As an example of helium gas reduction due to the high price and difficulty of obtaining helium gas in recent years, one method is to cool superconducting magnets and superconducting radio-frequency cavities, etc. directly with a small mechanical cryocooler instead of using liquid helium. In this case, since the cooling capacity per unit is small, multiple units are used in combination depending on the required cooling capacity, which causes problems in terms of power consumption （efficiency）, footprint, and maintenance costs. Therefore, we have been developing a large-capacity and high-efficiency 4KGM-JT（Gifford-McMahon-Joule-Thomson） cryocooler system in the 10 W class at 4.2 K, using our 4K-GM cryocooler as a pre-cooler. This paper describes the technical details used in this cryocooler system.
This paper discusses magnetic bearings made of permanent magnets（ PM） and ring-shape stacked superconductors. We evaluate basic electromagnetic characteristics such as thrust force and rotation characteristics via experiments. The thrust force using a cylindrical PM and the ring-shape stacked superconductors are compared with the PM and a ring-shape bulk superconductor. The experimental results showed that both thrust forces are almost the same as 14 N. On the other hand, the rotation stability of the bearings using the 1- or 3-stacked superconductors, or a bulk superconductor is evaluated via time constant. The experimental results indicated that the time constants of the magnetic bearing using 3-stacked superconductors and using a bulk superconductor are 3.85 s and 2.92 s, respectively. The results showed that 3-stacked superconducting magnetic bearing have a competitive with the magnetic bearings using bulk superconductors.