Cryocooler

Abstract

A cryocooler includes a cylinder that has a first thermal conductivity and that extends in an axial direction, a cooling stage that has a second thermal conductivity higher than the first thermal conductivity and that includes a stage end portion and a stage tubular portion connecting the stage end portion to the cylinder in the axial direction, a displacer that is capable of reciprocating in the axial direction in the cylinder, that forms an expansion space with the stage end portion, and in which the expansion space takes a maximum volume at a top dead center, and a ceramic-based magnetic regenerator material that is accommodated in the displacer and of which an axial position in the displacer is determined to overlap the stage tubular portion in the axial direction when the displacer is at the top dead center.

What is claimed is: 
     
         1 . A cryocooler comprising:
 a cylinder that has a first thermal conductivity and that extends in an axial direction;   a cooling stage that has a second thermal conductivity higher than the first thermal conductivity and that includes a stage end portion and a stage tubular portion connecting the stage end portion to the cylinder in the axial direction;   a displacer that is capable of reciprocating in the axial direction in the cylinder, that forms an expansion space with the stage end portion, and in which the expansion space takes a maximum volume at a top dead center of the displacer; and   a ceramic-based magnetic regenerator material that is accommodated in the displacer and of which an axial position in the displacer is determined to overlap the stage tubular portion in the axial direction when the displacer is at the top dead center.   
     
     
         2 . The cryocooler according to  claim 1 , further comprising:
 a metal-based magnetic regenerator material that is accommodated in the displacer and of which an axial position in the displacer is determined not to overlap the stage tubular portion in the axial direction when the displacer is at the top dead center.   
     
     
         3 . The cryocooler according to  claim 1 ,
 wherein the displacer includes a displacer cap that faces the stage end portion with the expansion space interposed therebetween.   
     
     
         4 . The cryocooler according to  claim 3 ,
 wherein the displacer cap is formed of a material having a high electrical resistivity compared to the cooling stage.   
     
     
         5 . The cryocooler according to  claim 3 ,
 wherein the displacer cap includes a regenerator body formed of the ceramic-based magnetic regenerator material.   
     
     
         6 . The cryocooler according to  claim 5 ,
 wherein the regenerator body is exposed to the expansion space.   
     
     
         7 . The cryocooler according to  claim 3 ,
 wherein the ceramic-based magnetic regenerator material is disposed adjacent to the displacer cap in the axial direction in the displacer, and   when a stroke length of reciprocation of the displacer in the axial direction is denoted by S, an axial height of the displacer cap is denoted by Hc, and an axial height from the displacer cap to an upper end of the ceramic-based magnetic regenerator material is denoted by Hs, the axial position of the ceramic-based magnetic regenerator material in the displacer is determined to satisfy Hs>S−Hc.   
     
     
         8 . The cryocooler according to  claim 7 ,
 wherein the axial position of the ceramic-based magnetic regenerator material in the displacer is determined to satisfy Hs>(¾)S−Hc.