VACUUM PUMPS AND HARDWARE OUTLINE Introduction Basic concepts

VACUUM PUMPS AND HARDWARE OUTLINE • Introduction • Basic concepts of vacuum • Vacuum Hardware (pumps, gauges) • Mass Spectrometry 1

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: Al 2 O 3, Zr Interaction of gas (CO 2, O) with getter surface (passivation or oxidation) Role of the surface morphology: surface area/bulk 2

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 3

Ultra High Vacuum Apparatus 4

Gas kinetics velocity distribution 1 D k. B = Boltzmann constant velocity distribution 3 D 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 5

Gas kinetics 6

Molecular speed T (°C) f(v) Gas kinetics Most probable Mean Quadratic mean Neon @ 300 K m. Ne = 20 • 1. 67 x 10 -27 kg 7

Gas kinetics for ideal gas N = total number of molecules n = N/V = number density (mol/cm 3) Consider n molecules with speed v moving towards a surface d. S Arrival rate R: number of particles landing at a surface per unit area and unit time on a surface d. S we take the molecules arriving with speed v x in a time dt Nmolecule = how many molecules in d. V? total number of molecules with speed v x hitting the unit surface in a time dt volume d. V = vdt cos d. S 8

Gas kinetics 9

Gas kinetics molecules arrival rate R at a surface (unit area, time) if Mr=relative Molar mass m = Mr • amu m. Ne = 20 • 1. 67 x 10 -27 g k. B = Boltzmann’s constant (J/K) T = Temperature (K) p = Pressure (torr) MOx =32 O 2 at p = 760 torr, 293 K R = 2. 75 1023 molecules s-1 cm 2 O 2 at p = 1 x 10 -6 torr, 293 K R = 3. 61 1014 molecules s-1 cm 2 10

Gas kinetics Mean free path 2 r 2 r The sphere with 2 r 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 11

Gas kinetics For different molecules A and B r. A r. B Mean free path 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 12

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Gas kinetics: why the UHV Residual Gas H 2 O CO O 2 CH 4 Solid Surface N 2 1 Monolayer ~ 1014 – 1015 atoms/cm 2 Bulk Solid Adsorbed Atoms & Molecules 14

Why the UHV O 2 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 10 -10 torr 100, 000 sec @ p = 1 x 10 -11 torr Utra High Vacuum (UHV): p < 10 -10 -10 -11 torr 15

Plots of relevant vacuum features vs. pressure 16

Gas flow through a pipe Pipe d d. V Throughput p p = pressure measured in the plane d. V = volume of matter crossing the plane d. V/dt = Volumetric flow rate (portata volumetrica) [Q] = [p][L]3[t]-1 Quantity of gas (the V of gas at a known p) that passes a plane in a known time at constant temperature For steady flow, Q is continuous, i. e. , it has the same value at every position along the pipe, reflecting the conservation of mass. Qin = Qout Particle flow rate: variation of number of molecules through an area 17

Gas flow through a pipe Throughput Mass flow rate M = total mass Variation of mass through an area M=molar mass Factors affecting the flow • Magnitude of flow rates • Pressure drop at the pipe ends • Surface and geometry of pipe • Nature of gases 18

Regimes of gas flow through a pipe Throughput d For < d viscous For d intermediate For > d molecular pipe Viscous The mol-mol collisions are dominant Friction force S = layer contact area dvx /dy = mol speed gradient = viscosity laminar turbulent 19

Regimes of gas flow through a pipe d pipe mass flow For a pipe with diameter d and section d 2/4 Q’ mass flow per unit section Reynolds number = viscosity Laminar: Re<1200 turbulent: Re>2200 20

Regimes of gas flow through a pipe Reynolds number Laminar : Q < 8 103 ( T/M)d [Pa m 3/s] : Q < 5. 88 104 ( T/M)d [Torr l 3/s] Turbulent: Q > 1. 4 104 ( T/M)d [Pa m 3/s] : Q > 1. 08 105 ( T/M)d [Torr l 3/s] 21

Regimes of gas flow through a pipe 22

Regimes of gas flow through a pipe 23

Regimes of gas flow through a pipe d For < d viscous For d intermediate For > d molecular Knudsen number = d/ intermediate 3 d/ 80 molecular d/ 3 For air at RT p in Torr, λ in cm Only for intermediate and molecular flow intermediate: 10 -2 p d 0. 5 molecular: p d 10 -2 24

Pipe conductance: gas flow across pipe Pressures at pipe ends For small aperture connecting two chambers N 2 , V 2 P 2 [C] = [L]3[t]-1 SI: m 3 s-1 cgs: lt s-1 N 1, V 1 P 1 Arrival rate R: number of particles landing at a surface per unit area and unit time 25

Pipe conductance Viscous and intermediate regime (Poiseuille law) Laminar Turbulent Molecular regime Long cylindrical pipe For air at 0 C: 11, 6 d 3/L [lt/s] Elbow pipe The molecules must collide with walls at least once before exiting Equivalent to a longer pipe 26

Pipe conductance: Pipe impedance: In parallel 27

In series Q 1 = Q 2 = QT or gas would accumulate 28

The Concept of Transmission Probability R 1 R 2 R 1 A = total number of molecules /s crossing the plane EN to enter the pipe They approach it from all directions within a solid angle 2π in the left-hand volume Few molecules (1) will pass right through the pipe without touching the sides The majority (2) collide with the wall at a place such as X and return to the vacuum in a random direction After collision the molecule may: (a) return to the left-hand volume (b) go across the pipe to Y, and then another “three-outcome” event (c) leave the pipe through the exit plane EX into the right-hand volume. These three outcomes occur with different probabilities 29

Relevant physical parameters of a pumping system Q= flow through aspiration aperture p = Vessel Pressure V = Vessel Volume p 0 = pressure at pump inlet p 0 C Pumping speed S = Q/p 0 [S] = [L]3[t]-1 Volumetric flow rate SI: m 3 s-1 In the presence of a pipe cgs: lt s-1 Q at the pump inlet is the same as Q in pipe Effective pumping speed in the vessel 30

Relevant physical parameters of a pumping system p 0 Q= flow through aspiration aperture p = Vessel Pressure V = Vessel Volume C Pumping speed S = Q/p 0 Effective pumping speed [S] = [L]3[t]-1 if C = S the Se is halved 31

Relevant physical parameters of a pumping system Q= flow through aspiration aperture p = Vessel Pressure V = Vessel Volume p 0 Sources of flow (molecules) Q 1 = True leak rate (leaks from air, wall permeability) Q 2 = Virtual leak rate (outgas from materials, walls) Outgas rate for stainless steel after 2 hours pumping: 10 -8 mbar Ls-1 cm-2 32

Pump-down equation for a constant volume system Q = Q 0 +Q 1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Short time limit True leak rate Only the gas initially present contributes Long time limit Virtual leak rate Other outgassing sources contribute 33

Pump-down equation for a constant volume system Q = Q 0 +Q 1 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 % V= 1000 L P 0 = 133 Pa S= 20 L/s t = 331, 6 s Vol of 1 m 3 = 103 L to be pumped down from 1000 mbar to 10 mbar in 10 min = 600 s 7. 5 L/s = 27 m 3/h 34

Pump-down equation for a constant volume system Q = Q 0 +Q 1 S = Pumping speed p = Vessel Pressure V = Vessel Volume Long time limit Virtual leak rate Other outgassing sources contribute dp/dt = 0 Ultimate pressure 35

Pressure versus distance 36

Differential pumping operate adjacent parts of a vacuum system at distinctly different pressures A, B to be maintained at pressures P 1 and P 2, P 1 >> P 2 A: gas in with flow QL gas to B with flow q Q 1 = flow pumped S 1 = Q 1/p 1 QL/p 1 B: gas in with flow q To keep pressure p 2 S 2 = Q/p 2 q = C(p 1 − p 2) C p 1 S 2 = Cp 1/p 2 The size of the aperture depends by its function conductance C is determined. 37 Modern Vacuum Physics, Ch. 5. 8

Example CVD coatings on panels Antireflective coatings, p-n junction growth for solar panels P 1 P 0 C P 2 C S 1 = Cp 0/p 1 P 0 C S 2 = Cp 1/p 2 S 3 = Cp 2/p 1 38

Gas-solid interaction H 2 O CO CO 2 N 2 H 2 inelastic trapped physical adsorption (shortened to Physisorption): bonding with structure of the molecule unchanged CH 4 He elastic Chemisorption: bonding involves electron transfer or sharing between the molecule and atoms of the surface Can be thought of as a chemical reaction O 2 39

Gas-solid interaction Physisorption CO Origin: Van der Waals forces H 2 O CH 4 CO 2 N 2 He Typical q: 6 - 40 k. J/mol = 0, 062 - 0, 52 e. V /molecule The well depth is the energy of adsorption E to be supplied to desorb the molecule 40

Gas-solid interaction Chemisorption CO Origin: Electron sharing or transfer between molecules and surface atoms H 2 O CH 4 CO 2 N 2 He Typical q: 40 - 1000 k. J/mol = 0, 52 - 10 e. V /molecule The well depth is the energy of adsorption P is a precursor state the molecules have to overcome 41

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? O 2 Surface atoms have Evib = h = 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 42

Gas-solid interaction probability per second that a molecule will desorb p(t) = probability that it is still adsorbed after elapsed t O 2 p(t+dt) = p(t) x (1 - dt) probability of not being desorbed after dt dp = p(t+dt) - p(t) = - dt p(t) average time of stay 43

Gas-solid interaction average time of stay At RT 1013 s− 1 Molecular dependance O 2 97 k. J / mol = 1 e. V / molecule Temperature dependance Note: Simple model Neglects all other interactions, surface diffusion, adsorption sites so a can change 44

Gas-solid interaction monolayer (ML): monomolecular layer adsorbed on a surface number of molecules in a monolayer: N 0 1015 na = number of adsorbed molecules per unit area fractional coverage s( ) = sticking coefficient Probability that, on striking the surface already having coverage θ, a molecule becomes adsorbed. 45

Gas-solid interaction For dilute adsorbed layers ( <<1) simple model for the equilibrium state rate of adsorption rate of desorption at equilibrium N 2 at p = 1 x 10 -5 mbar, 293 K R = 2. 9 1015 cm-2 s-1 seq= 0. 2 a = 5 x 10 -3 s na, eq = 1 x 1012 cm-2 = 0. 003 equation of state for the adsorbed phase Coverage is proportional to pressure 46

Desorption P = 1000 mbar P = 10 -7 mbar pumping Equilibrium Experimental relation Far from equilibrium till…. Gas flow /area = 0. 5 = 1 for metals = 0. 5 for elastomers =1 q 1 metals 1 x 10− 7 mbar L s− 1 cm-2 For 1 mbar L at RT Nat with q 1 metals, outgassing rate 1012 molec s− 1 cm-2 2. 6 x 1019 10 -3 ML /second of released gas, principally water vapor 47

Desorption How important is the molecule/surface interaction energy? H 2 O Rate of desorption N 2 integrate fall of pressure at RT decays exponentially from the initial state with a time constant equal to the stay time Simple model calculation idealized UHV system RT, V= 1 L, A = 100 cm 2 S = 1 L/s only gas source: initially complete ML of specified binding energy adsorbed at the wall H 2 O q 48

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 flowes: Permeation Adsorption Solubility Desorption 49

Gas-solid permeation p 1 = 1000 mbar p 2 = 1 x 10 -8 mbar H 2 O CO CO 2 CH 4 N 2 He O 2 Residual Gas 50

Gas-solid permeation p 1 = 1000 mbar Permeation is a complex process Adsorption p 2 = 1 x 10 -9 mbar Residual Gas Dissociation Solution into the solid Diffusion Recombination Desorption 51

Gas-solid permeation p 1 = 1000 mbar Permeation process can be quantified trough phenomenological quantities p 2 = 1 x 10 -9 mbar Residual Gas permeability =Q/(p 1 -p 2)A Q=flow trough wall A= unit area [Q] = [p][L]3[t]-1 =[L]3[t]-1[L]-2 m 3 s-1 m-2 ls-1 cm-2 52

Gas-solid permeation For a given gas A = wall area d = wall thickness cm 3 s-1 cm-2 Pa-1 depending on diffusion mechanisms He Kp = Permeability coefficient p = 13 mbar d = 1 mm m 3 s-1 m-1 Pa-1 53

Gas-solid permeation Metal – gas Kp Table of gas permeability Glass He, Ne, O 2 p H 2 , Ar, Metals Polymers No rare gas All gases p p 54

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) c = gas concentration Henry’s law Valid for low concentrations and for glass and plastic materials No formation of alloys 55

Solubility H 2 on metals For dissociated gas Interstitial or substitutional Sievert’s law Valid for low concentrations and for metals Note the high solubility of H 2 in Ti, Zr 56

Vacuum Pumps Throughput pumps • Pistons • Gears • Turbines • Jet stream Capture pumps • Cold traps • Ionization • Getters Differences: pressure range, speed, gas selectivity 57

Pressure Ranges Spanned by Different Vacuum Pumps More than one pump to HV and UHV 58

What pump to use? Pumping speed S = Q/p • Depends on the gas type • S varies with p p = inlet pressure S = [L]3[t]-1 Q=Q 0 cost pu. S = Q 0 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: 59

What pump to use? • Ultimate pressure • Time to reach the u. p. • Residual gas composition • Other (absence of magnetic fields) 60

Rotary Roughing Pump inlet Exhaust valve Oil Rotor blade Eccentric rotor Spring Cylindric body S: 2, 5 ÷ 102 m 3/h 0. 7 ÷ 28 l/s CR: 105 Starting operating pressure: 103 mbar 1 m 3/h = 0. 28 l/s Pu: 10 -2 mbar 61

Dual stage Rotary Roughing Pump inlet Exhaust valve Rotor blade Eccentric rotor Spring Pu: 10 -3 ÷ 10 -4 mbar Advantages Disadvantages • No saturation • Heavy duty • Low cost (2500 €) • Oil backstreaming • Need traps for oil vapor • Noisy 62

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 3. 3 104 Pa The vapor liquefies and does not reach P > 1 105 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 63

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 64

Diaphragm Pump Housing Valves Head cover Diaph. clamping disc Diaphragm supp. disc Connecting rod Eccentric bushing CR: 102 103 Starting operating pressure: 103 mbar Pu: ~ 1 mbar 65

Diaphragm Pump Advantages Oil-free No saturation Low cost Disadvantages High ult. pressure (4 mbar) Low pump speed Noisy 66

Root Pump 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 67

Root Pump S and CR of a root pump depend on the preliminary pump installed ahead patm pp pr root palette Sr Sp The gas flow is the same for both pumps Palette: 60 m 3/h = 16, 8 l/s Sr = 16, 8 x 40 = 672 l/s 68

Turbomolecular Pump S: 50 ÷ 5000 l/s CR: 105 109 Starting operating pressure: 10 -2 mbar Pu: 10 -10 mbar 69

Turbomolecular Pump Molecular speed distribution without blades (only v) Molecular speed distribution plus blade speed 70

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 71

Turbomolecular Pumping speed: depends on gas type After bake out Residual gas: H 2 72

Compression Ratio of a Turbomolecular Pump 73

Turbomolecular Pump Rotor suspension Ball bearings (lubricant required) Magnetic (lubricant absent) Advantages Disadvantages No saturation Clean (magnetic) UHV Any orientation Cost Delicate Quite noisy 70 l/s ~ 4000 € 250 l/s ~ 9000 € 2000 l/s ~ 20000 € 74

Molecular drag pump Turbo disk Threaded stator Safety ball bearing Magnetic bearing Cylindrical Rotor Operating principle: Same as turbo but different geometry Threaded stator No blades but threads Forevacuum flange (outlet) Gas entry Electrical socket Lubricant reservoir 75

Molecular drag pump S: 40 ÷ 100 l/s CR: H 2: 102 109 He: 103 104 Starting operating pressure: 1 -20 mbar Pu: 10 -7 mbar N 2: 107 109 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) 76

vapor diffusion pump Fluid is heated and ejected from nozzles at high speed baffle 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 77

vapor diffusion pump The pumping speed and the pressure strongly depends on oil type S: 20 ÷ 600 l/s Advantages No saturation Heavy duty Low cost Starting operating pressure: 10 -2 mbar Pu: 10 -9 mbar Disadvantages gas reaction Liquid vapor tension Contamination Needs water cooling 78

Getter pumps Pumping mechanism - Gas-surface chemical interaction - Chemisorption - Solution of gas inside material Sublimation getters The active material is sublimated by thermal heating Non evaporable getters The active material is constituted by porous medium 79

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 80

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 81

Pumping speed (l/s) Getter pumps area Adsorption probability Molecular weight (g) S strongly depends on gas sublimation > 103 l/s Non evaporable 800 - 2 x 103 l/s A’= sublimation, A=non evaporable Zr-Al S depends on active surface saturation 82

Getter pumps With a number of panels one can obtain S > 1 x 104 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 H 2 Heavy duty Low cost No contamination Disadvantages Saturation Metal vapours No rare gas pumping Pressure limit: 10 -10 ÷ 10 -12 mbar 83

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 84

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 H 2: accumulates into the cathodes Produce cathode vapors Depositing also on anodes to work as getters Need regeneration by annealing 85

Ion-getter pump S: 4 ÷ 1000 l/s Advantages Heavy duty No traps No contamination Any mounting position Silent Pressure limit: 10 -11 ÷ 10 -12 mbar Starting operating pressure: 10 -3 10 -4 mbar Disadvantages High magnetic fields Low pump S for H 2 Medium - high cost 86

Cryogenic pumps Pumping mechanism Liquid He cooled Cold walls - Gas – cold surface interaction - Physisorption, condensation Liquid N 2 cooled Adsorbing material Adsorbing pumps Pumping mechanism - Gas – cold surface interaction - Physisorption Adsorbing porous material High surface/volume ratio Zeolites Al 2 O 3, Si. O 2 H 2 O and N 2 pumping 87

Cryopump Pumping mechanism - Gas – cold surface interaction - Physisorption and condensation Metal wall 88

vapor pressure Cryopump The gas condensation if gas pressure > vapor pressure at wall T S: 4 ÷ 100 l/s Starting operating pressure: 10 -9 mbar Advantages Heavy duty No contamination Low cost Disadvantages Pressure limit: 10 -10 ÷ 10 -11 mbar Saturation Noisy Needs other UHV pumps 89

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 + 90

Ionization in gases Ionization energy Electron affinity Ion - Ion + - + Less probable e. V More probable 91

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 relative energy loss for electrons Very small 92

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 93

Elastic collision Apply external electric field E If e- has vin~ 0 Maximum kinetic energy of an emoving in a gas Depends on electric field and pressure 94

Ionization 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 + - 95

Ionization e- can ionize an atom But it can also - e- trapped inside atom with formation of negative ions - Due to practical measurements e- with Ek unit pressure unit lenght Specific ionization coefficient - Long path to produce more ions 96

Vacuum measurement Different types of vacuometers depending on pressure range Mechanical, thermal, ionization 97

Vacuum measurement Bourdon Mechanical Membrane tube Pin wheel index To vacuum 105 102 Pa (103 1 mbar) The tube curvature changes with pressure Needs calibration Precision: 1 -2% fsr 105 102 Pa (103 1 mbar) The membrane or bellow bends with pressure Needs calibration Precision: 1 -2% fsr 98

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 99

Thermal conductivity vacuometers Thermal dissipation unbalanced = cost Stephan-Boltzmann =wire emissivity contact dissipation radiative dissipation 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 100

Ionization vacuum gauges Hot cathode Cold cathode Based on gas ionization and current measurements 101

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 The gauge measure the total pressure Range: 10 -4 – 10 -12 mbar Sensitivity K = i e K depends on gas, gauge geometry, gauge potential 102 Usually one increases by designing the gometry

Ionization vacuum gauge 1 tesla Cold cathode 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 Range: 10 -4 – 5 x 10 -10 mbar Note: discharge starts only by mag field to avoid high E field - induced currents 103

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 = ionization multiplicity Specific mass of Ar+ = 40 Specific mass of Ar++ = 20 For a single molecule there are many peaks, depending on n 104

Mass Spectrometry Specific mass table 105

Mass Spectrometry detector To remove secondary electrons Faraday cup All ions measured No filaments Low sensitivity sturdy Amplifier time constant large Channeltron - electron multiplyer High sensitivity Delicate Fast response 106

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. 107

Quadrupole Mass Spectrometry (QMS) The forces are uncoupled along x, y, z axis Quadrupole potential -(U+Vcos( t)) U+Vcos( t) r 0 = rod separation (~3 mm) Superimpose an oscillating field Vcos( t) 108

Quadrupole Mass Spectrometry (QMS) ion equation of motion Constant speed along z Stability parameters 109

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 110

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 111

Quadrupole Mass Spectrometry (QMS) Reducing U relative to. V, 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 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 V=V 0 cos( t) 112

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 113

Quadrupole Mass Spectrometry (QMS) VAC > UDC Ion traveling in the z direction -(U+Vcos( t)) (y) 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. 114

Quadrupole Mass Spectroscopy (QMS) profiles of the residual gas H 2 O p ≈ 3 x 10 -7 mbar Before bake-out H 2 O CO+N 2 CO 2 p ≈ 5 x 10 -11 mbar After bake-out 115

VACUUM SEALING Low Vacuum Clamps Viton rings No bake at high temperatures Reusable 116

VACUUM SEALING UHV HV Plastic deformation and shear Bake at high temperatures Reusable (maybe once) 117

VALVES Diaphragm Butterfly 118

VALVES Stem All metal Dynamometric sealing 119

VALVES Leak Gate High conductance UHV to air compatible Large clearance for instruments Bakeable 120

FEEDTHROUGH Multi-pin for signal or Low currents Multi-pin for high currents 121

MANIPULATION Rotation 122