Pumps cryopump

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... 

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. 

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. 

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. 

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. 

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. 

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. 

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. 

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 ... 

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. 

The gas molecules entering the pump produce the area related theoretical pumping speed according the equation 2.29a with T = 293 K. The different pumping speeds have been combined for three representative gases H2, N2 and H2O taken from each of the aforementioned groups. Since water vapor is pumped on the entire entry area of the cryopump, the pumping speed... 

Cooldown time The cooldown time of cryopumps is the time span from start-up until the pumping etfect sets in. In the case of refrigerator cryopumps the cooldown time is stated as the time it takes for the second stage of the cold head to cool down from 293 K to 20 K. 

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. 

Capacity C (mbar I) The capacity of a cryopump for a certain gas is that quantity of gas (py value at T = 293 K) which can be bonded by the cryopanels before the pumping speed for this type of gas G drops to below 50 % of its initial value. 

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