Generation of vacuum (pumps): Vacuum (pressure) measurements:

Transcription

1 Generation of vacuum (pumps) p and measurements Generation of vacuum (pumps): ᴥ pumping p 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

2 Classification of pumps: processes 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 vv ) on datasheet you ll find K 0 for Q = 0 (ideal), direct use for P u, but for Q ǂ 0 check provided curves ask(p vv ) or K(Q) if available.

3 Rough or back pumping Systems Rotary mechanical pumps High compression factor (1 stage , 2 stage 10 7 ), but care on water problem and back-streaming

4 Oil lubricated system: Oil provides also good sealing and works as cooling liquid, but Oil is in the volume where gas is compressed C Solution: Ballast, in order to ingiect air at the exaust 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

5 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 Characteristics Curves: Two or more stage system

6 GAS FIXED SCROLL ORBITING SCROLL OUTLET 2 Dry Sytem INLET 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 i application, escludind load lock systems. Movement of gas in a scroll mechanism Maintenance after hours Dry pump scroll: characteristic curves

7 Dry piston pumps Maintenance after h 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.

8 High vacuum pumping systems Root Blowers There is no oil in the systems exposed to the vacuum

9 Roots pump Characteristics High pumping speed Low compression factor but: Compared with rough pumps If operated properly, the roots compress the oil in the outlet, then allowing oil free (?) vessel. Compared with a rough pump, high speed system and 10 time lower pressure Cryo- pumping p 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 P v not higher than Torr At T = 4.2 K H 2 gives P v 10-7 Torr

10 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 sat 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). Cryo-c condensa ation kt residence (sojourn) time t = t e (see desorption). tr 0 E Cr ry-osor rption Binding energy in cryosorption is 2 or 3 times bigger for heavy gases, 10 for H 2 and 30 for He.

11 Conclusions on cryosoprtion It may be concluded that at the boiling temperature of N 2 (77 K) all gases except He, H 2,D 2 and Ne may be cryosorbed. Cryotrapping Cryotrapping is sorption process by which non-condensables gases are trapped (buried) in the growing solid-liquid liquid condensation layer of a condensable gas microcrystallites, while others are incorporated within the crystallites. This method traps non-condensable molecules. Cryotrapping H 2 or He in N 2, or Ar & CO 2

12 Back pump required for starting ti 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 UHV pumps p

13 Turbo-molecular pumps Turbo molecular pumping Speed For pressure lower than --- we can assume in our calculation a constant tpumping speed, and we can be safe for stable working conditions.

14 Compression factor (K 0 ) K 0 M Turbo pumps compress better heavy molecules (oil ~ amu) For light molecules we have more backstreaming. Camparison between turbo pumps and available informations TurboVac T1600 TW1600 MagW1500S S N Ar He H K 0 N Ar He H P u < mbar < mbar mbar P out (N 2 )< 0.5 mbar < 8 mbar <0.2 mbar (air) <2.0 mbar (water)

15 Getter pumps: Sorption pumps -Ti-sublimation pumps Ti, sublimated in vacuum, reacts with gases and solidify On colder surfaces trapping the gases. Cooling the surface at lower temperature, pumping speed increases 2 or 3 order. Sputter getter ion pumps: combine penning process and Ti trapping Diode Triode

16 Ion sputter pumps Diode configuration is better for H 2 NEG pump (non evaporable getter) Chemical reaction. Reactive alloys of zirconium or titanium, configured in cartridge. Active gases (O 2, N 2, H 2 O, CO, CO 2 ) impinging on the cartridge surface are dissociated and permanently trapped, in the form of stable chemical compounds. Also hydrogen is very effectively pumped. p Hydrogen (and its isotopes) atoms diffuse inside the getter bulk and dissolve as a solid solution. Noble gases (and CH 4 at room temperature) are not pumped.

17 Vacuum monitoring and control Vacuum Gauges Direct measurements Indirect measurements

18 Direct measurements Capacitance Manometer Baratron Piezo Membranovac Diaphragm: High precision commercial gauges 0.15.%» mbar» » » » It measures the capacitance variaton due do the deformation of a wall (diaphragm). for the lowest pressure range displacement of f10-9 cm: thermal stability required. Indirect measurements (thermal conductivitivty 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 ) Heat transfer depends d on P if T and accomodation parameters are constants. t

19 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. 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: mbar

20 Thermocouple gauges Thermocouple gauges also rely on the dependence of P on the heat transfer. 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%. UHV gauges: ionization gauges 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 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.

21 Hot cathode ionization gauges Bombarding e - : i e Positive ion current i + =i p collected: i p = S i e 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 ) If normalized to Nitrogen S(N 2 ) = 1.00 P( x) = ( meter reading ) P Relative sensitity of gas ( x ) Relative Sensitivity respect to N 2 H He H 2 O 0.9 N Acetone 5

22 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. Vacuum Gauges calibration HV gauges are companies calibrated at the order 10 %, and there is also sensitivity problem on gas type. UHV gauges are calibrated in the order of 10-20%. Calibration procedure will be followed in the lab. 1253from M.H. Hablanian High-Vacuum Technology a practical Guide (Marcel Dekker Inc., New York 1997)

23 Back to the practice in lab Purpose p is to measure the conductance of the gaseous polarized Hydrogen, and check wich gauge to use for its stable working conditions. We have to work with Hydrogen then we have to to design the system for H 2 Estimated through-put for target thickness measurements: H in the range of mbar l/s of H it meand mbar l/s of recombined H 2. Partial pressure measurements M a s s S p e c t r o m e t r y

24 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. Ion source At 70 ev max σ ιonization

25 Ion seperation (filering m (amu)/q(e)) Dynamic or static : m d r e dt 2 = t 2 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 Most frequently used in vacuum, compact and easy

26 By sostitution: Mathieu-type type equation where + for x - for y In the stability vertex: q = we have m = V /( f r0 ) m in amu, r o in m, V in V and f = ω/2π Hz. but U V 1 = 2 then only a given ratio m/e is stable (stay on the axis of the quadrupole spectrometer. a q

27 Phenomenological description (fundamentals Leybold) U constant t Adding Vcos( ωt) Given M, ions on the axis f(v) Given U/V, i + f(m) Fragmentation-cracking By the electron excitation of molecule we have also fragmantation effects: Fragmantation-cracking pattern for H 2 O

28 Cracking pattern and isotopes xxx/yyy: xxx M(amu), yyy: relative abundance. Max is 100 Isotopic abundance Mass Spectra parent and sons

29 Air mass spectrum Ion Detection a) Faraday Cup and b) Secondary Electron Multiplier

30 ion

31 Or in other words, sensitivity of detection

32 Leak detection Leak detection (vacuum method) He Mass spectrometer usually B static tuned to He +

33 Leak check without the leak detector