Generation of vacuum (pumps) and measurements

Generation of vacuum (pumps) and measurements Generation of vacuum (pumps): ᴥ pumping systems general considerations on their use and match with physical quantities introduced for the dimensioning of vacuum systems Vacuum (pressure) measurements: ᴥ vacuum gauges general considerations and use: Total pressure gauges, Partial pressure measurements. Classification of pump systems: pressure ranges 1

Main characteristics of pumps Working pressure range Starting pressure Inlet pressure (max) Outlet pressure (max) Ultimate pressure (check the conditions). Pumping Speed S=S(p, M, or chemical) Throughput Q = Q(p in, p out, M, gas). Compression factor (K=P out /P in ) K=K(Q,p pv ) on datasheet you ll find K 0 for Q = 0 (ideal), direct use for P u, but for Q ǂ 0 check provided curves as K(p pv ) or K(Q) if available. Rough or back pumping Systems 2

Rotary mechanical pumps High compression factor (1 stage 10 5, 2 stages 10 7 ), but care on water problem and back-streaming Oil lubricated system: Oil provides also good sealing and works as cooling liquid, but Oil is in the volume where gas is compressed. 316 mbar @ 70 C Solution: Ballast, in order to inject air at the exaust 3

Oil pumps: the back-streaming problem Start HV pump, before, pump which compresses the oil in between HV pump And rotary pump Warning: when turbo has to be stopped, air has to be fed (automatic vent), during the deceleration of the turbo to avoid back streaming Characteristics Curves: one stage system Pumping speed Immediate use S pb (S back pump) Time of evacuation, Immediate use directly connected: P=P o e -(Sp/V) t with bellow or connection: parametrized (D 4 /L) plot S p vs t/v 4

Characteristic Curves: two stage system FIXED SCROLL ORBITING SCROLL GAS 1 7 6 OUTLET 2 Dry Sytem INLET 5 3 4 Gas displacement with rototraslation movement of a moveing spiral on a fixed spiral. P u Pirani measurement : 0,01 mbar Application: Oil free. Limited capacity of pumping vapors and chemical gases, liquids are dangerous for the pump. Useless for industrial application, escludind load lock systems. Movement of gas in a scroll mechanism Maintenance after 8.000-10.000 hours 5

Dry pump scroll: characteristic curves Non return check-valve Membrane dry pumps: Transfer gas pump which make use of the oscillation of a membranes or diaphragms. P u in system with at least two stage: 1 mbar Oil free pumps. Available model with teflon protection of volume and parts exposed to the vacuum for aggressive or corrosive chemical processes. 6

High vacuum pumping systems CryoCryopumping Based on the principles of cryocondensation, Cryo-sorption, and Cryotrapping: decreasing the surface temperature of systems exposed to the vacuum the processes are favored, depending on the physical and chemical properties of the gases present in the vacuum. At T < 20 K for most gases Pv not higher than 10-11 Torr At T = 4.2 K H2 gives Pv 10-7 Torr 7

Terms for cryopumping Cryo-condensation (interaction between molecules): for gases increasing the coverage of a surface a saturation equilibrium is reached between adsorption and desorption. ᴥ Corresponding gas pressure in vacuum: vapor pressure curve. p = Q/S + p vap Cryo-sorption (interaction of molecules with surfaces): submonolayer surface coverage experience attractive van der Waals forces exerted by cold surfaces: ᴥ as consequence H 2 can be cryosorbed at 20 K, and all gases may be cryosorbed at their own boiling temperature (1 bar). Cryotrapping: Cryotrapping is sorption process by which noncondensables gases are trapped in the growing solid-liquid condensation layer of a condensable gas microcrystallites, while others are incorporated within the crystallites. This method traps non-condensable molecules. Back pump required for starting pressure (< 10-3 ) mbar and regeneration. Careful use of oil pumps? Poisoning of surfaces by oils. High pumping speed for H 2 O and H 2 8

UHV pumps Turbo-molecular pumps 9

Turbo molecular pumping Speed For pressure lower than --- we can assume in our calculation a constant pumping speed, and we can be safe for stable working conditions. Compression factor (K 0 ) K 0 M Turbo pumps compress better heavy molecules (oil ~ 70-75 amu) For light molecules we have more backstreaming. 10

Camparison between turbo pumps and available informations TurboVac T1600 TW1600 MagW1500S S N 2 1550 1420 1220 Ar 1410 1200 1180 He 1300 --- 1150 H 2 720 --- 920 K 0 N 2 5 10 5 1 10 7 1.5 10 8 Ar 1 10 6 3 10 8 --- He 1 10 4 --- --- H 2 2 10 2 --- --- P u < 3 10-10 mbar < 3 10-10 mbar 1 10-10 mbar P out (N 2 )< 0.5 mbar < 8 mbar <0.2 mbar (air) <2.0 mbar (water) Sputter getter ion pumps: combine penning process and Ti trapping Diode Triode 11

Ion sputter pumps Diode configuration is better for H 2 Vacuum monitoring and control 12

Vacuum Gauges Direct measurements Indirect measurements Direct measurements Capacitance Manometer Baratron Piezo Membranovac Diaphragm: High precision commercial gauges 0.15.%» 1100-10 -1 mbar» 110 10-2» 11-10 -3» 1.1-10 -4» 0.11-10 -5» MKS willbe used(lowestp) It measures the capacitance variaton due do the deformation of a wall (diaphragm). for the lowest pressure range displacement of 10-9 cm: thermal stability required. 13

Indirect measurements (thermal conductivity Gauge) Thermal conductivity gauge Heat transfer depends on P, linear dependence for 0.01< K n <10. Low pressure λ = High Pressure kt 2( πd 2 p) 4 H = Aσε ( T T 1 + thermal leaks 2 4 1 ) Heat transfer depends on P if T and accomodation parameters are constants. Pirani Gauge Thermal conductivity gauge, with a Withstone bridge for a higher sensitivity. Vacuum vessel Let s start at a given P with the balanced bridge, if P increases, T of the filament R 2 decreases (due to a higher heat exchange), the bridge is unbalanced. We can feed more current to keep T =cost (method) Compensating tube is used for zeroing at a P < 10-4 mbar. Thermal conductivity dependes on the gas, the read-out is calibrated for N 2. 14

If R 1 R 4 = R 2 R 3, then I M = 0 T constant method: R 2 if P, therefore V dc (or V=I dc ) then T ) Accuracy 10%, in the range of more sensitivity, more or less for pressure in the range: 10-2 10 2 mbar Thermocouple gauges Thermocouple gauges also rely on the dependence of T on the heat transfer due to P Constant Current on a filament on which center a thermocouple si soldered. Therefore from the measurement of the T is possible to provide a measurement of P. Accuracy 30%. 15

In HV & UHV the particle density is so low, that it is non possible to detect the force exerted on a surface from the molecule impinging on it or the heat transfer. Hot Cathode gauge UHV gauges: ionization gauges It esploit the ionization of gas by electron bombardment and collection of the positive ion produced in the vacuum vessel The positive ion current collected is proportional to the density of particles in the vacuum vessel, to the electron current and to the ionization cross-section. Hot cathode ionization gauges Bombarding e - : i e 16

Positive ion current i + =i p collected: If normalized to Nitrogen S(N 2 ) = 1.00 P( x) = i p Relative Sensitivity respect to N 2 H 2 0.42-0.45 He 0.18 H 2 O 0.9 N 2 1.00 Acetone 5 = S i P i e electron bombardment current, P pressure, S sensitivity (depends on ionization cross-section), Fixing S for nitrogen to 1. S P(x) to be measured ( ) ( N2 ) P x = P ( N2 ) S ( x ) ( meter reading ) P Relative sensitity of gas e ( x ) Cold Cathode ionizzation gauges Penning Gauge High voltage (1 ( kv) discharge induced by cosmic rays. Thanks to the magnetic field the effective path length of the bombarding e - is larger. Disadvantages: higher self-pumping due to sputtering 0.1 ~ 0.5 l/sec. Advantages: pressure range that connect hot cathode and TC. 17

Partial pressure measurements M a s s S p e c t r o m e t r y Ion source An electron beam is required in order to ionized the gas in the vacuum system. Ion optic pieces in order to extract the ions from the ionizing volume and focus it in the filtering system. 18

Ion source At 70 ev max σ ιonization Ion seperation (filering m (amu ( amu)/q(e)) Dynamic or static : 2 m d r 2 e dt = E( r, t) + v( r, t) B( r, t) the second member of the ion motion equation is function of time or not. Dynamic E(r,t) quadrupole Static B( r ) sector 19

Most frequently used in vacuum, compact and easy By sostitution: Mathieu-type type equation where + for x - for y 20

In the stability vertex: q = 0.706 we have: 6 2 2 m = 13.8 10 V /( f r0 ), where m in amu, r o in m, V in V and f = ω/2π in Hz, and U 1 a = V 2 q only a given ratio m/e is stable (stay on the axis of the quadrupole spectrometer). Phenomenological description (Leybold manual) U constant Adding Vcos( ωt) given M, i + on the axis id f(v) Given U/V, i + f(m) II condition in which Ions are on axis for xz and yz 21

Fragmentation-cracking By the electron excitation of molecule we have also fragmantation effects: Fragmentation-cracking pattern for H 2 O Cracking pattern and isotopes xxx/yyy: xxx M(amu), yyy: relative abundance. Max is 100 Isotopic abundance 22

Mass Spectra parent and sons Air mass spectrum 23

Ion Detection a) Faraday Cup and b) Secondary Electron Multiplier 24

ion Or in other words, sensitivity of detection 25

Leak detection 26

Leak detection (vacuum method) He Mass spectrometer usually B static tuned to He + Leak check without the leak detector 27