A molecular drag pump varies from the TMP in that the momentum imparted to the incoming gas molecules is not by impact with a rotating blade, but by impact with a solid rotating surface--the molecules are 'dragged' along the surface.
A number of TMP manufacturers now offer what they refer to as hybrid TMPs--turbo pumps with integrated Holweck molecular drag pump stages.
This is a built-in mechanical system, based on the Holweck molecular drag pump principle, that provides a high compression ratio between the magnetic bearing chamber and the pump exhaust.
Molecular drag pumps, on the other hand, aren't much when it comes to speed, but can sustain high compression ratios at foreline pressures that exceed 1 torr even for light gases.
The resulting hybrid designs consist essentially of placing half of a Holweck molecular drag pump at the end of a conventional turbomolecular pump having the normal number of stages.
There are three turbine-type pumps on the market: axial-flow turbomolecular pumps, molecular drag pumps, and hybrids that combine the two types on the same axis.
Molecular drag pump
technology changed this by not only providing cleaner roughing, but also allowing roughing pressures of [10.sup.-4] to [10.sup.-6] torr to be routinely achieved.
Using a molecular drag pump
eliminates most of these problems and offers more design options for a leak detection system.
This idea, which was first implemented by molecular drag pumps
in the 1930s and 1940s and later refined in the first turbomolecular pump in 1958, has been so effective that few other solutions exist to provide this high level of vacuum performance in such a compact space.
Conversely, turbomolecular, turbo/ drag, and molecular drag pumps
are intended to allow the pumped gases to pass through the pump into a foreline where they are repumped by a backing pump and expelled to the atmosphere.
Because hydrocarbons in even small amounts (ppm levels) can dramatically affect product yields, oil-based diffusion pumps were replaced by dry turbo pumps, molecular drag pumps
, and hybrid pumps, and rotary vane roughing/backing pumps were replaced by membrane, scroll, or frictionless multistage Roots technologies.