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Ultra-high vacuum
Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressure lower than about 1×10−9 torrs (1×10−9 mbar; 1×10−7 Pa). 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:
Typically, UHV requires:
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. Helium could diffuse through the steel and glass from the outside air, but this effect is usually negligible due to the low abundance of He in the atmosphere.
Measurement of high vacuum is done using a nonabsolute gauge that measures a pressure-related property of the vacuum. See, for example, Pacey. These gauges must be calibrated. The gauges capable of measuring the lowest pressures are magnetic gauges based upon the pressure dependence of the current in a spontaneous gas discharge in intersecting electric and magnetic fields.
UHV pressures are measured with an ion gauge, either of the hot filament or inverted magnetron type.
In any vacuum system, some gas will continue to escape into the chamber over time and slowly increase the pressure if it is not pumped out. This leak rate is usually measured in mbar L/s or torr L/s. While some gas release is inevitable, if the leak rate is too high, it can slow down or even prevent the system from reaching low pressure.
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Ultra-high vacuum
Ultra-high vacuum (often spelled ultrahigh in American English, UHV) is the vacuum regime characterised by pressure lower than about 1×10−9 torrs (1×10−9 mbar; 1×10−7 Pa). 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:
Typically, UHV requires:
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. Helium could diffuse through the steel and glass from the outside air, but this effect is usually negligible due to the low abundance of He in the atmosphere.
Measurement of high vacuum is done using a nonabsolute gauge that measures a pressure-related property of the vacuum. See, for example, Pacey. These gauges must be calibrated. The gauges capable of measuring the lowest pressures are magnetic gauges based upon the pressure dependence of the current in a spontaneous gas discharge in intersecting electric and magnetic fields.
UHV pressures are measured with an ion gauge, either of the hot filament or inverted magnetron type.
In any vacuum system, some gas will continue to escape into the chamber over time and slowly increase the pressure if it is not pumped out. This leak rate is usually measured in mbar L/s or torr L/s. While some gas release is inevitable, if the leak rate is too high, it can slow down or even prevent the system from reaching low pressure.