Cryogenic cooling system

Abstract

A cryogenic cooling system (100), having a working region (101), a first cryocooler (102) having a first cooling stage (103), a first conduit (104) for passing a stream of fluid cooling medium towards the working region (101), at least one thermal coupling (105) of the first conduit (104) and the first cooling stage (103) for cooling the fluid cooling medium, a second cryocooler (106) having a second cooling stage (107), a first cold stage (108), at least one thermal coupling (110) of the second cooling stage (107) and the first cold stage (108) for cooling the first cold stage (108), and a first thermal coupling (111) of the first cooling stage (103) and the first cold stage (108), provided by a coupling element (109), for cooling the first cold stage (108), wherein the coupling element (109) has a controlled thermal coupling for reducing the first thermal coupling when a relevant temperature of the cryogenic cooling system (100) is below a threshold temperature.

1 . A cryogenic cooling system ( 100 ), comprising:
 a vacuum enclosure ( 121 ) and a working region ( 101 ) within the vacuum enclosure ( 121 ),   a first cryocooler ( 102 ) having a first cooling stage ( 103 ), the first cooling stage ( 103 ) being located in the vacuum enclosure ( 121 ),   a first conduit ( 104 ) for passing a stream of fluid cooling medium towards the working region ( 101 ),   at least one thermal coupling ( 105 ) of the first conduit ( 104 ) and the first cooling stage ( 103 ) for cooling the fluid cooling medium on its way towards the working region ( 101 ),   a second cryocooler ( 106 ) having a second cooling stage ( 107 ), the second cooling stage ( 107 ) being located in the vacuum enclosure ( 121 ),   a first cold stage ( 108 ) located in the vacuum enclosure ( 121 ),   at least one thermal coupling ( 110 ) of the second cooling stage ( 107 ) and the first cold stage ( 108 ) for cooling the first cold stage ( 108 ), and   a first thermal coupling ( 111 ) of the first cooling stage ( 103 ) and the first cold stage ( 108 ), provided by a coupling element ( 109 ), for cooling the first cold stage ( 108 ),   wherein the coupling element ( 109 ) comprises a controlled thermal coupling for reducing the first thermal coupling when a relevant temperature of the cryogenic cooling system ( 100 ) is below a threshold temperature.   
     
     
         2 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the working region ( 101 ) is arranged to receive an object to be cooled. 
     
     
         3 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the first cold stage ( 108 ) is thermally coupled to a heat radiation shield surrounding the working region ( 101 ) or is a part of a heat radiation shield surrounding the working region ( 101 ). 
     
     
         4 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the first cold stage ( 108 ) is a 4K stage of the cryogenic cooling system ( 100 ). 
     
     
         5 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the first cryocooler ( 102 ) comprises a first pulse tube, the second cryocooler ( 106 ) comprises a second pulse tube, the first cooling stage ( 103 ) is a lower temperature stage of the first pulse tube, and the second cooling stage ( 107 ) is a lower temperature stage of the second pulse tube. 
     
     
         6 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the coupling element ( 109 ) comprises a thermal conductor ( 202 ,  203 ) and the controlled thermal coupling comprises an actuator and a controller ( 800 ) for controlling the actuator, the thermal conductor ( 202 ,  203 ) being arranged to be moved between a contact state and a non-contact state by the actuator and the controller ( 800 ), wherein in the contact state the thermal conductor ( 202 ,  203 ) is in thermal contact with the first cold stage ( 108 ) and the first cooling stage ( 103 ), and in the non-contact state the thermal conductor ( 202 ,  203 ) is not in thermal contact with the first cooling stage ( 103 ) and/or the first cold stage ( 108 ). 
     
     
         7 . The cryogenic cooling system ( 100 ) according to  claim 6 , wherein the actuator comprises a piezoelectric actuator ( 201 ). 
     
     
         8 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the controlled thermal coupling ( 109 ) comprises a first material, wherein a thermal conductivity of the first material ( 301 ) is greater at a higher reference temperature above the threshold temperature than at a lower reference temperature below the threshold temperature. 
     
     
         9 . The cryogenic cooling system ( 100 ) according to  claim 8 , wherein the first material ( 301 ) comprises a superconducting material, wherein a critical temperature of the superconducting material is higher than the threshold temperature. 
     
     
         10 . The cryogenic cooling system ( 100 ) according to  claim 9 , wherein the superconducting material is niobium. 
     
     
         11 . The cryogenic cooling system ( 100 ) according to  claim 8 , wherein the controlled thermal coupling ( 109 ) further comprises a second material ( 401 ), wherein the second material ( 401 ) is in thermal contact with the first cooling stage ( 103 ) and the first material ( 301 ) is in thermal contact with the first cold stage ( 108 ), and the first material ( 301 ) and the second material ( 401 ) are thermally coupled. 
     
     
         12 . The cryogenic cooling system ( 100 ) according to  claim 11 , wherein the second material ( 401 ) is a thermal conductor at all temperatures. 
     
     
         13 . The cryogenic cooling system ( 100 ) according to  claim 11 , wherein the second material ( 401 ) comprises copper or silver and the first material ( 301 ) comprises stainless steel, graphite, and/or niobium. 
     
     
         14 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the controlled thermal coupling ( 109 ) comprises a heat exchange fluid. 
     
     
         15 . The cryogenic cooling system ( 100 ) according to  claim 14 , further comprising a gas-tight sleeve ( 601 ) enclosing the first cooling stage ( 103 ), wherein the sleeve is provided with a thermally conductive interface ( 602 ) coupled to the first cold stage ( 108 ) and the cryogenic cooling system further comprises a pump ( 603 ) for pumping the heat exchange fluid into and out of the sleeve. 
     
     
         16 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the controlled thermal coupling ( 109 ) has a first thermal conductance above the threshold temperature and a second thermal conductance below the threshold temperature and the first thermal conductance is greater than the second thermal conductance. 
     
     
         17 . The cryogenic cooling system ( 100 ) according to  claim 16 , wherein a ratio between the second thermal conductance and the first thermal conductance is 0.1 or less. 
     
     
         18 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the fluid cooling medium is helium-3, helium-4, or a mixture of helium-3 and helium-4. 
     
     
         19 . The cryogenic cooling system ( 100 ) according to  claim 1 , wherein the fluid cooling medium is helium in a dilution refrigerator