Abstract
Liquid Helium is used widely, from hospitals to characterization of materials at low temperatures. Many experiments at low temperatures require liquid Helium, particularly when vibration isolation precludes the use of cryocoolers and when one needs to cool heavy equipment such as superconducting coils. Here we describe methods to simplify the operations required to use liquid Helium by eliminating the use of high pressure bottles, avoiding blockage and improving heating and cooling rates. First we show a simple and very low cost method to transfer liquid Helium from a transport container into a cryostat that uses a manual pump having pumping and pressurizing ports, giving a liquid Helium transfer rate of about 100 liters an hour. Second, we describe a closed cycle circuit of Helium gas cooled in an external liquid nitrogen bath that allows precooling a cryogenic experiment without inserting liquid nitrogen into the cryostat, eliminating problems associated to the presence of nitrogen around superconducting magnets. And third, we show a sliding seal assembly and an inner vacuum chamber design that allows inserting large experiments into liquid Helium.
Highlights
Much of the innovative work in cryogenics is devoted into dry or closed-cycle cryogenic systems, driven by the development of thermoacoustic systems and the improvement of thermodynamic machines [1]
Large systems including superconducting magnets require most often liquid Helium, because cooling using the huge enthalphy of Helium gas is unmatched with respect to any other cooling method
Several problems are repeatedly mentioned. These are about handling of liquid Helium bottles and transfer methods, inserting and cooling to liquid Helium temperatures large inserts, often containing a dilution refrigerator unit, and precooling to liquid nitrogen temperatures
Summary
Much of the innovative work in cryogenics is devoted into dry or closed-cycle cryogenic systems, driven by the development of thermoacoustic systems and the improvement of thermodynamic machines [1]. There are many laboratory books and notes that are there to help the users, in addition to usual manuals provided by companies delivering cryostats [12, 13]. Most of these manuals explain transfer of liquid Helium and sample turn-around methods. These are about handling of liquid Helium bottles and transfer methods, inserting and cooling to liquid Helium temperatures large inserts, often containing a dilution refrigerator unit, and precooling to liquid nitrogen temperatures. We discuss possible solutions to these issues, which we developed in the use of a set of five large dilution refrigerator units that are equipped with Scanning Tunneling Microscopes. The improvements in sliding seal assembly and vacuum chamber have not been reported to our knowledge
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