-: Sublimation pumps :-
Applications: Sublimation pumps are frequently used in combination with a sputter-ion pump, to provide highspeed pumping for reactive gases with a minimum investment (Welch, 1991). They are more suitable for ultrahigh vacuum applications than for handling large pumping loads. These pumps have been used in combination with turbomolecular pumps to compensate for the limited hydrogen-pumping performance of older designs. The newer, compound turbomolecular pumps avoid this need.
Operating Principles: Most sublimation pumps use a heated titanium surface to sublime a layer of atomically clean metal onto a surface, commonly the wall of a vacuum chamber. In the simplest version, a wire, commonly 85% Ti/15% Mo (McCracken and Pashley, 1966; Lawson and Woodward, 1967) is heated electrically; typical filaments deposit ~1 g before failure. It is normal to mount two or three filaments on a common flange for longer use before replacement. Alternatively, a hollow sphere of titanium is radiantly heated by an internal incandescent lamp filament, providing as much as 30 g of titanium. In either case, a temperature of ~1500°C is required to establish a useable sublimation rate. Because each square centimeter of a titanium film provides a pumping speed of several liters per second at room temperature (Harra, 1976), one can obtain large pumping speeds for reactive gases such as oxygen and nitrogen. The speed falls dramatically as
the surface is covered by even one monolayer. Although the sublimation process must be repeated periodically to compensate for saturation, in an ultrahigh vacuum system the time between sublimation cycles can be many hours.With higher gas loads the sublimation cycles become more frequent, and continuous sublimation is required to achieve maximum pumping speed. A sublimator can only pump reactive gases and must always be used in combination with a pump for remaining gases, such as the rare gases and methane. Do not heat a sublimator when the pressure is too high, e.g., 10¯3 torr; pumping will start on the heated surface, and can suppress the rate of sublimation completely. In this situation the sublimator surface becomes the only effective pump, functioning as a nonevaporable getter, and the effective speed will be very small (Kuznetsov et al., 1969).
Applications: Sublimation pumps are frequently used in combination with a sputter-ion pump, to provide highspeed pumping for reactive gases with a minimum investment (Welch, 1991). They are more suitable for ultrahigh vacuum applications than for handling large pumping loads. These pumps have been used in combination with turbomolecular pumps to compensate for the limited hydrogen-pumping performance of older designs. The newer, compound turbomolecular pumps avoid this need.
Operating Principles: Most sublimation pumps use a heated titanium surface to sublime a layer of atomically clean metal onto a surface, commonly the wall of a vacuum chamber. In the simplest version, a wire, commonly 85% Ti/15% Mo (McCracken and Pashley, 1966; Lawson and Woodward, 1967) is heated electrically; typical filaments deposit ~1 g before failure. It is normal to mount two or three filaments on a common flange for longer use before replacement. Alternatively, a hollow sphere of titanium is radiantly heated by an internal incandescent lamp filament, providing as much as 30 g of titanium. In either case, a temperature of ~1500°C is required to establish a useable sublimation rate. Because each square centimeter of a titanium film provides a pumping speed of several liters per second at room temperature (Harra, 1976), one can obtain large pumping speeds for reactive gases such as oxygen and nitrogen. The speed falls dramatically as
the surface is covered by even one monolayer. Although the sublimation process must be repeated periodically to compensate for saturation, in an ultrahigh vacuum system the time between sublimation cycles can be many hours.With higher gas loads the sublimation cycles become more frequent, and continuous sublimation is required to achieve maximum pumping speed. A sublimator can only pump reactive gases and must always be used in combination with a pump for remaining gases, such as the rare gases and methane. Do not heat a sublimator when the pressure is too high, e.g., 10¯3 torr; pumping will start on the heated surface, and can suppress the rate of sublimation completely. In this situation the sublimator surface becomes the only effective pump, functioning as a nonevaporable getter, and the effective speed will be very small (Kuznetsov et al., 1969).
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