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Concept
Ultra-high vacuum
Summary
Ultra-high vacuum (UHV) is the vacuum regime characterised by pressures lower than about . UHV conditions are created by pumping the gas out of a UHV chamber. At these low pressures the mean free path of a gas molecule is greater than approximately 40 km, so the gas is in free molecular flow, and gas molecules will collide with the chamber walls many times before colliding with each other. Almost all molecular interactions therefore take place on various surfaces in the chamber.
UHV conditions are integral to scientific research. Surface science experiments often require a chemically clean sample surface with the absence of any unwanted adsorbates. Surface analysis tools such as X-ray photoelectron spectroscopy and low energy ion scattering require UHV conditions for the transmission of electron or ion beams. For the same reason, beam pipes in particle accelerators such as the Large Hadron Collider are kept at UHV.
Maintaining UHV conditions requires the use of unusual materials for equipment. Useful concepts for UHV include:
Sorption of gases
Kinetic theory of gases
Gas transport and pumping
Vacuum pumps and systems
Vapour pressure
Typically, UHV requires:
High pumping speed — possibly multiple vacuum pumps in series and/or parallel
Minimized surface area in the chamber
High conductance tubing to pumps — short and fat, without obstruction
Use of low-outgassing materials such as certain stainless steels
Avoid creating pits of trapped gas behind bolts, welding voids, etc.
Electropolishing of all metal parts after machining or welding
Use of low vapor pressure materials (ceramics, glass, metals, teflon if unbaked)
Baking of the system to remove water or hydrocarbons adsorbed to the walls
Chilling of chamber walls to cryogenic temperatures during use
Avoiding all traces of hydrocarbons, including skin oils in a fingerprint — gloves must always be used
Hydrogen and carbon monoxide are the most common background gases in a well-designed, well-baked UHV system. Both Hydrogen and CO diffuse out from the grain boundaries in stainless steel.
Official source
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