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UHV Vacuum Techniques: Basic Concepts Lecture Contents

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UHV Vacuum Techniques: Basic Concepts Lecture Contents UHV Vacuum Techniques: Basic Concepts Lecture Contents Lecture 1: Basics Lecture 2: Achieving Vacuum Aims: ► Levels of vacuum and ► Vacuum pumps subsequent applications ► ● What you ought to know Pump configurations ► Modelling gas flow about using vacuum ► Types of chambers and ► Pressure measurement - fittings ● What you might need atmosphere to UHV during your PhD ► Complete systems ● A quick starter for using vacuum equipment ● Cover entire range of vacuum - not just UHV References Why know about vacuum? ● Two excellent books: ● Nearly everyone here ● Many people here will use ► “Basic Vacuum Technology” , A. Chambers, R.K. Fitch and needs to know a little about vacuum systems: B.S. Halliday. vacuum: ► e.g. most research groups ► “A Users Guide to Vacuum Technology” , O'Hanlon. ► vacuum used very widely use vacuum to some in modern experimental extent physics ► ● Vacuum equipment manufacturers: need to understand the ► mostly simple physical systems ► Catalogues, websites principles involved ► might need to analyse ► e.g. Leybold, Edwards, Varian, Pffeiffer, Alcatel etc. ► what is achievable with what is happening or today's technology design modifications ► the basics - so its easier to ● Local knowledge (valuable, but finite resource!): go away and find out more ► workshops ► group technical support staff Historical Perspective What is the equipment like? BC 1657 1900 1950's 2000+ Ultra High Vacuum -10 Rough Vacuum (~10 mbar) Ancient Greeks (~10 -1 mbar) - Vacuum “inconcievable” Magdeburg Manufacture of Hemispheres Lightbulbs Experiment Ultra High Surface Mercury Vacuum treatment Sealed research to Pumps achieve XHV Hydrocarbon Sealed Pumps Some Definitions Assume the vacuum system is a sphere... Units: ► CHAMBER Usually use mbar (although Pa are SI, sometimes Torr) PUMP (GAS) ► 1000 mbar = 1 atm = 10 5 Pa = 760 Torr Levels of Vacuum: Kinetic Theory: reasonably valid For nitrogen at 295K: ► Low Vacuum - 1000 mbar to 1 mbar -1 -2 -1 ● Maxwell Boltzman Distribution P (mbar) n (litre ) λ J (cm s ) ► Medium Vacuum - 1 mbar to 10 -3 mbar 8kT 22 23 -3 -8 v= 1000 2.5x10 66 nm 2.9x10 ► High Vacuum - 10 mbar to 10 mbar m 19 20 ● 1 2.5x10 66 µm 2.9x10 ► Ultra High Vacuum - 10 -12 mbar to 10 -8 mbar Mean free path 1 -3 16 17 -12 = 1x10 2.5x10 66 mm 2.9x10 ► eXtreme High Vac. - below 10 mbar 2 d 2 n -6 13 14 ● Impingement Rate 1x10 2.5x10 66 m 2.9x10 nv J = 1x10 -10 2.5x10 9 660 km 2.9x10 10 4 Applications Elementary Gas Transport ● Conductance is a Low Vacuum: Ultra high vacuum (UHV): ● Throughput, Q, is the geometric property, which volume of gas passing ► mechanical handling ► keeping surfaces clean for represents the ability of a through an area per second hours (surface science, gas to flow from a pressure at a specific temperature epitaxial growth) gradient: and pressure: Medium vacuum: -10 ► space simulation (~10 Q mbar at 1000 km) dV C= ► industrial processes Q= p P P ► dt 2 1 ► vacuum drying/packaging achieving ultra high purities (e.g. for fusion) ► vacuum distillation ► units, e.g. mbar.l/s C Extreme high vacuum (XHV): ● p p High vacuum (HV, λ>d): Volumetric flow rate often 1 Q 2 ► storage rings called the speed, S: ► e-beams (welding, TV) dV Q ► ultra pure growth S= = ► vacuum evaporation or dt p S coating Modes of Gas Flow Molecular Flow For vacuum systems: ● dominant for HV and UHV and well ● Fluid flow is described by the understood Reynolds number and the ● Flow is only turbulent at very Knudsen Number (both high pressures and pumping ● particles flow in all directions to reach dimensionless) speeds (e.g. during initial dynamic equilibrium u D for round pipes evacuation, if unthrottled) R e= ● pumps 'wait and catch' gas particles - ρ = density ● Viscous flow and the high vacuum pumps do not 'suck' u = stream velocity transitional regime are Kn = D = diameter important at high pressures D λ = m.f.p. (above about 10 -3 mbar) PUMP ► gas can be 'sucked out' ● Turbulent flow if Re>2000, laminar if Re<1200. ● Molecular flow dominant below about 10 -3 mbar in ● Viscous flow if Kn<0.01, 'normal' sized chambers. molecular flow if Kn>1. Particles scattered from surface with ► gas flows through random cosine distribution to surface normal collisions with walls (Knudsen's cosine law) Molecular Conductance of an Aperture Conductance of Pipes Net flow through an aperture ● For short pipes is convenient ● Various methods for corresponds to the rate of impingement to reciprocally add the from both sides: calculating conductance of pipes give same results (see conductances for a long pipe and equivalent aperture: P1, n 1 P2, n 2 O'Hanlon for details) Q=k T J 1J 2 A ● k T For long pipes (L>>D): C 0×C L = p1 p2 A J A J C = 2 m 1 2 S C C D 3 2 k T 0 L C = L 6 L m 3 k T 12.4 D / L C 0= A ● C = 2 m S l/s 14D /3L 3 (D, L in cm) for nitrogen (air) ● C L = 12.4 D / L l/s ► conductance depends on (T/m) 1/2 at room temperature. (D, L in cm) for nitrogen (air) ► C = 11.8A l/s (A in cm 2) for nitrogen ● Accurate to about 10% 0 at room temperature. (air) at room temp. ► need to account for gas and temp. Combining Conductances Effect of Conductance on Speed of a Pump Conductance, C ● For conductances in series, ● For conductances in add reciprocally: parallel, add normally ► 1/C T = 1/C 1 + 1/C 2 + 1/C 3 ► CT = C 1 + C 2 + C 3 C1 CHAMBER PUMP PUMP C1 C2 C3 C2 C3 The molecular conductance ● this assumes the volumes Effective pumping speed at of the entrance aperture are independent - need to chamber is reduced by conductance determines maximum speed of be careful of any connecting pipe any pump Viscous and Transitional Flow Conductances of Complex Shapes Molecular Transitional Viscous ● Monte Carlo simulations ● higher pressures in pipes to required in general mechanical pumps mean transitional and viscous ● Refer to O'Hanlon flow may be important for simple 'standard' shapes ● conductance increases with pressure ● important in connections to ● Quite often, just need mechanical pumps - can approximate value use smaller connections ► can get quite a long ● accurate calculations way using simple complex approximations ● results tabulated - often in Approximate conductance is the conductance of the two apertures, mfr's catalogues added reciprocally... From BOC Edwards Catalogue A closer look at a generic system... Quantitative Description of Pumping Outgassing: gradual loss ● Constant volume system ● of particles adsorbed on CHAMBER governed by Ultimate pressure (dp/dt=0) walls of the chamber - Backstreaming: many pumps water is the major problem. pressure is simply can lose fluids into the vacuum dp system, causing contamination, V =Q Sp p = Q / S which can be difficult to remove. dt T T Outgassing ► in the case of a leak, PUMP Real Leak: a fine ● i.e. change in gas in chamber, Q ~Q passageway to the T L Leak Initial Air air outside. d(pV)/dt is load minus gas ● Process removed by pump For initial pumpdown, Q L is not important, so we obtain Virtual Leak Backstreaming ● Solving fully requires detailed knowledge of Q - not p = p 0 exp { -t / (v/S) } Evaporation T Virtual Leak: usually a generally available Evaporation: liquids (and greases) ► easy to calculate initial small trapped volume, will limit the vacuum until evaporated. which acts like a real ► pumpdown times using the (Clean components and WEAR evaporation and leak, but will deplete with GLOVES!) outgassing of range of pumping speed at the time. gases gives complex chamber behavior... QT = Q P + Q O + Q L + Q VL + Q E + Q B Limit of Pressure - Outgassing Measurement of Pressure ● Require a measurable property which changes (linearly) ● ● outgassing limits vacuum in a for UHV, must accelerate with pressure, preferably independent of gas type clean, leaktight HV/UHV degassing of water by baking chamber to ~200°c for ~24 hours ● No single principle is good between atm. and UHV. ● made up of general 'grot', ► desorption is activated greases, water vapour etc. with Boltzmann factor Pressure (mbar) 10 3 10 0 10 -3 10 -6 10 -9 10 -12 ► rule of thumb: rate doubles Capsule gauge for every extra 10°c Diaphragm Pirani (thermal cond.) ► can get to UHV in days Capacitance manometer instead of years Spinning rotor Penning Bayard-Alpert (Ionis.) ● what is the ultimate limit? Inverted Magnetron log(pressure) ► diffusion of H through Extractor Ion 2 RGA Mass Spec. log(time) chamber walls Vacuum Gauges Mechanical Total Pressure Gauges ● In practise, integrated ● Sense pressure by systems now widely available mechanical deformation ► almost 'plug and play' ● Measure total pressure - independent of gas type ► connect to PC for logging ● ► Capsule gauge: can link to pump Capsule Gauge controllers ► simple mechanical lever from expanding capsule to dial ● Composite gauge heads can Annular Diaphragm measure over wide ranges ► good for 1-1000 mbar electrode ► ● Diaphragm gauges combined pirani & ion Disc gauge ► electrode ● e.g. integrated ion gauge sense by mechanical pressure under ► measure from atm. to UHV and controller deflection measurement ► ► Still need care in operation sense by change in Chemical - e.g. responses different capacitance, very accurate and good to 10 -5 mbar, but Getter for different gases expensive Capacitance Manometer Pirani Gauge - Low to Medium Vacuum Bayard Alpert Ion Gauge - HV and UHV ● measures thermal ● hot filament emits electrons, conductivity from hot wire to RF RC which are attracted to grid and surroundings through vacuum spiral around ● typically set up in bridge V ● electron impact ionised arrangement with a residual gas inside grid compensating filament, and ● calibrated at HV positive gas ions reach collector and are detected by 30 V 180 V ● 'standard' backing line gauge, electrometer cost few £100 ● usable between about 10 -3 and ● needs calibrating to atm.
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