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Beam-Surface Interaction A Vacuum point of view

Beam-Surface Interaction A Vacuum point of view. F. Le Pimpec SLAC/NLC. Cornell May 2004. Dynamic Vacuum. You want to address the terms of this formula. How to measure the Pressure ?. Outline.  Measuring and Reaching XHV  XHV with Getters

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Beam-Surface Interaction A Vacuum point of view

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  1. Beam-Surface Interaction A Vacuum point of view F. Le Pimpec SLAC/NLC Cornell May 2004

  2. Dynamic Vacuum You want to address the terms of this formula How to measure the Pressure ? F. Le Pimpec - SLAC

  3. Outline  Measuring and Reaching XHV  XHV with Getters  Beam Interaction with Technical surfaces - Desorption Induced by Electronic Transition - Electron Cloud - Ion instabilities  Summary and Conclusion F. Le Pimpec - SLAC

  4. Reaching and Measuring XHV(10-12 Torr) Luminosity for accelerators Lifetimein storage rings Reaching XHV is commercially easier than measuring it A CERN modified Helmer gauge measured 10-14 Torr XHV is not official Pressure  10-7 Torr are called UHV F. Le Pimpec - SLAC

  5. Why Measure Total Pressure ? Partial Pressure gives information on the contents of the vacuum Total pressure can be computed from the partial P measurements Operational in the same range (UHV) The use of hot and cold gauge style device need calibration for every single species for accurate readings – chemistry sensitivity RGA’s electronics are sensitive to the beam passage ! And are still not cheap compared to gauges ! BA –SVT305 RGA F. Le Pimpec - SLAC

  6. Modify Extractor gauge with hidden collector (U. Magdeburg) UHV - XHV Total Pressure • Xray limitation due to the e- hitting the grid : Ions are desorbed from the Collector. Remedies : Modulation • ESD from the gauges elements – Reducing emission current : WrongThe grid will pump then release molecules • Installing a hot gauge in a small tube – Transpiration effectDespite a higher pressure thegauge will read lower. Solution: nude gauges – but sensitivity to stray ions from surroundings F. Le Pimpec - SLAC

  7. Kr Trace Ar Trace Pressure (mbar) UHV - XHV Partial Pressure  The instrument of predilection is the Quadrupole Mass Analyzer  The Ion source is identical to that of an ion gauge • Same ESD problem as for a gauge. ESD ion have higher energy than ionized gas  Need to apply RF on the rod • Resolution, and the price, is dependent on the RF supply  Sensitivity (A/Torr) is non-linear over few decades of pressure – space charge & collision at HV  At XHV range, there is no absolute calibration standard 10-7 10-4

  8. Distributed Pumping Reaching XHV in Static Vacuum Reaching UHV from high vacuum is easy : Sputter/getter Ion pump To reach XHV – Adding extra capture pumps  Cryopump : lump or distributed pumping (LHC cold bore)  Evaporable Getter : Ti sublimator (lump pumping)  Non Evaporable Getter pump (distributed pumping) Diode XHV is possible but is not easy to reach because of outgassing F. Le Pimpec - SLAC

  9. XHV Limit : Outgassing & Vapor Pressure At which temperature is my system going to be running ? To minimize outgassing : Find a material with a low D coefficient Provide a diffusion barrier Installed a vacuum “cryostat” Degass the material … After Honig and Hook (1969) Vapor Pressure : True also for getters and cryosystem F. Le Pimpec - SLAC

  10. C. Benvenutti Lump pumping LEP dipole chamber, getter St101 (ZrAl) (1989-2000) Inserted “linear” pump Inserted “total” pump (TiZrV) Surface pump / diffusion barrier Reaching Static XHV with NEG The LEP : 1st major success of intensive use of NEG pumps Thin film getter is the new adopted way of insuring UHV in colliders or SR light sources ~24 km of NEG  P~10-12 Torr range DAFNE ESRF SOLEIL DIAMOND RHIC LHC NLC/GLC ??... TiZrV NEG Coating Setup at CERN F. Le Pimpec - SLAC

  11. What are Getters ?Getters are Capture Pumps  Cryopumps and Sputter/getter-ion pumps are also capture pumps.  Differentiation is needed • Physical getters (Zeolite) • Work at LN2 temperature by trapping air gases (including water vapor). Cheap primary dry pump. • Recycling by warming up the zeolite • Chemical getters or simply : getters • Includes Evaporable and Non Evaporable Getter F. Le Pimpec - SLAC

  12. How do Getters Work? Dissociation of residual gases on a surface is not systematic Whatever the getter is, the same principle applies : The use of a clean surface to form chemicals bonds Covalent bond (sharing of the e-) Ionic bonds (1 e- is stolen by the most electro- elements (Mg+O-)) Metallic bonds (valence electrons shared) Tied bonds :Chemisorption≥ eV F. Le Pimpec - SLAC

  13. Photo courtesy of Thermionics Laboratory, Inc Ref. “Sorption of Nitrogen by Titanium Films,” Harra and Hayward, Proc. Int. Symp. On Residual Gases in Electron Tubes, 1967 Varian, Inc Titanium vs. Other Evaporable Getters for Accelerator Use Ba - Ca - Mg : High vapor pressure. Trouble if bake out is requested Zr - Nb - Ta : Evaporation temperature too high  Wide variations due to film roughness  For H2, competition between desorption and diffusion inside the deposited layers  Peel off of the film ~50m Typical required sublimation rate 0.1 to 0.5 g/hr F. Le Pimpec - SLAC

  14. Non-Evaporable Getters NEGs are pure metals or are alloys of several metals - Restoration is achieved by “activation” - heating of the substrate on which the getter is deposited. Joule or bake heating - During activation, atoms migrate from the surface into the bulk, except H2. - Heating to “very high” temperature will outgas the getter. This regenerates it but also damages the crystal structure. • : molecules.s-1.cm-2 : sticking coefficient P : Pressure (Torr) 1ML : ~1015 molecules.cm-2 F. Le Pimpec - SLAC

  15. Non-Evaporable Getters : Uses St 707 (ZrVFe) Pump cartridge for Ion Pump or as lump pumps Use of St 2002 pills to insure a vacuum of 10-3 Torr Application of NEG are rather wide : NEG is used in UHV (accelerators -tokamak) Used for purifying gases (noble gas) Used for hydrogen storage, including isotopes Lamps and vacuum tubes … F. Le Pimpec - SLAC Ref [7]

  16. What Makes NEG So Attractive? • AGREATMaterial • High distributed pumping speed • Initial photo, electro-desorption coefficient lower than most technical material (Al - Cu - SS) • Secondary Electron Yield (SEY) lower than that of common technical materials • Drawbacks • Needs activation by heating - Pyrophoricity (200°C to 700°C) • Does not pump CH4 at RT, nor noble gases • Lifetime before replacement (thin film) F. Le Pimpec - SLAC

  17. Pumping Speed 0.6 H2 Ti32Zr16V52 (at.%) 0.01 Sticking probability 0.005 0 100 350 2 Hours Heating T (°C) CERN/ESTgroup Pumping speed plots for getter are everywhere in the literature • From sample to sample, pumping speed plots vary • Many geometric cm2 are needed to see the pumping effects. Roughness (true geometry) • Temperature and/or time of activation is critical to achieve the pumping speed required • Capacity of absorption of the NEG is determined by its thickness

  18. Insuring Dynamic UHVBeam Interaction Dynamic Outgassing should be studied for every surfaces susceptible of being used No existing coherent theory  Source of gas are induced by photons (SR), electrons and ions bombardment F. Le Pimpec - SLAC

  19. CO NEG 0% Sat (13C18O) 13C18O SS NEG 100 % Sat (13C18O) CO Photodesorption hCO at c = 194 eV NEG St707 (Zr70V25Fe5) An activated NEG desorbs less H2 CO CH4 CO2 than a 300°C baked SSA saturated NEGdesorbs more CO than a baked Stainless Steel F. Le Pimpec - SLAC

  20. SS Cu Also True For Thin films TiZr and TiZrV F. Le Pimpec - SLAC

  21. CO NEG Sat (13C18O) 13C18O NEG Sat by CO Cu NEG 100 % NEG Sat (13C18O) CO Electrodesorption hCO at Ee- = 300 eV NEG St707 An activated NEG desorbs less H2 CO CH4 CO2 than a 120°C baked OFE Cu surface. A saturated NEGdesorbs less *C*O than a 120 °C baked OFE Cu surface F. Le Pimpec - SLAC

  22. Ion Desorption From Al surfaces F. Le Pimpec - SLAC (*) 300°C in the measurement of M.H. Achard

  23. Ion Desorption by Heavy Energetic Ions on Technical Surfaces 1.5 109 Pb53+ ions (per shot) under 89.2° grazing incidence and 4.2 MeV/u E. Mahner et al. NEG Ti30Zr18V52 F. Le Pimpec - SLAC Measure at CERN for the LHC

  24. Other Beam Interactions • Electron cloud & multipacting •  Free electron trapping in a p+ / e+ bunch • Ion instabilities – link to the pressure - Pressure bump - Fast beam-ion collective instability F. Le Pimpec - SLAC Electron Cloud

  25. NLC Fast Head tail straight 1012 SEY of technical surfaces baked at 350°C for 24hrs SEY & Electron Cloud Electron cloud can exist in p+ / e+ beam accelerator and arise from a resonant condition (multipacting) between secondary electrons coming from the wall and the kick from the beam, (PEP II - KEK B - ISR - LHC). M. Pivi F. Le Pimpec - SLAC

  26. NLC: 130 eV e-beam conditioning TiZrV coating Scheuerlein et al. Thin Film & Electron Cloud Low SEY : Choice for the NEG of the activation Temperature and time . Conditioning (photons e-ions) Contamination by gas exposure, or by the vacuum residual gas, increases the SEY; even after conditioning. Angles of incidence, of the PE, change the shape of the curve at higher energy Roughness changes the SEY of a material Variability from sample to sample TiN/SS F. Le Pimpec - SLAC

  27. Al disk with triangular shape 1 mm Real SEY Cu  = 60° Alternative Solution:Playing with Roughness Very rough surfaces emits less SE, because SE can be intercepted by surrounding “walls” Experiment SEY Al flat - grooved result Simulation F. Le Pimpec - SLAC G. Stupakov

  28. Ion Instability – Pressure Bumps • Ionized molecules are accelerated toward the wall by e+ /p beam • Linked directly to I Dependant on surface cleanliness Dependant on the beam pulse structure Ion impact energy as a function of beam current, LHC - Gröbner Runaway condition is possible above a certain threshold Surface with a low  Reduce the Pressure (S) Use of clearing electrodes F. Le Pimpec - SLAC

  29. Fast Ion Instability Fast ion instability can arise in e- beam accelerator from ionization and trapping of the residual gas. T. Raubenheimer The amplitude of displacement yb must be kept as small as possible due to requested luminosityDiminishing the pressure It is not, so far, a critical issue F. Le Pimpec - SLAC

  30. Conclusion Reaching and measuring static XHV is possible and will become necessary, as we push for higher luminosity A NEG barrier diffusion solution provides pumping speed, low (phe-i), low SEY and will insure dynamic UHV  Ion instability – Pressure reduction  Electron Cloud Issue The vacuum solution has to be beam-dynamic friendly  Wakefield (electrical conductivity) due to a film thickness or surface roughness (or both)  Lifetime of the solution (NEG) - % lifetime of the vacuum device  Heat Load in a cryogenic system (e-cloud) … F. Le Pimpec / SLAC-NLC

  31. Acknowledgement SLAC : R. Kirby, M. Pivi, T. Raubenheimer CERN : V. Baglin, JM. Laurent, O. Gröbner,A. Mathewson • … F. Le Pimpec - SLAC

  32. References • CAS Vacuum Technology: CERN 99-05 • H. Brinkmann –Leybold Vacuum • R. Reid – Daresbury Vac group • CERN – Colleagues & web site • P. Danielson : Vacuum Lab • USPAS - June 2002 • SAES getters • SLAC – colleagues • Web request for the beautiful pictures • … F. Le Pimpec - SLAC

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