Vacuum. Gas kinetics, pumping, gauges

Transcription

1 acuum Gas kinetics, uming, gauges

2 Kinetic Theory of Gases Microscoic iew of Gases Molecular elocities z y x z y x + + equialence of directions m N U m N K N U x gas : a energy of The internal

3 The Origin of ressure δm m x ressure results from multile collisions between molecules and walls of a ot containing gas. The momentum transfer to the wall: The total momentum transferred : dm N δm coll The ressure : Fx 1 dm A A dt m x N N ( m ( dt A m Adt N m 3 m x x U m N 1 N N x U 3 x The No.of collisions with the wall during the time N coll 1 N ( dta x dt : Units: 1 atm N/m a 1 Torr 1 mm Hg a 1 bar atm 750 Torr 1 si 51.7 Torr 68.9 mbar

4 The Meaning of Temerature T is a measure of the kinetic energy of gas molecules moement U Nm 3 N kt kt 1 3 U N ; U 3 K 3 Molecules transfer energy to iston to moe it ; <> dros and the temerature decreases df

5 The elocity Distribution of Gases r The number distribution N( : so that the number of molecules r r r with a elocity between and + d is r N( dxdydz, and r N( dxdydz N y z d x d z d y x r r N( The robability distribution ϕ( : N r 3 ϕ( d is the robability that a molecule r r r elocity is between and + d. r 3 ϕ( d, and r 3 ϕ( d 1 ϕ ( r ϕ( x ϕ( y ϕ( z, ostulate: Any way in which the total energy and momentum can be shared among the molecules is equally robable ϕ( x indeendent eents m m x ex π kt kt m ϕ( r π kt deriation is beyond the scoe of the course 3 ex m kt

6 symmetrical Maxwell Distribution Distribution of the absolute alues of elocities f ( 4π f 4 m m ( ϕ 4π ex π kt kt M ( ex π RT RT 3 M 3 Nm RT N kt M (Eq..1 in Ohring m ϕ( r π kt 3 ex m kt asymmetrical

7 Characteristic elocities M RT M RT d f d f M RT d f d f M RT rms m 3 3 ( ( 8 ( ( on now π N.B. it is in Ohring s book

8 Collisions in Gas r r r σ π (r σλ π (r Collision cross section dt N dt Mean-free ath: λ σ N kt σ Tye of acuum (Torr λ (air at 300K rough nm 50 µm medium µm 5 cm high cm 500 m ultra-high < 10-7 > 0.5 km comes from the fact that the target molecules moe as well λ ~ [Torr] [cm] for air

9 Collisions in Gas Different Molecules r 1 r σ σ σ 11 1 π σ π ( r1 4π r1 1 π ( r1 + r ( r 4π r r n1 N1 n N dt λ λ n σ + n σ n σ + n σ After collision: elastic scattering, absortion (nuclear fusion, dissociation, ionization. One can introduce an effectie cross-section for each of such rocesses.

10 Gas Imingement on urfaces Φ 0 dn x x Φ n M M x RT x ex dx n π RT RT π M 0 Φ [Torr] MT [ 1 cm s ] uming: 3 T & 3 cm A[cm ] M s & ( air at 300K 11.7 A liter/s

11 Gas Flow Regimes D characteristic dimension of the system (chamber or ie diameter, for instance Molecular flow: λ/d > 1 Intermediate flow: 0.01 < λ/d < 1 iscous flow: λ/d < 0.01

12 Conductance Is only a function of geometry at a gien temerature (not for the iscous flow which deends on ressure Molecular flow: Q C( 1 [Torr - liters / s] For a iscous flow: 4 184( D / L 1 + D.5 cm; L 100 cm; ( + / 380 Torr 1 C mol 1.9 liters/s; C isc 7300 liters/s Electrical analogy: Connection in series : 1 C 1 C C C + C 1 + C + C K Connection in arallel: C tot tot + K

13 uming eed The olume of gas assing the lane of the inlet ort er unit time at a gien ressure at the inlet. Q / Deends on a um but also on a ie line: Q isc ~ D 4 It is more efficient to increase D than! Refers to a gien lane, while conductance refers to an element across which a gien ressure difference is set. Q C( ; 1+ / C Q / ; Q / an effectie uming seed With outgassing: Q Q When Q 0, the minimum ressure 0 that a um can gie is reached and Q 0. The effectie uming seed is then Q/ (1 0 / 0 when 0 ;

14 um-down Time * ( Q dt t d Q ; Q * includes both the um and chamber outgassing + + t A B t Q t B t Q t dt Q t d dt Q t Q t d dt Q t d ex ex ( ( ln( ( ln( ( ( ( ( ( ( * * * * * * (0 ; 0 ; * * Q A Q init init t t init ex ( 0 0

15 ums ums Gas-transfer Entrament ositie dislacement Kinetic Absortion Rotary Mechanical Diffusion utter ion Roots Turbomolecular

16 Rotary Mechanical Rotary-iston: ~10400 liters/s Rotary-ane: ~350 liters/s Roots: ~1000 liters/s Torr Rotary ane

17 Roots um Dry um, minimum contamination Requires foreuming, suscetible to oerheating

18 Diffusion um Rough um required 5x Torr liters/s oil aors imart a referred direction to molecular motion

19 Turbomolecular um >10-10 Torr; ~10 3 liters/s 0-30 thousand reolutions er min ery clean (no oil contamination collision with turbine blades imart a referred direction to molecular motion

20 Cryo-um ery clean (no oil contamination Different absorbing surfaces: bare metal, coered with re-condensed gases, micro-orous charcoal or zeolite Fore-ums required (10-3 Torr uming seed is only limited by imingement rate: s (T(300/T 0.5 ( s is the saturation ressure of the umed gas s (0K for LN is Torr He, H is roblematic to um out CTI cryoums

21 Cryo-um

22 utter Ion um Gases are ionized and accelerated by high oltage Ions sutter Ti cathode Ti is deosited elsewhere and getters the gas molecules Fresh layer of Ti buries those inside the metal film

23 ressure Gauges: DIRECT READING Direct Reading (Dislacement of a wall olid Wall Liquid Wall Diahragm Bourdon Radiometer Liquid Manometer McLeod Comression Caacitance Diahragm U-tube Manometer train-gauge Diahragm

24 ressure Gauges: INDIRECT READING Indirect Reading (Gas roerty Momentum Transfer Energy Transfer Charge Generation Quartz Thermocoule Hot Cathode Cold Cathode Radioactie Rotating Conection Triode enning Alhatron irani Bayard-Alert Magnetron Inerted Magnetron Thermistor Extractor Double Inerted Magnetron chultz-hels

25 ressure Gauges: irani htt:// In a irani gauge, the reference filament is immersed in a fixed-gas ressure, while the measurement filament is exosed to the system gas. A current through the bridge heats both filaments. Gas molecules hit the heated filaments and conduct away some of the heat. If the gas ressures (or comosition around the filaments is not identical, the bridge is unbalanced and the degree of unbalance is a measure of the ressure..

26 ressure Gauges: enning Range Deendence Measuring mbar secific gas deendent Adantages - robust, no filament, oeration guaranteed - simle construction, suly and measurement - enningcel stays on room temerature; degassing generally not necessary Disadantages - discharge ower not linear with ressure - sensitiity secific gas deendent - enning cel and magnet are heay and extensie - with magnet not suitable for build in - magnetic disturbed field - at low ressure difficult to start - bad reroducibility in consequence of the getter ion um rincial;

27 ressure Gauges: Bayard-Alert In a Bayard-Alert gauge, a tungsten filament is heated to gie off electrons which are accelerated then to ~70 e. These energetic electrons ionize the residual gas molecules which they collide with. The ositie ions formed are then collected by a wire collector held at about The ion current at the collector wire aries with the gas density which is a direct measure of gas ressure. Ionization gauges are deendent on gas comosition Cannot gie accurate absolute ressure measurements. The ractical working range of the Bayard-Alert gauge is in the 10-4 Torr to Torr.

28 ressure Gauges: A COMARION

29 acuum ystems LN oil-aor tra

30 acuum Leaks ery leak tight <10-6 Torr l/s Adeqautely leak tight ~ 10-5 Torr l/s Not leak tight >10-4 Torr l/s Technical Data mallest detectable helium leak rate (acuum mode mallest detectable helium leak rate (niffer mode Max. detectable helium leak rate (acuum mode L 00 dry mbar x l x s -1 < 3 x mbar x l x s -1 < 1 x 10-7 mbar x l x s -1 1 x 10-1 Leybold Inficon UL 00 Leak Detector Mass ectrometer uming seed for helium at the inlet l/s 0.6 Leak rate measurement range mbar x l x s -1 1 x to 1 x 10-1 Time until ready for oeration minutes < 3 Mass sectrometer 180 magnetic sector field Detectable masses amu, 3 and 4 detector

31 Residual Gas Analysis Mass ectra secies fingerrint