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CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department

CLIC seminars. CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department. Content: Vacuum problems in CLIC. Known solutions: Bakeout. Lumped pumping. Linear pumping. Distributed pumping: NEG coatings. Amorphous carbon coatings.

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CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department

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  1. CLIC seminars CLIC Vacuum Issues (2) Paolo Chiggiato Vacuum, Surfaces and Coatings TE department • Content: • Vacuum problems in CLIC. • Known solutions: • Bakeout. • Lumped pumping. • Linear pumping. • Distributed pumping: NEG coatings. • Amorphous carbon coatings. • Scrubbing (conditioning). • Possible developments. Paolo Chiggiato,TE-VSC

  2. Vacuum problems in CLIC • Synchrotron radiation induced desorption (LEP). • Pressure instabilities (ISR). • Electron clouds and electron induced desorption (SPS). • Ion trapping (e- synchrotron light sources). • Limited vacuum conductance (undulator vacuum chambers, ESRF). • Each of these obstacles has been already surmounted in the past. The challenge for the CLIC vacuum is the simultaneous combination of some of these problems in the same beam pipe. DR vacuum requirements are the most demanding. • In this respect, the CLIC vacuum requirement is far to be conventional. As a consequence, non-conventional solutions have to be proposed. Some of them demand a vigorous development in the next years. Paolo Chiggiato,TE-VSC

  3. Vacuum problems in CLIC Pressure requirements (?) with beam P=10-9Torr P=10-9Torr P=10-8Torr P=10-8Torr P ≤ 10-9Torr P ≤ 10-9Torr P ≤ 10-9Torr Paolo Chiggiato,TE-VSC

  4. Synchrotron radiation bombardment Paolo Chiggiato,TE-VSC

  5. Vacuum problems in CLIC Pressure instabilities Paolo Chiggiato,TE-VSC

  6. Vacuum problems in CLIC Ion Trapping Paolo Chiggiato,TE-VSC

  7. Vacuum problems in CLIC Electron cloud phenomena dCR=1.1 Paolo Chiggiato,TE-VSC

  8. Vacuum problems in CLIC Limited Vacuum Conductance Paolo Chiggiato,TE-VSC

  9. Present Solutions: Bakeout • Any system requiring a pressure lower than 10-8Torr in a few days of pumping should be baked in-situ. • Effects of bakeout: • Lower outgassing: from roughly 10-10Torr l s-1 cm-2 of water vapor to 10-12-10-13Torr l s-1 cm-2 of H2 • Lower desorption yields: • Lower SEY: but always higher than the required threshold for DR. Electron energy: 500 eV Paolo Chiggiato,TE-VSC

  10. Present Solutions: Lumped Pumping Pumps located at regular distances from each other give rise to a parabolic pressure profile with minima at the pumps location. The average pressure is given by: The average pressure is quickly limited by the chamber conductance when increasing the spacing between the pumps: the benefit of increasing S is minimal. It is more efficient to increase the number of pumps i.e. decrease L. But this can be very expensive or not possible due to space limitation. for small aperture Q is the degassing rate per metre of pipe. C is the conductance of one metre of pipe, S is the pumping speed of each pump 10 Paolo Chiggiato,TE-VSC

  11. Present Solutions: Linear Pumping When a pumping speed exceeding a few hundreds of litres per second is required, linear pumping is the obvious choice. LEP vacuum chamber cross section PEP-II vacuum chamber Two kinds of linear pump have been used: Integrated ion pumps (they make use of the magnetic field of the machine bending magnets) NEG strips (St101 or St707) Paolo Chiggiato,TE-VSC

  12. Present Solutions: Distributed Pumping-1 • NEG films do not need space, electric power, insulation and feedthroughs(simplified design). • After activationa NEG film surface is very clean resulting in a large pumping speed and reduced degassing (both thermal and ion/radiation/electron induced). • NEG films trap the gas coming from the substrate material. • Low SEY. • The film coating is considered a part of the vacuum chamber. Paolo Chiggiato,TE-VSC

  13. Present Solutions: Distributed Pumping-2 About 15 Kg of Ti-V-Zrare spread over 6 Km of LHC beam pipe Paolo Chiggiato,TE-VSC

  14. Present Solutions: Distributed Pumping-3 Low SEY are obtained for Ti-Zr-V coatings after heating in vacuum. Lower SEY should be obtained by increasing the roughness of the coating. This is obtained by: Increasing the substrate roughness, for example through chemical attack. Optimizing the sputtering process. Paolo Chiggiato,TE-VSC

  15. Present Solutions: Distributed Pumping-4 Substrate temperature during coating 100 °C 300 °C SEY measurements expected in the next months In any case, for NEG films, the low SEY can be obtained only after in-situ activation at temperatures higher than 175°C. Paolo Chiggiato,TE-VSC

  16. Present Solutions: Amorphous Carbon Films-1 If those heating temperatures are unsafe, low SEY can be obtained thanks to another coating recently developed at CERN for that purpose, namely amorphous carbon thin film. • Advantages of magnetron-sputtered C films: • They do not need any in situ bakeout to attain the lowdmax . • Their dmax is lower than that of smooth TiZrVand scrubbed surfaces. • Multiple exposures to air do not increase the dmax if the samples are correctly stored. • Good adhesion, no loose dust C particles. • Resistive behavior: major impact on the impedance can be excluded. 0.2 μm 10 μm Paolo Chiggiato,TE-VSC

  17. Present Solutions: Amorphous Carbon-2 Courtesy of Mauro Taborelli and Christina Yin Vallgren Paolo Chiggiato,TE-VSC

  18. Present Solutions: Amorphous Carbon-3 Unbaked Baked at 150°C for 24h Compared to uncoated stainless steel: H2: about 5 times lower CO2: at least a factor of 5 higher The water vapor outgassing rate is higher than that of uncoated stainless steel by a factor of 20 after 100 h of pumping. Paolo Chiggiato,TE-VSC

  19. Present Solutions: Amorphous Carbon-4 Angle of incidence = 25 mrad Critical Energy 20.5 KeV Angular acceptance 4.234 mrad Photon Flux (E>10eV) 2.94x1015 photons (s mA)-1 Beam Energy 6 GeV Typical Beam Current 185 mA Angle of incidence = 25 mrad C coated chamber 31 The system is bakes at 300°C (24h). The sample is not baked. The sample is separated from the rest of the system by a gate valve (at the diaphragm position, not pictured in the drawing); it is pumped by an auxiliary TMP during the bakeout of the system. At the end of the bakeout, the gate valve is opened. Paolo Chiggiato,TE-VSC

  20. Present Solutions: Amorphous Carbon-5 The photon desorption yield of the unbaked C coated sample is lower than that of uncoated stainless steel. Courtesy of Roberto Kersevan Paolo Chiggiato,TE-VSC

  21. Present Solutions: Amorphous Carbon-6 Courtesy of Pedro Costa Pinto Paolo Chiggiato,TE-VSC

  22. Present Solutions: Scrubbing (Conditioning)-1 Beam conditioning reduces both desorption yields and SEY • Desorption yields, except for H2O, have a power law dependence on the dose D of bombarding particles: where a is found to vary between 0.6 and 1. The samples were baked at 150 C for 24 h and at 300 C for 2 h a=0.77 E=300 eV J. Gomez-Goñi and A. G. Mathewson J. Vac. Sci. Technol. A 15, 3093 (1997) Paolo Chiggiato,TE-VSC

  23. Present Solutions: Scrubbing (Conditioning)-2 Noël Hilleret EPAC 2000 Paolo Chiggiato,TE-VSC

  24. Implementation in CLIC NEG coating + auxiliary IP or scrubbing Lumped pumps Lumped pumps NEG coating + auxiliary IP NEG coating + auxiliary IP --NEG coating (possibly rough) + auxiliary IP -- or --a-C and lumped getter pumps + auxiliary IP if only low T baking is allowed Paolo Chiggiato,TE-VSC

  25. Possible Developments • Rough NEG coatings for lower SEY. • Lower activation temperature NEG coatings. • Coating in restricted geometries (two halves solution) • Optimization of a-C film outgassing. • Miniaturization of lump pumps. Paolo Chiggiato,TE-VSC

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