1. Thin Films Applied To Superconducting RF 1 VACUUM ARC DEPOSITION IN INTERIOR CAVITIES Physical and Engineering Principles and Ideas for Interior Implementations Raymond L. Boxman Electrical Discharge and Plasma Laboratory School of Electrical Engineering Tel-Aviv University

2. Thin Films Applied To Superconducting RF 2 Background and Objectives Vacuum Arc Deposition –(a.k.a. cathode arc deposition, arc evaporation) –Most popular method for applying hard coatings in tool industry –…but less well known than other PVD (e.g. sputtering, e-beam evaporation) and CVD methods Objectives of this lecture: –Review: Physics of vacuum arc Engineering issues in vacuum arc deposition –Suggest implementations with interior cavity

3. Thin Films Applied To Superconducting RF 3 Outline I. Physics of the Vacuum Arc –The Arc Discharge –Cathode Spots and Cathode Spot Plasma Jets Observations Theory –Macroparticles II. Vacuum Arc Engineering –Arc Ignition –Cathode Spot Confinement and Motion –Heat Removal –Macroparticle Control III. Suggestions for Coating Interior Cavities

4. Thin Films Applied To Superconducting RF 4 I. Physics of the Vacuum Arc – The Arc Discharge D.C. Discharges –Corona High V, Low I At sharp point –Glow Discharge V ~ 100’s V, I ~mA’s Cathode fall 150-550 V, depends on gas and cathode material –Arc 10’s of volts, A-kA Cathode spots

5. Thin Films Applied To Superconducting RF 5 Difference between Glow and Arc – cathode electron emission process Glow ‘individual’ secondary emission of electrons by: –Ions (depends on ionization energy, not kinetic energy) –Excited Atoms –Photons Not enough! –Multiplication in avalanche near cathode –Need high cathode drop (100’s of V’s) –Used in sputtering to accelerate bombarding ions into ‘target’ cathode Arc Collective electron emission –Current at cathode concentrated into cathode spots –Combination of thermionic and field emission of electrons –Can get sufficient electron current –Low cathode voltage drop (10’s of V’s) –High temp. in cathode spot gives high local evaporation rate – used in vacuum arc deposition

6. Thin Films Applied To Superconducting RF 6 Cathode Spot Characteristics Diam:  m’s Lifetime: ns’s to  s’s –Extinguish, reignite at adjacent location –Apparent ‘random walk’ motion –In B field, “retrograde motion” in -J  B direction

7. Thin Films Applied To Superconducting RF 7 Cathode Spot Plasma Jets ~Fully Ionized –Multiple ionizations common Z av (Ti) ~2 Ion directed kinetic energy 20-150 eV –Flow velocity ~10 4 m/s ~cos  distribution T i, T e ~few eV Supersonic ions, thermal electrons I i  -0.1 I arc, I e  1.1 I arc

8. Thin Films Applied To Superconducting RF 8 Cathode Spot Theory Two Approaches: –Quasi-continuous (~steady state) –Explosive Emission Quasi-continuous approach: –Must account simultaneously for: Cathode heating (for e - and atom emission) Electron emission Atom emission High ion energy / plasma velocity

9. Thin Films Applied To Superconducting RF 9 Beilis Model: Cathode Spot & Cathode Plasma Jet Cathode Cathode Spot Region Hydrodynamic Plasma Flow SHEATH Electron relaxation zone. Ion diffusion Kinetic flow Knudsen Layer Plasma Jet Expansion  Acceleration Region e  i e  a

10. Thin Films Applied To Superconducting RF 10 Beilis Model TF emission of electrons Evaporation of atoms Acceleration of electrons into vapor –Collisionless sheath –Collisionless Knudsen layer –Electrons loose energy to vapor in relaxation zone

11. Thin Films Applied To Superconducting RF 11 Beilis Model – cont ’ d Back-flow of electron and ions to cathode –Heats cathode spot Joule heating under cathode surface Joule heating of plasma Hydrodynamic plasma expansion

12. Thin Films Applied To Superconducting RF 12 Beilis Model – Hydrodynamic Plasma Expansion Like in jet engine – conversion of thermal  directed kinetic energy But plasma heated all along length –Continuous heating, conversion into kinetic energy, so T i ~3ev, E i ~20-150eV

13. Thin Films Applied To Superconducting RF 13 Explosive Electron Emission (Mesyats et al.) Cathode spot is a sequence of explosion of protuberances

14. Thin Films Applied To Superconducting RF 14 EEE (Mesyats et al.) – cont ’ d Each explosion creates further protuberances, which can then explode Idea supported by high resolution laser shadowgraphs, showing short life time and small dimensions, spike noise in ion current, etc.

15. Thin Films Applied To Superconducting RF 15 Macroparticles

16. Thin Films Applied To Superconducting RF 16 Macroparticles Spray of liquid metal droplets from the cathode spot small fraction of cathode erosion for refractory metals large fraction of cathode erosion for low melting point cathode materials exponentially decreasing size distribution function most mass in the 10-20  m diam range preferentially emitted close to cathode plane Downward pressure from plasma jet on liquid surface

17. Thin Films Applied To Superconducting RF 17 II. Vacuum Arc Engineering Coating forms on any substrate intercepting part of plasma jet In vacuum, coating composition  cathode composition In reactive gas background, can form compounds (nitrides, oxides, carbides, etc.)

18. Thin Films Applied To Superconducting RF 18 II. Vacuum Arc Engineering Arc Ignition Cathode Spot Confinement and Motion Heat Removal Macroparticle Control

19. Thin Films Applied To Superconducting RF 19 Arc Ignition Problem: extremely high voltage needed to break-down vacuum gap (~100 kV/cm) Drawn-arc (most common) –Trigger electrode, mechanically operated –Connected to +voltage (e.g. anode via current limiting resistor) –Momentary contact with cathode –Arc ignited when contact broken Current transfers to main anode Breakdown to trigger electrode –Vacuum gap –Sliding spark Laser ignition

20. Thin Films Applied To Superconducting RF 20 Controlling Cathode Spot Location and Motion Objectives: –Locate CS’s on ‘front’ surface of cathode Maximize plasma transmission to substrates Prevent damage to cathode structure –Methods: Magnetic Field (retrograde and “acute angle” motion Passive border Strellnitski shield Pulsed arc

21. Thin Films Applied To Superconducting RF 21 Magnetic Control of Cathode Spots

22. Thin Films Applied To Superconducting RF 22 Passive Border

23. Thin Films Applied To Superconducting RF 23 Strelnitski Shield

24. Thin Films Applied To Superconducting RF 24 Pulse Control Basic Idea: arc duration shorter than CS travel time to edge –Short Pulse –Laser Ignition –Long Pulse - Long Cathode –Active detection of CS location – quench arc when CS reaches edge

25. Thin Films Applied To Superconducting RF 25 Heat Removal Total power P = V arc I arc –V arc ~20-40 V –I arc ~ 50-1000 A –P > 1 kW Distribution –~1/3 in cathode –~2/3 in anode –Substrate:

26. Thin Films Applied To Superconducting RF 26 Heat Removal from Cathode Cool cathode important to –minimize MP generation –Prevent cathode damage In best case, C.S.’s rapidly moved around to give on average a uniform heat flux on cathode surface S=P/A

27. Thin Films Applied To Superconducting RF 27 Heat Removal from Cathode, cont ’ d Then average surface Temp (far from C.S.) given by h c – contact heat transfer coefficient h w – heat transfer coefficient to water

28. Thin Films Applied To Superconducting RF 28

29. Thin Films Applied To Superconducting RF 29 Substrate Temperature Control T s critical in determining coating properties Measure with IR radiation detector T s determined by balance between heating and cooling processes Often use heat flux from process to control T s –Vary bias or arc current

30. Thin Films Applied To Superconducting RF 30 Macroparticle Control 3 Approaches –Ignore Get good results (e.g. with tool coatings) despite (or because of?) MPs –Minimize MP Production/Transmission –Remove MPs

31. Thin Films Applied To Superconducting RF 31 Minimize MP Production/Transmission Choose refractory cathode material –“Poison” (i.e. nitride) cathode surface Operate at ‘higher’ N 2 background pressure Low cathode temperature –direct cooling –lower current (  lower deposition rate) Place substrates where plasma flux max, MP flux min

32. Thin Films Applied To Superconducting RF 32

33. Thin Films Applied To Superconducting RF 33 Macroparticle Removal Filtered Vacuum Arc Deposition –Use magnetic field to bend plasma beam around an obstacle which blocks MP transmission

34. Thin Films Applied To Superconducting RF 34 VENETIAN BLIND

35. Thin Films Applied To Superconducting RF 35 Two quarter-torus filtered arcs at Tel Aviv University

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38. Thin Films Applied To Superconducting RF 38 Filtered Arc – Advantages and Disadvantages Advantages –High quality, very smooth coatings –‘almost’ MP free –Can achieve higher deposition rate than other ‘high quality’ techniques Disadvantages –Usually poor plasma transmission Material utilization efficiency low –Much slower than unfiltered arc deposition –Bulky equipment

39. Thin Films Applied To Superconducting RF 39 Other Arc Modes Hot Anode Vacuum Arc –Crucible anode Hot Refractory Anode Vacuum

40. Thin Films Applied To Superconducting RF 40 10  m

41. Thin Films Applied To Superconducting RF 41 III. How can we coat the inside of:

42. Thin Films Applied To Superconducting RF 42 Approach 1: Ignore MPs

43. Thin Films Applied To Superconducting RF 43 Approach 1: Ignore MPs Cavity serves as vacuum chamber and anode Various techniques for magnetically controlling c.s. motion

44. Thin Films Applied To Superconducting RF 44 Approach 2: Miniature Filter: Example – Welty Rect. Filter

45. Thin Films Applied To Superconducting RF 45 Approach 2: Miniature Filter: Another Example Progress in Use of Ultra-High Vacuum Cathodic Arcs for Deposition of Thin Film Superconducting LayersProgress in Use of Ultra-High Vacuum Cathodic Arcs for Deposition of Thin Film Superconducting Layers J.Langner, M.J.Sadowski, P.Strzyzewski, R.Mirowski, J.Witkowski, S.Tazzari, L.Catani, A.Cianchi, J.Lorkiewicz, R.Russo, T.Paryjczak, J.Rogowski, J.Sekutowicz Presentation 28 Sept at XXXIII-ISDEIV in Matsue, Japan Showed use of a cylindrical “Venetian Blind” filter to deposit Nb inside cavity!

46. Thin Films Applied To Superconducting RF 46 Approach III. Beilis “ black-body ” HRAVA deposition device

47. Thin Films Applied To Superconducting RF 47 Interior Coatings - Considerations Use cavity as vacuum chamber –Need complicated end seal to allow for electrical connections (main arc and trigger), cooling water, in some cases motion –Cooling can be applied directly to outside of tube Fitting everything into cavity – difficult! Integrity, lifetime? Triggering – not shown

48. Thin Films Applied To Superconducting RF 48 Summary and Conclusions VAD uses inherent properties of cathode spot plasma jets to rapidly deposit dense, high quality coatings Successful implementation requires “plasma engineering” to: –Confine cathode spots on desired surface –Remove process heat –Control macroparticle contamination

49. Thin Films Applied To Superconducting RF 49 Summary and Conclusions, cont ’ d Several approaches exist for efficiently and rapidly coating interior of RF cavities –But with technical difficulties