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The development of ultrahigh‐vacuum techniques had a profound effect on vacuum chamber design. It stimulated equally important improvements in pump design and in the understanding of some basic aspects of pump performance as well as their limitations. Design improvements also helped to clarify certain elusive concepts, for example, the meaning of the term ‘‘ultimate pressure’’ of a pump. Regarding partial pressures, the ordinary dual‐stage sliding‐vane mechanical pumps have been demonstrated to have full pumping speed at pressures as low as 10-10 Torr for gases nearly absent at the discharge. Modern vapor‐jet pumps (diffusion pumps) appeared in the 1960’s with improvements in all performance aspects. Using high‐performance pumping fluids, pressures near 1×10-9 Torr were achieved with only ambient temperature baffles, and near 1×10-11 Torr with liquid nitrogen cryobaffles. Even the extreme high‐vacuum range was obtained with vapor‐jet pumps assisted by gaseous helium‐ or liquid helium‐cooled traps. However, the fear of accidental backstreaming events shifted the users to turbomolecular pumps which, after nearly 50 years of stagnation, finally became a major pumping choice. Turbopump improvements are continuing to this day driven by the need for cleaner pumping methods and rapid achievement of lower pressures. The trend is to optimized staging arrangements to increase the limiting pressure ratios (particularly for hydrogen), the introduction of light‐weight, ceramic, grease‐lubricated bearings, magnetic bearings, and at least ten times higher permissible discharge pressures which allow entirely oil‐free backing pumps, the use of smaller backing pumps, and the achievement of an ultrahigh pressure range even in the presence of light gases such as hydrogen.
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films (Volume:12 , Issue: 4 )
Date of Publication: Jul 1994