Vacuum cryopump

This paper describes a newly developed and tested multipurpose vacuum cryopump, designed to satisfy the following requirements ... 

Such vessels can also be baked at a temperature of several hundred degrees, to drive off any gas adsorbed on metal surfaces. The pumping function of an ion gauge was developed into efficient ionic pumps and turbomolecular pumps , supplemented by low-temperature traps and cryopumps. Finally, sputter-ion pumps, which rely on sorption processes initiated by ionised gas, were introduced. A vacuum of 10 "-10 Torr, true UHV, became routinely accessible in the late 1950s, and surface science could be launched. 

For the transfer of LN2, a simple thick-walled rubber tube or a low-conductivity metallic tube which can be bent to the required shape may be used. In the latter case, a convenient covering, e.g. a flexible foamed neoprene, is needed. Less frequent is the use of double-walled metallic tubes. In this case, the vacuum enclosure must contain some charcoal active as a cryopump. 

Cryopumps adsorb (freeze) residual gas to a surface cooled to the temperature of liquid nitrogen. They are highly efficient and silent and provide clean vacuum, but cannot be operated without interruptions to recover the adsorber. Cryopumps are typically operated in combination with turbo pumps because they are only started after high vacuum conditions are reached. Otherwise, the adsorber would soon be saturated. 

This effect has been utilized for a long time in condensers (see 2.1.5) mainly in connection with chemical processes previously the baffle on diffusion pumps used to be cooled with refrigerating machines. Also in a sealed space (vacuum chamber) the formation of condensate on a cold surface means that a large number of gas molecules are removed from the volume they remain located on the cold surface and do not take part any longer in the hectic gas atmosphere within the vacuum chamber. We then say that the particles have been pumped and talk of cryopumps when the pumping effect is attained by means of cold surfaces. 

Three differing capacities of a pump for the gases which can be pumped result from the size of the three surfaces (baffle, condensation surface at the outside of the second stage and sorption surface at the inside of the second stage). In the design of a cryopump, a mean gas composition (air) is assumed which naturally does not apply to all vacuum processes (sputtering processes, for example. See 2.1.9.6 Partial Regeneration ). 

Considering the position of the cryopanels In the cryopump, the conductance from the vacuum flange to this surface and also the subtractive pumping sequence (what has already condensed at the baffle can not arrive at the second stage and consume capacity there), the situation arises as shown In Fig. 2.69. 

Crossover value The crossover value is a characteristic quantity of an already cold refrigerator cryopump. It is of significance when the pump is connected to a vacuum chamber via an HV / UHV valve. The crossover value is that quantity of gas with respect to T =293 K which the vacuum chamber may maximally contain so that the temperature of the cryopanels does not increase above 20 K due to the gas burst when opening the valve. The crossover value is usually slated as a pV value in in mbar I. 

The crossover value and the chamber volume V result in the crossover pressure p to which the vacuum chamber must be evacuated first before opening the valve leading to the cryopump. The following may serve as a guide ... 

For the production of pressures in the ultrahigh vacuum region, sputter-ion, and sublimation pumps, as well as turbomolecular pumps and cryopumps, are used in combination with suitable forepumps. The pump best suited to a particular UHV process depends on various conditions (for further details, see Section 2.5). 

Cryopump System operating down to 2,5 K, 1962 Trans. Ninth National Vacuum Symposium, American Vacuum Society, 216-219, The Macmillan Company, New York... 

CRYOPUMP. 11) An exposed surface refrigerated to cryogenic temperature for the purpose of pumping gases in a vacuum chamber by condensing the gas and maintaining the condensate at a temperature such that the equilibrium vapor pressure is equal to or less than the desired ultimate pressure in the chamber. 

Cryopumping Procedure for attaining ultrahigh vacuum by the freezing of residual gases in a chamber. 

It is now increasingly common to have high-vacuum systems based on turbomolecular pumps (TMPs), turbomolecular pumps + cryo-surfaces, or cryopumps. Diffusion pumps were extensively used in the past and remain fairly widespread in laboratory applications. However, for industrial purposes their use appears to be restricted to systems requiring very high pumping speeds (5o= thousands of Ls ), particularly where substantial amounts of particulates are handled. 

Crossover pressure. If a refrigerator cryopump is attached to a vacuum system via a valve and the pump is already cold, the crossover pressure... 

Efficient high-vacuum pumps generally do not operate well near atmospheric pressure. Thus the vacuum system must have a mechanical vacuum pump to evacuate the system to a pressure where the high-vacuum pumps are effective. Mechanical pumps require routine maintenance, such as ballasting and replacing the pump oil. The diffusion pump is the least expensive and most reliable high-vacuum pump. Turbomolecular pumps and cryopumps are also used on mass analyzers. The high-vacuum pumps also require... 

Vacuum pumps are generally divided into 13 categories according to the working principle, as listed in Table 2.5. They include water jet pump, water ring pump, steam ejector, oil-sealed rotaiy pump, Roots pump, vacuum diffusion pump, oil vapom booster pump, sputtering-ion pump, radial field pump, titanium sublimation pump, sorption pump, molecular pump and cryopump [9],... 

Figure 3.24 is reprinted from Vacuum, Vol. 37, W G Baechler, Cryopumps for research and industry, p. 21-29,1987, with permission from Elsevier. 

The vacuum in the mass spectrometer itself is maintained by a cryopump or by turbomolecular pumps backed by a rotary pump. The aperture of the skimmer is again below 1 mm. 

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