Refrigerator

A refrigerator for cooling a sample (16,17) comprising a reservoir (1) for storing gaseous .sup.4 He when in use; a cooler (13) for cooling gaseous .sup.4 He from the reservoir; and a helium vessel (18,19) for containing .sup.4 He, the .sup.4 He in the helium vessel being in fluid communication with the reservoir (1) via the cooler (13). The sample (16,17) is mounted, in use, in thermal contact with the .sup.4 He in the helium vessel whereby the .sup.4 He in the helium vessel provides a path for heat to transfer from the sample to the cooler.

FIELD OF THE INVENTION 
The present invention relates to a refrigerator for cooling a sample. 
DESCRIPTION OF THE PRIOR ART 
A number of refrigerators are known for cooling a sample to very low 
temperatures. 
In one known refrigerator a sample is immersed in a bath of liquid .sup.4 
He. Heat energy is efficiently removed from the sample by conduction and 
convection in the liquid .sup.4 He, and absorbed by the latent heat of 
evaporation of the .sup.4 He. However this system suffers from a number of 
problems. Firstly, as the .sup.4 He evaporates the bath must be regularly 
refilled with liquid .sup.4 He from a storage dewar to ensure that the 
sample remains fully immersed. Secondly, a large volume of liquid .sup.4 
He is required to fully immerse the sample, and to minimise the required 
refill frequency. Thirdly, if the sample heats up suddenly sudden 
evaporation of liquid .sup.4 He can cause a dangerous increase in pressure 
above the liquid. Fourthly, when the bath is full it is not possible to 
invert the system. 
In an attempt to solve these problems, a "cryogen free" system has been 
developed by Oxford Instruments and is sold as the Tesla 78 mm room 
temperature bore cryofree magnet ref Cryof5/78. The cryofree system 
comprises a cooling engine which provides a cold stage at a temperature of 
4.2 K. The sample is shielded from room temperature radiation by a 
radiation shield and a vacuum space. The sample and radiation shield are 
connected to the cold stage by thermally conductive links, such as copper 
flanges. Although no helium cryogen is required, this system suffers from 
a number of problems. Firstly, the system takes a long time to cool down 
from room temperature. Secondly, the system cannot absorb large or sudden 
heat loads efficiently due to the limited finite refrigeration power of 
the cooling engine. Thirdly, if the cooling engine suffers a loss of power 
the system warms up very quickly. Fourthly, conventional cooling engines 
cannot cool the sample down to liquid .sup.4 He temperatures (ie. of the 
order of 4 K). 
SUMMARY OF THE INVENTION 
In accordance with a first aspect of the present invention there is 
provided a refrigerator for cooling a sample comprising a reservoir for 
storing gaseous .sup.4 He when in use; a cooler for cooling gaseous .sup.4 
He from the reservoir; and a helium vessel in fluid communication with the 
reservoir via the cooler, wherein the helium vessel contains .sup.4 He in 
use, and wherein the sample is mounted, in use, in thermal contact with 
the .sup.4 He in the helium vessel whereby the .sup.4 He in the helium 
vessel provides a path for heat to transfer from the sample to the cooler. 
In accordance with a second aspect of the present invention there is 
provided a method of cooling a sample comprising providing a reservoir of 
gaseous .sup.4 He; cooling gaseous .sup.4 He from the reservoir with a 
cooler; providing .sup.4 He in a helium vessel; mounting a sample in 
thermal contact with the .sup.4 He in the helium vessel; and transferring 
heat from the sample to the cooler along a path provided by the .sup.4 He 
in the helium vessel. 
The present invention provides a number of significant advantages over 
conventional refrigerators. Firstly the refrigerator utilizes the cooling 
properties of .sup.4 He whilst using a significantly smaller volume of 
.sup.4 He than in a conventional liquid .sup.4 He bath system. Secondly 
the helium vessel and reservoir together form a closed volume which does 
not require refilling with .sup.4 He. Thirdly if the sample heats up 
quickly then .sup.4 He can expand safely from the helium vessel into the 
reservoir without creating a dangerous increase in pressure. 
The cooler may cool the .sup.4 He down to approximately 20 K, above the 
boiling point of .sup.4 He. In this case all of the .sup.4 He in the 
refrigerator will be gaseous. However preferably the cooler has sufficient 
cooling power to cause gaseous .sup.4 He to condense and flow into the 
helium vessel. This takes advantage of the more efficient heat transfer 
properties of liquid .sup.4 He. For instance the liquid .sup.4 He can wet 
the sample and thus removes "hot spots" on the sample more efficiently. In 
addition the condensed .sup.4 He adds significantly to the cold thermal 
mass hence improving immunity to temperature fluctuations. 
The sample may be housed in the helium vessel, ie. in contact with the 
.sup.4 He. Alternatively the sample may be housed in a sample chamber 
outside the helium vessel, and thermally connected to the .sup.4 He in the 
helium vessel by a thermally conductive link. 
Any suitable cooler may be used but typically the cooler comprises a 
cooling engine such as a closed-cycle cryogenic cooler. In a preferred 
example the cooler comprises a Gifford-McMahon cycle cryogenic cooler. 
Typically the refrigerator further comprises a radiation shield which 
shields the sample from external radiation. In this cases the cooler 
preferably also cools the radiation shield. 
Due to the reduced volume of .sup.4 He (when compared with a conventional 
helium bath) the refrigerator can take a long time to cool down from room 
temperature. Therefore preferably the refrigerator further comprises a 
liquid nitrogen precooling system for precooling of the refrigerator with 
liquid nitrogen. 
The liquid nitrogen precooling system may precool the radiation shield 
and/or the sample. 
The cool down time can also be reduced by providing one or more additional 
coolers for cooling the sample and/or the radiation shield. 
Typically the volume of the helium vessel is chosen such that the total 
volume of .sup.4 He in use in the helium vessel is less than 5 liters, and 
preferably less than 2 liters. If the sample is housed in the helium 
vessel then this volume will be the volume between the sample and an inner 
periphery of the helium vessel. In this case the area of the sample which 
is contacted by helium is typically greater than 1 m.sup.2. In a preferred 
example the volume is between 1 and 2 liters. 
In the case where the cooler condenses the .sup.4 He, then equivalently the 
volumes of the helium vessel and the cryogen reservoir are chosen such 
that the total volume of liquid .sup.4 He which flows into the helium 
vessel does not exceed 5 liters and preferably does not exceed 2 liters. 
In a preferred example the condensed volume is between 1 and 2 liters. 
In a preferred embodiment a solid high thermal conductivity link is also 
provided to give an additional path for heat to transfer from the sample 
to the cooler. 
Any sample may be cooled by the refrigerator but preferably the sample 
comprises a superconducting magnet. The efficient thermal transport 
properties of .sup.4 He are particularly suited to this application where 
it is important to cool localised hot spots which may be caused by eddy 
current heating. In addition if the magnet quenches the refrigerator can 
easily absorb the sudden heat load and the evaporating .sup.4 He can 
expand safely into the reservoir without creating a dangerous increase in 
pressure. 
The magnet may be formed of a material comprising Nb.sub.3 Sn. However this 
material is expensive and preferably the magnet is formed of a material 
comprising NbTi.

DETAILED DESCRIPTION OF THE EMBODIMENT 
Referring to FIG. 1, a helium gas cylinder 1 supplies helium gas to a 1200 
liter capacity gas storage tank 2 under the control of a helium gas feed 
valve 3. The gas storage tank 2 stores enough gas at room temperature to 
condense into 1-2 liters of liquid helium at 4.2 K. The gas storage tank 2 
is housed in a control/service cabinet 4. A liquid nitrogen dewar 5 
supplies liquid nitrogen to the control/service cabinet 4 via a nitrogen 
feed valve 6. 
A superconducting magnet (not shown in FIG. 1) is cooled by the system and 
housed in outer vacuum casing 8. Primary cooling power is provided by a 4 
K Gifford-McMahon cycle cryogenic cooler comprising a cold head 13 and a 
compressor 14. An example of a suitable 4 K cryogenic cooler is the 
Sumitomo Heavy Industries, Ltd SRDK-408DW rare-earth enhanced cryocooler, 
comprising a model RDK-408D cold head, and a model CSW71B compressor. 
Secondary cooling power is provided by a 20 K Gifford-McMahon cycle 
cryogenic cooler comprising a cold head 12 and compressor 15. All gas and 
electrical supply lines are contained in an armoured caterpillar conduit 
11, as illustrated in more detail in FIG. 2. 
Referring to FIG. 2, the compressors 14, 15 are connected to the cold heads 
12,13 by respective helium flex lines 66,67. The flexlines 66,67 supply 
helium to the cold head from the compressor, and also return helium to the 
compressor from the cold head. 
The flow of helium in helium gas supply line 23 is regulated by valves 
24,25 which are controlled by gas controller 26. These valves are normally 
open to enable rapid expansion during magnet quench. 
To accelerate the cool-down from room temperature to 4.2 K, liquid nitrogen 
can also be fed to pre-cooling heat exchangers which are in thermal 
contact with the magnet and radiation shield (described below). The liquid 
nitrogen is fed from liquid nitrogen supply line 40 under control of 
liquid nitrogen controller 41. The liquid nitrogen controller 41 receives 
liquid nitrogen from dewar 5 via feed line 42, and vents nitrogen gas via 
vent 43. Nitrogen is returned to the nitrogen controller 41 by return line 
44. 
The compressors 14,15 and controllers 26,41 are controlled by a PC 45. The 
pressure of the gas storage tank 2 can be monitored by a pressure gauge 46 
and the pressure of the helium compartments 34,35 (shown in FIGS. 4 and 5) 
can be monitored by a pressure gauge 47. 
Referring now to FIGS. 3-5, the superconducting magnet comprises a pair of 
serially connected superconducting NbTi magnet coils 16,17. Each coil 
16,17 is wound in a U-shaped groove running round the edge of a respective 
aluminium former 50,51. The aluminium formers 50,51 are each bolted and 
welded to a respective stainless steel or copper magnet vessel 18,19. A 
superconducting magnet switch (not shown) is also housed in one of the 
magnet vessels 18,19. The magnet vessels 18,19 are suspended on struts 
21,22 inside a radiation shield 20. The radiation shield 20 shields the 
magnet and superconducting switch from 300 K radiation. The radiation 
shield 20 is housed inside an outer vacuum casing 8 which provides vacuum 
insulation and acts as the external interface of the system. 
Each coil 16,17 has an inner diameter of 1890 mm, an outer diameter of 1960 
mm and a width (from left to right in FIGS. 4 and 5) of 200 mm. Therefore 
the area of magnet which is wetted by liquid helium is 1.96 
m.times..pi..times.0.2 m=1.23 m.sup.2. 
The coils generate a magnet field of 0.25 T. 
Liquid nitrogen from supply line 40 is fed into an inlet port 53 (FIG. 3) 
in the turret containing the 20 K cooler. Inlet port 53 leads to a heat 
exchanger comprising a continuous length of 6-10 mm OD tube which is wound 
inside the radiation shield 20 as indicated at 54, and wound outside the 
magnet vessels 18,19 as indicated at 55. Liquid nitrogen from the heat 
exchanger exits to return line 44 via outlet port 56 (FIG. 3). 
Referring to FIG. 5 (which shows the 4 K cryocooler cold head 13) the cold 
head 13 has a first cold station 27 and a second cold station 28 provided 
in a cooling chamber 29. The cryocooler produces continuous closed-cycle 
refrigeration at temperatures depending upon the heat load imposed, in the 
range of 25 K to 40 K for the first stage cold station 27 and in the range 
of 3.5 K to 4.2 K for the second stage cold station 28. The cold heads 
12,13 each have conductive connections with the radiation shield 20, and 
therefore cool the radiation shield by conduction. 
The helium supply line 23 is connected to the cold head 13 via input 7. The 
input 7 is in fluid communication with the cooling chamber 29. The cooling 
chamber 29 is also in fluid communication with a supply line 30 which 
leads to a T-junction 31. The supply lines 32,33 from the T-junction 31 
each communicate with helium chambers 34,35 between the magnets 16,17 and 
the inner periphery of their respective magnet vessel 18,19. Therefore the 
helium chambers 34,35 are each in fluid communication with the storage 
tank 2 via the 4 K cold head 13. The space between the magnet coils 16,17 
and the inner periphery of their respective magnet vessel is of the order 
of 1-5 mm, and the volume of each helium chamber 34,35 is of the order of 
1 liter. 
A solid high thermal conductivity link is provided between the cold stage 
28 and the coils 16,17 by a number of strands of copper braid 57 which are 
attached to a copper flange 58. 
The cold head 13 also contains current leads (not shown) to run the magnet, 
and leads (not shown) for operating the superconducting switch. The 
current leads for the magnet can be either brass or preferably high 
temperature superconductor. 
The system is cooled down from room temperature by the following method. 
1. First it is necessary to reduce the pressure in the outer vacuum casing 
8 to 1.times.10.sup.-4 mbar using a suitable vacuum pump (not shown). 
After the initial pump-down the vacuum integrity is maintained by sorption 
pumps (not shown). 
2. The cryocoolers 12-15 are switched on. 
3. The liquid nitrogen supply valve 60 is opened. 
4. The liquid nitrogen controller 41 pumps liquid nitrogen around the 
magnet vessels 18,19 and the radiation shield 20 via the pre-cooling heat 
exchangers 54,55. This cools the radiation shield 20 and the 
superconducting magnet coils 16,17. 
5. The liquid nitrogen supply valve 60 is closed and the return valve 61 is 
opened. 
6. The liquid nitrogen is removed from the system by purging from the vent 
side with helium gas. This siphons the residual liquid nitrogen from the 
pre-cooling heat exchangers back to the liquid nitrogen storage dewar 5. 
It is necessary to remove the liquid nitrogen as it will become frozen 
solid on the radiation shield 20 and magnet coils 16,17 when they cool 
below the solidification point. In addition Nitrogen has a significant 
thermal mass which is not really useful so cold head cooling power is 
wasted cooling it down. 
7. The magnet is cooled by conduction through solid thermally conductive 
attachments with the cold heads 12,13 (i.e. through the flange 58 and 
braids 57) and through convection and conduction in the helium gas 
surrounding the magnet--ie. the helium gas in the helium chambers 34,35 
and along supply lines 31,32,33 provides a path for transfer of heat from 
the magnet coils 16,17 to the cooling chamber 29. 
8. As the second cold stage 28 reduces below the helium saturation 
temperature at the current gas pressure, condensation of liquid helium 
occurs at the second cold stage 28. Liquid helium then runs along supply 
line 31 and passes into the helium chambers 34,35 via supply line 32,33. 
9. As the temperature reduces further and condensation of liquid helium 
continues, the magnet coils 16,17 and superconducting switch (not shown) 
are first wetted by the liquid helium and eventually fully immersed in 
liquid helium. 
10. Condensation continues until the pressure becomes equal to the helium 
saturation pressure at which point condensation will cease. At this point 
the gas storage tank 2 will be at room temperature at a reduced pressure. 
11. The magnet coils 16,17 and superconducting switch are now at 
approximately 4 K and can be operated in a conventional way. 
The time required to completely cool the system from room temperature to 
4.2 K using liquid nitrogen pre-cooling of the system is approximately 24 
hours per tonne of magnet. 
The pressure of the storage tank 2 vs temperature of the magnet is shown in 
FIG. 6. As can be seen, the pressure is greater than 1 bar and typically 
lies between 1 and 2 bar. The pressures will be substantially the same 
everywhere in the helium circuit (i.e. in the storage tank 2, the cooling 
chamber 29, the supply lines 30,32,33 and the helium chambers 34,35) 
during normal operation. However a separate pressure gauge 47 for the 
helium chambers 34,35 is required when the helium gas supply line 23 is 
disconnected during servicing. Before reconnection the pressures in the 
helium chambers 34,35 and in the storage tank 2 are equalised with 
reference to the pressure gauges 46,47. 
Any hot spots on the magnet coils 16,17 (which may be generated by eddy 
currents etc.) are rapidly cooled by conduction and convection in the 
liquid helium and by the latent heat of evaporation of the liquid helium. 
Any helium evaporated in the process will be recondensed later when the 
magnet is in persistent mode. 
In the event of a power failure (or if the 4 K cryocooler 13,14 must be 
replaced or repaired), the approximate time to magnet quench will be of 
the order of one hour depending on the total helium volume and cryostat 
configuration. While the power is reconnected (or the 4 K cryocooler 13,14 
is replaced or repaired) the magnet can be maintained in persistent mode 
while at a stationary preselected field and will continue to operate as 
long as it is kept cold.