Beam-Surface Interaction A Vacuum point of viewF. Le PimpecSLAC/NLCCornell 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 Beam Interaction with Technical surfaces - Desorption Induced by Electronic Transition - Electron Cloud - Ion instabilities Summary and Conclusion

Reaching and Measuring XHV(10-12 Torr) Luminosity for accelerators Lifetime in storage ringsReaching XHV is commercially easier than measuring itA CERN modified Helmer gauge measured 10-14 TorrXHV is not official Pressure 10-7 Torr are called UHV

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Torr

Vacuum Gauges

Spinning Rotor gauge

Penning gauge

Hot Cathod Ionization gauge, Bayard Alpert

Cold Cathod Discharge gauge

Extractor - Ionization gauge, Modified Bayard Alpert

Vacuum Pumps

Cryopump

Diffusion pump

Turbomolecular pump

Titanium Sublimation pump

Ion Sputter pump

Non Evaporable Getter pump & Cryogenic pump

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Why Measure Total Pressure ?Partial Pressure gives information on the contents of the vacuumTotal pressure can be computed from the partial P measurementsOperational 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 RGAs electronics are sensitive to the beam passage ! And are still not cheap compared to gauges !RGABA SVT305

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 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

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 standard10-710-4

Reaching XHV in Static Vacuum Reaching UHV from high vacuum is easy : Sputter/getter Ion pumpTo 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)XHV is possible but is not easy to reach because of outgassingDiode

XHV Limit : Outgassing & Vapor PressureTo 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)At which temperature is my system going to be running ?Vapor Pressure :True also for getters and cryosystem

Reaching Static XHV with NEG~24 km of NEG P~10-12 Torr rangeThin film getter is the new adopted way of insuring UHV in colliders or SR light sourcesThe LEP : 1st major success of intensive use of NEG pumpsDAFNEESRFSOLEILDIAMONDRHICLHCNLC/GLC ??...TiZrV NEG Coating Setup at CERN

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 : gettersIncludes Evaporable and Non Evaporable Getter

How do Getters Work ?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 eVDissociation of residual gases on a surface is not systematic

Titanium vs. Other Evaporable Getters for Accelerator Use Wide variations due to film roughness For H2, competition between desorption and diffusion inside the deposited layers Peel off of the film ~50mBa - Ca - Mg : High vapor pressure. Trouble if bake out is requestedZr - Nb - Ta : Evaporation temperature too highTypical required sublimation rate 0.1 to 0.5 g/hr

Non-Evaporable Getters- 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.NEGs are pure metals or are alloys of several metals : molecules.s-1.cm-2 : sticking coefficientP : Pressure (Torr)1ML : ~1015 molecules.cm-2

Non-Evaporable Getters : UsesSt 707 (ZrVFe)Ref [7]Use of St 2002 pills to insure a vacuum of 10-3 TorrPump cartridge for Ion Pump or as lump pumpsApplication of NEG are rather wide :NEG is used in UHV (accelerators -tokamak)Used for purifying gases (noble gas)Used for hydrogen storage, including isotopesLamps and vacuum tubes

What Makes NEG So Attractive? A GREAT MaterialHigh distributed pumping speedInitial photo, electro-desorption coefficient lower than most technical material (Al - Cu - SS)Secondary Electron Yield (SEY) lower than that of common technical materials DrawbacksNeeds activation by heating - Pyrophoricity (200C to 700C)Does not pump CH4 at RT, nor noble gasesLifetime before replacement (thin film)

Pumping SpeedPumping 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 requiredCapacity of absorption of the NEG is determined by its thicknessTi32Zr16V52 (at.%)CERN/EST groupH22 Hours Heating T (C)1003500.60.010.005Sticking probability0

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

Photodesorption hCO at c = 194 eVAn activated NEG desorbs less H2 CO CH4 CO2 than a 300C baked SS A saturated NEG desorbs more CO than a baked Stainless Steel NEG St707 (Zr70V25Fe5)

Also True For Thin films TiZr and TiZrV

Electrodesorption hCO at Ee- = 300 eVNEG St707An activated NEG desorbs less H2 CO CH4 CO2 than a 120C baked OFE Cu surface. A saturated NEG desorbs less *C*O than a 120 C baked OFE Cu surface

Ion Desorption From Al surfaces(*) 300C in the measurement of M.H. Achard

Ion induced desorption yieldA.G. MathewsonM.H. AchardM.H. Achard-R. Calder-A.G. MathewsonM.P. Lozano197619761978200115N2+ at 2 keV15N2+ at 2 keVK+ at 2 keVK+ at 1.4 keVAr+ at 3 keVAluminiumas receivedH24.5 102.33.6 - 10184 - 7CH40.55 0.950.20.3 - 0.910.5 0.8CO7 102.53 - 10.570.9 1.5CO21.8 3.20.51 - 3.71.21 2.5

Aluminium after 24 hours baking at 2000C (*)H23.2 43.22.5CH40.22 0.230.20.32CO2.8 2.92.21.5CO20.75 - 10.180.35

Ion Desorption by Heavy Energetic Ions on Technical SurfacesE. Mahner et al.1.5 109 Pb53+ ions (per shot) under 89.2 grazing incidence and 4.2 MeV/uNEG Ti30Zr18V52Measure at CERN for the LHC

Other Beam InteractionsElectron Cloud Electron cloud & multipacting Free electron trapping in a p+ / e+ bunchIon instabilities link to the pressure- Pressure bump- Fast beam-ion collective instability

SEY & Electron CloudElectron 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

Thin Film & Electron CloudLow 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 energyRoughness changes the SEY of a materialTiN/SSVariability from sample to sample

Alternative Solution:Playing with RoughnessVery rough surfaces emits less SE, because SE can be intercepted by surrounding wallsSimulationG. StupakovSEY Al flat - grooved resultExperiment

Ion Instability Pressure BumpsIonized molecules are accelerated toward the wall by e+ /p beamLinked directly to IDependant on surface cleanlinessDependant on the beam pulse structureIon impact energy as a function of beam current, LHC - Grbner Runaway condition is possible above a certain thresholdSurface with a low Reduce the Pressure (S)Use of clearing electrodes

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

ConclusionReaching 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

AcknowledgementSLAC : R. Kirby, M. Pivi, T. Raubenheimer

CERN :V. Baglin, JM. Laurent, O. Grbner, A. Mathewson

ReferencesCAS Vacuum Technology: CERN 99-05H. Brinkmann Leybold VacuumR. Reid Daresbury Vac groupCERN Colleagues & web siteP. Danielson : Vacuum LabUSPAS - June 2002SAES getters SLAC colleagues Web request for the beautiful pictures