Turbomolecular vacuum pumps are used in applications requiring a clean high or ultra-high vacuum for example in research & development of nuclear fusion, or in industrial fields like the coating industry and in analytical instrumentation.  

In principle, the turbomolecular pump is a turbine rapidly rotating in a housing where the stages of the turbine are equipped with a number of rotor blades. Located between the rotating rotor blades are stationary stator disks with blades arranged in the opposite direction.  

By means of a momentum transfer from the rotating rotor blades to the gas molecules, the gas molecules initially non directional thermal motion is changed into a directional motion from the inlet flange of the pump in the axial direction towards the fore vacuum flange.  

In the molecular flow range (i.e. at pressures below 10^-3 mbar (0.75 x 10^-3 Torr)) the mean free path of the gas molecules is larger than the spacing between the rotor and the stator blades (typically a few tenths of a millimetre). Correspondingly the molecules chiefly collide with the optically dense rotor blades, resulting in a highly efficient pumping action. In the laminar (constant) flow range (i.e. at pressures over 10^-1 mbar (0.75 x 10^-1 Torr)) the effect of the rotor is impaired by frequent collisions between molecules themselves.  

For this reason, a turbomolecular pump is not capable of pumping gases at atmospheric pressure.