Abstract
This chapter discusses the cooling performance and other features of two new types of He3 refrigerators that have been built at OIS. These refrigerators are constructed using the well-know phenomena of adiabatic expansion of gas and adsorption pumping. The first type operates in a liquid helium environment at 4.2K. The second design is cryogen free and uses a Pulse Tube Cryocooler. This ULT system is embodied in a commercial He3 Cryofree refrigerator Heliox AC-V, developed by Oxford Instruments. It is important to mention the reliability of the expansion refrigerator. Because current cryocoolers produce cooling down to <3K, this causes an extra liquid fraction by direct condensation into the He3 pot. However this temperature is close to the He3 critical point so any failure in the pulse tube and subsequent deterioration of the minimum temperature achieved by the PTR will completely stop the operation of a traditional He3 refrigerator. For these He3 systems this limit is much higher at 4.2K or even 5K, where adiabatic expansion of He3 still produces liquefaction of He3 and hence ultra low temperatures..
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