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

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

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

Reaching and Measuring XHV (10-12 Torr) Luminosity for accelerators Lifetime in 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

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

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 : Wrong The grid will pump then release molecules Modify Extractor gauge with hidden collector (U. Magdeburg) Installing a hot gauge in a small tube – Transpiration effect Despite a higher pressure the gauge will read lower. Solution: nude gauges – but sensitivity to stray ions from surroundings F. Le Pimpec - SLAC

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 Ar Trace Kr Trace Pressure (mbar) 10-7 10-4

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 Distributed Pumping XHV is possible but is not easy to reach because of outgassing F. Le Pimpec - SLAC

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

Reaching Static XHV with NEG The LEP : 1st major success of intensive use of NEG pumps LEP dipole chamber, getter St101 (ZrAl) (1989-2000) Inserted “linear” pump Inserted “total” pump (TiZrV) Surface pump / diffusion barrier Lump pumping C. Benvenutti 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

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

The use of a clean surface to form chemicals bonds 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

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 Photo courtesy of Thermionics Laboratory, Inc Varian, Inc Ref. “Sorption of Nitrogen by Titanium Films,” Harra and Hayward, Proc. Int. Symp. On Residual Gases in Electron Tubes, 1967  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

Non-Evaporable Getters NEGs are pure metals or are alloys of several metals : molecules.s-1.cm-2 : sticking coefficient P : Pressure (Torr) 1ML : ~1015 molecules.cm-2 - 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. F. Le Pimpec - SLAC

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]

What Makes NEG So Attractive? A GREAT Material 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

Pumping Speed 0.6 H2 Ti32Zr16V52 (at.%) 0.01 Sticking probability 0.005 100 350 2 Hours Heating T (°C) CERN/EST group 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

Insuring Dynamic UHV Beam 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

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

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

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

Ion Desorption From Al surfaces Ion induced desorption yield A.G. Mathewson M.H. Achard M.H. Achard-R. Calder-A.G. Mathewson M.P. Lozano 1976 1978 2001 15N2+ at 2 keV K+ at 2 keV K+ at 1.4 keV Ar+ at 3 keV Aluminium as received H2 4.5 – 10 2.3 3.6 - 10 18 4 - 7 CH4 0.55 – 0.95 0.2 0.3 - 0.9 1 0.5 – 0.8 CO 7 – 10 2.5 3 - 10.5 7 0.9 – 1.5 CO2 1.8 – 3.2 0.5 1 - 3.7 1.2 1 – 2.5 Aluminium after 24 hours baking at 2000C (*) 3.2 – 4 3.2 0.22 – 0.23 0.32 2.8 – 2.9 2.2 1.5 0.75 - 1 0.18 0.35 F. Le Pimpec - SLAC (*) 300°C in the measurement of M.H. Achard

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

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

NLC Fast Head tail straight 1012 SEY & Electron Cloud NLC Fast Head tail straight 1012 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). SEY of technical surfaces baked at 350°C for 24hrs M. Pivi F. Le Pimpec - SLAC

Thin Film & Electron Cloud NLC: 130 eV e-beam conditioning 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 Scheuerlein et al. TiZrV coating Variability from sample to sample TiN/SS F. Le Pimpec - SLAC

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 Al disk with triangular shape 1 mm Real SEY Cu  = 60° Simulation F. Le Pimpec - SLAC G. Stupakov

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

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

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

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

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