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Basics of Vacuum Technology. Unit: Pa = N/m 2 = J/m 3. Three states of residual gas. d = typical distance between walls < l > = average collision distance. Viscous state : < l > < d/100 Collisions, energy transfer through collisions, viscous flow, diffusion

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Basics of vacuum technology l.jpg
Basics of Vacuum Technology

Unit: Pa = N/m2 = J/m3


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Three states of residual gas

  • d = typical distance between walls

  • <l> = average collision distance

  • Viscous state: <l> < d/100

    • Collisions, energy transfer through collisions, viscous flow, diffusion

  • Intermediate state: d/100 < <l> < d

  • Molecular state: d < <l>

    • Molecules collide with the walls, heat transfer from wall to wall with molecules, no viscosity



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Residual gas

  • Initially air

  • Rough vacuum: mainly air

  • Intermediate vacuum: gas starts to get out from surfaces (outgassing)

  • High vacuum: Mainly gases from surfaces, typically 70 – 90 % water

    • Gradually water decreases and CO + CO2 increase

  • Ultra vacuum: mainly Hydrogen

  • Depends on Pump/pre-pump: most pumps pump easier heavy gases

  • Nobel gases difficult to pump

  • Principally residual gas is NOT thin air


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  • Note!

  • The composition of residual gas depends on

  • how long time the vessel has been open

  • how it was vented

  • what molecules can have attached on the surface (e.g. humidity -> water)

  • possible leaks

  • Etc.


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Vacuum forces

  • Practically “zero” pressure on the vacuum side and 1 atm on the pressure side.

    • e.g. 10 cm x 10 cm square flange:

This is important when designing vacuum vessels/chambers!


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Some constants and equations

Avogadro constant

1 mol ideal gas in NTP = 22.4 l

molar gas constant

Equation of state for residual gas in terms of moles

Boltzmann constant

Equation of state for residual gas in terms of number of molecules


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Number of gas molecules

One cubic-cm in NTP

10-7 mbar = 10-5 Pa: N = 2.5 x 109

(molecules in cm3)


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Energy in residual gas

Maxwell-Boltzmann theory:

Energy distribution of translation energy

Average translation energy of a gas molecule



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Energy

Translation energy

Rotation energy (for a 2-atom molecule)


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Velocity

Velocity distribution for gas molecules

Average velocity

Most probable velocity



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Application in energy saving windows and thermal insulators: which gas?

Average velocity of some molecules at different temperatures


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Molecules on the surface which gas?

Molecules of the residual gas stick on the surface

Collision rate on unit area

Time for one molecular layer formation

= residual gas molecule diameter


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In ultra vacuum, Hydrogen is the dominating gas. At a pressure of 10-5 Pa and at 20 oC the molecule layer formation time is


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Collision distance pressure of 10

We get also from the M-B distribution for the average collision distance or mean free path:

For a Nitrogen molecule at 20 oC and 100 kPa



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If a fast electron, ion or molecule moves in a residual gas the residual gas molecules can be considered to be at rest. This leads to

Fast particle

Electron

So, the mean free path for an accelerated ion at 10-5 Pa = 10-7 mbar is 905 m

This was the criterion for JYFL vacuum level!


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Vacuum criteria the residual gas molecules can be considered to be at rest. This leads to

  • For beam transport and accelerators:

    • Mean free path = trajectory length

    • Decay time due to collisions

  • For “clean” manufacturing (crystal growth, surface manipulation, electronics:

    • Time for building a molecular layer

  • What else?


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Vacuum pumps the residual gas molecules can be considered to be at rest. This leads to

Capacity of a vacuum pump

Pumping speed

Transmission of the pump


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Conductance of the flow channel the residual gas molecules can be considered to be at rest. This leads to

Q = transmission of the flow channel

P1 = Pressure at inlet

P2 0 Pressure at outlet

C is called the conductance of the flow channel


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= resistance the residual gas molecules can be considered to be at rest. This leads to

Conductance for a long round tube in incompressible flow

Conductance for a long round tube in laminar flow

Conductance for a long round tube in molecular flow

Note! This does NOT depend on pressure