Pressure fluctuations in systems evacuated by diffusion pumps

• Published in: Vacuum Vol. 16; no. 11; pp. 619 - 621
• Main Authors: Bance, UR, Harden, EH
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1966
• ISSN: 0042-207X; 1879-2715
• DOI: 10.1016/0042-207X(66)91423-0

The problem of pressure fluctuations occuring in systems pumped by diffusion pumps has been reported by several investigators, various conclusions have been reached. Such fluctuations can exhibit different characteristic symptoms; in work carried out by the authors, two distinct types of fine side fluctuations have been identified, the more common taking the form of relatively high pressure (10 −5 torr or greater) repetitive pulses, the other, generally noticeable only at pressures below 10 −7, exhibiting a random pressure movement particularly observable when pumping the lighter gases such as hydrogen (which forms the major residual in ultra-high vacuum systems), so reducing the effective pumping speed for such gas and limiting attainable ultimates. In this paper experiments are discussed which demonstrate that the random lower pressure fluctuations are due to problems of jet system design, particularly associated with overcooling of the top jet cowl rim, and simple modifications to the upper jet system are suggested which have been shown to improve the pumping stability by a factor of 10 or 20 in a typical 3 in. stainless steel jet diffusion pump.