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a thermostat in which the working unit or the object under study is maintained at temperatures below 120°K (cryogenic temperatures) by an external cooling source. The coolant is usually a liquefied or solidified gas with low liquefaction and freezing temperatures (such as nitrogen, hydrogen, helium). The temperature of an object placed in the cryostat is controlled by changing the vapor pressure over the coolant in the cryostat or by heating the vapors of the coolant. Cryostats are classified according to the type of cooling agent used (nitrogen, helium, and hydrogen), the type of material used (glass, metal, or plastic), and the function (for electronic, optical, and other research; superconducting magnets; and radiation detectors).
Cryostats of all types require shielding of their working chambers from the influx of heat from the surrounding medium. The thermal insulation requirements for the working modules of cryostats increase with decreasing boiling point and heat of vaporization of the coolant used. Cryostats filled with liquid nitrogen or oxygen frequently use high-vacuum thermal insulation, similar to that used in the widely known Dewar flasks and domestic vacuum bottles. However, ordinary high-vacuum insulation is inadequate for helium cryostats. Therefore, to decrease the flux of radiant energy from the exterior walls of the cryostat, it is necessary to reduce their temperature; this is done through cooling of the walls by an auxiliary coolant (for example, liquid nitrogen) or by installation in the thermal insulation space of screens that reflect radiation.
Glass cryostats are widely used in laboratory practice. They are simple to fabricate and are transparent, which makes possible direct observation of experiments. A general-purpose glass helium cryostat usually consists of two Dewar flasks, one inside the other. The inner flask is filled with liquid helium, and the outer flask is filled with liquid nitrogen. Low mechanical strength is a disadvantage of glass cryostats.
Metal cryostats are more reliable in use; the most versatile cryostats of this type use liquid helium as the basic coolant. In a metal helium cryostat with additional liquid-nitrogen cooling, the helium container of the cryostat is surrounded on all sides by a copper screen. High vacuum, which is maintained during operation by an adsorbent, is generated in the space between the helium container and the jacket. A bellows is built into the cryostat to compensate for the thermal stresses arising between the interior cells and the enclosure. Cryostats are constructed from copper, stainless steel, or aluminum alloys. The surfaces of the cryostat cells are polished on the vacuum side to reflect heat.
Metal cryostats designed for optical studies contain windows, as well as rotation devices, which may be used for changing the position of the sample. Cooling of the screens in helium and hydrogen cryostats is performed by vapors of the main coolant rather than liquid nitrogen. Cryostats are widely used in cryogenic engineering.