A method may produce a two-stage heat regenerating cryogenic refrigerator including a vacuum vessel, first and second cylinder disposed in the vessel, the second cylinder coaxially connected to the first cylinder, and first and second regenerator respectively disposed in the first and second cylinder. The method may include: accommodating a first heat regenerating material (HRM) in the first regenerator; and filling a plurality of HRM particles in the second regenerator. The HRM particles may be a second HRM, each of the HRM particles including an oxide or oxysulfide heat regenerating substance having a maximum value of specific heat at a temperature of 20 K of 0.3+ J/cm3.Math.K and Ca, Mn, Mg, Be, Sr, Al, Fe, Cu, Ni, and/or Co. Each of the HRM particles may include a first and second region, the second region being closer to an HRM particle outer edge than the first region.

An expander unit of a cryogenic refrigerator device includes a moving assembly with a porous regenerative heat exchanger configured to move back and forth along a longitudinal axis. A magnetic spring assembly includes a stationary magnetic assembly fixed to the cold finger base that includes one or more magnetic rings fixedly arranged about a bore. A movable magnetic assembly includes one or more movable magnetic rings fixed to the moving assembly. An outer lateral dimension of each of the movable magnetic rings is less than an inner lateral dimension of the bore. The stationary magnetic assembly and the movable magnetic assembly are configured such that, when the moving assembly is displaced along the longitudinal axis from an equilibrium position, attractive and repulsive forces between the movable magnetic assembly and the stationary magnetic assembly yield a restoring force that is directed to restore the moving assembly to the equilibrium position.

Provided is a group of rare-earth regenerator material particles having an average particle size of 0.01 to 3 mm, wherein the proportion of particles having a ratio of a long diameter to a short diameter of 2 or less is 90% or more by number, and the proportion of particles having a depressed portion having a length of 1/10 to of a circumferential length on a particle surface is 30% or more by number. By forming the depressed portion on the surface of the regenerator material particles, it is possible to increase permeability of an operating medium gas and a contact surface area with the operating medium gas.

Integral linear cryogenic Stirling refrigerator comprised of the free piston positive displacement pressure wave generator, the moving assembly of which is connected to the free piston displacer by the dynamic spring-mass-spring mechanical phase shifter the mechanical properties of which (spring rates and weight) are selected to provide a predetermined phase lag of motion of the displacer piston relative to the moving assembly of pressure wave generator.

A cryocooler includes: a housing furnished with a housing bottom surface; a displacer furnished with a displacer upper surface between the housing bottom surface and which an upper gas chamber is formed, and being enabled to reciprocate axially with respect to the housing; a housing gas flow path formed in the housing and opening onto the upper gas chamber; a displacer upper gas flow path formed in the displacer and opening onto the upper gas chamber; and a gas-guiding flow channel formed in at least either the housing bottom surface or the displacer upper surface constituting a portion of the upper gas chamber, and interconnecting the housing gas flow path and the displacer upper gas flow path when the displacer is positioned at top-dead center in its axial reciprocation.

A reduction in a permeability of refrigerant gas is suppressed while increasing a filling factor of regenerator material particles with respect to a stage of a cold head. A cold head includes a stage including regenerator material particle groups, and a metal mesh material partitioning the regenerator material particle groups. The metal mesh material has quadrangular mesh holes each having a length of a long side of 1/10 or more and or less of each of average particle sizes of the regenerator material particle groups.

A heat regenerating material particle of an embodiment contains a heat regenerating substance having a maximum value of specific heat at a temperature of 20 K or less is 0.3 J/cm.sup.3.Math.K or more, and one metal element selected from the group consisting of calcium (Ca), magnesium (Mg), beryllium (Be), strontium (Sr), aluminum (Al), iron (Fe), copper (Cu), nickel (Ni), and cobalt (Co). The heat regenerating material particle includes a first region and a second region, the second region is closer to an outer edge of the heat regenerating material particle than the first region, and the second region has a higher concentration of the metal element than the first region.

A reduction in a permeability of refrigerant gas is suppressed while increasing a filling factor of regenerator material particles with respect to a stage of a cold head. A cold head includes a stage including regenerator material particle groups, and a metal mesh material partitioning the regenerator material particle groups. The metal mesh material has quadrangular mesh holes each having a length of a long side of 1/10 or more and or less of each of average particle sizes of the regenerator material particle groups.

Provided is a cold storage material having a large thermal capacity in a ultra-low temperature range of 10 K or less and being highly durable against thermal shock and mechanical vibration. The cold storage material contains a rare earth oxysulfide ceramic represented by the general formula R.sub.2O.sub.2S (wherein R is one or more kinds of rare earth elements selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), and Al.sub.2O.sub.3 having a specific surface area of 0.3 m.sup.2/g to 11 m.sup.2/g is added to the cold storage material.

A cooling device implementing a stirling-type thermodynamic cycle includes a compressor with a reciprocating piston driven by an electric motor rotating about an axis via a crankshaft. The electric motor comprises an internal stator and an external rotor and is connected to the crankshaft via a link with at least one degree of freedom in rotation about the axis of the electric motor.