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VACUUM PUMPS AND HARDWARE
OUTLINE Introduction Basic concepts of vacuum Vacuum Hardware (pumps, gauges) Mass Spectrometry
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Research applications: impact on everyday life
GETTERS NEED OF VACUUM TV TUBES LCD BACKLIGHT GAS LIGHTS (NEON, HIGH POWER LAMPS) DEWAR (FOR DRINKS) Getters are stripes of material adsorbing the gas Active material: alkali (Cs, Rb), rare earths (Yb, Lu), Hg Support: Al2O3, Zr Interaction of gas (CO2, O) with getter surface (passivation or oxidation) Role of the surface morphology: surface area/bulk
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Basic concepts of vacuum
UHV Apparatus Gas Kinetics Vacuum concepts Vacuum Pumps Vacuum Gauges Sample Preparation in UHV Cleaving Sputtering & Annealing Fracturing Exposure to gas/vapor Evaporation/Sublimation
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Ultra High Vacuum Apparatus
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Gas kinetics velocity distribution 1D kB = Boltzmann constant
probability of finding a particle with speed in the element dv around v Maxwell-Boltzmann distribution probability density of finding a particle with speed in the element dv around v
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Gas kinetics T (°C) Molecular speed Most probable Mean Quadratic mean
f(v) Molecular speed Most probable Mean Quadratic mean 300 K mNe = 20 • 1.67 x kg
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Gas kinetics
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number of particles landing at a surface per unit area and unit time
Gas kinetics for ideal gas N = total number of molecules n = N/V = number density (mol/cm3) Consider n molecules with speed v moving towards a surface dS dS Arrival rate R: number of particles landing at a surface per unit area and unit time on a surface dS we take the molecules arriving with speed vx in a time dt total number of molecules with speed vx hitting the unit surface in a time dt how many molecules? volume = vdt cosdS
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Gas kinetics
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molecules arrival rate R at a surface
Gas kinetics molecules arrival rate R at a surface (unit area, time) if M=Molar mass m = M • amu mNe = 20 • 1.67 x g kB = Boltzmann’s constant (J/K) T = Temperature (K) p = Pressure (torr) MOx =32 g/mol O2 at p = 760 torr, 293 K R = molecules s-1cm2 O2 at p = 1 x 10-6 torr, 293 K R = molecules s-1cm2
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Gas kinetics: why the UHV
Residual Gas H2O CO O2 CO2 CH4 Solid Surface N2 1 Monolayer ~ 1014 – 1015 atoms/cm2 Bulk Solid Adsorbed Atoms & Molecules
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Gas kinetics Mean free path 2r 2r
The sphere with 2r is the hard volume The surface of the sphere is the effective section or cross section for impact The number of impacts per unit time is
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Gas kinetics Mean free path For different molecules A and B rA rB
p in torr is so large that the collisions with walls are dominant with respect to molecular collisions 2 depends on the fact that we did not consider the presence of other molecules
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Why the UHV O2 at p = 1 x 10-6 torr, 293 K R = 3.61 1014
Sticking probability = 1 1 monolayer of atoms or molecules from the residual gas is adsorbed at the surface in: 1 sec @ p = 1 x 10-6 torr 10 sec @ p = 1 x 10-7 torr 100 sec @ p = 1 x 10-8 torr 1,000 sec @ p = 1 x 10-9 torr 10,000 sec @ p = 1 x torr 100,000 sec @ p = 1 x torr Utra High Vacuum (UHV): p < torr
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Plots of relevant vacuum features vs. pressure
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Gas flux through a pipe d pipe [Q] = [p][L]3[t]-1
p = pressure measured in the plane dV = volume of matter crossing the plane dV/dt= Volumetric flow rate (portata) [Q] = [p][L]3[t]-1 Throughput Volumetric flux: variation of number of molecules through an area
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Variation of mass through an area
Gas flux through a pipe M = total mass Volumetric flux Mass flux Variation of mass through an area M=molar mass Factors affecting the flux Magnitude of flow rates Pressure drop at the pipe ends Surface and geometry of pipe Nature of gases
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Regimes of gas flux through a pipe
Throughput d pipe For < d viscous For d intermediate For > d molecular Viscous S = layer contact area dvx /dy = mol speed gradient The mol-mol collisions are dominant Friction force = viscosity laminar turbulent
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Regimes of gas flux through a pipe
d pipe Volumetric flux mass flux For a pipe with diameter d and section d2/4 Q’ mass flux per unit section = viscosity Reynolds number Laminar: Re<1200 turbulent: Re>2200
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Regimes of gas flux through a pipe
Reynolds number Laminar: Q < (T/M)d [Pa m3/s] Turbulent: Q > (T/M)d [Pa m3/s]
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Regimes of gas flux through a pipe
d For < d For d For > d viscous intermediate molecular Only for intermediate and molecular flux Knudsen number = d/ intermediate 3 d/ 80 d/ 3 molecular 10-2 p d 0.5 p d 10-2 For air at RT
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Pipe conductance: Pipe impedance: In parallel [C] = [L]3[t]-1
Flux across pipe Pipe conductance: [C] = [L]3[t]-1 Pressures at pipe ends Pipe impedance: SI: m3s-1 cgs: lt s-1 In parallel
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In series Q1 = Q2 = QT or gas would accumulate
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Pipe conductance Viscous and intermediate regime (Poiseuille law)
Laminar Turbulent Molecular regime Long cylindrical pipe For air at 0 C: 11,6 d3/L [lt/s] Elbow pipe The molecules must collide with walls at least once before exiting Equivalent to a longer pipe
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Relevant physical parameters of a pumping system
Q= flux through aspiration aperture p = Vessel Pressure V = Vessel Volume p0 = pressure at pump inlet p0 Pumping speed S = Q/p0 [S] = [L]3[t]-1 C Volumetric flow rate SI: m3s-1 cgs: lt s-1 In the presence of a pipe Q at the pump inlet is the same as Q in pipe Effective pumping speed in the vessel
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Effective pumping speed
Relevant physical parameters of a pumping system p0 Q= flux through aspiration aperture p = Vessel Pressure V = Vessel Volume C Pumping speed S = Q/p0 Effective pumping speed [S] = [L]3[t]-1 if C = S the Se is halved
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Sources of flux (molecules)
Relevant physical parameters of a pumping system Q= flux through aspiration aperture p = Vessel Pressure V = Vessel Volume p0 Sources of flux (molecules) Q1 = True leak rate (leaks from air, wall permeability) Q2 = Virtual leak rate (outgas from materials, walls) Outgas rate for stainless steel after 2 hours pumping: 10-8 mbar Ls-1 cm-2
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Pump-down equation for a constant volume system
Q = Q0 +Q1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Short time limit Long time limit True leak rate Only the gas initially present contributes Virtual leak rate Other outgassing sources contribute
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Pump-down equation for a constant volume system
Q = Q0 +Q1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Short time limit True leak rate Suppose: Constant S Q = 0 Time needed to reduce p by 50 % Vol of 1 m3 = 103 L to be pumped down from 1000 mbar to 10 mbar in 10 min = 600 s V= 1000 L P0 = 133 Pa S= 20 L/s t = 331,6 s 7.5 L/s = 27 m3/h
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Pump-down equation for a constant volume system
Q = Q0 +Q1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Long time limit Virtual leak rate Other outgassing sources contribute dp/dt = 0 Ultimate pressure
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Pressure versus distance
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Differential pumping operate adjacent parts of a vacuum system at distinctly different pressures A, B to be maintained at pressures P1 and P2, P1 >> P2 A: gas in with flux QL gas to B with flux q Q1 = flux pumped S1 = Q1/p1 QL/p1 B: gas in with flux q To keep pressure p2 S2 = q/p2 q = C(p1 − p2) C p1 S2 = Cp1/p2 The size of the aperture depends by its function conductance C is determined. Modern Vacuum Physics, Ch. 5.8
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Example CVD coatings on panels
Antireflective coatings, p-n junction growth for solar panels P0 P1 P2 P1 P0 C C C S2 S3 S1 S1 = Cp0/p1 S2 = Cp1/p2 S3 = Cp2/p1
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Gas-solid interaction
H2O CO elastic inelastic trapped CO2 physical adsorption (shortened to Physisorption): bonding with structure of the molecule unchanged Chemisorption: bonding involves electron transfer or sharing between the molecule and atoms of the surface Can be thought of as a chemical reaction CH4 N2 O2 He H2
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Gas-solid interaction
Physisorption CO Origin: Van der Waals forces H2O CH4 CO2 O2 N2 Typical q: kJ/mol = 0, ,52 eV /molecule He H2 The well depth is the energy of adsorption E to be supplied to desorb the molecule
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Gas-solid interaction
Chemisorption CO Origin: Electron sharing or transfer between molecules and surface atoms H2O CH4 CO2 O2 N2 Typical q: kJ/mol = 0, eV /molecule He H2 The well depth is the energy of adsorption P is a precursor state the molecules have to overcome
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How does this affect vacuum?
Gas-solid interaction How does this affect vacuum? Molecule trapped in the adsorbed state at temp. T potential well of depth q Dilute layer (no interactions with other mol.) How long does it stays? O2 Surface atoms have Evib = h = KBT = KBT/h At RT = 0.025/(6.63 × 10−34 ÷ 1.6 × 10−19) = 6 × 1012 s−1 1013 s−1 = number of attempts per second to overcome the potential barrier and break free of the surface. probability that fluctuations in the energy will result in an energy q Boltzmann factor probability per second that a molecule will desorb
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Gas-solid interaction
probability per second that a molecule will desorb p(t) = probability that it is still adsorbed after elapsed t p(t+dt) = p(t) x (1-dt) O2 probability of not being desorbed after dt dp = p(t+dt) - p(t) = - dt p(t) average time of stay
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Temperature dependance
Gas-solid interaction average time of stay At RT 1013 s−1 Molecular dependance O2 97 kJ / mol = 1 eV / molecule Temperature dependance Note: Simple model Neglects all other interactions, surface diffusion, adsorption sites so a can change
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Desorption Outgassing rate 1012 molec s−1cm-2 P = 1000 mbar
pumping Equilibrium Far from equilibrium till…. Experimental relation Gas flux /area = 0.5 = 1 for metals = 1 = 0.5 for elastomers q1 5x10−8 mbar L s−1cm-2 1 mbar L Nat 2.6x1019 Outgassing rate 1012 molec s−1cm-2
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How important is the molecule/surface interaction energy?
Desorption How important is the molecule/surface interaction energy? H2O Rate of desorption N2 integrate fall of pressure at RT Simple model calculation idealized UHV system RT, V= 1 L, A = 100 cm2 S = 1 L/s only gas source: initially complete ML of specified binding energy adsorbed at the wall q
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Outgassing Gas is continuously released, (at relatively small rates) from walls Principally water vapor Limit to attainable vacuum achievable in reasonable times (hours) ∼10−6 mbar Origin of fluxes: Permeation Adsorption Solubility Desorption
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Gas-solid permeation p1 = 1000 mbar p2 = 1x10-8 mbar H2O CO CO2 CH4 N2
He H2 Residual Gas
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Gas-solid permeation p1 = 1000 mbar p2 = 1x10-9 mbar Permeation is a
complex process Adsorption Residual Gas Dissociation Solution into the solid Diffusion Recombination Desorption
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Gas-solid permeation p1 = 1000 mbar p2 = 1x10-9 mbar Residual Gas
Permeation process can be quantified trough phenomenological quantities Residual Gas permeability =Q/(p1-p2)A Q=flux trough wall A= unit area [Q] = [p][L]3[t]-1 =[L]3[t]-1[L]-2 m3s-1m-2 ls-1cm-2
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depending on diffusion
Gas-solid permeation For a given gas A = wall area d = wall thickness depending on diffusion mechanisms He Kp = Permeability coefficient p = 13 mbar d = 1 mm cm3s-1cm-2 Pa-1 m3s-1m-1Pa-1
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Gas-solid permeation Metal – gas Kp Table of gas permeability Glass
Metals Polymers He, H2, Ne, Ar, O2 No rare gas All gases p p
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Solubility Is the quantity of substance A that can be dissolved in B at given T and p For a gas Gas quantity dissolved in solid volume unit at standard conditions For undissociated molecular gas (interstitial) Henry’s law c = gas concentration Valid for low concentrations and for glass and plastic materials No formation of alloys
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Solubility H2 on metals For dissociated gas
Interstitial or substitutional Sievert’s law Valid for low concentrations and for metals Note the high solubility of H2 in Ti,Zr
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Vacuum Pumps Throughput pumps Capture pumps Pistons Cold traps Gears
Turbines Jet stream Cold traps Ionization Getters Differences: pressure range, speed, gas selectivity
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Pressure Ranges Spanned by Different Vacuum Pumps
More than one pump to HV and UHV
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What pump to use? Pumping speed S = Q/p Compression ratio:
p = inlet pressure S = [L]3[t]-1 Depends on the gas type S varies with p Q=Q0 cost puS = Q0 For a pressure range where S does not depend on p, i.e. the pumping speed is constant This can be used to measure S or to estimate the time to reach pu Compression ratio:
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What pump to use? Ultimate pressure Time to reach the u.p.
Residual gas composition Other (absence of magnetic fields)
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Rotary Roughing Pump S: 2,5 ÷ 102 m3/h CR: 105 0.7 ÷ 28 l/s
inlet Exhaust valve Oil Rotor blade Eccentric rotor Spring Cylindric body S: 2,5 ÷ 102 m3/h 0.7 ÷ 28 l/s CR: 105 Starting operating pressure: 103 mbar Pu: 10-2 mbar 1 m3/h = 0.28 l/s
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Dual stage Rotary Roughing Pump
inlet Exhaust valve Rotor blade Eccentric rotor Spring Pu: 10-3 ÷ 10-4 mbar Advantages No saturation Heavy duty Low cost (2500 €) Disadvantages Oil backstreaming Need traps for oil vapor Noisy
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Rotary Roughing Pump: gas ballast
CR=105 Op. temp T 70 °C The gas can liquefy inside the rotation chamber Vapor pressure Pump water vapor at 70 °C when P reaches Pa The vapor liquefies and does not reach P > Pa So the exhaust valve does not open The vapor remains inside the pump and is mixed with oil Decrease pump speed, and can damage the rotor by increasing the friction
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Rotary Roughing Pump: gas ballast
Solution: gas ballast NO gas ballast Gas ballast The valve is set to decrease the CR to 10 liquid Ballast valve The vapor does not liquefy
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Diaphragm Pump CR: 102 103 Starting operating pressure: 103 mbar
Housing Valves Head cover Diaph. clamping disc Diaphragm Diaphragm supp. disc Connecting rod Eccentric bushing CR: 102 103 Starting operating pressure: 103 mbar Pu: ~ 1 mbar
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High ult.pressure (4 mbar)
Diaphragm Pump Advantages Oil-free No saturation Low cost Disadvantages High ult.pressure (4 mbar) Low pump speed Noisy
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Root Pump Advantages Disadvantages Oil-free No saturation
Eight-shaped rotor turning in opposite direction Clearance between rotors ~ 0.3 mm No lubricants CR depends on clearance Advantages Oil-free No saturation High throughput Disadvantages Need prevacuum Medium cost delicate
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Root Pump patm pp pr S and CR of a root pump depend
on the preliminary pump installed ahead root palette Sr Sp The gas flux is the same for both pumps Palette: 60 m3/h = 16,8 l/s Sr = 16,8 x 40 = 672 l/s
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Turbomolecular Pump S: 50 ÷ 5000 l/s CR: 105 109 Starting operating
pressure: 10-2 mbar Pu: mbar
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Turbomolecular Pump Molecular speed distribution
without blades (only v) Molecular speed distribution plus blade speed
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Turbomolecular Pump Principle of operation Molecular regime
Low pressure side High pressure side The speed distribution (ellipse) depends on the angle between V and blade The pumping action is provided by the collisions between blades and molecules
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Turbomolecular Pump Pumping speed: depends on gas type After bake out
Residual gas: H2
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Compression Ratio of a Turbomolecular Pump
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Turbomolecular Pump Rotor suspension
Ball bearings (lubricant required) Magnetic (lubricant absent) Advantages No saturation Clean (magnetic) UHV Any orientation Disadvantages Cost Delicate Quite noisy 70 l/s ~ 4000 € 250 l/s ~ 9000 € 2000 l/s ~ €
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Molecular drag pump Operating principle: Same as turbo but
Safety ball bearing Turbo disk Magnetic bearing Threaded stator Cylindrical Rotor Operating principle: Same as turbo but different geometry Threaded stator No blades but threads Forevacuum flange (outlet) Gas entry Lubricant reservoir Electrical socket
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Molecular drag pump CR: H2: 102 109 He: 103 104 S: 40 ÷ 100 l/s
N2: 107 109 S: 40 ÷ 100 l/s Starting operating pressure: 1-20 mbar Pu: 10-7 mbar They are use in combination with turbo in a single mounting so Higher backing vacuum pressure Use a low CR backing pump (i.e. membrane for clean operation)
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vapor diffusion pump baffle
Fluid is heated and ejected from nozzles at high speed due to the nozzle shape and pressure difference between inside and pump cylinder. Fluid speed up to Mach 3-5 The gas molecules are compressed to the pump base through collisions with oil vapor baffle
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The pumping speed and the pressure strongly depends on oil type
vapor diffusion pump The pumping speed and the pressure strongly depends on oil type Starting operating pressure: 10-2 mbar S: 20 ÷ 600 l/s Pu: 10-9 mbar Disadvantages gas reaction Liquid vapor tension Contamination Needs water cooling Advantages No saturation Heavy duty Low cost
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Getter pumps Pumping mechanism - Gas-surface chemical interaction
- Chemisorption - Solution of gas inside material Sublimation getters Non evaporable getters The active material is sublimated by thermal heating The active material is constituted by porous medium
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Sublimation getter pumps
Pumping mechanism - Gas-surface chemical interaction - Chemisorption - Solution of gas inside material Sublimation getters Ti or Ti – Mo filaments The material form a thin film on the pump walls that becomes the active layer The molecules are chemisorbed on the film
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Non evaporable getter pumps
Pumping operation Cartridge of porous material (Zr-16%Al) Activated by heating (750 °C) and kept at operating T 300 °C to increase molecule diffusion Problem: saturation of getter material requires cartridge change
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Getter pumps S strongly depends on gas sublimation Non evaporable
area Pumping speed (l/s) Adsorption probability Molecular weight (g) S strongly depends on gas sublimation Non evaporable 800- 2x103 l/s > 103 l/s A’= sublimation, A=non evaporable Zr-Al S depends on active surface saturation
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Getter pumps With a number of panels one can obtain S > 1x104 l/s
Stripes of active material Plus: Wall cooling Gas-surface weak interaction Physisorption and diffusion into the bulk But if warmed it releases the gas Advantages Pump H2 Heavy duty Low cost No contamination Disadvantages Saturation Metal vapours No rare gas pumping Pressure limit: 10-10 ÷ mbar
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Ion-getter pump Pumping mechanism - Gas-surface chemical interaction
- Chemisorption - Solution of gas inside material - Ionization of gas molecules - Burying inside the active material Ion-getter with cathodic grinding Ti ~1 Tesla 7 KV
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Basic processes occurring within a single cell
e- ionize molecules Secondary e- ionize molecules Ions are accelerated to cathodes produce secondary e- grind up cathode material make craters Ions buried into cathode material Produce cathode vapors Depositing also on anodes to work as getters H2: accumulates into the cathodes Need regeneration by annealing
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Ion-getter pump Starting operating pressure: 10-3 10-4 mbar
S: 4 ÷ 1000 l/s Pressure limit: 10-11 ÷ mbar Advantages Heavy duty No traps No contamination Any mounting position Silent Disadvantages High magnetic fields Low pump S for H2 Medium - high cost
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Cryogenic pumps Adsorbing pumps Liquid He cooled Cold walls
Pumping mechanism - Gas – cold surface interaction - Physisorption, condensation Liquid N2 cooled Adsorbing material Adsorbing pumps Pumping mechanism - Gas – cold surface interaction - Physisorption Adsorbing porous material High surface/volume ratio Zeolites Al2O3, SiO2 H2O and N2 pumping
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Cryopump Pumping mechanism - Gas – cold surface interaction
- Physisorption and condensation Metal wall
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Cryopump The gas condensation if gas pressure > vapor pressure
at wall T S: 4 ÷ 100 l/s Starting operating pressure: 10-9 mbar Pressure limit: 10-10 ÷ mbar Advantages Heavy duty No contamination Low cost Disadvantages Saturation Noisy Needs other UHV pumps
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Ionization of a molecule (atom) from collisions with e-
Ionization in gases Type of collisions: neutral Molecule – electron neutral Molecule – ions neutral molecule – neutral molecule (Penning) radiation absorption neutral Molecule – hot metal surface Ionization of a molecule (atom) from collisions with e- - - + - - Ion - Ion +
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Ionization in gases Ionization energy Electron affinity Ion + Ion - -
Less probable - More probable eV
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Atom or neutral molecule – electron collision
Collision type: elastic atom excitation molecule dissociation - Ionising ( e) Elastic collision Considering the relative speed and energy conservation In the collision the kinetic energy of electrons (and of the molecules) remains almost unchanged Very small relative energy loss for electrons
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Elastic collision e- suffers very small energy loss for each elastic collision e- mean free path e = average space between two elastic collisions e- collision rate e = collisions number per unit time number of collisions Total energy loss
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Elastic collision Apply external electric field E If e- has vin~ 0
Maximum kinetic energy of an e- moving in a gas Depends on electric field and pressure
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Ionization probability i = ionizing collisions/total collisions
if Ionization energy e- can ionize an atom - But it can also - Increase the atom kinetic energy - Excite an e- to unoccupied bound states + Ionization probability i = ionizing collisions/total collisions -
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Ionization e- can ionize an atom e- trapped inside atom
with formation of negative ions - But it can also Due to practical measurements Specific ionization coefficient - e- with Ek unit pressure unit lenght Long path to produce more ions
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Vacuum measurement Different types of vacuometers depending on pressure range Mechanical, thermal, ionization
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Vacuum measurement Mechanical Bourdon Membrane 105 102 Pa
tube Pin wheel index To vacuum 105 102 Pa (103 1 mbar) 105 102 Pa (103 1 mbar) The tube curvature changes with pressure Needs calibration Precision: 1-2% fsr The membrane or bellow bends with pressure Needs calibration Precision: 1-2% fsr
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Thermal conductivity vacuometers
Pirani heated filament The filament temperature, and hence the resistance depends on heat dissipation in the gas, i.e. on the gas pressure Pressure variation means T variation i.e. resistance variation. This is measured through the W. bridge V variation
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Thermal conductivity vacuometers
unbalanced Thermal dissipation contact dissipation radiative dissipation = cost Stephan-Boltzmann =wire emissivity Kgas= gas thermal conductivity Kf= wire thermal conductivity =coefficient For small p, the reference bridge is Hence The pressure is obtained by measuring the Wheatstone voltage In general it depends on the gas type
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Based on gas ionization and current measurements
Ionization vacuum gauges Hot cathode Cold cathode Based on gas ionization and current measurements
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Ionization vacuum gauge
I+ = ion current i = specific ionization coefficient I- = electron current from filament Sensitivity K = σi · λe e = electron mean free path I+ = I- i e p Directly proportional to pressure Sensitivity K = i e The gauge measure the total pressure K depends on gas, gauge geometry, gauge potential Range: 10-4 – mbar Usually one increases by designing the gometry
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Ionization vacuum gauge
Cold cathode 1 tesla electrons from gas or field emission similar to the behavior inside the ion getter pumps Less precise due to problem of discharge current at low pressure No filament so less subject to Filament faults Note: discharge starts only by mag field to avoid high E field - induced currents Range: 10-4 – 5 x10-10 mbar
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Mass Spectrometry Need to distinguish the intensity of specific gas molecules Collect molecules Molecule ionization Separation of different molecules Current measurement Specific mass = ion mass (a.u.)/ion charge = n = ion ionization multiplicity Specific mass of Ar+ = 40 For a single molecule there are many peaks, depending on n Specific mass of Ar++ = 20
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Mass Spectrometry Specific mass table
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Mass Spectrometry detector Faraday cup All ions measured
To remove secondary electrons Faraday cup All ions measured No filaments Low sensitivity sturdy Channeltron - electron multiplyer High sensitivity Delicate Fast response Amplifier time constant large
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Quadrupole Mass Spectrometry (QMS)
Storing system Detector (Channeltron) Analyser (Quadrupole field) Ion source (filament) Vacuum Chamber Quadrupole field between the rods Ions of varying mass are shot axially into the rod The applied quadrupole field deflects the ions in the X and Y directions, causing them to describe helical trajectories through the mass filter.
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Quadrupole Mass Spectrometry (QMS)
The forces are uncoupled along x,y,z axis -(U+Vcos(t)) Quadrupole potential U+Vcos(t) r0 = rod separation (~3mm) Superimpose an oscillating field Vcos(t)
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Quadrupole Mass Spectrometry (QMS)
ion equation of motion Constant speed along z Stability parameters
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Quadrupole Mass Spectrometry (QMS)
Solved numerically for different a and q Ions oscillate in the xy plane Only some e/m values reach detector Solutions inside are real (stable trajectory) All solutions outside are imaginary and give increasing oscillation amplitudes Neutralization of the ions on the rods
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Quadrupole Mass Spectrometry (QMS)
Zoom to region I fixed U, V and the overall ion motion can (depending on the values of a and q) result in a stable trajectory causing ions of a certain m/z value to pass the quadrupole Stable solutions for The line shrink to one point Only one ion with m/e ratio can reach detector
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Quadrupole Mass Spectrometry (QMS)
Reducing U relative toV, an increasingly wider m/z range can be transmitted simultaneously. Zoom to region I Work line q for The line enter the stable solutions region V=V0cos(t) All the ions with a/q on the line will reach detector the width q of the stable region determines the resolution. By varying the magnitude of U and V at constant U/V ratio an U/V = constant scan is obtained ions of increasingly higher m/e values to travel through the quadrupole
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Quadrupole Mass Spectrometry (QMS)
How to select different m/e ratios Change U, V keeping the ratio constant This results in different m/e values allowed to pass the quadrupole In the U,V space a changing in m/e ratio means moving along the straight line
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Quadrupole Mass Spectrometry (QMS)
VAC > UDC -(U+Vcos(t)) (y) Ion traveling in the z direction X Motion will tend to be stable for (t) >0 and unstable (t) <0 U+Vcos(t) (x) Greatest effect on the lighter ions (smaller m/e) ejected to the electrodes Heavier ions will be less perturbed and stay in a stable transmitting trajectory. The pair of X rods acts as a high pass filter. Y Motion will tend to be stable for (t) <0 and unstable (t) >0 Greatest effect on the heavier ions (larger m/e) ejected to the electrodes Lighter ions will be less perturbed and stay in a stable transmitting trajectory. The pair of Y rods acts as a low pass filter. the two pairs act as a band pass filter, low and high mass ions outside the band being rejected, with limits determined by the values of U, V, and ω.
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p ≈ 3x10-7 mbar Before bake-out p ≈ 5x10-11 mbar After bake-out
Quadrupole Mass Spectroscopy (QMS) profiles of the residual gas H2O p ≈ 3x10-7 mbar Before bake-out H2 H2O CO+N2 p ≈ 5x10-11 mbar After bake-out CO2
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VACUUM SEALING Low Vacuum Clamps Viton rings No bake at high temperatures Reusable
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VACUUM SEALING UHV HV Plastic deformation and shear
Bake at high temperatures Reusable (maybe once)
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VALVES Diaphragm Butterfly
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VALVES Stem All metal Dynamometric sealing
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Large clearance for instruments
VALVES Leak Gate High conductance UHV to air compatible Large clearance for instruments Bakeable
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FEEDTHROUGH Multi-pin for high currents Multi-pin for signal or
Low currents
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MANIPULATION Rotation
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