Cryogenic refrigerator, and method for cooling down cryogenic refrigerator

Abstract

A cryocooler includes: a cold head including a first cooling stage and a second cooling stage cooled to a lower temperature than the first cooling stage; a heater thermally coupled to the first cooling stage; a first temperature sensor that measures a first temperature of the first cooling stage; and a controller configured to perform a cool-down operation of the cold head to cool the first cooling stage from an initial temperature to a first final target temperature and to cool the second cooling stage from the initial temperature to a second final target temperature lower than the first final target temperature, in which the controller is configured to, during the cool-down operation, acquire the first temperature from the first temperature sensor, and control the heater such that the first temperature follows an intermediate target temperature lower than the initial temperature and higher than the first final target temperature.

What is claimed is: 
     
         1 . A cryocooler comprising:
 a cold head including a first cooling stage and a second cooling stage cooled to a lower temperature than the first cooling stage;   a heater thermally coupled to the first cooling stage;   a first temperature sensor that measures a first temperature of the first cooling stage; and   a controller configured to perform a cool-down operation of the cold head to cool the first cooling stage from an initial temperature to a first final target temperature and to cool the second cooling stage from the initial temperature to a second final target temperature which is lower than the first final target temperature,   wherein the controller is configured to, during the cool-down operation,
 acquire the first temperature of the first cooling stage from the first temperature sensor, and 
 control the heater such that the first temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature. 
   
     
     
         2 . The cryocooler according to  claim 1 ,
 wherein the controller is configured to change the intermediate target temperature during the cool-down operation.   
     
     
         3 . The cryocooler according to  claim 2 , further comprising:
 a second temperature sensor that measures a second temperature of the second cooling stage,   wherein the controller is configured to, during the cool-down operation,
 acquire the second temperature of the second cooling stage from the second temperature sensor, and 
 set the intermediate target temperature based on the second temperature of the second cooling stage. 
   
     
     
         4 . The cryocooler according to  claim 3 ,
 wherein the controller is configured to set the intermediate target temperature to the second temperature or a temperature which is higher than the second temperature.   
     
     
         5 . The cryocooler according to  claim 1 ,
 wherein the controller is configured to compare the first temperature of the first cooling stage to the intermediate target temperature, turn on the heater when the first temperature falls below the intermediate target temperature, and turn off the heater when the first temperature exceeds the intermediate target temperature.   
     
     
         6 . The cryocooler according to  claim 1 , further comprising:
 a second temperature sensor that measures a second temperature of the second cooling stage,   wherein the controller is configured to, during the cool-down operation,
 acquire the second temperature of the second cooling stage from the second temperature sensor, and 
 end the control of the heater in which the first temperature of the first cooling stage follows the intermediate target temperature when the second temperature of the second cooling stage falls below a predetermined temperature threshold. 
   
     
     
         7 . The cryocooler according to  claim 1 ,
 wherein the controller is configured to maintain the intermediate target temperature constant during the cool-down operation.   
     
     
         8 . A cool-down method for a cryocooler, the cool-down method comprising:
 performing a cool-down operation of a cold head to cool a first cooling stage of the cold head from an initial temperature to a first final target temperature and to cool a second cooling stage of the cold head from the initial temperature to a second final target temperature which is lower than the first final target temperature; and   during the cool-down operation, applying a heat load to the first cooling stage such that a temperature of the first cooling stage follows an intermediate target temperature which is lower than the initial temperature and higher than the first final target temperature.