New spiral molecular drag stage design for high compression ratio, compact turbomolecular-drag pumps

Currently, commercially available turbo-drag pumps for high vacuum systems are based on either Gaede- or Holweck-type molecular drag pumping stages used in series downstream of axial bladed stages to extend the maximum compression ratio up to the 10 mbar foreline pressure range. Modern Gaede-type molecular drag stages use a disk-shaped impeller, allowing a very compact design, and the maximum compression ratio is limited by the leakage effect to about 10 per stage. Holweck stages are able to supply a high pumping speed, thanks to the presence of many channels in parallel and a high compression ratio, but this is obtained with the use of a less compact drum-shaped impeller. In this article, a new spiral molecular drag stage design is presented with the advantages of both high compression ratio and pumping speed per stage and very compact design: a stage occupying the very small axial space of one Gaede can supply the same compression ratio and pumping speed of a Holweck stage of the same diameter and peripheral speed in a much smaller axial space. The new spiral drag stage allows the design of very compact, high compression ratio turbo-drag pumps. The comparison of a 700 l/s turbo-drag pump implementing the new spiral molecular drag pump design with the existing Gaede- and Holweck-based products of the same pumping speed is presented, showing the performance advantages of the new design.