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:
Process load in a vacuum system
The process loads in a vacuum system can be summarised in the diagram below:
- Process load
- Outgassing
- Leaks
- Back-streaming
- 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
- External atmosphere permeation
- Permeation
- Outer surface
- Inner surface
- Diffusion
- Vaporisation
- 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:
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.
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
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:
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