Vacuum pump system

A vacuum pump system for a multi-stage gas inlet system having a plurality of vacuum chambers said vacuum pump system comprising:

• a multi-stage turbomolecular pump with each separate stage having stator means, rotor means supported on a common shaft, an inlet side, and an outlet side;
• at least one pumping stage located downstream of said turbomolecular pump and having a rotor, which is supported on said common shaft, an inlet side and an outlet side, said turbomolecular pump and said at least one pumping stage forming a first pumping unit;
• a dry pumping stage located downstream of said at least one pumping stage and discharging against atmosphere;
• and
• a plurality of suction connections connecting the inlet and outlet sides of the stages of said multi-stage turbomolecular pump and said at least one pumping stage with respective vacuum chambers of the gas inlet system,
• wherein said suction connections are dimensioned and arranged with regard to pressure ratios and suction capacities of individual stages of said turbomolecular pump and said at least one pumping stage according to the equation:
• ##EQU4## wherein:
• K.sub.34 is a pressure ratio between adjacent first and second stages (103 and 104),
• S.sub.3 and S.sub.4 --are suction capacities of said adjacent stages (103, 104), respectively,
• C.sub.34 --is a transfer conductance between said stages (103 and 104),
• KO.sub.34 --is an inside pressure ratio of second stage (104) at zero throughput, and
• S.sub.31 is an inner suction capacity of first stage (103),
• whereby a return flow from the outlet side to the inlet side of each stage of said multi-stage turbomolecular pump and of said at least one pumping stage is small in comparison with a gas flow between two vacuum chambers connected with respective inlet and outlet sides.