Cryopumping Temperatures Below

Cryopump (vacuum technology) 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 Vacuum pump. 

Cryopumping to generate ultrahigh vacuum involves the use of cold surface below 25 K. At these temperatures all gases other than helium, hydrogen, and neon are condensed when they make contact with the cold surface. At pressures above 1 x 10" torr a relatively high concentration of gas molecules limits the useful life of a cryopump due to condensate build-up and consequent surface temperature rise. At lower pressures the useful life of the cryopump is rapidly extended. Below 1 x torr the cryopump limitation is its tem-... 

Pumpdown. Chamber pumpdown was not a prime test objective, although every effort was made to reach lowest possible pressures. A sudden, accidental increase in cryopump cold-plate temperature, from 25° to29°K, provided an opportunity to observe cryopump response. The sequence of events is reported in Table III. Due to the sudden release of cryodeposits, chamber pressure rose from 1 x 10" to 5 x 10" torr. As soon as the original error was corrected and the cold plate was cooled below 25 °K, chamber pressure dropped below its original level, to 2 x 10" torr, within 60 sec. 

Temperature-dependent NMR spectra of monolayers of CgDe on a-BN were measured at 52.7 MHz from 80 to 293 K. Below 225 K, the largely different linewidths of the singlet spectra recorded appear to be mainly determined by the adsorbent particle morphology (averaging of the nuclear quadrupole interaction upon surface diffusion). Above 225 K, the linewidths are considered to be dominated by isotropic molecular reorientation motions [56] also compare [57]. Graphitic a-BN is used as sorbent layer on sorption surfaces for cooling traps and cryopumps [58]. 

The wet disks are immediately immersed into the vapor of refluxing isopropanol. Once they reach the reflux temperature, as noted by a reduction in the rate of alcohol condensation on the disks, they are removed into the room air where they rapidly become dry and slowly cool to room temperature. At this point, the disks are individually weighed to the nearest 0.1 mg. The disks are then loaded into a metal tray with large circular regions on their bottom faces exposed and the tray placed in the load lock of a vacuum chamber. The load lock is pumped to about 10 torr and then the tray is translated into the chamber and the load lock sealed off from the chamber. The chamber is pumped to about 10 torr with a cryopump. The major residual gas is water from the rotatable rubber seal used between the evacuated space in the chamber and the water flow path into and out of the rotatable copper crucible mentioned below. Titanium vapor is sublimed from a Ti ingot by bombardment with about 8 keV electrons while the periphery of the ingot is cooled by sparse physical contacts with the water-cooled copper crucible in which it rests. The Ti vapor condenses in the line of sight from the source onto the exposed bottoms of the disks. The thickness of the deposited titanium, is monitored by a calibrated quartz crystal balance close to the quartz disks. Typically (iji 5 x 10 cm. 

Cryopumping. A simpler method for inducing a vacuum is known as cryopumping. A hermetically sealed insulation space is first filled with a gas, most commonly carbon dioxide. When the inner vessel of the container is filled with a cryogenic liquid, the gas molecules in the annular space condense and freeze on the inner wall, creating a vacuum in the insulation space. Carbon dioxide molecules, for example, will freeze out on a surface cooled below 200 K, thus reducing the gas pressure in a sealed insulation space. Tests have shown that, with one wall of insulation exposed to liquid hydrogen temperature, a 2.7 x 10 MPa pressure can be achieved by this technique. 

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