Vacuum Techniques old

8/7/2019 Vacuum Techniques old

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Vacuum Techniques

Introduction

Vacuum technology is necessary to cryogenics. The vacuum techniques are needed, for

example, to eliminate gas convection in cryostats, to open thermal switches, to decrease thepressure above the surface of a liquid to lower its temperature , to circulate helium in dilution

refrigerators, for the purpose of leak detection . The ultimate performance of any refrigerator

depends on the right choice of pumps and connecting tubes.

A cryogenist must be sufficiently experienced in vacuum physics to be able of projecting

leak tight vacuum equipments, of avoiding gas leaks, or if they occur, of locating and getting

rid of them. Hence, the cryogenist must know how to solder and glue. As a matter of fact,

cryogenic equipments undergo very high mechanical stress due to great thermal gradients and

due to different expansion coefficients of the various materials used. A peculiar characteristic

of vacuum technologies as applied to cryogenics is that vacuum equipments are built and

tested at room temperature, then evacuated and cooled. Defects that may develop cannot befixed at low temperature. Moreover, some problems that occur at low temperatures disappear

when the equipment has been warmed up again: hence, they cannot be easily located.

Let us remember also that cold surfaces adsorb gases: if a small leak to atmosphere exists,

air will condense on cooled surfaces. If the amount of gas adsorbed is large, during warm up,the pressure in the vacuum space may become very high. A release valve must be therefore

present in the system.

A container is said to be under vacuum when the inside pressure is lower than the outer

one, usually the atmospheric pressure. If no container exists, vacuum is a space region at a

pressure lower than the atmospheric pressure.

Vacuum may be natural or artificially produced. Natural vacuum, for example, occurs on

the lunar surface or in the interstellar space where one should however speak of numerical

density of particles (1 particle/cm3) instead of pressure. In the intergalactic space, thedensity is around 1 particle/m3. Natural vacuum laboratories in use are, for example, the

Space Shuttle or Space Stations, but we will deal only with artificial vacuum, produced by

pumps inside a container.

Vacuum can be divided into three regions depending on the pressure of the gas:

low or rough vacuum (from the atmospheric pressure down to about 1 Pa);

high vacuum (1 Pa10-4

Pa);

ultrahigh vacuum (below 104

Pa).

Such more or less arbitrary classification reflects three different physical situations.

In the low vacuum range, the number of molecules in the volume is much larger than that

adsorbed on the internal surface of the container. In the high-vacuum range, the mean free

path of the molecules is of the order or larger than the dimensions of the container, and most

of molecules are on the container inner walls. In the ultrahigh vacuum range, the flux of

molecules onto an initially clean surface is so low that there is enough time to carry out

experiments before it is covered by a monomolecular layer of gas.

The simplest vacuum system consists of a vacuum chamber, an interconnecting tube

(Vacuum line or pipe) and a vacuum pump which produces a pressure gradient along the

tube.

To obtain a high or ultrahigh vacuum, both the chamber and the tube must be clean,since impurities (like water, with its high vapour pressure, and its unavoidable presence


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in the air) slacken the reaching of the final vacuum.

Let us also anticipate that, if a vacuum chamber initially contains air at the atmospheric

pressure , during the pumping process the composition remains approximately the same in the

low-vacuum range. Then, the composition changes, becoming usually richer in light

molecules.

Vapour pressure

Any liquid or even solid material always produces a gaseous phase in equilibrium with

the denser phase. The pressure of the gaseous phase is called vapour pressure. The final

vacuum in an evacuated chamber is often controlled by the vapour pressure of the most

volatile material present in the system.

The dependence on temperature of the vapour pressure p can be approximately

expressed as:

p exp (L(T)/RT)where L(T) is the molar latent heat of evaporation.

Mean free path and viscosity

In the kinetic theory, the gas molecules are represented by hard spheres colliding

elastically with each other and with the container walls.An important parameter that can be

calculated by this model is , the mean free path of a molecule between collisions. The mean

free path of molecules is:


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where d is the equivalent hard-sphere diameter of the molecule (d = 0.22nm for He 4 [6]) and

n is the number of molecule for unit volume.

Flow regimes in vacuum

When a gas is removed from a container through a tube, the type of gas flow depends on

pressure. In the low vacuum range, the gas flow is controlled by the collisions among

molecules (viscous flow). If some molecules are removed from a region (by a pump), other

molecules will refill the empty region. In this situation, the diameter of the pumping line is

not crucial. In the high- and ultrahigh-vacuum range (molecular range), the mean free path is

larger than the dimensions of the container, and interactions among molecules are rare. The

pumping tube diameter must be large to increase the probability that molecules reach the

pump. For a tube of radius r, the flow is viscous if /r < 10-2 and is molecular if /r > 1.Gas flowin vacuum systems may fall into one of the three regimes.

These are:1.Viscous Flow:

Distance between molecules is small; collisions between molecules dominate; flow

through momentum transfer

2.Transition Flow:

Region between viscous and molecular flow

3.Molecular Flow:

Distance between molecules is large; collisions between molecules and wall dominate;

flow through random motion


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Vacuum pumps

The main components of a vacuum system are the pumps. The types of pumps most

commonly used in low-temperature experiments are:

1. Roughing pumps: pressure range 1051 Pa

Rotary vane pumps; Booster (Roots) pumps; Scroll pumps;

Sorption pumps (cryopumps).

2. High-vacuum pumps: pressure range 100106

Pa

Oil diffusion pumps;

Turbomolecular pumps;

Molecular drag pumps.

To achieve very low pressures (high vacuum), at least two pumps (one roughing pump and

a high-vacuum pump) are connected is series. To get high pumping speed, a parallel ofpumps both in rough and in high vacuum is sometimes used. Nowadays, the vacuum industry

offers integrated vacuum systems containing several vacuum pumps in order to cover an

extended range of operating pressure with a compact apparatus.

In the following sections, the functioning principles of the single pumps used in cryogenics

will be described.

1.Rotary vane oil-sealed mechanical pump

A volume of gas is enclosed in the space bounded by the rotor, the stator and the two vanes

(see Fig. ). The pump removes the gas by compressing it to a pressure slightly higher than the

atmospheric pressure. This overpressure opens the spring-loaded outlet valve, and the gas

escapes to the atmosphere. A thin film of oil makes the final seal; therefore the ultimate


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pressure depends also on the oil vapour pressure. With one stage, the lowest attainable

pressure is about 10-2

torr and with two stages in series 10-3

torr.

Due to the friction of the sliding vanes, the biggest pump size available (water cooled) is

about 400 l/s. Smaller pumps (0.55 l/s) are air cooled.

Condensation of vapours must be avoided since it causes a deterioration of the oil: water

vapour condense at 234 torr at 70C, which is the typical working temperature of the pump.

To avoid condensation, most pumps have a ballast valve in the high-pressure stage. After

the gas has been closed off from the inlet, some gas is bled in through the ballast valve. This

causes the discharge valve to open sooner, with a reduction of the compression ratio. Rotarypiston pumps have similar characteristics.

Most mechanical pumps exhibit vibration that may represent a crucial drawback for very

low-temperature refrigerators.

Another drawback of oil-sealed pumps is the back-streaming of oil vapour into the

roughing line, which may occur at low pressure. Contamination by back-streaming oil can be

drastically reduced by using proper traps like molecular sieve traps with zeolite

The speed of rotary vane pumps is nearly constant from 1 atm down to 102

torr. To obtain

higher speeds in the 10210

3torr range, booster pumps are used in series with rotary vane

pumps.

2.Booster(Roots ) pump

The booster (blower or Roots) pumps are high-throughput pumps with a low compression

ratio. A booster pump is schematically shown in Fig. below. The gap between the two

counter-rotating rotors is of the order of 0.1 mm. The rotation (about 50 rps) of the rotors,

mounted on parallel shafts, is synchronized by gears to avoid contact. No oil is used to seal

this gap. The starting pressure of booster pumps is about 10 torr. Heating of the pump

becomes excessive when it is operated continuously at high pressure. Therefore, a bypass is

used for high-pressure roughing. Booster pumps with throughput up to 600 l/s are available.

To overcome the problem of the low compression ratio, multistage systems are produced in

which several booster units are cascaded. The system presents both an overall high

compression ratio and a high throughput. Such multistage system is capable of reaching 102


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torr and works from the atmospheric pressure without the need of rouging pump as in the

case of a single booster pump. The throughput can be up to about 300 l/s.

A system of cascaded booster pumps has the characteristics of a medium-size rotary vane

pump but does not present the drawback of the back-streaming. For this reason, it finds

application in cryogenics, for example, for the circulation of the He mixture in dilution

refrigerators.

3.Scroll pumps

A scroll vacuum pump uses two interleaved Archimedean spiral-shaped scrolls to pump or

compress gases (see Fig. below). One of the scrolls is fixed, while the other orbits

eccentrically without rotating, thereby trapping and compressing gases between the scrolls

and moving it towards the outlet.

The device was first patented in 1905 but did not become efficient for 50 years.

Also for these pumps, multistage systems (e.g. Varian TriScroll Pump) are used which

allows for final pressure around 102

torr and pumping speed of about 30m3/h. The typical

application of scroll pumps systems is the backing for turbomolecular pumps.


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There are two main drawbacks in scroll pumps: the production of Polytetrafluoroethylene

(PTFE) powder dust due to the friction between the fixed and the moving part; moreover,

since in the present commercial versions the motor is not in vacuum, the tightness is not the

best.

4.Sorption pumps

In a sorption pump, the gas is trapped within the adsorbing material (zeolites or active

charcoal) called molecular sieve. Zeolites are porous aluminium silicates which adsorb large

amount of gas when cooled to low temperature (usually 77K). The pump is filled with zeolite

and put in a bucket containing liquid nitrogen.

Gases that are condensable at 77K are trapped by cryocondensation. (Cryosorption is

instead the trapping of gas with a lower melting temperature inside the pores of the molecular

sieve that has a huge surface/volume ratio, typically about 700m2/cm

3.)

If gases like H2, He and Ne are to be trapped, the pump must be cooled to 4.2 K. Ultimate

pressure of the order of 105

torr can be achieved.

When cooled with liquid He, this type of pump can reach pressure below 108

torr.

The sorption pumps are clean but are one shot, that is, two pumps in parallel and

connected

by valves alternatively are needed for a continuous pumping. When the first pumpis saturated, the second pump is started, while the first is regenerated: removing the liquid

nitrogen, the trapped gas is expelled through the blow-off valve. The pump (with zeolite) is

heated to 200300C to remove water vapour. Charcoal pumps are heated to about 100C.

Another advantage of this kind of pumps is the very low level of vibration.

5.Oil diffusion pumps

In a diffusion pump, the dense oil vapour produced by the boiler (see Fig. below) is ejected

into the vacuum at high (or even supersonic) speed through the nozzles.

It collides with the gas communicating a large downward momentum to the gas

molecules. The top jet operates at the lowest pressure. The body of the pump is typically


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cooled by water or air. When the vapour reaches the cooled walls, it condenses back into a

fluid which drops into the pump boiler for reboiling.

A cold cap is usually mounted on the top of the pump assembly to prevent vapour from

reaching the vacuum chamber. A thermal protection switch is often used. The maximum

working pressure of a diffusion pump is about 103

torr. The ultimate pressure of a diffusion

pump can be around 109

torr and heavily depends on the oil vapour pressure (p < 108

torr at

room temperature for very good oils). Pumps with very large pumping speed (up to 104 l/s)are commercially available.

A drawback of oil diffusion pumps is the so-called back-steaming. It is the flow of a small

quantity of oil vapour towards the inlet of the pump. A water-cooled baffle like that shown in

Fig. can be put above the inlet. Baffles are made up of arrays of optically dense fins cooled

by a continuous water flow. A baffle always reduces the pumping speed.

To reduce to a minimum the amount of fluid reaching the vacuum chamber, the use of a

liquid N2-cooled baffle is the best choice.


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A diffusion pump needs a forepump (usually a rotary pump). Oil can also migrate from the

forepump into the vacuum chamber through the diffusion pump, when the rotary pump works

in the molecular regime (102 torr).If an operating diffusion pump is exposed to atmosphere, even for a short time, a strong

oxidation of the fluid takes place. In extreme cases, combustion or explosion is possible.

The pressure in the foreline must be kept below a value called critical forepressure. If thispressure is exceeded, oil vapour will be injected into the vacuum chamber in great amounts.

A dumping of the pump can occur. For this reason, the size of the rotary pump must be

properly chosen.

Note that a decrease of the pumping speed, due to a reduction of the heater power to the

boiler, decreases the critical forepressure.

6. Turbomolecular pumps

Turbomolecular (turbo) pumps are very clean (especially magnetically levitated version)

mechanical pumps, with pumping speed up to more than 7000 l/s.

The pumping action is due to a high-speed rotating surface that transfers momentum to the

gas molecules (see Fig. below).

With metal gaskets and moderate bakeout, turbo pumps can reach pressure below 109

torrwithout traps. They can be started at a pressure up to 1 torr. The time required to reach full


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pumping speed (1 min) is much shorter than for diffusion pumps. Also these pumps must bebacked by a primary pump.

The turbo pumps are made up of 1040 rotating (rotor) and fixed (stator) disks

alternatively arranged. Each disk has 2060 blades with proper tilt.

The motor (usually working in vacuum) is moved by a special current power supply. The

rotor turns at 104

105

rpm, usually a multiple of the line frequency. The pumping speed of aturbo pump unit depends on its rotational speed. High-speed turbo pumps need more frequent

maintenance interventions. In some turbo pumps, a low-speed mode allows operation up to

101

torr. However, full rotational speed is achieved at pressuresbelow 103

torr. Low-level

vibrations in kHz range are always produced due to residual unbalance of the rotor.

The pumping speed of a turbo pump is almost constant over a wide range of pressure and

depends on the gas species.

Light gases with higher thermal velocity are pumped less than heavier ones. This is why

turbo pumps produce an (almost) oil-free vacuum. The lubrication of turbo pumps is made

with a special vacuum grease. Pumps with magnetically levitated rotor are available, but they

are more expensive.

Pressure gauges

The precise measurement of the total or partial pressure of gas is an extremely arduous

task. Pumps, pressure gauges and vacuum chamber walls deeply influence the total pressure

and composition of gas under measure; selective pumping, chemical reactions and physical

processes continuously change the gas parameters, and in most cases only an approximate

knowledge of the gas pressure is possible. Fortunately, the order of magnitude of pressure is

often an adequate information in most problems. An accuracy of 10% is usually considered

quite good. A further difficulty occurs when the pressure of a gas at low temperature is to be

known: the gage is usually at room temperature, and gradients of temperature and pressure

occur along the tube connecting the measuring head to the low-temperature gas.

1. Total-pressure gages

We shall describe only a few of the most commonly used total-pressure gages. All gages

described, except the McLeod and the diaphragm gage, measure density rather than pressure.

2. McLeod gage

The McLeod gage is shown in Fig.below. By rising the mercury reservoir, the gas in the

volume V at the pressure p to be measured is trapped in the bulb B.

A further rising of the reservoir causes a compression of the gas in the capillary C (closed).

Capillary D is open and connected to the vacuum system. The difference dh between the two

mercury heights corresponds to a pressure difference dp = gdh (dh in mm givesnumerically dp in torr); is the density of mercury. If the compression of the gas in B and C

is isothermal, we can write:

The McLeod gage is a primary vacuometer; moreover its readings are independent of the

type of gas, except condensable vapours. It covers a wide range of pressures with a good

accuracy (104

torr around 0.1 torr, 2107

torr around 106

torr) .

The main drawback is represented by the fact that a continuous monitoring of the pressure

is impossible. Bakeable McLeod gages have been built .

The main application of this vacuometers is for calibration of other gages.


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Scheme of a McLeod pressure gauge

3. Bourdon gage

It is a simple and reliable gage covering six decades of pressure (not with one gage).

Below 1 torr, its sensitivity is very low. The gage is usually made up of a bended flexible

metallic tube (see Fig.below) of elliptic cross-section. When pressure inside the tube is

different for the outside (atmospheric) pressure, the tube bends. A gear and lever system

moves the needle.

Bourdon-type pressure gauge

4. Diaphragm gage

It measures differential pressures. A thin diaphragm separates two chambers (see

Fig. below). One of the two chambers is kept at a constant pressure. A pressure difference

causes the bending of the diaphragm which forms, together with a fixed electrode, a

capacitor. The change in capacitance is measured by a very sensitive capacitance meter

(measurement of _C 1019 Farad are possible).

Pressure down to 104

torr can be measured. The accuracy of the diaphragm gage is

limited by the dependence of the capacitance on temperature (typical 1% of full scale).

The time response can be around 103

s. Bakeable units are commercially available.


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Diaphragm capacitance pressure gauge

5. Thermal conductivity gages

In these gages, a wire inside the gas whose pressure is to be measured, is electrically

heated by a constant power (see Fig. below). As the gas density decreases, the heat loss from

the filament to the envelope walls decreases and hence the filament temperature increases

(not linearly). The temperature (200300C) is read by a thermocouple in thermal contactwith the wire.

Thermal Conductivity gauge

Pirani gage is similar: here the change in temperature of the wire is measured by the

variation of the wire electrical resistance. The lowest measurable pressure is around 10

3

torr.The calibration (usually for dry air) is a function of the heat conductivity of the gas.

6. Hot cathode ionization gageThis is a high-vacuum gage. Figure shows the triode and inverted configuration gage

with indicative biasing. As shown in Fig, electrons emitted by the filament are accelerated

towards the grid; they collide with the gas molecules and ionize them. The ions are attracted

by the collector. The ion current is a measure of the gas density. The ratio of the ion current

to the pressure is called sensitivity of the gage (ampere/torr). It depends primarily on the

geometry of the gage, biasing and ionization cross-sections of the gas. The ionization

efficiency of electrons for some gases as a function of energy is shown in Fig.


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When the grid is bombarded by electrons, soft X-rays are emitted that, in turn, produce

photoelectrons at the collector. This phenomenon limits the minimum pressure that can be

measured to about 108

torr.

The inverted configuration of Fig. (b) reduces the contribution of the photoelectron

current, extending the range to about 1010

torr.

Due to the presence of a high-temperature filament, frequent recalibrations are necessary.

Ion Gauges:(a)convenyional ion gauge: (b)inverted ion gauge(Baayard-Allpert gauge)

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Vacuum Techniques old