Cryopumps

As you may have observed water condenses on cold water mains or windows and ice forms on the evaporator unit in your refrigerator. This effect of condensation of gases and vapors on cold surfaces, water vapor in particular, as it is known in every day life, occurs not only at atmospheric pressure but also in vacuum. 

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. 

Cryo engineering differs from refrigeration engineering in that the temperatures involved in cryo engineering are in the range below 120 K ( -153 °C). Here we are dealing with two questions  

The entire scope of a refrigerator cryopump is shown in Fig. 2.65 and consists of the compressor unit (1) which is linked via flexible pressure lines (2) - and thus vibration-free - to the cryopump (3). The cryopump itself consists of the pump casing and the cold head within. Helium is used as the refrigerant which circulates in a closed cycle with the aid of the compressor. 

10-100 W the second stage operates at temperatures between 8 and 20K with cooling powers in the range 2 to 10 W) are more convenient to use in routine applications for pumping speeds up to 104Ls 1. 

1 Heat Loads in Cryogenic Systems. In any cryogenic system, the heat load transferred to the cold surface determines the refrigeration capacity required to maintain the low temperature. Analysis has shown that the thermal loads received by a surface at cryogenic temperatures are due to  

An unshielded cryosurface (area Ac, temperature Tc, emissivity ec) receives from the surrounding vessel wall (corresponding values Aw, Tw, w). 

With cryosurfaces, it is convenient to reduce e as much as possible by polishing or gold-plating (s 0.03). The accumulation of condensate changes s considerably, however, and, for condensate films a few millimetres thick, it can increase significantly and approach 0.9 (s for a black body =1). 

This involves the heat that must be extracted from the gas to cool it to the trapping temperature and the enthalpy of condensation (or heat of adsorption) that must also be removed. This quantity increases with gas throughput. 

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Modem UHV chambers are constmcted from stainless steel. The principal seals are metal-on-metal, thus the use of greases is avoided. A combination of pumps is nomially used, including ion pumps, turbomolecular pumps, cryopumps and mechanical (roughing) pumps. The entire system is generally heatable to 500 K. This bakeout for a period of... 

Another big advance in the appHcation of ms in biotechnology was the development of atmospheric pressure ionization (API) techniques. There are three variants of API sources, a heated nebulizer plus a corona discharge for ionization (APCl) (51), electrospray (ESI) (52), and ion spray (53). In the APCl interface, the Ic eluent is converted into droplets by pneumatic nebulization, and then a sheath gas sweeps the droplets through a heated tube that vaporizes the solvent and analyte. The corona discharge ionizes solvent molecules, which protonate the analyte. Ions transfer into the mass spectrometer through a transfer line which is cryopumped, to keep a reasonable source pressure. 

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. 

Each test electrode was transferred to the EC chamber and subjected to electrochemical stabilization. The EC chamber was then evacuated rapidly by two sorption pumps and a cryopump to transfer the electrode to the XPS chamber again. 

Figure 5. Front end of triple a quadrupole mass spectrometer with an ion source and reaction chamber (see Figure 4) mounted in front of an evacuated space containing skimmer cone CB, AC only quadrupole lens Qo, and first quadrupole Qi. The second and third quadrupoles, Q2 and Q3, are not shown. CR denotes cryopumping surfaces. IQ is an interquad lens.

The thermochemical determinations involving ions obtained via ES, described in this work, provide an illustration of opportunities presented by the ES method. However, these are only the beginning of what we believe should become an important area in gas-phase ion chemistry. The ion equilibria measurements, so far performed in our laboratory, were restricted by the limited upper temperature (T < 500 K). This restriction was due to the cryopumping used and could be easily overcome and the temperature extended upwards by -200 K by using conventional pumping. 

If gases like H2, He and Ne are to be trapped, the pump must be cooled to 4.2 K. Detailed information on cryopumping can be found in ref. [10]. Ultimate pressure of the order of l(U5torr can be achieved. 

R.A. Haefer Cryopumping, theory and practice, Clarendon Press, Oxford (1989)... 

To improve the cryopumping action, a getter pump can be attached to the cold wall of a cryostat. 

Referring to Fig. 5.6, the 3He refrigerator [25] contains a pump P and an evaporator E. They are connected by a stainless steel tube T internal to the copper support C. The latter is in good thermal contact with the working plane B of a pumped 4He cryostat (for example that of Fig. 5.3) not shown in figure. The tube is connected to a charcoal cryopump P linked to the 4lie bath by a thermal connection L. A thermometer Th monitors the temperature of the pump. A thermal shield (not shown), at the temperature of the 4He bath, surrounds the refrigerator. 

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. 

Pumps which pump vapors by means of condensation (condensers) and pumps which pump permanent gases by way of condensation at very low temperatures (cryopumps)... 

NEG pumps are mostly used in combination with other UHV pumps (turbomolecular and cryopumps). Such combinations are especially useful when wanting to further reduce the ultimate pressure of UHV systems. 

In continuous flow cryopumps the cold surface is designed to operate as a heat exchanger. Liquid helium in sufficient quantity is pumped by an auxiliary pump from a reservoir into the evaporator in order to attain a sufficiently low temperature at the cold surface (cryopanel). 

Today refrigerator cryopumps are being used almost exclusively (cold upon demand). These pumps operate basically much in the same way as a common household refrigerator, whereby the following thermodynamic cycles using helium as the refrigerant may be employed ... 

The series-manufactured refrigerator cryopumps from LEYBOLD use a two-stage cold head operating according to the Gifford-McMahon principle (see Fig. 2.67). In two series connected stages the temperature of the helium is reduced to about 30 K in the first stage and further to about 10 K in the... 

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