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Limitations on ultimate vacuum achieved in a turbomolecular pumped system

At high and ultra high vacuum the ultimate pressure achieved by a turbomolecular pumped system is limited by the conditions both at its inlet and exhaust. Here we describe why.

What is the gas load process in HV and UHV systems?

The process loads in a vacuum system can be summarised in the diagram below

Turbo molecular pumper system diagram

  1. Process load
  2. Outgassing
  3. Leaks
  4. Back-streaming
  5. Initial gas

In HV, UHV and XHV, the dominant gas load is from outgassing which is generally a summation of surface desorption, chamber wall bulk-diffusion and external atmosphere permeation.

Electron photon ion

  1. Permeation
  2. Outer surface 
  3. Inner surface
  4. Diffusion
  5. Vaporisation
  6. Desorption

In these vacuum ranges there is a corollary to a ‘process load’, for example in accelerators/colliders where surface ‘stimulated desorption’ from electron, photon and ion impacts can be a significant source of gas.

The total or ultimate pressure Ptot above a turbomolecular pump is: 

Ultimate pressure turbomolecular pump formula

where PTi is the partial pressure of the ith gas; this is determined by the turbomolecular pumps backing line partial pressure (PTbi) via the Compression Ratio (CRi) of the turbomolecular pump for each of the ith gases.

Formula partial pressure of ith gas

Outgassing (and other gas sources) in the chamber of the ith gas provide an additional load PQi.

where Qi is the gas load of the ith gas and Si is the turbomolecular pump’s speed for the ith gas.

However there are complicating factors

PQi and PTi are functionally dependent since:

Si and CRi are both dependent on inlet and backing pressure and flow

Formula for Limitations on ultimate vacuum achieved in a turbomolecular pumped system article

where So is the zero flow speed and Kmax is the ‘zero’ flow compression ratio. Hence ignoring outgassing and other gas loads, the expected ultimate pressure is

Formula for Limitations on ultimate vacuum achieved in a turbomolecular pumped system article