generation of vacuum (pumps) and measurements · start hv pump, before, pump which compresses the...

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�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.

Generation of vacuum (pumps) and Generation of vacuum (pumps) and Generation of vacuum (pumps) and Generation of vacuum (pumps) and

measurementsmeasurementsmeasurementsmeasurements

Vacuum technology

Classification of pump systems: Classification of pump systems: Classification of pump systems: Classification of pump systems:

pressure rangespressure rangespressure rangespressure ranges

Vacuum technology

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Main characteristics of pumpsMain characteristics of pumpsMain characteristics of pumpsMain characteristics of pumps

Vacuum technology

�Working pressure range

�Starting pressure

�Inlet pressure (max)

�Outlet pressure (max)

�Ultimate pressure (check the conditions).

�Pumping Speed S=S(p, M, or chemical)

�ThroughputQ = Q(pin, pout, M, gas).

�Compression factor (K=Pout/Pin) K=K(Q,ppv) on datasheet you’ll find K0 for Q = 0 (ideal), direct use for Pu, but for Q ǂ 0 check provided curves as K(ppv) or K(Q) if available.

Vacuum technology

Rough or back pumping SystemsRough or back pumping SystemsRough or back pumping SystemsRough or back pumping Systems

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Vacuum technology

Rotary mechanical pumpsRotary mechanical pumpsRotary mechanical pumpsRotary mechanical pumps

High compression factor (1 stage 105 , 2 stages 107), but …care on water problem and back-streaming

Vacuum technology

Oil lubricated system:Oil lubricated system:Oil lubricated system:Oil lubricated system:

316 mbar@ 70 °C

Solution: Ballast, Solution: Ballast, Solution: Ballast, Solution: Ballast,

in order to in order to in order to in order to injectinjectinjectinject air at the exaust air at the exaust air at the exaust air at the exaust

Oil provides also good sealingand works as cooling liquid, but

Oil is in the volume where gas is compressed.

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Vacuum technology

Oil pumps: the backOil pumps: the backOil pumps: the backOil pumps: the back----streaming streaming streaming streaming

problemproblemproblemproblemStart 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 Characteristics Curves: one stage Characteristics Curves: one stage Characteristics Curves: one stage

systemsystemsystemsystem

Vacuum technology

Pumping speed Immediate use

Spb (S back pump)

Time of evacuation, Immediate use directly connected:

P=Poe -(Sp/V)·t

with bellow or connection:parametrized (D4/L) plot Sp vs t/V

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Vacuum technology

Characteristic Curves: Characteristic Curves: Characteristic Curves: Characteristic Curves: twotwotwotwo stage stage stage stage systemsystemsystemsystem

Dry Dry Dry Dry SytemSytemSytemSytem

Vacuum technology

Movement of gas in a scroll mechanism

12 3

456

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INLETOUTLET

FIXED SCROLL

ORBITING SCROLL

GAS

• Gas displacement with roto-traslation movement of a

moveing spiral on a fixed spiral.

• Pu 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.

Maintenance after 8.000-10.000 hours

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Vacuum technology

Dry pump scroll: characteristic curvesDry pump scroll: characteristic curvesDry pump scroll: characteristic curvesDry pump scroll: characteristic curves

Vacuum technology

Membrane dry pumps:Membrane dry pumps:Membrane dry pumps:Membrane dry pumps:

• Transfer gas pump which make use of the oscillation of a

membranes or diaphragms.

• Pu 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.

Non return check-valve

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Vacuum technology

High vacuum pumping systemsHigh vacuum pumping systemsHigh vacuum pumping systemsHigh vacuum pumping systems

Vacuum technology

CryoCryoCryoCryo----

pumpingpumpingpumpingpumpingBased on the principles of

cryocondensation,Cryo-sorption,

andCryotrapping:

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

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�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 + pvap

�Cryo-sorption (interaction of molecules with surfaces): submonolayer surface coverage experience attractive van der Waals forces exerted by cold surfaces:

ᴥ as consequence H2 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 non-condensables 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.

Terms for Terms for Terms for Terms for cryopumpingcryopumpingcryopumpingcryopumping

Vacuum technology

AA 2011/2012 Vacuum technology

High pumping speed for H2O and H2

Back pumprequired for

starting pressure (< 10-3 ) mbar and

regeneration.

Careful use of oil pumps?Poisoning of surfaces by

oils.

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Vacuum technology

UHV pumpsUHV pumpsUHV pumpsUHV pumps

TurboTurboTurboTurbo----molecularmolecularmolecularmolecular pumpspumpspumpspumps

Vacuum technology

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Vacuum technology

Turbo molecular pumping SpeedTurbo molecular pumping SpeedTurbo molecular pumping SpeedTurbo 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 (KCompression factor (KCompression factor (KCompression factor (K0000)))) MK ∝0

Turbo pumps compress better heavy

molecules (oil ~ 70-75 amu)

For light molecules we have more

backstreaming.

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Vacuum technology

Camparison between turbo pumps and available Camparison between turbo pumps and available Camparison between turbo pumps and available Camparison between turbo pumps and available

informationsinformationsinformationsinformations�TurboVac T1600 TW1600 MagW1500S

�S�N2 1550 1420 1220�Ar 1410 1200 1180�He 1300 --- 1150�H2 720 --- 920

�K0

�N 2 5· 105 1· 107 1.5 ·108

�Ar 1· 106 3· 108 ---�He 1·104 --- ---�H2 2· 102 --- ---

�Pu < 3 ·10-10 mbar < 3 ·10-10 mbar 1 ·10-10 mbar

�Pout (N2)< 0.5 mbar < 8 mbar <0.2 mbar (air)

<2.0 mbar (water)

Vacuum technology

Sputter Sputter Sputter Sputter –––– getter ion pumps: getter ion pumps: getter ion pumps: getter ion pumps:

combine penning process and Ti trapping combine penning process and Ti trapping combine penning process and Ti trapping combine penning process and Ti trapping

Diode

Triode

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Ion sputter pumpsIon sputter pumpsIon sputter pumpsIon sputter pumps

Vacuum technology

Diode configuration is better for H2

Vacuum monitoring and control Vacuum monitoring and control Vacuum monitoring and control Vacuum monitoring and control

Vacuum technology

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Vacuum GaugesVacuum GaugesVacuum GaugesVacuum Gauges

Vacuum technology

� Direct measurements

� Indirect measurements

Vacuum technology

Direct measurementsDirect measurementsDirect measurementsDirect 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 will be used (lowest P)

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.

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Vacuum technology

Indirect Indirect Indirect Indirect measurementsmeasurementsmeasurementsmeasurements

((((thermalthermalthermalthermal conductivityconductivityconductivityconductivity Gauge)Gauge)Gauge)Gauge)� Thermal conductivity gauge � Heat transfer depends on P, linear dependence for 0.01< Kn <10.

Heat transfer depends on P if T and accomodation parameters are constants.

leaks thermal

)( 41

421

+−= TTAH σε

High PressureLow pressure)(2

2 pd

kT

πλ =

Vacuum technology

Thermal conductivity gauge, with a Withstone bridge for a higher sensitivity.

PiraniPiraniPiraniPirani Gauge Gauge Gauge Gauge

Let’s start at a given P with the balanced bridge, if P increases, T of the filament R2 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 N2.

Vacuum vessel

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Vacuum technology

Accuracy 10%, in the range of more sensitivity, more or less for pressure in the range:

If R1 R4 = R2R3, thenIM = 0

T constant method: R2 ↓ if P ↑ , therefore Vdc (or V=Idc) ↑ then → T ↑ )

10-2 ÷102 mbar

Vacuum technology

Thermocouple gaugesThermocouple gaugesThermocouple gaugesThermocouple gauges

Thermocouple gauges also rely on the dependence of T on the

heat transfer due to P

Constant Current on a filament on Constant Current on a filament on Constant Current on a filament on Constant Current on a filament on

which center which center which center which center

a thermocouple si soldered.a thermocouple si soldered.a thermocouple si soldered.a thermocouple si soldered.

Therefore from the measurement of the T is possible to provide a

measurement of P.

Accuracy 30%.

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� 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.

Vacuum technology

UHV gauges: UHV gauges: UHV gauges: UHV gauges:

ionization gaugesionization gaugesionization gaugesionization gauges

Hot cathode ionization gaugesHot cathode ionization gaugesHot cathode ionization gaugesHot cathode ionization gauges

Vacuum technology

Bombarding e- : ie

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Vacuum technology

� P(x) to be measured ( ) ( )( ) ( )2

2 NPxS

NSxP =

H2 0.42 - 0.45

He 0.18

H2O 0.9

N2 1.00

Acetone 5

Relative Sensitivity respect to N2

( )( )xgasofsensititylative

readingmeterPxP

Re)( =

� If normalized to Nitrogen S(N2) = 1.00

� Positive ion current i+=i p collected:

PiSi ep ⋅⋅=ie electron bombardment current, , , , P pressure, Ssensitivity (depends on ionization cross-section), Fixing S for nitrogen to 1.


Disadvantages: higher selfDisadvantages: higher selfDisadvantages: higher selfDisadvantages: higher self----pumping due to sputtering 0.1 ~ 0.5 l/sec.pumping due to sputtering 0.1 ~ 0.5 l/sec.pumping due to sputtering 0.1 ~ 0.5 l/sec.pumping due to sputtering 0.1 ~ 0.5 l/sec.

Advantages: pressure range that connect hot cathode and TC. Advantages: pressure range that connect hot cathode and TC. Advantages: pressure range that connect hot cathode and TC. Advantages: pressure range that connect hot cathode and TC.

Penning Gauge

Thanks to the magnetic field the effective path length of the bombarding eThanks to the magnetic field the effective path length of the bombarding eThanks to the magnetic field the effective path length of the bombarding eThanks to the magnetic field the effective path length of the bombarding e---- is larger.is larger.is larger.is larger.

Cold Cathode ionizzation gaugesCold Cathode ionizzation gaugesCold Cathode ionizzation gaugesCold Cathode ionizzation gauges

High voltage (High voltage (High voltage (High voltage (1 kV) discharge induced by

cosmic rays.

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M

a

s

s

S

p

e

c

t r

o

m

e

t

r

y

Partial pressure measurementsPartial pressure measurementsPartial pressure measurementsPartial pressure measurements

�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.

AA 2011/2012 Vacuum technologyAA 2011/2012 Vacuum technology

Ion sourceIon sourceIon sourceIon source

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Ion sourceIon sourceIon sourceIon source

At 70 eV max σιonization

�Dynamic or static :

the second member of the ion motion equation is function of time or not.

Ion Ion Ion Ion seperationseperationseperationseperation ((((fileringfileringfileringfilering m (m (m (m (amuamuamuamu)/q(e)))/q(e)))/q(e)))/q(e))


),(),(),(2

2

tttdt

d

e

mrBrvrE

r ∧+=

Dynamic E(r,t) quadrupole Static B( r ) sector

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Most frequently used in vacuum, compact and easyMost frequently used in vacuum, compact and easyMost frequently used in vacuum, compact and easyMost frequently used in vacuum, compact and easy

where

By By By By sostitutionsostitutionsostitutionsostitution: Mathieu: Mathieu: Mathieu: Mathieu----type equationtype equationtype equationtype equation


+ for x

- for y

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� In the stability vertex: q = 0.706 we have:

where m in amu, ro in m, V in V and f = ω/2π in Hz,

and

only a given ratio m/e is stable (stay on the axis of the quadrupole spectrometer).


),/(108.13 20

26 rfVm ⋅=

q

a

V

U

2

1=

Vacuum technology

Phenomenological Phenomenological Phenomenological Phenomenological descriptiondescriptiondescriptiondescription ((((LeyboldLeyboldLeyboldLeybold manualmanualmanualmanual) ) ) )

U constant

Adding Vcos( ωt)

givenM, i+ on the axis id f(V)

GivenU/V, i+ f(M)

II condition in whichIons are on axis for xzand yz

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�By the electron excitation of molecule we have also fragmantation effects:

FragmentationFragmentationFragmentationFragmentation----crackingcrackingcrackingcracking


Fragmentation-cracking pattern for H2O


Cracking pattern and isotopesCracking pattern and isotopesCracking pattern and isotopesCracking pattern and isotopes

Isotopic abundanceIsotopic abundanceIsotopic abundanceIsotopic abundance

xxx/yyy:xxx M(amu),

yyy: relative abundance.Max is 100

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Mass Spectra parent and sonsMass Spectra parent and sonsMass Spectra parent and sonsMass Spectra parent and sons

Air mass spectrumAir mass spectrumAir mass spectrumAir mass spectrum


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Ion Detection Ion Detection Ion Detection Ion Detection


a) Faraday Cup

and

b) Secondary Electron Multiplier


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ion

Or in other words, sensitivity of Or in other words, sensitivity of Or in other words, sensitivity of Or in other words, sensitivity of

detection detection detection detection

Vacuum technology

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Leak detectionLeak detectionLeak detectionLeak detection

Vacuum technology

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Leak detectionLeak detectionLeak detectionLeak detection

(vacuum method)(vacuum method)(vacuum method)(vacuum method)

Vacuum technology

He

Mass spectrometer usuallyB static tuned to He+

Vacuum technology

Leak check without the leak detectorLeak check without the leak detectorLeak check without the leak detectorLeak check without the leak detector

generation of vacuum (pumps) and measurements · start hv pump, before, pump which compresses the...

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