Turbomolecular pump
From Freepedia
Turbomolecular pumps use a rapidly spinning turbine rotor to transport gas from the inlet of the pump towards the exhaust, in order to create or maintain a vacuum. Within normal operating ranges the gas is transported molecular, laws of fluid dynamics don't apply at high vacuum environment. 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.
The maximum compression varies linearly with circumferential rotor speed. In order to obtain extremely low pressures down to 1 micropascal, rotation rates of 20,000 to 90,000 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.
