Vacuum Technology and Vacuum Engineering……

In many plasma physics applications it is necessary to evacuate one or more regions of avacuum enclosure to pressures substantially lower than atmospheric pressure.

The requirements can vary from low vacuum atmospheric pressure (= 760 Torr) to 25Torr, to high vacuum 10-3 Torr to 10-6 Torr, to ultra high vacuum (10-9 Torr) and beyond.

Very different techniques are required to attain each of these vacuum conditions.Some of the principles involved will be covered in this section.Emphasis will be given to the strategies applied on the chambers used in this course.

Start with ����� Molecular FundamentalsAt S.T.P. there are some 2.5. 1019 molecules per ccAverage mean free path between the particles at S.T.P. is 660 Š(6.6 10-6 cm)

AT ANY PRESSURE P(Torr) AND TEMPERATURE T (º K) the number density (n)and mean free path (λ) of particles may be calculated using the relationships:

( )rateincidence

MT

Px

pathfreeMeanPT

x

densityNumberTP

xn

=Φ

=

=

−

21

22

220

18

105.3

103.2

106.9

δλ

Molecular diameter in cm is denoted by δ. Rate at which molecules strike

unit area of a surface in units of #/cm2 as derived from kinetic theory is Φ.

The accompanying Tables 1.1, 1.2 and 1.3 lists a number vacuum andmolecular characteristics important in vacuum engineering considerations.Note the connection between n and λ and time to form a monolayer in Table

1.1.Table 1.3 shows that at pressures between 10-3 and 10-4 Torr there are moremolecules on the walls than in the volume.

The mean free path concept is extremely important in vacuum engineeringconsiderations. It defines the boundary between two types of gas flow.

At high pressures when λ is much LESS than the dimensions of the

container, the molecules are in a constant state of inter-collision, and gasbehavior is dominated by the interactions between the particles. Suchinteractions result in viscous force and cause good communicationsthroughout the gas. Called VISCOUS FLOW conditions.

When λ becomes much LARGER than the dimensions of the container,

the molecules collide then more frequently with the walls than with eachother. The statistical motion of the independently moving molecules thengoverns gas behavior. Called MOLECULAR FLOW conditions.

Between these extremes we have Slip Flow conditions.Vacuum pumps like vacuum gauges operate in limited pressure ranges.

In general a pump that operates in the viscous flow domain will not operatein the molecular flow region and vice versa.

We start with a consideration of commonly used terms

GAS FLOW, CONDUCTANCE, PUMP THROUGHPUT, and SPEED

For compressible gases the Volumetric flow rate indicatesNOTHING about the QUANTITY of gas flowing unless both Tand P are specified.

One normally talks about “PUMPING SPEED”, S, (units are #liters/sec or volume per unit time) in place of volumetric flow.

The product SP at the same point yields the throughput Q at thatpoint. (units Torr liter/sec). To convert to mass flow rate use therelationship 1.7 10-4 Torr liter = 1 gram molecular weight.

Conductance is defined as gas throughput Q per unit pressure dropQ/∆P (= F liter/sec). See chapter 3 for different geometries.

MECHANICAL PUMPS-- see Chapter 10–

Pressures of 10-2 to 10-4 Torr--- Viscous flow

Used extensively for initial system exhaust and for backing tolower pressures other pumps that are incapable of operatingdirectly to atmospheric pressure.Such pumps employ an oil sealed rotor that turns off center withina cylindrical stator. The interior of the pump is divided into twovolumes by spring-loaded vanes attached to the rotor. Gas from thepump inlet enters one of these volumes and is compressed andforced through a one-way valve to the exhaust. Oil is introducedinto the chamber to provide sealing of the sliding surface (vanes)and the stator. The thin film of oil is maintained by the oilreservoir on top. The gas pressure in the final stage of compressionexceeds atmospheric pressure and in conjunction with the oil isswept out of the chamber through some form of check valve.

Two stage versions employ two rotors.Note the improvement in base pressure attained.

Pumps revolve at few hundred revolutions/minute.Usual capacity 10’s to 1000 cubic feet per minute.

Ballast valves are employed to reduce the detrimental effects ofwater and other condensable vapors limiting the ultimate pressurethat can be reached. Gas leaked into the compression chamber,raises the pressure and reduces the condensation.

BIG problem with such pumps is back streaming. Recall at the lowestpressure they are actually operating in the molecular flow regime.OILLESS options

Roots pumps handle large gas flows-

inlet

vanes

TURBOMOLECULAR PUMPS—(high speed molecular bat).

Functioning as a molecular turbine and operating in the molecular flowregime, this type of pump has almost completely replaced diffusion pumpsin many plasma applications. These pumps cannot operate directly againstatmospheric pressure and must be backed with a mechanical pump to lowerthe exhaust pressure at the exit into the molecular flow domain.The pump exploits momentum transfer from a series of high speed rotatingblades (rotors) orientated to impart a significant component of velocity to thegas molecules in the direction of the pump exhaust. The rotors operate atspeeds between 24, 000 to 60,000 rpm and the edge of the rotor is thenmoving at molecular speeds.

.

Multiple blades distribute compression ratio.

Provide roughly same pumping speed for allgases but the log of the compression ratiovaries roughly with (molecular weight)_

Compression ratio of 107 for N2 would beonly102 for H2.Compression ratio of 107 for N2 is for anexhaust pressure of 0.1 Torr. Increasepressure to 1 Torr Ratio becomes only 10.

Cryogenic Pumping.

At sufficiently Low Temperaturescryocondensation, cryoabsorption, and crotrappingcan be exploited for pumping.Cryocondensation requires that the pumping surface temperature be lowenough to condense the vapor.

CRYO PUMP.

Any surface will act as a pump for a gas that condenses in contact with thesurface. Cryopumps rely on this and use a closed-cycle helium refrigeratorto cool the active surfaces. Their working temperatures are below 20 ºK .Usually such pumps have two stages of pumping. Stage 1 is usually a metalsurface maintained between 30 to 100 ºK. This sector traps water vapor,carbon dioxide, and the major components of the air.Stage 2 operates between 10 and 20 ºK and is coated with a cryoabsorbantmaterial such as charcoal to pump neon, hydrogen and helium. These pumpsare very effective in pumping condensable gases but their performance withhelium is critically dependant on the history and quality of the charcoal.In practice these pumps are used to keep a chamber at low pressure—theywill not handle gas loads.

Big advantage is that it provides for an oil freeenvironment.Cost and maintenance must be factored intoadoption considerations.High rotation speeds and close tolerancesdemand careful mounting.Small particles can create enormous damage tosuch pumps and may dictate pump mounting.Bringing chamber up to atmospheric pressuremust be done with care.

Motor and bearings are the critical elements of any turbo pump.Motor drive and control electronics are a significant fraction of the total cost of the pump.Active electronic control is required to maintain rotor speed over a wide range of loads and inorder to protect the pump if overload occurs. Bearings can be grease packed and water cooled,lubricated with oil, air or magnetic field supported.

Sorption Pumps

are closed pumps that rely on cooled surfaces of materials like Zeoliteor activated charcoal to pump chambers into the 10-3 Torr range.There are non refrigerated cooled pumps.

DIFFUSION PUMPS

Use a stream of high velocity vapor as the driving force. Generally the pump is operatedin the molecular flow region and is capable of obtaining a maximum pumping speed atinlet pressures lower than 10–4 Torr.The basic components are the boiler (bottom),

the jet assembly ( 1, 2, 3 in (a)), the casing and the working fluid.

A stream of vapor at supersonic velocities comes oof the jet in the molecular flow region. The vaporstream imparts a forward (downward) component velocity to the gas molecules to be pumped , andthey are carried along toward the roughing pump.The vapor stream is recovered by condensing uponcollisions with the cooled walls of the pinup. Whetwo or three jets are used in series the pumps areca11ed fractionating, Working fluids includedistilled mineral oils, synthetic oils.

Disadvantages include oil breakdown also oil getspumped away.