Gas-driven cryocooler

Abstract

A gas-driven cryocooler is capable of executing an initial cooling for cooling from an initial temperature to a cryogenic temperature and a steady-state operation for maintaining the cryogenic temperature following the initial cooling. The gas-driven cryocooler includes a compressor, a cold head, and a flow restrictor that is connected between the compressor and the cold head and that operates to restrict a flow rate of a working gas to the cold head during the initial cooling, as compared with a flow rate during the steady-state operation.

What is claimed is: 
     
         1 . A gas-driven cryocooler configured to execute an initial cooling for cooling from an initial temperature to a cryogenic temperature and a steady-state operation for maintaining the cryogenic temperature following the initial cooling, the gas-driven cryocooler comprising:
 a compressor;   a cold head; and   a flow restrictor that is connected between the compressor and the cold head and that operates to restrict a flow rate of a working gas to the cold head during the initial cooling, as compared with a flow rate during the steady-state operation.   
     
     
         2 . The gas-driven cryocooler according to  claim 1 ,
 wherein the compressor includes a discharge port and a suction port for the working gas,   the cold head includes:
 a displacer that extends in an axial direction, 
 a displacer cylinder that accommodates the displacer to be reciprocable in the axial direction and that forms an expansion chamber between the displacer and the displacer cylinder, 
 a main pressure switching valve that alternately connects the expansion chamber to the discharge port and the suction port, 
 a drive piston that extends from the displacer to a side opposite to the expansion chamber in the axial direction, 
 a drive chamber that accommodates the drive piston to be reciprocable in the axial direction, and 
 an auxiliary pressure switching valve that alternately connects the drive chamber to the discharge port and the suction port to generate a differential pressure between the expansion chamber and the drive chamber and reciprocate the drive piston in the axial direction by the differential pressure, and 
   the flow restrictor is connected between the auxiliary pressure switching valve and the drive chamber and operates to restrict a flow rate of the working gas to the drive chamber during the initial cooling, as compared with a flow rate during the steady-state operation.   
     
     
         3 . The gas-driven cryocooler according to  claim 1 ,
 wherein the flow restrictor includes a flow control valve that opens at a smaller opening degree during the initial cooling, as compared with an opening degree during the steady-state operation.   
     
     
         4 . The gas-driven cryocooler according to  claim 3 ,
 wherein the flow control valve includes a gas pressure actuator that drives the flow control valve such that the flow control valve opens at a smaller opening degree during the initial cooling, as compared with an opening degree during the steady-state operation.   
     
     
         5 . The gas-driven cryocooler according to  claim 4 ,
 wherein the flow restrictor includes a gas volume thermally coupled to a low temperature portion of the cold head,   the flow control valve includes a valve body that defines an opening degree of the flow control valve, and   the gas pressure actuator includes a pressure chamber connected to the gas volume and drives the valve body in response to a pressure in the pressure chamber.   
     
     
         6 . The gas-driven cryocooler according to  claim 4 ,
 wherein the flow control valve includes a valve body that defines an opening degree of the flow control valve, and   the gas pressure actuator includes a pressure chamber connected to the compressor and drives the valve body in response to a pressure in the pressure chamber.   
     
     
         7 . The gas-driven cryocooler according to  claim 4 ,
 wherein the flow control valve includes a valve body stopper that defines a maximum opening degree of the flow control valve by the gas pressure actuator.