Vacuum Science and Technology in Accelerators

1. Vacuum Science and Technology in Accelerators Ron Reid Consultant ASTeC Vacuum Science Group (r.j.reid@dl.ac.uk) Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

2. Session 6 Processing Techniques for Vacuum Componentsand Systems Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

3. Aims • To understand why we need to adopt a rigorous cleaning and processing strategy for modern accelerators • To understand some of the techniques involved • To understand some of the aspects of quality control for vacuum Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

4. Why Do We Need To Clean For Vacuum? • We may not need to! • It depends on what we need vacuum for • Vacuum regime required • Ultimate pressure • Cleanliness • So we need to make a proper assessment of the real requirements of the application • But in general, modern accelerators require good, “clean” vacuum Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

5. Some Reasons for Cleaning • Irrespective of application - manufacturer desires attractive appearance! • Characteristics of a surface (surface properties) may be altered by ‘contamination’ at the surface. • Processes may be poisoned by ‘contaminants’ Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

6. Why Clean Accelerator Vacuum Systems? • Transport of particles and EM radiation • Loss reduction • Reduce scattered radiation for health & safety • Maintain clean in-vacuum surfaces • Prevent particle target poisoning • Maintain efficient optical properties for EM radiation transport • Protect gun cathodes from poisoning/degradation • To provide a controlled atmosphere Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

7. Requirements for UHV/XHV • Minimise desorption • Remove ‘contaminants’ (i.e. components with high outgassing/vapour pressure) • Deplete reservoirs • Bulk gases • Surface overlayers (e.g. adventitious graphite) • Provide barriers Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

8. How do we know if a surface is clean? • Phenomenologically • Measure outgassing (thermal desorption) • Measure stimulated desorption (according to requirements of system) • In each case total and partial pressure measurements useful • Characterise surfaces • Surface analysis Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

9. Cleaning and Processing • Cleaning of accelerator components is generally only performed during the manufacturing or pre-installation programme. • Processing may be performed during manufacture or pre-installation, but may also be carried out in situ, either as part of the installation programme or periodically during operations. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

10. A Distinction • Differentiate between • Cleaning • Removal of unwanted components • Passivation • Formation of barriers • Low sticking probabilities Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

11. Some examples of cleaning processes • Solution • Water based • Solvent based • Alcohols • Chlorinated hydrocarbons • Freons • Hydrofluoroether • Non flammable ethers • Detergents • Aqueous • Alkaline degreasers Etchants • Acids • Alkalis Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

12. A note on surface preparation • There is a temptation to polish surfaces on the assumption that if it looks nice it must be better. • But, for vacuum use • Avoid grinding • Shot blasting • Mechanical polishing • Such processes drive contaminants into the surface and they can be difficult to remove. • But sometimes they may be necessary for other reasons Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

13. Some examples of passivation • Air Baking • Electropolishing • Glow Discharge • But note that all of these have some cleaning effect! Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

14. Another note on surface preparation • Electropolished surfaces look very nice. However, unless essential for other reasons, it is best to avoid it for vacuum use. • The polishing process involves immersion in a mixed acid solution (often containing things like orthophosphoric acid). It results in a heavily modified sub-surface layer which contains a lot of hydrogen and acid residues which can increase outgassing. • Baking at 450oC is usually required to restore the initial (untreated) outgassing rate. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

15. Hydrophile Hydrophobe Science of Cleaning • Solvent- A solvent is a substance that dissolves another substance or substances to form a solution (a homogeneous mixture). The solvent is the component in the solution that is present in the largest amount or is the one that determines the state of matter (i.e. solid, liquid, gas) of the solution. • Surfactants - surface active agent Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

16. Science of Cleaning • Penetrating and Wetting agents Surfactants change the chemical composition of the hydrophobic and hydrophilic ends of the molecule, opening up the possibility of • Detergency • Foaming • Emulsifying • Solubilising • Dispersing • Chelators - remove the hardness in water • Saponifiers - convert animals fats into natural soaps Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

17. Typical Cleaning Agents Agent Examples Advantages Disadvantages Disposal Water Cheap, readily Need de-min for cleanliness. To foul drain available Not a strong solvent Alcohols Ethanol, methanol, iso-propanol Relatively cheap and Need control – affect workers; Evaporate or controlled readily available. some poisonous; some disposal. Quite good solvents flammable; stringent safety precautions. Organic Acetone, ether, Good solvents, Either highly flammable or Usually evaporate Solvents benzene evaporate easily with carcinogenic low residue. CFC’s FreonTM Excellent solvents; Banned Strictly controlled, must (CFC-113) evaporate easily with not be allowed to low residue evaporate Chlorinated Trichloroethylene (TrikeTM) Excellent solvents. Strictly controlled Toxic, requires stringent safety precautions. hydrocarbons Non-toxic. Low boiling point. Low residue Detergents Aqueous solutions, Require careful washing and To foul drain and non toxic. Cheap and drying of components. Can dilution readily available. leave residues. Moderate solvents. Alkaline AlmecoTM Aqueous solutions, Can leave residues and may Requires degreasers sodium non- toxic. Moderate throw particulate precipitates neutralisation, then hydroxide solvents dilution to foul drain. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

18. Some actual cleaning processes Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

19. Current legislative situation • Current Legislation • Vienna Convention - 1985 • Montreal Protocol - 1987 • Amendments - most recent 2000 • Protection of the Ozone Layer • CFC’s • HCFC’s • Carbon Tetrachloride • Methyl Chloroform • Other halogenated hydrocarbons Solvent Emissions Regulations 2004; • Why Change? • Reclassification of 1,1,2-Trichloroethylene (TrikeTM) • Improved Health and Safety regulations Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

20. Cleaning Techniques Ultrasonic cleaning - widely used Surface Cavitation bubbles Ultrasonic Waves Contaminant Heaters Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

21. Cleaning Plant Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

22. Cleaning Plant Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

23. What strategy should be adopted? • The least that is proved to be effective for the task in hand • But understand what is required and the limitations of each process • Design for cleaning • Pay enormous attention to detail • Pay enormous attention to health and safety! Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

24. About Bakeout • Bakeout to moderate temperatures (250oC) is an efficient way of reducing outgassing, especially of water. • In an accelerator, bakeout has to be undertaken with care to ensure temperature gradients are minimised and damage does not occur. • Vessel supports must be designed to accommodate the movements due to thermal expansion and contraction (and so that vessels get back to where they started!) • Bellows are used to accommodate these movements. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

25. About Bakeout • With stainless steel, in machines the most common way to bake is to use wrapped heater tapes and bands with ceramic blanket insulation. • For simpler vessels, close wrapped Kapton insulated printed heaters with superinsulation can be used to reduce the overall thickness to less than 1mm. • For aluminium where temperatures used are less than 180oC, superheated water or wrapped film are used. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

26. About Bakeout • Bakeout is best performed into external pumps, e.g. turbo pump sets. • Towards the end, during cooldown, hot filament gauges should be degassed, in situ TSPs and NEG pumps carefully degasssed and conditioned and ion pumps “flashed” for conditioning. • Bakeout should be monitored rather than for fixed times – terminating when water in the rga spectrum falls to a predefined level. • Heating and cooldown rates must be carefully controlled. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

27. Quality Control • Accelerator builders are, in general, always building high precision prototypes which must work to a stringent specification. • To achieve this, good quality control or quality assurance is essential. • QA systems such as those set up under standards like ISO 9001 are well established and the mechanical aspects of vessel and component manufacture (e.g. materials, dimensions, tolerances) will fall under their aegis. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

28. Quality Control • Vacuum aspects of quality control are much more nebulous • The system builder needs to specify exactly what is wanted, how it is to be measured and how it is to be assessed. • There are no “standard” standards. • It is also likely that contractors will need to be educated and vacuum equipment may need to be supplied. • Trained vacuum inspectors will also need to be available. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

29. Quality Control • First define your standards. Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

30. Quality Control • General vacuum specification • Materials • Techniques • Processes • Handling • Inspection • (In addition to vessel drawings, mechanical specification, etc.) Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

31. Quality Control • Assessment (Tests) • Leak test • Performance test • Base pressure • Outgassing rate • Cleanliness Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

32. Leak tests • Specify a realistic leak rate • Specify testing method Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

33. Leak Detection Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

34. In a sealed chamber, G P Outgassing test • Rate of Rise (gas accumulation) Nemanic & Setina Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007

35. Cleanliness Test • Usually specified in terms of an rga spectrum. For example Notes The residual gas spectrum shall have been recorded over 1 –200 amu The spectrum shall have been corrected for sampling error, mass discrimination and species relative sensitivities These limits are expressed in terms of percentages of the total pressure in the system, The definition of “general contaminants” is the sum of the partial pressures of all peaks present in the residual gas spectrum of mass to charge ratio (amu) equal to 39, 41-43 and 45 and above Also to be excluded from this summation are any peaks related to the rare gases xenon (i.e. 132, 129, 131) and krypton (i.e. 84, 86, 83) Vacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007