Abstract: An expander generates cold by expanding a refrigerant gas in a cryogenic refrigerator. A compressor compresses the refrigerant gas returning from the expander. Pipes are connected to the expander and the compressor and circulate the refrigerant gas between the expander and the compressor. A determiner determines whether or not a change cycle of the pressure of the refrigerant gas flowing in the pipes is in a predetermined range. The determiner may determine whether or not the change cycle of the pressure of a low-pressure pipe in which a low-pressure refrigerant gas flows toward the compressor from the expander is in a predetermined range.

Abstract: An expander generates cold by expanding a refrigerant gas in a cryogenic refrigerator. A compressor compresses the refrigerant gas returning from the expander. Pipes are connected to the expander and the compressor and circulate the refrigerant gas between the expander and the compressor. A determiner determines whether or not a change cycle of the pressure of the refrigerant gas flowing in the pipes is in a predetermined range. The determiner may determine whether or not the change cycle of the pressure of a low-pressure pipe in which a low-pressure refrigerant gas flows toward the compressor from the expander is in a predetermined range.

Abstract: A regenerative refrigerator includes a cylinder; a displacer provided in the cylinder; a groove pattern formed on the exterior circumferential surface of the displacer or the interior circumferential surface of the cylinder to form a first gas passage connecting one end and the other end of the exterior or interior circumferential surface, and including a groove having at least part thereof extending along a direction to cross the axial directions of the displacer to cause gas flowing from the one end to the other end in the gap between the exterior and interior circumferential surfaces to actively exchange heat with the cylinder and the displacer; a second gas passage to and from an expansion space; and a regenerator material formed of bismuth granules and provided in at least part of the second gas passage, wherein the lowest attainable temperature is 5 K to 15 K in an unloaded state.