UHV Requirements for Accelerator and Synchrotron Radiation...

May 24th-26th MEDSI 2006, Himeji, Japan 1

UHV Requirements for Accelerator and UHV Requirements for Accelerator and Synchrotron Radiation Source EngineeringSynchrotron Radiation Source Engineering

E. TrakhtenbergE. TrakhtenbergAdvanced Photon SourceAdvanced Photon Source

Argonne National LaboratoryArgonne National Laboratory

USAUSA


First Approach to this ThemeFirst Approach to this Theme


The term “vacuum” refers to a given space filled with gas at a pressure below atmospheric. (American Vacuum Society-1958)

So, the difference between vacuum and high pressure is only in the sign.

But that is actually a very important difference:


100 liters volume

100 At pressure

100 liters volume

10-6 Torr vacuum

Leak 1 cm3 per hour

Pressure will change only 0.66•10- 5 %

Vacuum will change to ~10-2 Torr

After one hour

100 liters volume

100 At pressure

Leak 1 cm3 per hour


Vacuum UnitsMain units for pressure and vacuum

Is Pa (Pascal). 1Pa=N/M2

Other units:

Millibar (1 millibar = 100 Pa)

Torr (1 Torr = 133.3 Pa)

Lb/in2 (psi) (1 psi = 6894.7 Pa)


Vacuum system CalculationsVacuum system Calculations

Numerous software packages can be used to perform vacuum system calculationsSome companies can provide the complete service – see OXFORD DANFYSIK’s web page.

“GRAHAM ENGINEERING ANSWERS” is just one of several examples – www.graham-mfg.com

Their popular Vacworks Software includes the complete Vacuum System Design.

There are some simple formulas which can help one to make preliminary estimations.

http://www.graham-mfg.com/


Gas Flow and Conductance - 1Gas flow Q through the cross-section (number of molecules) is equal

to the volumetric flow speed multiplied by pressure:

Q=P • dV/dt

Conductance of the tube connecting two parts of a vacuum system is the ratio of the gas flow to the differential pressure:

C = Q/ΔPor

Q = C • ΔP = C(P2 – P1)

If P2 ≤ P1 (practical case) Q ~ C • P2


Gas Flow and Conductance - 2There is an obvious analogy with electricity: C corresponds to electrical conductance and ΔP to the voltage. So we will have the same rules for the conductance in parallel and series connections:

C = C1 + C2 (for the parallel connection)1/C = 1/C1 + 1/C2 (for the series connection)

Here are two simple, easy-to-remember formulas:

Conductance of a round tube of length L, where L is more than 10 times D, its

diameter, for nitrogen: C = 12.2 D3/LConductance of a diaphragm or a very short tube, for nitrogen, where F is the

square area: C = 12.2 F (cm2)


Effective Pumping Speed

One has to take into account that the effective pumping speed might be very different from the pump capacity. For example, if the conductivity of a connecting line is equal to the pump capacity, the effective pumping speed will be only 50% of nominal.

(1/C = 1/C1 + 1/C2)

That is very true especially for a long object with a small aperture, like an ID vacuum chamber.

REMEDY:Use multiple small pumps located along the vacuum chamber (BESSY-II);

Use a distributed pumping source like NEG (nonevaporated getter) strips (APS) or NEG-coated chambers (ESRF).


From discrete pumping From discrete pumping to linear pumpingto linear pumping

Courtesy of Michael Ferris from ‘Saes Getter”


From linear pumping From linear pumping to integrated pumpingto integrated pumping

Courtesy of Michael Ferris from ‘Saes Getter”


ID vacuum chamber at the APS ID vacuum chamber at the APS

BPM location

Welding joint

NEG support slot


Major sources of residual outgassing

Diffusion of the gases through the wall material

Evaporation of the wall material

Surface contamination (physical, chemical adsorption)

Gases dissolved in the vacuum-chamber material


OUTGASSING AND PUMPINGOUTGASSING AND PUMPING

Outgassing of the inner surface in the vacuum system is: Q=qo•ΣF

qo - Specific residual outgassingΣF - Total surface of all in-vacuum components


Pumping Balance - I


Pumping Balance - II

Pressure inside the chamber:

SeffQQQ 321 ++


Specific Residual Outgassing

Diffusion of gases through the material should only be taken into account for very thin metallic walls, very high outside pressures, or high temperatures.Gas diffusion through Viton should always be taken into account.To avoid the sizable impact of material evaporation on the vacuum, restrict the use of any Zinc-containing alloys inside the vacuum chamber.

Surface contamination can be drastically decreased by proper cleaning.


How to Decrease Residual Outgassing

1. High quality of the surface finish for ultra-high vacuum application (32 micro inches or better). It provides a double advantage: decreases the actual surface and simplifies the cleaning process;

2. Proper choice of the surface finish technique;(avoid grinding, polishing and so on – replace by diamond cutting);

3. Wide use of an electrochemical polishing.

4. Only water solvable cooling liquids – no oil!.


Gas Photodesorbtion under Synchrotron Radiation

For a properly cleaned and baked surface, gas photodesorbtion is the major contributor to surface outgassing;That is why in a synchrotron-radiation storage ring, the vacuum level with beam is about one order of magnitude worse than without beam;The use of vacuum-melted metals, especially for the for photon absorbers, is highly recommended;The use of ante-chambers in ID vacuum chambers is a good practice. It prevents the synchrotron radiation from striking the vacuum chamber wall;Increasing the local pumping in the photon absorber area helps too.


The Importance of Vacuum Hygiene

Outgassing from a single finger print (before baking) is:10-5 sec

Torrl∗

For a 1•10-10 Torr vacuum, a pumping speed of about 10000 L/sec would be required just to pump the gas coming from this one finger print (!?)


Cleaning Facility at Argonne National LaboratoryCleaning Facility at Argonne National Laboratory--II


Cleaning Facility at Argonne National LaboratoryCleaning Facility at Argonne National Laboratory--IIII


Materials for UltraMaterials for Ultra--High Vacuum Applications: MetalsHigh Vacuum Applications: Metals

Stainless steel - (316, 316L where magnetic permeability close to 1 is required, 304 or equivalent for general applications). For parts with very thin walls, it is recommended to use vacuum molten and degassed stainless steel;

Aluminum - pure aluminum and different Al alloys (e.g. 6061 if parts should be welded);

Copper – Oxygen-Free copper, especially for components that will require brazing or welding. One of the best high-quality OFHC suppliers is “Hitachi Cable”. They can provide a variety of vacuum degassed profiles;

Zirconium OFHC Copper and GlidCop - for extreme heat load applications;

Bronze – Different kinds, but without Zn. Brass is restricted for ultra-high-vacuum applications;

Regular steel – only if it is really necessary, and then with the Ni or Cr plating;

Tungsten (vacuum compatible) – for components of safety shutters and targets;

Molybdenum or Tantalum – for high-temperature applications and other special purposes.


Materials for UltraMaterials for Ultra--High Vacuum Applications: High Vacuum Applications: NonNon--MetalsMetals

Glass – different kinds (Corning types 7056 – the most common, Pyrex – for viewports and electrical feedthrough). Quartz viewports are used for high-temperature and VUV applications;

Sapphire – viewports for up to 450 C° baking temperature:

Ceramics - different kinds – for spacing insulators, electrical feedthroughs (especially for high voltage, RF feedthroughs) and so on.

Teflon and Viton – in small quantities as a seal material and insulating spacers. It is not recommended to use this material inside mirror chambers or near high voltage components.


Consideration of atmospheric pressure in vacuum systemsConsideration of atmospheric pressure in vacuum systems

This picture shows the end of the IXS beam line at the APS. The total length is 8.25 m, and the end ellipse size is 1.45m by 0.43 m. Multiple ribs are used to prevent the vacuum tube from collapsing, and are a significant part of the whole system cost.

Courtesy of Bran Brajuskovic

Consideration of atmospheric pressure in vacuum systemsConsideration of atmospheric pressure in vacuum systems


The most popular seal types for ultraThe most popular seal types for ultra--high vacuum high vacuum componentscomponents

• ConFlat flanges (rotatable and non rotatable);

• EVAC chain-clamp metal seals;

• “HelicoFlex” seals;

• Indium wire seals;

• Wire seals (copper and aluminum wires).


ConFlat Flange Basics ConFlat Flange Basics

Data Base for ConFlat Flanges: http:/www.aps.anl.gov/asd/me/Calculators/FlangeConv.html

CF seals work very well up to 8” diameter;

Force to seal is about 25-30 kg/mm;

No scratches permitted on the conical surfaces;

Up to this diameter CF flanges from all popular vendors are pretty much the same;

Maximal baking temperature is ~250 C°

For flanges with a bigger diameter, flatness of the flanges becomes very important – in welded chambers, after awhile it is not so easy to reseal the joint due to residual stresses.

One of the ways to fix the problem is to use a thicker gasket, vacuum annealed and silver coated;

To decrease the friction inside the threads of the tightening bolts, some sources recommend to use MoS2.


EVAC Aluminum Metal SealsEVAC Aluminum Metal Seals

Very convenient to use in narrow areas;

Aluminum knife-edges are compressed by the sealing surfaces of the mating flanges when the chain clamp is tightened;

For LCLS break section aluminum seal was replaced by the copper one;

“Nor-Cal Products” is a provider of such flanges;

For complete information use:

www.n-c.com

LCLS undulator break section


““HelicoFlexHelicoFlex”” SealSeal

Our experience with the Helicoflex seal:

Advantages: very compact, sealing forces are relatively small;

Disadvantages: quite expensive, requires extremely high surface finish (better than 8 micro inches), sensitive to the baking cycles. (note: Inappropriate for SRF systems due to particulates.)


Indium wire sealIndium wire sealMajor advantages:Tightening forces are small – about 1kg/ mm length of the seal;

Compression easily breaks an oxide layer;

Can be used to join metals, glass and ceramics;

Indium seals are especially useful at cryogenic temperatures;

For practical considerations, indium is non toxic;

One doesn’t need to join the ends of the indium wire – it is enough to make an intersection. For rectangular openings it is easy just to place four straight wires with intersections in the corners;

Indium wire is reusable – a simple fixture with different end pieces can provide wire of different diameters.

Major disadvantages:Melting temperature of indium is 157 C°, so the maximum baking temperature is 110-120 C°;

Small yield strength makes indium seals sensitive to the outside load – better to use tightening bolts with Belleville washers.


VATSEAL VATSEAL –– the metal seal for vacuum and cryogenicthe metal seal for vacuum and cryogenic

It is a very compact seal and is used often to replace elastomer seals. It is a reliable seal, and at the same time creates a low-resistance RF contact.•Material: silver-plated copper;•Maximal baking temperature: up to 300 C°;•Sealing forces (minimum): 200 kg per cm sealing line;•Sealing surface requirements: flatness over 50 mm – 0.02mm, surface finish – Ra = 0.2µ or better.


Seals using copper or aluminum wireSeals using copper or aluminum wireCopper wire seals are recommended for 250-600 mm tube O.D. – see for example www.mdcvacuum.com

Aluminum wire is recommended for rectangular chambers of any size.

The front opening could be sealed with Al. wire (1100 type). Four straight wires can be used for such a seal and simply crossed in the corners.

One can see the holes for the pins to locate the wires.

Courtesy of ”Johnson UltraVac”

http://www.mdcvacuum.com/


PumpingPumping

The most suitable pumps for ultra-high vacuum applications are:• Getter-ion pumps (“Varian”, “Kurt J. Lesker”, “Physical Electronics”) – it is

better to use them with Ti-Sublimation Inserts;

• High-performance Maglev Hybrid Turbomolecular Pumps and pumping

systems (“Alcatel”, “Pfeifer Vacuum”, “Leybold”, “Shimadzu Mag-Lev”);

• Nonevaporated getter (NEG) strips and pumps (“Saes Getter”);

• Standard Mechanical Oil-free pumps for rough pumping (multiple vendors).


How an Ion How an Ion –– Getter Pump worksGetter Pump works


BuiltBuilt--in Getterin Getter--Ion Pumps Ion Pumps

Storage ring magnet pole

Storage ring magnet pole

Built-in getter-ion pump

Good field region

*All Russian storage rings (“VEPP-2M”,”VEPP-3” and “VEPP-4”) have such getter-ion pumps

*M.D. Malev, E.M. Trakhtenberg, "Built-in getter-ion pumps", Vacuum V.23, 1973, p. 403.


How to decrease How to decrease ““virtual leaksvirtual leaks”” and outgassingand outgassing

• Avoid all kinds of “pockets” in the design;

• Avoid blind holes – make them through holes if possible, or at least vent them;

• Avoid large-area flat-to-flat contacts – instead, have flats with recesses and venting grooves, but be reasonable;

Note: It is especially important when virtual leaks can decrease pump down speed.


Special Technologies for UltraSpecial Technologies for Ultra--HighHigh--Vacuum ApplicationsVacuum Applications

• Arc Welding for Vacuum Applications

• E-Beam Welding

• Laser Welding

• Diffusion Welding

• Vacuum Brazing

• Explosion Bonding


Welding for UltraWelding for Ultra--HighHigh--Vacuum ApplicationsVacuum Applications

• For stainless steel: use TIG welding with argon;

• For copper: use TIG welding with helium;

• All welding should be done from the inside, preferably without a filler;

• The last one or two joints could be done from outside with full penetration;

• The parts to be joined should be of about the same thickness;

• Parts should be carefully cleaned before welding

• The gap between the welded parts should not exceed 50-80 microns;

• Welded joints should be clean (and dry) before leak testing;

• For complex components with the many joints, baking in a vacuum furnace is recommended before leak testing.


Welding for UltraWelding for Ultra--HighHigh--Vacuum Applications: Vacuum Applications: A design exampleA design example

All welding joints are made from the inside. Only the last (top disc) is welded from the outside, and it is a full-penetration weld.


Welding of Rectangular ChambersWelding of Rectangular Chambers

WRONG RIGHT WRONG RIGHT

On the left sketch location of the last welding joint is too far from the previously welded joints from inside.

On the right sketch this location is shifted more close to the previously welded joints-intersection of the welding joints is more secure.


HighHigh--Quality Welding Joints on APS Vacuum ChambersQuality Welding Joints on APS Vacuum Chambers

Major requirements:

1. Full penetration

2. No underbead


Robotic Welding Machine at the APS Robotic Welding Machine at the APS


• Recommended for materials and joints like tungsten and stainless steel or tungsten and copper that can’t be joined by standard welding due to big difference in the thermal expansions coefficients.

• Should be done in vacuum at an elevated temperature equal to 70% of the melting temperature of the component with the lowest melting temperature;

• Pressure should be applied across the joint for two to three hours to achieve 80% of the yield stress of the material with the lower yield stress;

• The easiest way to apply such a pressure is to use bellows with the proper cross-section.

It is possible using diffusion welding to make very exotic multilayer combinations of different materials, including ceramics.

Diffusion Welding Diffusion Welding


Atlas Atlas Core TechnologiesCore TechnologiesSS/AL Bond InterfaceSS/AL Bond Interface

Patent# 5836623Patent# 5836623

Stainless

Aluminum

Stainless

CopperTitanium

AL/SS Bond Interface

•• Diffusion Inhibiting LayersDiffusion Inhibiting Layers–– Copper and Titanium InterlayerCopper and Titanium Interlayer–– Enables Bonding AL/SSEnables Bonding AL/SS

•• Vacuum:Vacuum:–– <1x10<1x10--1010cc He/Seccc He/Sec

•• Thermal:Thermal:–– Peak 500C at weld upPeak 500C at weld up–– 00--250C Operational250C Operational

•• MechanicalMechanical–– Tensile 38,000 Psi.Tensile 38,000 Psi.–– Shear 30,000 Psi.Shear 30,000 Psi.

Stainless Knife-edge Aluminum Body


Atlas Flanges All Sizes & Types Atlas Flanges All Sizes & Types

•• All CF sizesAll CF sizes–– USUS–– Metric Metric –– ISOISO

•• Flush Mount, Tube Flush Mount, Tube Mount Weld NeckMount Weld Neck

•• Atlas ATAtlas AT--VCRVCR•• Wire SealWire Seal•• Quick DisconnectQuick Disconnect•• RectangularRectangular•• Custom:Custom:

–– Beam Tube, Beam Tube, Chamber and Chamber and InterfacesInterfaces


How to avoid waterHow to avoid water--toto--vacuum joints vacuum joints –– Example IExample I

A gap between the water-tight joint and the vacuum-tight joint creates the necessary separation

Water-tight joint

Vacuum-tight joint

Separating gap


How to avoid waterHow to avoid water--toto--vacuum joints vacuum joints –– Example IIExample II

Gap between water-tight joint and vacuum-tight joint creates the necessary separation

Water-tight joint

Vacuum-tight joints

Separating gap

Final water-tight welding joint


Separating windows in the beam lines and front ends Separating windows in the beam lines and front ends -- II

APS Beryllium Window Design in Collaboration with APS Beryllium Window Design in Collaboration with ““Brush WellmanBrush Wellman””

It is good practice to use separating Be windows between the storage ring and a beam line during the commissioning and initial operation stages;

Do not expose Beryllium to air when the synchrotron radiation beam is coming through It is very important during the shielding verification process;

Make proper pumping, and don’t expose Be windows to atmospheric pressure.

Quality of the brazing should be carefully inspected, especially the absence of brazing filler on the beryllium surface

Courtesy of Oliver Schmidt


Separating windows in the beam lines and front ends Separating windows in the beam lines and front ends -- IIII

View from 6” flange side

APS Diamond Window Design in Collaboration with Diamond MaterialAPS Diamond Window Design in Collaboration with Diamond Materialss

Courtesy of Yiefei Jaski

6” CF flange

4.5” CF flange


BellowsBellows

Bellows are used very often in vacuum installations as assembly compensator components, mechanical motion feedthroughs, spacers between outside heavy components like pumps and the main chamber, and so on.

Connecting cuffs on the bellow are rather thin, and its welding to the main parts can create some problem;

If possible, it is better to use the standard bellow assembly with attached flanges;

For low range motion and for compensation use formed bellows – they are cheaper and more reliable.

Formed bellow Welded bellow Deposited bellow


An Example of Bellow assembly DesignAn Example of Bellow assembly Design

Transition spacer between bellow and shaft helps make welding more reliable using standard TIG welding;

Machining of the blank CF flange as shown serves the same purpose.


Major Advantages of Vacuum or Hydrogen Brazing Major Advantages of Vacuum or Hydrogen Brazing in the Furnacein the Furnace

No fluxNo flux;;No oxidation of the joining parts;No oxidation of the joining parts;

Parts are automatically cleaned; Parts are automatically cleaned;

Capillary forces could more easily suck in the brazing Capillary forces could more easily suck in the brazing

alloy (for vacuum brazing) alloy (for vacuum brazing)

Uniform heating or cooling without stresses;Uniform heating or cooling without stresses;

Good control of the temperature ramping.Good control of the temperature ramping.

NOTE: Parts should be carefully shielded from the oven heaters and numerous thermocouples should be used


Vacuum/Hydrogen Brazing Furnace at APSVacuum/Hydrogen Brazing Furnace at APS


Example of a Vacuum Brazing FixtureExample of a Vacuum Brazing Fixture1. All fixture parts for brazing of stainless steel or copper components should be made of stainless steel to keep thermal expansion coefficients the same.

2. Use wedges instead of nuts to hold the parts together.

3. To hold parts together during brazing, one can use inserts with a lower thermal expansion coefficient, like molybdenum, in the clamping rods.

Molybdenum insert

Stainless steel rodWedges


Brazing cycle for Au/Cu 50/50 brazing fillerBrazing cycle for Au/Cu 50/50 brazing filler

Initial slow temperature rise to 850Initial slow temperature rise to 850°°CC;;

Holding time at this temperature approximately one hour;Holding time at this temperature approximately one hour;

Spike temperature to 990Spike temperature to 990°°CC; ;

Holding time at this temperature 2Holding time at this temperature 2--3 minutes;3 minutes;

Cooling cycle with a ramp rate of 100Cooling cycle with a ramp rate of 100°°C/minC/min..

NOTE: Parts should be carefully shielded from the oven heaters and numerous thermocouples should be used

For stainless steel/copper joints, Gold/Copper brazing filler was used with very good results


Photon Shutter Brazing JointsPhoton Shutter Brazing Joints

The total number of brazing joints is 16: two halves, two flangThe total number of brazing joints is 16: two halves, two flanges, four es, four fittings and eight plugs;fittings and eight plugs;

All brazing joints were done in one shot;All brazing joints were done in one shot;

There are no waterThere are no water--toto--vacuum joints.vacuum joints.


What was not covered in this presentation:What was not covered in this presentation:

•• Leak checking;Leak checking;

•• Friction and motion in vacuum;Friction and motion in vacuum;

•• Different types of mechanical and Different types of mechanical and

electrical feedthroughs; electrical feedthroughs;

•• Valves;Valves;

•• Vacuum gauges and Residual Gas Vacuum gauges and Residual Gas

Analyzers (RGA)Analyzers (RGA)


Recommended literature:Recommended literature:

1.1.* * A. Roth. A. Roth. ““Vacuum TechnologyVacuum Technology””, 1990 Elsevier Science;, 1990 Elsevier Science;

2. 2. American Vacuum Society.American Vacuum Society. ““Glossary of Terms used in Vacuum TechnologyGlossary of Terms used in Vacuum Technology””, 1958 , 1958 PergamonPergamon Press, New York;Press, New York;

3. 3. A.E. Barrington.A.E. Barrington. ““High Vacuum EngineeringHigh Vacuum Engineering””,1963, Prentice,1963, Prentice--Hall, Hall, EngelwoodEngelwood Cliffs, Cliffs, New York;New York;

4. 4. Diels, K. and R.J. Diels, K. and R.J. JaeckelJaeckel.. ““LeyboldLeybold Vacuum HandbookVacuum Handbook””, 1966, , 1966, PergamonnPergamonn Press, Press, Oxford;Oxford;

5. 5. Erikson, E.D., et al.Erikson, E.D., et al. ““Vacuum outgassing of various materialsVacuum outgassing of various materials””, 1984, Journal of , 1984, Journal of Vacuum Science & Technology.Vacuum Science & Technology.

6. Catalogs of MDC, Kurt J. 6. Catalogs of MDC, Kurt J. LeskerLesker Co., Varian Vacuum Products.Co., Varian Vacuum Products.

*Note:*Note: A. RothA. Roth’’s book contents a huge list of references about all components os book contents a huge list of references about all components of f vacuum science and technologyvacuum science and technology


Ancient civilizations believed that the Earth rested on three whales…..


I believe that synchrotron radiation engineering is based on three main pillars...

Vacuum Heat transfer

I have tried to highlight just one of these pillars

Mechanical Engineering

UHV Requirements for Accelerator and Synchrotron Radiation...

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