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A turbomolecular pump is used to push gas from the inlet of the pump to the exhaust. By using a rapidly spinning turbine rotor a vacuum is created between the inlet and the exhaust.

The ultimate goal of a turbomolecular pump is to end up with only one molecule of the gas. This molecule can then be used to do research on the gas. In practice the pump ends up with a small number of molecules.
To reach such a high level of vacuum the pump often uses multiple stages consisting of rotor/stator pairs which are mounted in series. Every stage extracts more molecules and then passes the gas on to the next stage. A mechanical vacuum pump is then used to reduce the exhaust pressure.

Of course there is a maximum compression. This varies linearly with the circumferential rotor speed. To create a vacuum with extremely low presures on the order of 10-10 torr rotation rates of 20,000 to 30,000 revolutions per minute are often nescesary.
The precise rotor speed necessary to create this kind of vacuum is determined by the molecular weight of the gas. The compression ratio varies exponentially with the square root of the molecular weight, thus heavy molecules are pumped more efficiently than light molecules. As a result there are two light gasses that can not be pumped vacuum by a turbomolecular pump: hydrogen and helium.

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