Lecture 10
May 19, 2004
Fabrication, Processing and Vacuum Techniques
Erik Jongewaard
Stanford Linear Accelerator Center enj@slac.stanford.edu
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• Vacuum
– A discussion of the vacuum requirements of microwave tubes
• Materials
– Common materials used in microwave tubes and the reasons for their selection
• Fabrication Techniques
– Fabrication techniques for individual parts and assemblies made from these parts
• Processing
– Vacuum processing of the completed microwave tube
2

• Pressure measured in units of Torr
(equivalent to mm Hg, 760 Torr = 760 mm
Hg = 1 atm)
• High vacuum: ~10 -3 to ~10 -8 torr
• Ultrahigh vacuum: < 10 -8 torr
3

• When the mean free path is of the order of the size of the vacuum enclosure collisions with the enclosure walls dominate, this is called the molecular flow regime.
• For a klystron with a 1 cm drift tube molecular flow occurs at pressures below ~5x10 -3 torr.
• At 10 -9 torr the mean free path is ~50 km
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• The interior space of a microwave tube must have a low enough density of gas molecules to allow free passage of electrons
• Reliable cathode emission requires low partial pressures of cathode “poisons”
• Higher gradients (DC and RF) are supported at lower pressures
5

• Proper selection of materials
– Use low vapor pressure materials
– Use clean materials
• Use clean forming and joining techniques
• Chemical cleaning
– Removes surface contamination (cutting fluids, surface oxides, etc.)
• High temperature processing, vacuum bakeout
– Drives off adsorbed and absorbed contaminants and gasses
• Appendage pumping
– Maintains low pressure during periods of tube outgassing
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• Microwave tube materials must be vacuum compatible
– Low vapor pressure at operating and bakeout temperatures
– Few inclusions or stringers that can lead to real or virtual vacuum leaks
• Materials must meet tube operational requirements
– Electrical, thermal conductivity
– Expansion match adjacent materials
– Strength
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(A representative, not exhaustive list)
• OFE Cu, 99.99% pure, oxygen-free copper
– High thermal and electrical conductivity
– Brazes easily
– Very low strength when annealed
– Used extensively for RF and high heat flux surfaces
• Cupronickel, 70% Cu, 30% Ni alloy
– Brazes and welds easily with proper chemistry
– Moderate strength
– Used primarily for weld flanges and compliant members brazed to ceramics
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• Monel 404, ~45% Cu, ~55% Ni alloy
– Acceptable brazability
– Moderate strength
– Thermal expansion between copper and iron
– Used for spacers and cavities in PPM structures
• Austenitic Stainless Steel (primarily 304L), 8-
12% Ni, 18-20% Cr, balance Fe alloy
– Brazes and welds easily
– High strength
– Poor thermal and electrical conductivity
– Used extensively for tube structural components
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• Core Iron, pure Fe
– Brazes and welds well
– Magnetically soft
– Non-monotonic thermal expansion
– Used for magnetic field shaping components
• Ferritic Stainless Steel (430), 16-18% Cr, balance Fe alloy
– High strength
– Low thermal and electrical conductivity
– High microwave loss
– Used for loss cavities and loads
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• Molybdenum
– Refractory high strength material
– Acceptable brazing
– Used for cathode heaters, support structures and components subject to pulsed heating loads
• Tungsten
– Refractory high strength material
– Primarily used for dispenser cathode matrix
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• Braze Materials
– Copper
• Least costly
• Excellent wetting of stainless steels
• Low vapor pressure
– Gold-copper alloys
• Costly, dependent upon gold fraction
• Alloyed with other materials to tailor properties
• Wide range of melting temperatures allows step brazing
• Low vapor pressure
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• Braze Materials
– Copper-silver alloys
• Less costly than gold-copper alloys
• Lower melting temperature ranges than goldcopper alloys
• high vapor pressure, can be a problem for high temperature bakeout at low pressures
• Not used in the klystron department
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• Aluminum Oxide, Al
2
O
3
– Hard, strong dielectric
– Transparent (mostly) to microwaves
– Used for HV seals, insulators, and RF windows
• Titanium Nitride, TiN
– Low secondary electron yield
– Deposited by reactive sputtering or evaporation
– Mutipactor and avalanche breakdown suppressor
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• Usually the desired function makes the material selection obvious
• Sometimes the tradeoffs are more subtle and an optimal selection of material must be arrived at through the use of a “figure of merit”
15

OFE Copper
Porous Tungsten
317L Stainless Steel
304L VAR Stainless
Steel
304L Stainless Steel
Cupronickel
Moly-Manganese
Ceramic Metalization
Aluminum Oxide
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OFE Copper
304L Stainless Steel
Core Iron
404 Monel
Gold-Copper Braze Alloy
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1. Raw material testing and certification
• Materials are tested for standards conformance.
2. Part fabrication
• Manufacture part using appropriate forming technique
3. Inspection
• 100% inspection of critical components
4. Cleaning/plating
• Parts chemically cleaned and etched to remove surface contamination and oxides
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5. Pre-braze assembly
• Parts are assembled with appropriate braze alloy using gloves in a clean environment to prevent contamination
6. Braze
• Assemblies are brazed in dry or wet H
2 according to materials used atmosphere
7. He leak check
• Verify hermeticity of assembly before proceeding with further assembly
8. Post-braze machining as required
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• Conventional machining
– Can be used on the widest variety of materials
– Most flexibility of part size and shape
– Can easily make vacuum compatible parts with proper control of surface finish and cutting fluids
– Most klystron parts are formed this way
• EDM (Electrical discharge machining)
– Good for hard to machine materials and difficult geometries
– Requires special cleaning/polishing to remove surface contamination
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• Hydroforming
– Relatively inexpensive tooling
– Good for thin walled axisymmetric parts such as weld flanges and bowl shapes such as UHF cavities
• Forging
– Used to make near net shape parts for subsequent machining
– Cross forging used to reduce size of inclusions in stainless steels
• Electroforming
– Can form complex geometries in one piece
– Difficult to get low oxygen content in copper deposits
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• Vacuum considerations may dictate the use of nonstandard parts
For example: rolled screw threads may have trapped contaminants from the rolling operation, use cut screw threads in critical locations such as the electron gun.
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23

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1. Degrease
• Removes surface oils leftover from fabrication
2. Alkaline rise
• Heavy duty detergent, removes more surface grime
3. Acid etch (one or more steps)
• Removes surface oxide film and varying amounts of base metal
4. Rinses (several steps of water, solvent rinses)
• Removes traces of etching solution and leaves surface chemically clean
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• Furnace Brazing
– Clean, vacuum compatible
– Choice of atmosphere (wet H
2 materials to be brazed
, dry H
2
, vacuum) dependent upon
– Used for most tube subassemblies
• Diffusion bonding
– Used where dimensional control is critical such as x-band accelerator structure assembly
• Welding
– GTAW (Gas tungsten arc welding)
• Used to join subassemblies (CuNi weld flange)
• Used to tack weld parts for braze
– RSW (Resistance spot welding)
• Used to used for attaching contact tabs and other non-critical joints
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Wet
Dry
Cu braze
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28

29

30

31

32

33

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• Since the gun is both the hottest part of an operating klystron and the most sensitive to pressure it requires special processing:
– Vacuum fire all gun components, typically at
800 °C
– Assemble gun stem and cathode and place in bell jar
– Raise temperature of cathode and outgass gun components
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36

1.
RF on, raise temp to 980 °C, keep pressure < 10 -6 torr
2.
Stabilize at 980 °C until pressure drops < 5x10 -8 torr
3.
Raise temp to 1030 °C, hold until pressure slope flattens
4.
Lower temp to 980 °C, hold until pressure slope flattens
5.
RF off, maintain 980 °C until pressure slope flattens
6.
Raise temp to 1030 °C, wait 1 hour after pressure peak, cool
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38

39

40

1.
Attach and start pumping tube
2.
Close and pump down vessel, ramp filament 2 A/hr to
12 A after tube pressure < 5x10 -6
3.
Once tube pressure < 3x10 -6 and vessel pressure
< 5x10 -5 start oven
4.
Ramp oven 15 °C/hr to 550°C as long as tube pressure
< 3x10 -5 and vessel pressure < 5x10 -5
5.
Ramp filament 1 A/hr to 17 A as long as tube pressure
< 3x10 -5
6.
Bake until pressure stabilizes
7.
Cool oven 15 °C/hr to 430 °C and wait 6 hours
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8.
Raise filament to 18 A providing tube pressure <
5x10 -8 hold until tube pressure < 10 -8 or stable
9.
Cool at 15 °C/hr until oven off, ramp filament down at 1
A/hr
10. RGA leak check
11. Remove vessel
12. Ramp filament 1 A/hr to 19 A hold until pressure drops
13. After 1 hour minimum at 19 A emission check at 1 kV
14. Cool
15. Pinch off
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Typical 5045 Bakeout
600
500
400
300
200
100
0
0
Tube temperature
Tube pressure
Vessel pressure
Window pressure
2 4 6
Time (Days)
8
1.00E-03
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
1.00E-09
10
1.00E-10
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600
500
400
300
200
100
0
0 2
75XP3-4A Bakeout
Tube temperature
Tube pressure
Vessel pressure
Window pressure
4 6 8 10
Time (Days)
12 14 16 18
1.00E-03
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
1.00E-09
20
1.00E-10
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45

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[3]
[4]
[1]
[2]
[5]
[6]
American Welding Society Committee on Brazing and Soldering, Brazing
Handbook, American Welding Society, 1991.
Dushman, S., Scientific Foundations of Vacuum Technique, 2nd ed., J. M. Lafferty, Ed., John
Wiley & Sons, Inc., 1962.
Kohl, Walter H., Handbook of Materials and Techniques for Vacuum Devices, American
Institute of Physics, 1995.
Rosebury, Fred, Handbook of Electron Tube and Vacuum Techniques, American Institute of
Physics, 1993.
Thornburg, D. L., Thall, E. S., Brous, Dr. J., A Manual of Materials for Microwave Tubes,
WADD Technical Report 60-325, Radio Corporation of America, 1961.
Varian Vacuum Products Division, Basic vacuum Practice, Varian Associates, Inc., 1986.
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Thanks to the following for help in preparing this talk
Chris Pearson
Chuck Yoneda
John Van Pelt
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