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Low cost, mechanically refrigerated diffusion pump baffle for ultrahigh vacuum chambers
1.A baffle limits backstreaming by condensing pump fluid vapors. Strictly speaking, a baffle is different from a trap, which serves to lower chamber pressures by pumping on condensable vapors such as water. A baffle that is cryocooled to liquid nitrogen temperatures is effectively a trap. For more details, see J. F. O’Hanlon, A User’s Guide to Vacuum Technology (Wiley, New York, 1980).
2.M. J. Coggiola, R. L. Leon, and R. L. Scally, Rev. Sci. Instrum. 54, 894 (1983).
3.Santovac 5 is a five-ring polyphenyl ether diffusion pump fluid of atomic mass 446 and is currently produced and marketed by Santovac Fluids, Inc., St. Charles, MO 63301 (it was previously marketed by Monsanto).
4.L. M. Ulvick and M. P. D’Evelyn, Rev. Sci. Instrum. 61, 3921 (1990).
5.L. H. Dubois, J. Vac. Sci. Technol. A 6, 162 (1988).
6.Varian model VHS-6. Its relatively low cost, low maintenance requirements, and our need for high pumping speeds for all gases, including noble gases, dictated our choice of a diffusion pump, as opposed to an ion pump, etc.
7.Micromaze foreline trap, Kurt J. Lesker Co., Clairton, PA 15025.
8.Inland Vacuum Industries, Churchville, NY 14428-0373.
9.Polycold Systems Inc., San Rafael, CA 94903-2264. The P-20 uses 450 W of power; its physical size is only 13.25 in.13.75 in. 12.5 in.
10.UTI model 100C mass spectrometer, UTI Instruments, a division of MKS Instruments, Inc., Andover, MA 01810.
11.Indicators of a leak tight ultrahigh vacuum chamber are: small mass 32 signal and a mass 14 signal that is smaller than the mass 12 signal
12.A mass spectrum of Santovac 5 was provided by Monsanto.
13.Professor Steven Sibener, University of Chicago, personal communication.
14.The vapor pressure curve of Santovac 5 was obtained by extrapolation of vapor pressure data (25 °C to 300 °C) provided by Santovac Fluids, Inc. Similarly, the vapor pressure curve of Inland 19 mechanical pump fluid was obtained by extrapolation of vapor pressure data (0–160 °C) provided by Inland Vacuum Industries. The vapor pressure curve of ice (water) was obtained from the CRC Handbook of Chemistry and Physics, 74th ed. (Chemical Rubber, Boca Raton, FL, 1993–1994).
15.See a discussion of the Clausius–Clapeyron equation in, e.g., R. A. Alberty and R. J. Silbey, Physical Chemistry, 2nd ed. (Wiley, New York, 1997).
16.Manufacturers specify the “pour point” of their pump fluids (−15 °C for Inland 19 and 5 °C for Santovac 5) and indicate that the freezing points would be slightly below the pour points.
17.The slopes of the curves are given by where is either the enthalpy of vaporization or sublimation and R is the gas con-stant. At a given temperature, implying that since all the quantities are positive in value. Therefore, the vapor pressure curve of a solid is expected to be steeper than that for a liquid.
18.Polycold Systems Technical Note, “Water Vapor Cryopumping.”
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