vacuum science and technology in accelerators · for vacuum use, the most common getter pump is the...

Lecture 3Vacuum Science and Technology in Accelerators

Cockcroft Institute Lectures - 2010R J Reid

Vacuum Science and Technology in

Accelerators

Ron Reid

Consultant

ASTeC Vacuum Science Group

([email protected])

Lecture 3Vacuum Science and Technology in Accelerators

Cockcroft Institute Lectures - 2010R J Reid

Session 4

The Production of Vacuum

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R J Reid Vacuum Science and Technology in Accelerators

Cockcroft Institute Lectures - 2010

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 accelerators

Mechanical Pumps

• Turbomolecular Pumps

Ion Pumps

Getter Pumps

• Evaporable

• Non evaporable

Cryopumps

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Mechanical 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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Mechanical 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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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

exp2

bV MK

TkN

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Turbomolecular 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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Ion Pumps

Based on Penning Cell

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Ion Pumps

Based on Penning Cell

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Ion Pumps

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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Ion PumpsUsing a heavier cathode e.g. Tantalum increases

reflected neutrals

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Ion Pumps

Triode Pumps use a different design

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Ion Pumps

Starcell configuration

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Ion Pumps

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Ion PumpsCurrent (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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Ion Pumps

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Ion Pumps

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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Ion Pumps

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Getter 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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Getter Pumps

For vacuum use, the most common getter pump

is the titanium sublimation pump

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Getter Pumps

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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Getter 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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Getter Pumps

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Cryogenic Pumps

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Cryogenic Pumps

There are two major classes of such pumps

• Liquid Pool

• Liquid helium temperature (~4K)

• Closed cycle

• Refrigerator (~12K)

• Supplemented by cryosorption

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Cryogenic Pumps

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Cryogenic Pumps

vacuum science and technology in accelerators · for vacuum use, the most common getter pump is the...

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