenum Anode and Cathode
Gap :~ 20 m
10 June 05

11. Field evaporation ↔ Tensile Strength Measured tensile strength of Mo*:
Local Field
Field evaporation ↔ Tensile Strength
Elocal = β∙Eapplied
Measured tensile strength of Mo*:
750 MPa
Elocal ≈ 11 GV/m (max 21 GV/m)
before-mentioned local field, which is the value one should use when discussing the tolerance of the materials itself. For example, there have been claims that breakdown can be triggered by field evaporation, i. e. microparticles being torn off the material, thereby contributing to the plasma eventually leading to breakdown. For Molybdenum we found that the avarage local field is appr. 11 GV/m with maximum value reaching above 20 GV/m, and using this simple relation between the tensile stress and the field, one finds that it corresponds to around 520 MPa and nearly 2 GPa. The tensile strength of this particular sample has been measured here at MME to be around 750 MPa. So, the conclusion is that there is a possible that field evaporation can occur.
However, I will show that this cannot be the whole story behind the initialization of breakdown.
tensile ≈ 520 MPa (max ~1950 MPa)
10 June 05

12. Automatic Spark Conditioning
Single measurement up to breakdown
Results of Spark Conditioning
10 June 05

13. Automatic Spark Conditioning
Molybdenum (Mo) – Tip and Sample
Spark Scan
Histogram
10 June 05

14. Scanning Electron Microscopy
Molybdenum surface after ~ 1600 sparks
Average energy per spark: 0.8 J
14 m
1.3 mm
Depth Profile / Inside Spot
Outside spot
Inside spot
8 m
8 m
Local Melting smoothes out the surface
Number of micro-protrusions strongly reduced
10 June 05

15. Electro Discharge Machined (EDM)
Rolled / Chem. cleaned Mo
EDM Mo
10 June 05

16. Gas Experiments From literature:
Pressure effect on the breakdown field significant for large gaps
Breakdown field deteriorates at pressures typically above 10-2 mbar
R. Hackham and L. Altcheh, J. Appl. Phys., 46, (1975)
10 June 05

17. Clear effect on pressure also at high vacuum / small gap (!)
Gas Experiments
Baked system
Laboratory Air
Unbaked system
Dry Air
Clear effect on pressure also at high vacuum / small gap (!)
10 June 05

18. The saturated breakdown field is reduced by ~150 MV/m
Gas Experiments
From ~1×10-9 mbar to ~ 1×10-6 mbar
The saturated breakdown field is reduced by ~150 MV/m
Damaged tip?
500 MV/m
k ≈ 6000 MV/m (mbar)-1
n ≈ 1/4
10 June 05

19. Spark Conditioning vs. Pressure Rise
Mo Tip & Sample
Pav ~ 4×10-8 mbar
At start: Follows the amount of energy deposited over the gap
Slight decrease in pressure rise during spark
Pressure rise typically in the order of a few percentage per spark
10 June 05

20. Comparison Molybdenum - TungstenW:
10 June 05

21. Comparisons - 30 GHz measurements
420
400‡ Mo
340
350‡ W
260
170† Cu
Max. surface field in RF
[MV/m]
(DC) E
breakd
sat
OBS!
Not saturated!
DC and RF breakdown measurements give similar breakdown fields
Superior behavior of both Mo and W with respect to Cu.
†M. Kildemo, S. Calatroni and M. Taborelli, PRST-AB, 7, (2004)
10 June 05
‡Saturation breakdown field measured at (2-4)×10-8 mbar

22. Future Work Installing new sampleholder and manipulator
XYZ movements
E-beam heating → ~ 1000oC
“In-situ” annealing
several samples
more experiments to study the effect of residual gas on the Mo breakdown field
pressure rise studies at UHV (~1×10-9 mbar)
other materials: CuZr, Ti, Mo-Re alloys,W-films,…
10 June 05

23. Conclusions
Surface preparation techniques influence the conditioning speed
There is a residual gas effect on the breakdown field also for high vacuum and small gaps
From 1×10-9 mbar to 1×10-6 mbar a reduction in the saturated breakdown field of ~150 MV/m is seen
Molybdenum shows higher DC saturation breakdown field than Tungsten after extensive spark conditioning
Similar to the results found for RF
10 June 05

24. Gonzalo Arnau Izquierdo Ahmed Cherif Ana Sousa E Silva
Contributors
Morten Kildemo
Sergio Calatroni
Mauro Taborelli
Holger Neupert
Gonzalo Arnau Izquierdo
Ahmed Cherif
Ana Sousa E Silva
Alessandra Reginelli
10 June 05

25. Movement of electrons in RF
Phase of RF field
z [mm]
t [ps]
ω = 2π∙33×109 s-1
φ = π/2
q = 1.6×10-19 C
E0 = 330×106 MV/m
Me = 9.1×10-31 kg
φ = 0
z0 = 0
10 June 05

26. Depth profile / ca. 100 sparks
10 June 05

27. After ~1600 sparks and exposed to air
XPS
After ~1600 sparks and exposed to air
Carbon reduced
Oxygen unchanged
Molybdenum peaks stronger
10 June 05
Alessandra Reginelli TS/MME

28. Vickers ↔ Tensile Strength
C. Bourgés Monnier, “Propriétés du molybd è ne et des alliages à base de molybdene”, Ecole des mines, Paris
10 June 05

29. 10 June 05

30. Details of experimental setup
HV supply
0 to + 12 kV
HV switching electronics
Q-meter
UHV
Tip
Sample PC
A-meter
Scope
10 June 05