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A high power, low heat leak dilution refrigerator for use with nuclear cooling
Physica 107B (1981) 561-562 North-Holland Publishing Company
JD 3
A HIGH POWER, LOW HEAT LEAK DILUTION REFRIGERATOR FOR USE WITH NUCLEAR COOLING
D.I. Bradley, T.W. Bradshaw I, A.M. Gu~nault, V. Keith, B.G. Locke-Scobie 2, I.E. Miller, G.R. Pickett and W.P. Pratt, Jr. 3 Department of Physics, University of Lancaster, Lancaster, LAI 4YB/ U.K.
We describe a large dilution refrigerator with a circulation rate of 600 ~mole/sec which has a base temperature around 3 mK and is designed to precool a nuclear cooling stage to 6 mK. The design is intended to reduce the heat leaks to the nuclear sample to a minimum. The cryostat is constructed in such a way that almost all the components can be replaced without disturbing the rest of the cryostat or the wiring.
At Lancaster we have built a very large dilution refrigerator for use with nuclear cooling experiments. One of the principal design considerations has been the minimising of heat leaks to the experimental region. Since the. machine is rather complex we have tried to make each component as independent of the others as possible so that each part may be replaced or removed without disturbing the rest (especially the experimental leads and filling tubes). The cryostat is c. 2.5 m long and is supported on a double re-entrant concrete block weighing ~20 tons. The main block is floated on air springs. All pumping lines are embedded in it and isolated from the outside by flexible metal bellows. The cryostat hangs from a smaller 4 ton block isolated from the main block by a rubber pad and rubber sections in the pumping tubes. The cryostat insert is' 250 mm in diameter and is constructed as rigidly as possible. A rigid triangle of supports is carried right down to 3 mK by a series of stainless steel, coopernickel and Vespel tubes. These can be seen in Fig.l denoted by (i). The 4.2K flange (2) is of stainless steel and carries a stainless steel vacuum jacket which supports the 7T magnetizing solenoid. The flange is furnished with three SQUIDs, a number of individually-shielded twisted pairs of leads (3) and a number of r.f. lines and unshielded lines. The 1.2K plate (4) carrying the continuously filled ~He pot (5), is of copper, is demountable and carries two independent sets of condensers for the refrigerator concentrated phase. The still (6) has a large surface area for the liquid and has a film burner (which is never used). The still normally operates at around O.5K, the temperature being regulated rather than a fixed power being dissipated. The still carries a copper-plated stainless steel radiation shield which is not vacuum tight but is line-of-sight tight and makes thermal contact with the still through a cone joint (7). This shield carries the final-field solenoid for the experimental region. Below the still is a large diameter coiled-tubular
0378-4363/81/0000-0000/$02.50
© North-Holland Publishing Company
Fig. i.
The upper part of the refrigerator
5 61
562
heat exchanger (8), consisting of 9 m of coiled 4 mm Cu-Ni tubing, carrying the concentrated phase, threaded inside 5 m of 5 mm Cu-Ni tube carrying the dilute phase. The exchanger is wound as a four-layer pancake with the coldest temperatures at the top. The tubular heat exchanger alone takes the mixing chamber to 17 mK. Below the coiled tube heat exchanger is the 20 mK plate (9), in Fig.2, which is thermally anchored to the dilute stream via a sintered silver pad. The plate carries a similar heat shield to that of the still. The heat shields, 4.2K vacuum jacket and nuclear specimen are all well enough aligned that there is no contact between them at the lower end. There are six small (iO) and four large (ii) sintered-silver heat exchangers, the latter being made of a graded mixture of 7OO~ and < IU Ag powder sintered on to pure O.i mm silver foil. The heat exchanger stack is flexibly mounted at the top and rigidly supported from the mixing chamber. The mixing chamber (12) is of epoxy and has a cone-joint demountable tail (13) giving access to a volume of 7 cms x ~3 cms in the mixing chamber for thermal contact, heat exchangers, etc. In addition, two smaller, demountable cone-joints on the sides of the mixing chamber (14) allow subsidiary ("parasite") experiments to be incorporated at mixing chamber temperatures. The temperature of the refrigerator is monitored by a series of carbon resistors mounted at strategic points, those at lower temperatures being furnished with low-pass filters (15). The temperature of the mixing chamber is monitored by a CMN thermometer via a SHE SQUID RLM bridge. The nuclear stage is rigidly attached to the base of the mixing chamber and when not in direct thermal contact does not significantly degrade the performance of the refrigerator. An aluminium heat switch allows a specimen of iOO cm 3 of dilute phase and attendant copper refrigerant to be cooled after magnetization to 6.5T to 15 mK in about 4 h, to iO mK in 20 hrs to 8 m K in about 5 days. The inner helium volume of 6 cm 2 has a residu@l heat leak of a few pW when the refrigerant is essentially demagnetized to zero field. We would like to acknowledge the very great assistance freely given by the Grenoble group especially Professor G. Frossati during the design and construction of this machine. This work is supported by the Science Research Council. Present addresses 1
University of Exeter, U.K.
2
SRC, Daresbury Laboratory, Warrington, U.K.
3
Michigan State University, East Lansing, Mich., U.S.A.
Fig.2
The lower part of the refrigerator
JD 3
A HIGH POWER, LOW HEAT LEAK DILUTION REFRIGERATOR FOR USE WITH NUCLEAR COOLING
D.I. Bradley, T.W. Bradshaw I, A.M. Gu~nault, V. Keith, B.G. Locke-Scobie 2, I.E. Miller, G.R. Pickett and W.P. Pratt, Jr. 3 Department of Physics, University of Lancaster, Lancaster, LAI 4YB/ U.K.
We describe a large dilution refrigerator with a circulation rate of 600 ~mole/sec which has a base temperature around 3 mK and is designed to precool a nuclear cooling stage to 6 mK. The design is intended to reduce the heat leaks to the nuclear sample to a minimum. The cryostat is constructed in such a way that almost all the components can be replaced without disturbing the rest of the cryostat or the wiring.
At Lancaster we have built a very large dilution refrigerator for use with nuclear cooling experiments. One of the principal design considerations has been the minimising of heat leaks to the experimental region. Since the. machine is rather complex we have tried to make each component as independent of the others as possible so that each part may be replaced or removed without disturbing the rest (especially the experimental leads and filling tubes). The cryostat is c. 2.5 m long and is supported on a double re-entrant concrete block weighing ~20 tons. The main block is floated on air springs. All pumping lines are embedded in it and isolated from the outside by flexible metal bellows. The cryostat hangs from a smaller 4 ton block isolated from the main block by a rubber pad and rubber sections in the pumping tubes. The cryostat insert is' 250 mm in diameter and is constructed as rigidly as possible. A rigid triangle of supports is carried right down to 3 mK by a series of stainless steel, coopernickel and Vespel tubes. These can be seen in Fig.l denoted by (i). The 4.2K flange (2) is of stainless steel and carries a stainless steel vacuum jacket which supports the 7T magnetizing solenoid. The flange is furnished with three SQUIDs, a number of individually-shielded twisted pairs of leads (3) and a number of r.f. lines and unshielded lines. The 1.2K plate (4) carrying the continuously filled ~He pot (5), is of copper, is demountable and carries two independent sets of condensers for the refrigerator concentrated phase. The still (6) has a large surface area for the liquid and has a film burner (which is never used). The still normally operates at around O.5K, the temperature being regulated rather than a fixed power being dissipated. The still carries a copper-plated stainless steel radiation shield which is not vacuum tight but is line-of-sight tight and makes thermal contact with the still through a cone joint (7). This shield carries the final-field solenoid for the experimental region. Below the still is a large diameter coiled-tubular
0378-4363/81/0000-0000/$02.50
© North-Holland Publishing Company
Fig. i.
The upper part of the refrigerator
5 61
562
heat exchanger (8), consisting of 9 m of coiled 4 mm Cu-Ni tubing, carrying the concentrated phase, threaded inside 5 m of 5 mm Cu-Ni tube carrying the dilute phase. The exchanger is wound as a four-layer pancake with the coldest temperatures at the top. The tubular heat exchanger alone takes the mixing chamber to 17 mK. Below the coiled tube heat exchanger is the 20 mK plate (9), in Fig.2, which is thermally anchored to the dilute stream via a sintered silver pad. The plate carries a similar heat shield to that of the still. The heat shields, 4.2K vacuum jacket and nuclear specimen are all well enough aligned that there is no contact between them at the lower end. There are six small (iO) and four large (ii) sintered-silver heat exchangers, the latter being made of a graded mixture of 7OO~ and < IU Ag powder sintered on to pure O.i mm silver foil. The heat exchanger stack is flexibly mounted at the top and rigidly supported from the mixing chamber. The mixing chamber (12) is of epoxy and has a cone-joint demountable tail (13) giving access to a volume of 7 cms x ~3 cms in the mixing chamber for thermal contact, heat exchangers, etc. In addition, two smaller, demountable cone-joints on the sides of the mixing chamber (14) allow subsidiary ("parasite") experiments to be incorporated at mixing chamber temperatures. The temperature of the refrigerator is monitored by a series of carbon resistors mounted at strategic points, those at lower temperatures being furnished with low-pass filters (15). The temperature of the mixing chamber is monitored by a CMN thermometer via a SHE SQUID RLM bridge. The nuclear stage is rigidly attached to the base of the mixing chamber and when not in direct thermal contact does not significantly degrade the performance of the refrigerator. An aluminium heat switch allows a specimen of iOO cm 3 of dilute phase and attendant copper refrigerant to be cooled after magnetization to 6.5T to 15 mK in about 4 h, to iO mK in 20 hrs to 8 m K in about 5 days. The inner helium volume of 6 cm 2 has a residu@l heat leak of a few pW when the refrigerant is essentially demagnetized to zero field. We would like to acknowledge the very great assistance freely given by the Grenoble group especially Professor G. Frossati during the design and construction of this machine. This work is supported by the Science Research Council. Present addresses 1
University of Exeter, U.K.
2
SRC, Daresbury Laboratory, Warrington, U.K.
3
Michigan State University, East Lansing, Mich., U.S.A.
Fig.2
The lower part of the refrigerator