Turbomolecular pumps use a rapidly spinning turbine rotor to push gas from the inlet of the pump towards the exhaust, in order to create or maintain a vacuum. Most turbomolecular pumps employ multiple stages consisting of rotor/stator pairs mounted in series. Gas captured by the upper stages is pushed into the lower stages and successively compressed to the level of the fore-vacuum pressure. A mechanical vacuum pump is needed to reduce the exhaust pressure and start a turbomolecular pump..
The maximum compression varies linearly with circumferential rotor speed. In order to obtain extremely low pressures on the order of 10 micropascals, rotation rates of 20,000 to 50,000 revolutions per minute are often necessary. Unfortunately, the compression ratio varies exponentially with the square root of the molecular weight of the gas. Thus, heavy molecules are pumped much more efficiently than light molecules. Most gases are heavy enough to be well pumped but it is difficult to pump efficiently hydrogen and helium.
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