Vacuum Science and Technology in Accelerators

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Vacuum Science and Technology in Accelerators

• Ron Reid
• Consultant
• ASTeC Vacuum Science Group
• (r.j.reid_at_dl.ac.uk)

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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 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.

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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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Turbomolecular pump principle
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Turbomolecular 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 a12 is the forward transmission probability
• and a21 is the reverse transmission probability
• It can be shown that
• where Vb is the blade velocity

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Turbomolecular pumps
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Turbomolecular 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

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Turbomolecular pumps

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

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Turbomolecular Pumps
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Turbomolecular Pumps

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

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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 Pumps
Using 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 Pumps

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

1.05 lt n lt 1.2
Pump life depends on quantity of gas pumped gt 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