Closed Cycle 1 K Refrigeration System

Abstract

A closed-cycle refrigerator provides cooling to extremely low temperatures, particularly in the range of 0.5 K to 2.0 K. A 4 K pulse-tube cryocooler cold head or G-M cryocooler cold head liquefies helium in a first cooling chamber at a pressure at approximately 1 atmosphere. Liquid helium flows from the first cooling chamber, through a Joule-Thomson valve, and into a second cooling chamber under a pressure differential created by a pump. Helium vapor extracted from the second cooling chamber by the pump is routed back to the first cooling chamber to be re-condensed. This closed-cycle design provides continuous cooling below 2 K. Cryocooler cold head cold sections have no physical contact with subsequent cooling elements, such as the first and second cooling chambers to reduce vibration transfer. In some embodiments the cryocooler cold head is connected to a vacuum chamber via a vibration damping coupler to further reduce vibration transfer.

What is claimed is: 
     
         1 . A refrigeration system for cooling a heat load comprising:
 a) a first cooling chamber having an inlet, and an outlet at a bottom;   b) a cryocooler cold head having a hot section, a cold section at least partially surrounded by the first cooling chamber, the cold section comprising at least one cooling stage thermally coupled to a condenser, such that when the condenser is cooled by the cryocooler cold head to a first working temperature, helium proximal to the condenser is condensed on the condenser and drips as a liquid into the bottom of the first cooling chamber;   c) an expansion valve having an inlet coupled to the outlet of the first cooling chamber and an outlet, such that liquid helium is expanded and cooled by expansion as the helium flows from the first cooling chamber through the expansion valve;   d) a second cooling chamber having an inlet coupled to the outlet of the expansion valve, the helium from the expansion valve outlet being collected in the second cooling chamber; and   e) a pump having an input coupled to the second cooling chamber and an output coupled to the inlet of the first cooling chamber;   such that the pump creates a closed cycle in which liquid helium in the first cooling chamber is drawn from the first cooling chamber through the expansion valve, cooled by expansion, and collected in the second cooling chamber, and helium vapor in the second cooling chamber is returned to the first cooling chamber.   
     
     
         2 . The refrigeration system of  claim 1 , wherein the cryocooler cold head is selected from the group consisting of: a) a Gifford-McMahon type cryocooler cold head; and b) a pulse-tube type cryocooler cold head. 
     
     
         3 . The refrigeration system of  claim 1 , wherein the cryocooler cold head is a pulse-tube type cryocooler cold head, and the refrigeration system further comprises a compressor coupled to the cryocooler cold head by a rotary valve that is physically separate from the cryocooler cold head and the compressor, such that vibration created by the rotary valve is decoupled from the cryocooler cold head. 
     
     
         5 . The refrigeration system of  claim 1 , wherein the condenser operates at a working temperature of 5 Kelvin or less. 
     
     
         6 . The refrigeration system of  claim 1 , wherein the first cooling chamber has a working pressure of two atmospheres or less. 
     
     
         7 . The refrigeration system of  claim 1 , wherein the expansion valve comprises a control input for adjusting a flow rate through the expansion valve. 
     
     
         8 . The refrigeration system of  claim 1 , wherein a working temperature of the second cooling chamber is 2.17 Kelvin or less. 
     
     
         9 . The refrigeration system of  claim 1 , in which the cold head has a mounting flange disposed between the cryocooler cold head hot section and the cryocooler cold head cold section, further comprising:
 a vacuum chamber having a vacuum chamber flange and surrounding at least the cryocooler cold head cold section, the first cooling chamber, the expansion valve and the second cooling chamber; and   a vibration damping coupler disposed between the cryocooler cold head flange and the vacuum chamber flange, such that vibration from the cryocooler cold head is decoupled from the vacuum chamber.   
     
     
         10 . The refrigeration system of  claim 1 , in which the output of the first cooling chamber is coupled to the input of the expansion valve through a counter-flow heat exchanger, and helium is drawn from the first cooling chamber by the pump flowing through the counter flow heat exchanger before reaching the expansion valve. 
     
     
         11 . The refrigeration system of  claim 10 , in which the counter-flow heat exchanger is exposed to helium flowing out of the second cooling chamber to the pump, such that heat is transferred from helium flowing to the expansion valve from the first cooling chamber to helium flowing out of the second cooling chamber to the pump. 
     
     
         12 . The refrigeration system of  claim 1 , wherein the pump is an oil-free dry type pump. 
     
     
         13 . A refrigeration system for cooling a heat load comprising:
 a) a first cooling chamber having an inlet, and an outlet at a bottom;   b) a cryocooler cold head having a hot section, a cold section at least partially surrounded by the first cooling chamber, the cold section comprising at least one cooling stage thermally coupled to a condenser, such that when the condenser is cooled by the cryocooler cold head to a first working temperature, helium proximal to the condenser is condensed on the condenser and drips as a liquid into the bottom of the first cooling chamber;   c) an expansion valve having an inlet coupled to the outlet of the first cooling chamber and an outlet, such that liquid helium is expanded and cooled by expansion as the helium flows from the first cooling chamber through the expansion valve;   d) a second cooling chamber having an inlet coupled to the outlet of the expansion valve, the helium from the expansion valve outlet being collected in the second cooling chamber;   e) a pump having an input coupled to the second cooling chamber and an output coupled to the inlet of the first cooling chamber;   f) a first radiation shield surrounding at least the second cooling chamber and the expansion valve, the radiation shield being thermally coupled to the first cooling chamber adjacent a colder end of the first cooling chamber; and   g) a second radiation shield surrounding at least the first radiation shield and the first cooling chamber, the second radiation shield being thermally coupled to the first cooling chamber;   such that the pump creates a closed cycle in which liquid helium in the first cooling chamber is drawn from the first cooling chamber through the expansion valve, cooled by expansion, and collected in the second cooling chamber, and helium vapor in the second cooling chamber is returned to the first cooling chamber.   
     
     
         14 . The refrigeration system of  claim 13 , further comprising a counter-flow heat exchanger coupled to the input of the expansion valve and the outlet of the first cooling chamber, such that helium is drawn from the first cooling chamber by the pump flowing through the counter-flow heat exchanger in a first direction before reaching the expansion valve, and helium is drawn by the pump from the second chamber through the counter-flow heat exchanger in a second direction before reaching the pump. 
     
     
         15 . The refrigeration system of  claim 13 , further comprising a counter-flow heat exchanger having a first flow channel in a first direction coupled to the input of the expansion valve and the outlet of the first cooling chamber, such that helium is drawn from the first cooling chamber by the pump flowing through the counter-flow heat exchanger first flow channel in a first direction before reaching the expansion valve, and a second flow channel in a second direction, such that helium is drawn by the pump from the second chamber through the second flow channel in a second direction before reaching the pump; wherein flow in the second flow channel second direction is constrained by a volume of the second cooling chamber, and the expansion valve is disposed within a volume of the second cooling chamber.