Development of a Small Helium Liquefier

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9400122 Silvera In order to provide liquid helium for a cryogenic laboratory with experiments focused on spin-polarized atomic hydrogen, superfluid helium, and high pressure studies of molecular hydrogen, a small system capable of liquefying at least 24 liters/day with peaks of 40/day is being developed. Experiments are carried out in three dilution refrigerators, 3He refrigerators, and pumped 4He cryostats. The sub-kelvin studies are mainly on the properties of spin-polarized atomic hydrogen, the only atomic system that remains gaseous to T=0K. Experiments include the development of a microwave atom trap working at low temperature, compression of a gas of hydrogen in intense magnetic fields, the formation of a two-dimensional gas on a surface of superfluid helium, and the study of cryogenic hydrogen maser. This research is aimed at observing quantum degenerate properties of hydrogen and to study H-H and H-He interactions. The ultimate goal is to attain Bose- Einstein condensation and superfluidity in a gas; the cryogenic maser is expected to surpass current standards in the measurement of time and frequency. Experiments at high pressure on molecular hydrogen and its isotopes, D2 and HD are designed to achieve pressures in the 2-4 megabar. New Phases have been observed at megabar pressures. The ultimate goal is to make and study metallic hydrogen, which has been predicted to be a room temperature superconductor. a rigorous test of metallization is to show that the electrical conductivity remains finite in the limit that temperature approaches zero, requiring liquid helium temperatures and the combination of low temperature techniques with ultra high pressure diamond anvil cells. The principal liquefier consists of a Gifford-McMahon cryocooler using a newly developed rare earth metal generator to precool compressed helium gas which is liquefied by expansion through a Joule-Thompson valve. The estimated production is a conservative 1 liter/hour. The liquefier is designed to operate as a turn-key system under computer control, requiring little daily maintenance. Two special cold traps for purifying the helium of air impurities will be computer monitored for alternate switching into the gas stream as they saturate and for automatic reactivation. A second system for use in high helium consumption cryogenic experiments will reliquefy or absorb the heat load on the helium reservoir by submerging a refrigerated probe in the helium bath. This system will be capable of reducing helium consumption by 16 liters/day and can greatly reduce the demand on the principal liquefier in peak periods of helium use. This system has potential to impact the commercial use of liquid helium as in magnetic resonance imaging, industrial research laboratories using cryogenic systems, as well as university teaching laboratories. ***