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.

In this paper we will link the local structure of amorphous carbon and the semi-empirical model which assumes the density of π and π* states as having Gaussian shapes and to be symmetrical with respect to Fermi level. We will show that the effect of charge transfer from sp3 to sp2 sites due their difference in electron-negativity, when coupled with the disorder effect (taken into account by assuming Gaussian distribution of the amount of charge transfer) lead to a sum of Gaussian symmetrical bands. Moreover, we will show that the sp2 pairs are responsible for most of the joint density-of-states (i.e. the imaginary part of the dielectric constant) in the visible region. The limits of the approach will be discussed as well.