Adsorption pumps

He refrigerators fall into two categories (1) refrigerators with external adsorption pump and (2) refrigerators with internal adsorption pump. Hereafter, we will describe a refrigerator of the second type. 

Adsorption pumps (see Fig. 2.59) work according to the principle of the physical adsorption of gases at the surface of molecular sieves or other adsorption materials (e.g. activated AljOj). Zeolite 13X is frequently used as an adsorption material. This alkali aluminosilicate possesses for a mass of the material an extraordinarily large surface area, about 1000 m /g of solid substance. Correspondingly, its ability to take up gas is considerable. 

The adsorption of gases at surfaces is dependent not only on the temperature, but more importantly on the pressure above the adsorption surface. The dependence is represented graphically for a few gases by the adsorption isotherms given in Fig. 2.60. In practice, adsorption pumps are... 

The ultimate pressure attainable with adsorption pumps is determined in the first instance by those gases that prevail in the vessel at the beginning of the pumping process and are poorly or not at all adsorbed (e.g. neon or helium) at the zeolite surface. In atmospheric air, a few parts per million of these gases are present. Therefore, pressures < 10 mbar can be obtained. 

If pressures below 10 mbar exclusively are to be produced with adsorption pumps, as far as possible no neon or helium should be present in the gas mixture. 

To pump out larger vessels, several adsorption pumps are used in parallel or in series. First, the pressure is reduced from atmospheric pressure to a few millibars by the first stage in order to capture many noble gas molecules of helium and neon. After the pumps of this stage have been saturated, the valves to these pumps are closed and a previously closed valve to a further adsorption pump still containing clean adsorbent is opened so that this pump may pump down the vacuum chamber to the next lower pressure level. This procedure can be continued until the ultimate pressure cannot be further improved by adding further clean adsorption pumps. 

Instead of rotary pumps, large water jet, steam ejector, or water ring pumps can be used. For batch evacuation, and the production of hydrocarbon-free fore vacuum for sputter-ion pumps, adsorption pumps (see Section 2.1.8.1) are suitable. If the use of oil-sealed rotary vane pumps cannot be avoided, basically two-stage rotary vane pumps should be used. The small amount of oil vapor that backstreams out of the Inlet ports of these pumps can be almost completely removed by a sorption trap (see Section 2.1.4) Inserted In the pumping line. 

Absolutely oil-free vacua may be produced in the medium vacuum region with adsorption pumps. Since the pumping action of these pumps for the light noble gases is only small, vessels initially filled with air can only be evacuated by them to about 10 2 mbar. Pressures of lO mbar or lower can then be produced with adsorption pumps only if neither neon nor helium is present in the gas mixture to be pumped. In such cases it can be useful to expel the air in the vessel by first flooding with nitrogen and then pumping it away. 

The remaining vacuum pumps to be discussed in this chapter fall into a group which remove gas particles from systems by sorption effects such as adsorption, chemisorption/gettering and implantation. They tend to be used on systems where any contamination of the vacuum by pump fluids, lubricants, etc. must be avoided. However, those pumps that remove gas particles exclusively by temperature-dependent gas adsorption on molecular sieves or A1203 (adsorption pumps) will not be discussed. 

In the He refrigerator, the He is condensed into a copper sponge, which is initially precooled by the surrounding helium bath. After thermally isolating the copper sponge and condensate from the helium bath, the temperature of the He is lowered to about 0.3 K by reducing the vapor pressure of the He with an adsorption pump. The cycle is repeated when all of the He is removed from the copper sponge. 

Capturing and keeping the gas molecules (adsorption pump, absorption or reaction pump) - e.g. cryopump, sorption pump, ion pump, getter pump, absorption pump, getter pump. 

Adsorption pump, vacuum (vacuum technology) A capture-type vacuum pump that pumps by cryocondensation or cryotrapping on a surface of temperature less than — 150°C. See also Vacuum pump. 

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