A cryocooler includes a cold head including a displacer movable in an axial direction, a drive piston connected to the displacer to move the displacer in the axial direction, an expansion chamber formed with the displacer, a piston drive chamber formed with the drive piston, a spool valve including a valve drive chamber, a spool that moves between a first position and a second position in response to a pressure of the valve drive chamber, and a pressure control mechanism configured to control a pressure of the valve drive chamber so that the spool reciprocates between the first position and the second position, and to generate a pressure fluctuation having an opposite phase to the pressure fluctuation in the expansion chamber in the piston drive chamber in synchronization with the reciprocation of the spool.

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.

A thermoacoustic energy conversion system includes a closed circumferential encasing filled with a working fluid through which an acoustic wave can propagate in a propagation direction in use of the system, and at least one assembly of two heat exchangers with a regenerator sandwiched there-between arranged in said encasing. The at least one assembly is arranged substantially parallel to a local longitudinal axis of the encasing.

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 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 refrigerator includes a regenerator, a low-temperature end heat exchanger, a pulse tube, a high-temperature end heat exchanger, and a phase adjustment mechanism, connected in that order. A draft tube is provided inside the regenerator, paralleling the regenerator's axis, and the draft tube can extend into the low-temperature end heat exchanger.

The present invention provides a regenerator material whose filling rate can be improved and falls within a suitable range, and which can thus easily reduce the pressure loss of a refrigerant gas in the regenerator; and a method for producing the regenerator material. The regenerator material of the present invention comprises a sintered body of rare earth element-containing particles, wherein the sintered body has a porosity of 30 to 40%. The regenerator material production method of the present invention comprises the step of sintering rare earth element-containing starting material particles, wherein D50 and D90/10 of the starting material particles are respectively 100 to 320 m and 1.5 to 2.5; wherein D10, D50, and D90 indicate the average particle sizes that respectively correspond to the 10th, 50th, and 90th percentiles of the total number of particles in the particle size distribution curve.

The thermoacoustic energy converting element part is provided with a plurality of through holes extending along a direction to penetrate the thermoacoustic energy converting element part to form travelling routes of acoustic waves. The thermoacoustic energy converting element part includes a wall surrounding each of the through holes to extend in an extending direction of the through hole and configured to exchange heat with the fluid. The through hole includes a hole that has a hydraulic diameter of 0.4 mm or smaller, and an open area ratio of the through holes in the thermoacoustic energy converting element part is 60% or higher. Thermal conductivity of the thermoacoustic energy converting element part in fluid atmosphere is 0.4 W/m/K or lower, and heat capacity of the thermoacoustic energy converting element part at 400 C. in the fluid atmosphere is higher than 0.5 J/cc/K.

A regenerator of a cryo-cooler uses helium both as a working gas and as a heat storage material. The regenerator includes cells whose exterior sides form flow channels through which the working gas flows. Each cell has connected first and second cavities enclosed by a heat-conductive cell wall. The cavities contain helium that is used to store heat. Each cells is shaped as a disk. The working gas flows both through the flow channels and around the regenerator so as to exchange heat with the helium in the cavities via the heat conducting cell wall. Each cell has a pressure-equalizing opening through the cell wall whose diameter is smaller than the thickness of the cell wall. The diameter of the pressure-equalizing opening is dimensioned to permit the pressure of the helium contained in the cell to change by a maximum of 20% during any working cycle of the cryo-cooler.

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.