Lecture 3 R J ReidVacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007 Vacuum Science and Technology in Accelerators Ron Reid

R J Reid

Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007

Lecture 3

Vacuum Science and Technology in Accelerators

Ron Reid

ConsultantASTeC Vacuum Science Group

([email protected])

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Session 3

The Production of Vacuum

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Aims• To demonstrate the main types of vacuum

pump used in accelerators• To understand the pumping mechanisms

involved• To understand the advantages and

limitations of each type of pump

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Pumping for acceleratorsMechanical Pumps

• Turbomolecular Pumps

Ion Pumps

Getter Pumps• Evaporable• Non evaporable

Cryopumps

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The Production of VacuumMechanical pumps

Mechanical pumps (displacement pumps) remove gas atoms from the vacuum system and expel them to atmosphere, either directly or indirectly

In effect, they are compressors and one can define a compression ratio, K, given by

K is a fixed value for any given pump for a particular gas species when measured under conditions of zero gas flow.

out

in

PK

P

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The Production of VacuumMechanical pumps

Here, we will only look at the turbomolecular pump.Turbo pumps cannot pump from atmosphere and cannot eject to atmosphere, so they require roughing (forevacuum) pumps to reduce the pressure in the vacuum system before they can be started and backing pumps to handle the exhaust.There are many types of roughing and backing pumps. Most accelerators now use clean (dry) pumps to avoid oil contamination in the system.

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Turbomolecular pump principle

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Turbomolecular pump principle

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The Production of VacuumTurbomolecular pump principle

To maximise the compression ratio, blade tip velocities need to be comparable to molecular thermal velocities.

For a single blade, at zero flow

where α12 is the forward transmission probability

and α21 is the reverse transmission probability

It can be shown that

where Vb is the blade velocity

12

21

out

in

PK

P

0

exp2bV M

KTkN

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The Production of VacuumTurbomolecular pumps

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Turbo pumps come in a wide range of speeds – from a few l sec-1 to many thousands of l sec-1 and operate from 10-3 mbar to lower than 10-9 mbar

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Operation can be extended to higher pressure by adding a drag stage

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Turbomolecular Pumps

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The choice of bearing type is important• Oil sealed• Greased• Greased ceramic ball• Magnetic

Turbomolecular Pumps

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The Production of VacuumIon Pumps

Based on Penning Cell

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Based on Penning Cell

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Pumping in the basic diode Penning cell

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Ion Pumps

The Diode pump has poor pumping speed for noble gases

Remedies• Differential Ion; Noble Diode

• “Heavy” cathode

• Triode• Special Anode shape e.g. Starcell

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Using a heavier cathode e.g. Tantalum increases reflected neutrals

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Ion PumpsTriode Pumps use a different design

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Ion PumpsStarcell configuration

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Current (per cell) – and hence pumping speed – depends on voltage, magnetic field, pressure and history.

nI kP 1.05 < n < 1.2

Pump life depends on quantity of gas pumped

> 20 years at 10-9 mbar

Prone to generate particulates

Leakage current unpredictable, so pressure indication below 10-8 mbar unreliable

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Diode Differential Diode

Starcell Triode

Voltage +7kV +7kV +2-5kV -5kV

Pumping Speed (Active gases)

Highest Good Good Lowest

Pumping Speed (Noble gases)

Lowest Good Higher Highest

Starting Pressure Lowest Lowest Good Highest

UHV Low Low Good Highest

Cost Lowest Higher Low Highest

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The Production of VacuumGetter Pumps

When a gas molecule impinges on a clean metal film, the sticking probability can be quite high.For an active gas with the film at room temperature, values can be between 0.1 and 0.8. These fall with coverage.For noble gases and hydrocarbons sticking coefficients are very low (essentially zero)Evaporated films, most commonly of titanium or barium, are efficient getters and act as vacuum pumps for active gases.

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For vacuum use, the most common getter pump is the titanium sublimation pump

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An important class of getter pumps are the Non Evaporable Getters (NEGs)

These are alloys of elements like Ti, Zr, V, Fe, Al which after heating in vacuo present an active surface where active gases may be gettered

Traditionally, the getters take the form of a sintered powder either pressed into the surface of a metal ribbon or formed into a pellet

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Getter Pumps

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Getter Pumps

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The Production of VacuumGetter pumps

In recent times, thin films of getter material have been formed on the inside of vacuum vessels by magnetron sputtering

These have the advantage of • pumping gas from the vacuum chamber by

gettering • and of stopping gases from diffusing out of

the walls of the vessels

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Getter Pumps

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Cryogenic Pumps

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Cryogenic PumpsThere are two major classes of such pumps

• Liquid Pool• Liquid helium temperature (~4K)

• Closed cycle• Refrigerator (~12K)• Supplemented by cryosorption

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The Production of VacuumCryogenic Pumps

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The Production of VacuumCryogenic Pumps

Documents

Lecture 3 R J ReidVacuum Science and Technology in Accelerators Cockcroft Institute Lectures - 2007 Vacuum Science and Technology in Accelerators Ron Reid