ABSTRACT
The cryogen-free systems have different structures from the “wet” systems.
Instead of having a “1 K pot” in the “wet” systems the cryogen-free systems normally employees Joule-Thompson unit for gas (helium in our discussion) condensation. An introductory theory of Joule-Thomson effect as well as the practical engineering details of the Joule-Thomson unit is presented.
Most of the cryogen-free systems are pre-cooled by the cryo-cooler with heat switch.
Three commonly used heat switches employed in cryogen-free systems are introduced in this chapter. They are mechanical heat switch, gas gap heat switch, and superconducting heat switch respectively.
The superconducting heat switch is exclusively used for the nuclear demagnetization refrigerator in order to reach 1.0 mK or lower temperatures. The reason we included the superconducting heat switch in our discussion is that developing cryogen-free nuclear demagnetization refrigerator is emerging from the horizon.
A brief discussion of the ultra-high vacuum (UHV) technologies is presented because UHV compatible cryogen-free systems is demanded.
