lecture 3 r j reidvacuum science and technology in accelerators cockcroft institute lectures - 2007...
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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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 3
Session 3
The Production of Vacuum
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Pumping for acceleratorsMechanical Pumps
• Turbomolecular Pumps
Ion Pumps
Getter Pumps• Evaporable• Non evaporable
Cryopumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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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.
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Turbomolecular pump principle
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Turbomolecular pump principle
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumTurbomolecular pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumTurbomolecular pumps
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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumTurbomolecular pumps
Operation can be extended to higher pressure by adding a drag stage
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Turbomolecular Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
The choice of bearing type is important• Oil sealed• Greased• Greased ceramic ball• Magnetic
Turbomolecular Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon Pumps
Based on Penning Cell
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon Pumps
Based on Penning Cell
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon Pumps
Pumping in the basic diode Penning cell
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon Pumps
Using a heavier cathode e.g. Tantalum increases reflected neutrals
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Ion PumpsTriode Pumps use a different design
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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The Production of Vacuum
Ion PumpsStarcell configuration
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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The Production of VacuumIon Pumps
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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumIon 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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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The Production of VacuumIon Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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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.
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumGetter Pumps
For vacuum use, the most common getter pump is the titanium sublimation pump
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumGetter 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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Getter Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Getter Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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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
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumGetter Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of Vacuum
Getter Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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The Production of Vacuum
Cryogenic Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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The Production of Vacuum
Cryogenic PumpsThere are two major classes of such pumps
• Liquid Pool• Liquid helium temperature (~4K)
• Closed cycle• Refrigerator (~12K)• Supplemented by cryosorption
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
Lecture 4
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The Production of VacuumCryogenic Pumps
R J Reid
Vacuum Science and Technology in AcceleratorsCockcroft Institute Lectures - 2007
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The Production of VacuumCryogenic Pumps