Turbo molecular pump

A turbo molecular pump including a magnetic bearing system comprising:

• a rotor including an impeller connected to a shaft and a motor;
• a stator positioned between the shaft and the impeller;
• radial magnetic bearings provided between said rotor and said stator, said radial magnetic bearings including passive magnetic bearings each having a permanent magnet and provided on said rotor and said stator respectively, said passive radial magnetic bearings each having ring permanent magnets magnetized axially and disposed concentrically so as to radially oppose each other, pairs of concentrically disposed passive radial magnet bearings are provided between said impeller and said stator and between said shaft of said rotor and said stator, respectively, at axially spaced positions, said ring permanent magnets, disposed on the rotor, is slightly deviated in axial direction from the ring permanent magnets disposed on the stator;
• an active axial magnetic bearing provided between said rotor and said stator and disposed in as vicinity where the impeller is connected to said shaft;
• and
• emergency bearings provided between said rotor and said stator and are disposed on a low vacuum side with one emergency bearing being positioned near said active axial magnetic bearing, and a diameter of said shaft is made smaller than an inside diameter of said emergency bearing near said active axial magnetic bearing.