Vacuum pump cryopump

Vacuum pump, cryopump A capture-type pump that operates by condensation and/or adsorption on cold surfaces. Typically there are several stages of cold surfaces and one of the stages will have a temperature below 120 K. See also Cryopanel Cryosorption pump. 

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],... 

In the case of the exhaust plume tests, the exhaust gas must be allowed to fully expand throughout the pertinent region of interest. This test requirement places some unusual demands upon the facility s vacuum pumping system. The requirements for the pumping of a directional free-molecule flow will be discussed in this paper, and a cryopump design technique will be presented which satisfies the exhaust plume test objectives. 

In space simulation, environmental testing, and other related fields there is a need for compact, high-speed vacuum pumps capable of operation over a wide span of subatmospheric pressure. The cryopump can fulfill these requirements. 

Introduction of the Si specimens into a vacuum chamber should be done carefully. When the specimen is in the atmosphere, H Si(lll) and organic-modified Si are not positively contaminated within a short period. The largest chance of contamination is when the surface is in the stage of initial vacuum pumping. The residual gas in the vacuum generated by oil rotary pump is water and oil, and the latter causes severe hydrocarbon contamination. It is necessary to use an oil-free pump, such as liquid-nitrogen cooled sorption pump or helium cooled cryopump, for evacuation from atmospheric pressure. After quick, clean evacuation from the atmospheric pressure to Torr, the... 

The student should be aware that it takes special vacuum equipment to maintain an ultrahigh vacuum. First, a clean surface must be inside a special vacuum chamber that has no leaks—which is easier said than done. In addition, special vacuum pumps must be used to get to such high vacuums and stay there. The normal oil-filled rotary vacuum pump can only maintain a vacuum of about 10 torr or so, a full five orders of magnitude higher than what is necessary for ultrahigh vacuum. Special vacuum pumps (like turbomolecular pumps, titanium sublimation pumps, or liquid-helium-based cryopumps) are needed and can be extremely expensive. 

From the operational point of view, reliable vacuum systems are a prerequisite for mass spectral measurements. In most cases, manufacturers apply differential stage pumping to achieve the required pressure range(s). Rotary pumps are used to provide an initial vacuum of approximately 10 to 10 Torr. High-vacuum pumps such as diffusion pumps (10 to 10 Torr), turbomolecular pumps (10 to 10 Torr), and cryopumps (10 to 10 Torr) are used to reduce pressure ftorther. Adequate knowledge in vacuum technology is essential in instrument design however, this is also beyond the scope of this chapter. 

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. 

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. 

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). 

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