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VacuumVacuum

PhysicsPhysics, Technology and, Technology and TechniquesTechniques ofofthethe

Kai Schwarzwlder,

Institut fr Physik Universitt Basel

October 6th 2006

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OutlineOutline

Introduction and basics

Measuring pressure

Pumps

Accessories

Leaks

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DefintionDefintion of of VacuumVacuum

A vacuum is a volume of space which is substansively empty of matter,

[wikipedia]

Real world vacuums are not perfect: they contain matter!

Pressure p describes how close to perfect vacuum (p=0) a space is.

Different units of pressure:

LeaksAccessoriesPumpsMeasuring pressureIntroduction and basics

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PressurePressure unitsunits

pounds per square-inch=6.89 10-1 1lbf/in21psi

(physical) atmosphere=1.01 103 760Torr1atm

technical atmosphere=9.81 102 10mH2O1at

medicine=1.33 100 1mmHg1Torr

SI=10-2 1N/m21Pa

cgs=1 1kDyn/cm21mbar

mbarDef.Unit

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Classification of Vacuums

10-11 / 10-17Space: Moon / interstellar

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pressure gauge(s)

pump(s)

housing

hoses, tubes, flanges, valves

solvants (cleaning)

baking facility (UHV)

leak detector

Equipment and Proceeding

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PressurePressure GaugesGauges

Pressure range of some gauges

Pressure / mbar

10410310210110010-110-210-310-410-510-610-710-810-910-1010-1110-1210-13

U-tube

diaphragm

Pirani

hot filament ion ization

cold cathode discharge


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DiaphragmDiaphragm GaugeGauge

pressure-dependent capacitance

independent of gas type sensitivity approx. independent

of diaphragm material and

thickness

can withstand bakeout


C

vacuum

ref. pressure

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ThermalThermal ConductivityConductivity GaugeGauge

heat conductance

jE~(T2-T1)

reduce radiation and convection

temperature reservoir


vacuum

IU

T1

T2

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Hot FilamentHot Filament GaugeGauge

electron emission

Barkhausen oscillations around anode grid

ionize atoms on their trajectory

X-ray emission of accelerated electrons

depends on gas type (ionization energy)


vacuum

-10V

+200V

0V

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ColdCold CathodeCathode DischargeDischarge GaugeGauge

electronelectron fieldfield emissionemission

sensitive to gassensitive to gas typetype


1...2kV

B

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RootsRoots BlowerBlower(Drehkolbenpumpe)(Drehkolbenpumpe)

~2500 rpm

1mbar...10-4mbar

vibrations (like every mech. pump)

no oil or lubrication


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Membrane PumpMembrane Pump(Membranpumpe)(Membranpumpe)

5 mbar

robust

no oil

backing pump


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RotaryRotary VaneVane PumpPump(Drehschieberpumpe)(Drehschieberpumpe)

350-700rpm

...10-1mbar

2 loads in 1 turn

oil to seal


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TurbomolecularTurbomolecularPumpPump(Turbomolekularpumpe)(Turbomolekularpumpe)

...60.000rpm

...10-9mbar

momentum transfer

magnet bearings

=>less friction, no oil

expensive


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OtherOtherMechanicalMechanical PumpsPumps

Scroll PumpScroll Pump


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(Oil(Oil --) Diffusion Pump) Diffusion Pump((l((l--)Diffusionspumpe))Diffusionspumpe)

10-10...10-2mbar

backing system: 10-1mbar

cheap

oil contamination

durable and reliable

no vibrations


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CryoCryo andand SorptionSorption PumpPump

CryoCryo PumpPump HV or UHV

Condensation of atoms on cold

wall

=> atoms are trapped

saturation at higher pressure

depends on gas typesboiling/condensation temperature

regeneration: re-evaporation of

trapped atoms (at RT)

SorptionSorption PumpPump 10-7mbar

3 phases

Sorption

Desorption

regeneration

material with large surface (coal,zeolite,...)


These two types are often combined.

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GetterGetterPumpPump

Certain materials are evaporated.

(barium, aluminium, titanium, sodium,...)

The evaporated atoms react with / bind residual gas atoms.

The getter atom condenses on a surface.


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Ion PumpIon Pump

Emitted electrons ionize gas

atoms.

ions accelerated to first cathode

Atoms are sputtered.

acceleration towards second

cathode

Atoms are buried in bulk of

second electrode.

Even noble gases are pumped.


e emission cathode

anode

2nd cathode

B

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AccessoriesAccessories

valves

wearing down by moving parts

bakeable

most critical part (leaks)

O-rings outgas (organic)

sharp edge cut into copper

Indium wire to seal joints


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LeakLeak typestypes andand sourcessources

cryo leaks: temperature dependent expansion

virtual leaks: e.g. outgasing

permeation leaks (rubber)

misfit joints

damaged O-rings

leaking cooling tubes

contamination / insufficient preparation

untight valves


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LeakLeak DetectionDetection andand LocalisationLocalisation II

Time dependence can give a hint. log-lin plot; log-log plot

Rate-of-rise: all valves closed

cavities are seperated

cavity with fast pressure rise is suspect

difficult to interpret (normal outgasing)

Bubble detection: pressurized system

spray soap solution on housing, tubes, etc.

the stronger the leak the more bubbles...


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LeakLeak DetectionDetection andand LocalisationLocalisation IIII

He mass spectrometer leak detector MSLD

He is detected with mass spectroscopy

(partial pressure of He)

Under-pressure-method

He is sprayed on set up

He penetrates leaks

He can be detected in a connected MSLD

pressurized method He is pumped in the system

He penetrates ou of leaks

Sniffer MSLD detects He

State-of-the-art

insensitive to source of He(damaged He-cooling lines, He in atmosphere, use 3He)


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LeakLeak DetectionDetection andand LocalisationLocalisation IIIIII

Pictures ofPictures ofHeHe--MSLDMSLD


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