Westerberg, L

CERN Document Server

267THE BEST LAID SCHEMES ...Lars WesterbergThe Svedberg Laboratory, Uppsala University, Uppsala, SwedenAbstractA list of recommendations for better vacuum practice is presented in thischapter.1. INTRODUCTION"The best laid schemes o'mice and men gang oft a-gley" (Rabbie Burns)The title of this chapter refers to a poem by the Scottish poet Rabbie Burns, who when farmingdestroyed a mice nest with his plough. He felt so sorry for this that he wrote the poem. The conclusionwe can draw is that even with the best preparations something can, and will often, go wrong becausesome not so obvious fact was forgotten. Much of the advice in this chapter is certainly availableelsewhere in the literature, but some may be compiled here for the first time.The topics to be covered in this chapter are recommendations concerning the choice ofmaterials with correct properties, corrosion resistance, joining dissimilar material, rolled versusforged material, weld preparation, virtual leaks, inadequate heat treatment, magnetic welds, commonerrors and design mistakes. The material is presented in the form of recommendations of what to door not to do. Clearly, deviations from these principles have caused, and will cause, trouble.2. RECOMMENDATIONS2.1 Design• Make sure to match pumping speed and conductance when designing a vacuum system.• Computer simulations are of great help in optimising the performance of a vacuum system.• Mechanical stability calculations help to avoid such unwanted surprises as collapsed chambersafter the first pumpdown.• A chamber which can not be made stable enough against deformation during pump down can bereinforced by external bridge-like constructions.2.2 Choice of materials with correct properties• Select materials with suitable mechanical properties and good weldability.• Use 316LN steel where low magnetic permeability and vacuum firing is required.• Use pyrolytic carbon in UHV (density 2.2). Sintered carbon (density around 1.7) contains a lot ofair and must be avoided.• Materials with too high vapour pressure, Cd (in plated steel screws), Zn (in brass), S and Se (in303 stainless steel) etc. should never be used in vacuum  systems!• Cast material should be used for flanges since rolled flange material can give rise to virtual leaks.See further below.• Avoid bare insulating surfaces in an accelerator. Charged structures may deflect the beam. Sparkscan cause trouble.• Charged Vespel may carbonise on the  surface when a spark occurs.• It is possible to coat ceramic tubes by a thin layer of sputtered Au or sublimated Ti to avoidcharging of the surface.2682.3 Corrosion resistance• 304 series steel is not acid proof.• Certain combinations of metals and chemicals such as fluorine solutions may give rise tocorrosion.• Corrosion of high-voltage feedthroughs (e.g. for ion pumps) in high humidity areas can beavoided by heating the feedthroughs slightly above air temperature.2.4 Joining dissimilar materials• Aluminium and stainless steel can be joined by aluminium knife-edge seals, like the LEP seal atCERN. The SMC company has a full range of aluminium Conflat-type seals and aluminiumflanges, which is easily combined with standard stainless steel flanges. It is also possible to usespring-loaded seals like Helicoflex.• Titanium/stainless steel joints can be obtained by explosion welding, performed by specialcompanies. We have, however, bad experience for such welds since three out of four leaked. Theywere replaced by brazed joints Ti/Cu/stainless steel.2.5 Machining• When machining do not use silicon oils. They can not be cleaned away.• Glass-bead blasting reduces the surface but may bury oil and dirt.• Glass-bead blasting as well as electropolishing should be followed by vacuum firing.2.6 Brazing and welding• Virtual leaks can be created by improper weld preparations.• Improper welding may create magnetic welds.2.7 Virtual leaks• One of the most common sources of virtual leaks is threaded holes for screws which have notbeen ventilated properly. Drill a 1 mm hole through the centre of the screw or, drill a hole fromthe bottom, or mill a narrow gap all the way down to the bottom on one side of the hole for thescrew as shown in Fig. 1.• Closed volumes can occur if welds are improperly done. See Section 6 of the Engineering chapterin these proceedings.• Rolled instead of forged flange material can lead to virtual leaks due to silicate enclosures asshown in Fig. 2.vent hole closed volumeCu GasketCF FlangeFig 1  The screw to the right has a bored throughventilation hole to prevent virtual leaks and slowpump-downFig. 2  Possible virtual leaks at the knifeedge of a Conflat flange2692.8 Flanges and gaskets• Improper torque applied to the bolts of a Conflat flange may cause a leak. Use a torque wrenchand apply the recommended torque stepwise after completing a full turn of all the bolts.• Do not re-use Conflat-type gaskets.2.9 Vacuum furnace heat treatment• It is important to have a sufficiently low pressure in the furnace, at least 10-3 Pa.• Magnetic welds can occur if, during the heat treatment (vacuum firing), the temperature does notrise fast enough in the temperature range 500 – 800 °C.• If a leak occurs during baking to 450 oC the chamber can be coated internally by copper oxides ifOFHC copper seals are use. Use instead silver plated OFHC copper or Oxygen-Free Silver (OFS)seals2.10 Baking• Inadequate bakeout temperature will not give significantly improved vacuum.• Non-baked areas defeat the purpose of the bakeout due to the large difference in outgassing ratebetween baked and unbaked stainless steel.• Crossed heating tapes give a too-high temperature spot and will eventually cause a short circuit.• Magnetic baking equipment will move in a magnetic field when the field is cycled and willeventually be damaged.• Poor contact between the thermocouple and the chamber will result in excessive bakeouttemperature, which can cause severe damage.• Inadequate heat insulation gives too low temperature.• Not degassing filaments of sublimation pumps results in excessive outgassing when the pump isoperated and therefore very fast saturation of the sublimated layer.2.11 Vacuum pumps• Diaphragm or membrane pumps an cause damage when the membrane beaks down. Ask themanufacturer for the recommended interval between membrane replacements.• Ion pumps exposed to a high noble-gas load release previously pumped gases.2.12 Contamination• Cleanliness is a must! No fingerprints. Use clean gloves! Use head cover.• Mount equipment in a clean tent where an overpressure is established by using a sufficiently largeair cleaner with HEPA filter. A movable clean bench can easily be combined with a tent. The tentis made from metal bars and clean plastic sheets. The over pressure prevents dust from entering.• When transporting or storing vacuum chambers protect sensitive parts such as bellows fromdamage.• Protect sealing surfaces by covering flanges with Al foil and protective covers after cleaning. Donot dismount until installation.• Beware of dust particles!• Beware of metal particles! They can stick in the seal of an all-metal gate valve resulting inexpensive repairs.• Ventilate vacuum chambers with clean nitrogen gas. Air contains water.• During leak detection connect the leak detector between the turbo pump and the fore pump forlowest contamination risk and highest leak sensitivity. This can be done without stopping theturbo pump if two manual valves are installed in this position.2703. REFERENCES FOR FURTHER INFORMATIONThe International Union of Vacuum Science Technique and Applications (IUVSTA) is aninternational organisation with 30 member national vacuum societies all over the world. It has eightdivisions: Applied Surface Science, Electronic Materials & Processing, Nanometer Structures, PlasmaScience and Technology, Surface Science, Thin Films, Vacuum Metallurgy and Vacuum Science. OnIUVSTA's home page, http://www.vacumm.org/iuvsta you find information about its vacuumcongress (IVC), conference series like ECOSS, ICTF and EVC as well as workshops and schools. Onthe Vacuum Science division page (http://www.vacumm.org/iuvsta/vsd/biblio.htm) you find a list ofvacuum books, both historical and modern ones. There are links to national vacuum societies whereyou can find information about meetings, and courses they organise. Most vacuum societies publish ajournal or a news letter. By joining your national vacuum society you can find very useful contacts.Good Luck!

The best laid schemes

Abstract

Similar works

Full text

Available Versions

CERN Document Server