How do ultra-high vacuum systems work?
Working principles for ultra-high vacuum
The boundary between the high and ultrahigh vacuum region cannot be precisely defined with regard to the working methods. In practice, a border between the two regions is brought about because pressures in the high vacuum region may be obtained by the usual pumps, valves, seals, and other components, whereas for pressures in the UHV region, another technology and differently constructed components are generally required. The “border” lies at a few 10-8 mbar. Therefore, pressures below 10-7 mbar should generally be associated with the UHV region.
The gas density is very small in the UHV region and is significantly influenced by outgassing rate of the vessel walls and by the tiniest leakages at joints. Moreover, in connection with a series of important technical applications to characterize the UHV region, generally the monolayer time (see also equation 1.21) has become important. This is understood as the time τ that elapses before a monomolecular or monatomic layer forms on an initially ideally cleaned surface that is exposed to the gas particles. Assuming that every gas particle that arrives at the surface finds a free place and remains there, a convenient formula for τ is
p in mbar
Therefore, in UHV (p < 10-7 mbar) the monolayer formation time is of the order of minutes to hours or longer and thus of the same length of time as that needed for experiments and processes in vacuum. The practical requirements that arise have become particularly significant in solid-state physics, such as for the study of thin films or electron tube technology.
Differences between high and UHV (Ultra-High Vacuum) systems
A UHV system is different from the usual high vacuum system for the following reasons:
- the leak rate is extremely small (use of metallic seals),
- the gas evolution of the inner surfaces of the vacuum vessel and of the attached components (e.g., connecting tubulation; valves, seals) can be made extremely small,
- suitable means (cold traps, baffles) are provided to prevent gases or vapors or their reaction products that have originated from the pumps used from reaching the vacuum vessel (no backstreaming).
To fulfill these conditions, the individual components used in UHV apparatus must be bakeable and extremely leaktight. Stainless steel is the preferred material for UHV components.
The construction, start-up, and operation of an UHV system also demands special care, cleanliness, and, above all, time.
The assembly must be appropriate; that is, the individual components must not be in the least damaged (i.e. by scratches on precision worked sealing surfaces). Fundamentally, every newly assembled UHV apparatus must be tested for leaks with a helium leak detector before it is operated. Especially important here is the testing of demountable joints (flange connections), glass seals, and welded or brazed joints.
After testing, the UHV apparatus must be baked out. This is necessary for glass as well as for metal apparatus. The bake- out extends not only over the vacuum vessel, but frequently also to the attached parts, particularly the vacuum gauges. The individual stages of the bake-out, which can last many hours for a larger system, and the bake-out temperature are arranged according to the kind of plant and the ultimate pressure required.
If, after the apparatus has been cooled and the other necessary measures undertaken (e.g., cooling down cold traps or baffles), the ultimate pressure is apparently not obtained, a repeated leak test with a helium leak detector is recommended.
Alto vuoto, ultra alto vuoto e alto vuoto estremo: principi fondamentali
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