Field of the Invention
[0001] The present invention pertains generally to coupling assemblies for thermally connecting
a cryocooler with an apparatus that is to be cooled. More particularly, the present
invention pertains to a method for cooling a superconducting device by using a sleeve
assembly which thermally interconnects two stages of a cryocooler with two different
components of a superconducting device simultaneously. The present invention particularly,
though not exclusively, pertains to a method for using a sleeve assembly to thermally
disconnect the pulse tube, two stage cryocooler from a superconducting device without
compromising the thermal condition of the superconducting device.
Background of the Invention
[0002] It is well known that superconductivity is accomplished at extremely low temperatures.
Even the so-called high temperature superconductors require temperatures which are
as low as approximately twenty degrees Kelvin. Other not-so-high temperature superconductors
require temperatures which are as low as approximately four degrees Kelvin.
[0003] In any case, there are numerous specialized applications for using superconducting
devices that require low temperatures. One specialized application, for example, involves
medical diagnostic procedures using magnetic resonance imaging (MRI) techniques. When
used for medical diagnosis, MRI techniques require the production of a very strong
and substantially uniform magnetic field. If superconducting magnets are used to generate
this strong magnetic field, some type of refrigeration apparatus will be required
to attain the low operational temperatures that are necessary.
[0004] To attain the low operational temperatures that are necessary for a superconducting
device, the refrigeration apparatus typically includes separate cryogenic units or
cryocoolers that are thermally connected with the superconducting device. During operation
of the superconducting device, such a connection is essential. There are times, however,
when it is desirable for the cryocooler to be selectively disconnected or disengaged
from the superconducting device. For example, during repair or routine maintenance
of the cryocooler in a refrigeration apparatus, it is much easier to work on the cryocooler
when it is disconnected from the superconducting device it has been cooling. Importantly,
when so disengaged, the cryocooler can be warmed to room temperature for servicing.
Any disengagement of the cryocooler from the superconducting device, however, must
allow for a reengagement. Further, it is desirable that the superconducting device
be held at a very low temperature during disengagement.
[0005] As it is known to persons skilled in the pertinent art, new generation cryocoolers,
such as "Pulse Tubes", cannot be "gutted" out and rebuilt as can the older generation
cryocoolers. Instead, these pulse tube cryocoolers must either be entirely replaced
or warmed to room temperature for servicing. It is, therefore, necessary for these
new generation cryocoolers to use a refrigeration apparatus or a sleeve to cool a
superconducting device. Because the entire pulse tube needs to be removed for servicing,
the pulse tube cryocoolers cannot be directly and permanently bolted to the sleeve
and, thus, the superconducting device. Further, the pulse tube internals cannot be
removed independently as they can in many Gifford McMahon (GM) two stage cryocoolers.
[0006] For an effective thermal connection, it is known that the efficacy of heat transfer
from one body to another body is dependent on several factors. More specifically,
the amount of heat (Q) that is conductively transferred through a solid body or conductively
transferred from one body to another body through a gas or liquid can be mathematically
expressed as:

[0007] In the above expression, k is the coefficient of thermal conductivity; A is the solid
bodies cross-sectional area, or the surface area in contact between the two bodies
for gas or liquid conduction; L is the solid bodies thermal length or the gap distance
between the bodies; and ΔT is the temperature differential across the solid or between
the two bodies. From this expression, it can be appreciated that in order to effectively
cool one body (e.g. a superconducting device) with another body (e.g. a cryocooler)
the transfer of heat, Q, must be accomplished. When the temperature differential between
the bodies is desired to be very low, and for a given coefficient of thermal conductivity,
it is necessary that the ratio of A/L be sufficiently high.
[0008] For any two separate bodies that are in contact with each other, even though they
may be forced together under very high pressures, there will always be some average
gap distance, L, between the interfacing cross-sectional surface areas of the bodies.
For the case wherein there is a vacuum in the gaps, the gaps can create undesirable
thermal insulators. Accordingly, it may be beneficial to have these gaps filled with
a gas, such as helium. If this is done, heat transfer between the bodies in contact
can result from a) solid conduction where there is actual contact between the bodies;
b) molecular/gas conduction across the helium-filled gaps; and possibly c) liquid
conduction in gaps where the gas has liquefied.
[0009] In light of the above, it is an object of the present invention to provide a method
for cooling two components of a superconducting device by using a sleeve assembly
that thermally interconnects two stages of a pulse tube cryocooler with the superconducting
device. Another object of the present invention is to provide a method for cooling
a superconducting device by using a sleeve assembly which allows the pulse tube, two
stage cryocooler to be thermally disengaged from the superconducting device while
the very low temperature of the superconducting device is substantially maintained.
Still another object of the present invention is to provide a method for cooling a
superconducting device which is effectively easy to implement and comparatively cost
effective.
Summary of the Invention
[0010] The present invention is directed to a method for cooling a superconducting device
by using a sleeve assembly which thermally interconnects a pulse tube, two stage cryocooler
with a superconducting device. For the present invention, the sleeve assembly has
a heat transfer cylinder, a heat transfer receptacle and a midsection which interconnects
the heat transfer cylinder with the heat transfer receptacle.
[0011] In more detail, the midsection of the sleeve assembly is hollow and elongated and
defines a passageway between the heat transfer cylinder and the heat transfer receptacle.
The heat transfer cylinder of the present invention is also hollow and is annular-shaped,
having an inner surface and an outer surface. The heat transfer receptacle is formed
with a recess and has an inner surface and an outer surface. Importantly, the inner
surface of the heat transfer receptacle that defines the recess is tapered. Both the
heat transfer cylinder and heat transfer receptacle are preferably made of copper,
aluminum or any other high thermal conductivity material. Furthermore, the midsection
of the sleeve assembly is preferably made of stainless steel or any other low thermal
conductivity material known in the art.
[0012] The structure of the sleeve assembly is dimensioned for the engagement with a cryocooler
which includes a cooling element and a tapered cooling probe. As contemplated for
the present invention, the cryocooler is moveable relative to the sleeve assembly
between a first configuration wherein the cryocooler is engaged with the sleeve assembly,
and a second configuration wherein the cryocooler is disengaged from the sleeve assembly.
Specifically, the two stages of the cryocooler will thermally engage and disengage
with the two components of the superconducting device simultaneously through the sleeve
assembly.
[0013] In operation, the sleeve assembly is engaged with the cryocooler when the cryocooler
is juxtaposed with the sleeve assembly to establish thermal communication between
the cryocooler and the superconducting device through the sleeve assembly. In more
detail, when juxtaposed, the tapered cooling probe of the cryocooler is urged against
the heat transfer receptacle of the sleeve assembly to establish thermal communication
therebetween. As stated above, the inner surface of the heat transfer receptacle is
tapered for mating engagement with the tapered cooling probe of the cryocooler. This
engagement, however, will not be perfect. Always, there is an average gap distance
between the inner surface of the heat transfer receptacle and the tapered cooling
probe of the cryocooler. As contemplated for the present invention, this gap distance
varies within the range between zero and approximately two thousandths of an inch
(0 -.002 inches). Importantly, under these conditions, the gap ratio, A/L, in the
above expression for Q will be in the range between approximately 10,000 in
2/in to approximately 50,000 in
2/in. Consequently, there can be effective heat flow, Q, even though the temperature
differential, ΔT, between the heat transfer receptacle and the tapered cooling probe
is small.
[0014] When the cryocooler is engaged with the sleeve assembly (first configuration), the
cooling element of the cryocooler is positioned at a very small gap distance from
the inner surface of the heat transfer cylinder. Importantly, this gap distance needs
to be small enough to establish effective thermal communication between the cooling
element and the heat transfer cylinder. For the present invention, this gap distance
will vary within the range between approximately one thousandth of an inch to approximately
five thousandths of an inch (.001 - .005 inches). Although the gap ratio, A/L, in
this case will be higher than it is for the receptacle/probe interface, there will
still be effective heat flow, Q.
[0015] In order for the cryocooler and sleeve assembly to move between the first (engaged)
and second (disengaged) configurations, an expandable bellows is provided which joins
the heat transfer cylinder of the sleeve assembly with the room temperature section
of the cryocooler and creates an enclosed chamber therebetween. In operation, the
bellows allows the cryocooler to be separated from the sleeve assembly with a space
therebetween which will maintain a gaseous thermal insulation between the cryocooler
and the sleeve assembly. Stated another way, there will be sufficient thermal insulation
between the sleeve assembly and the cryocooler to maintain the sleeve assembly at
a substantially same low temperature when the cryocooler is disengaged from the sleeve
assembly and is warmed to room temperature.
[0016] It is important for the sleeve assembly to maintain two substantially low temperatures
for it to continually cool the two separate components of the superconducting device.
To do this, the sleeve assembly of the present invention is operationally connected
to the superconducting device by a proximal conductor and a distal conductor. In more
detail, the proximal conductor is attached between the outer surface of the heat transfer
cylinder and a thermal shield of the superconducting device to establish thermal communication
therebetween. Further, the distal conductor is attached between the outer surface
of the heat transfer receptacle and the superconducting wires of the superconducting
device to establish thermal communication therebetween.
[0017] By way of a pipe, helium gas is pumped selectively into and from the chamber of the
sleeve assembly. As contemplated for the present invention, the introduction of helium
gas into the space between the cryocooler and the sleeve assembly will prevent a vacuum
from forming when the cryocooler is disengaged and displaced from the sleeve assembly.
Also, helium gas is useful to establish molecular conduction between the sleeve assembly
and the cryocooler for an effective thermal connection therebetween when these two
components are engaged with each other.
[0018] In accordance with another aspect the invention provides a method for cooling a superconducting
device comprising the steps of providing a cooling means formed with a probe, connecting
a receptacle in thermal communication with said superconducting device, selectively
juxtaposing said probe of said cooling means with said receptacle to establish thermal
communication therebetween to draw heat from said superconducting device, through
said receptacle, and into said cooling means to cool said superconducting device,
and maintaining a thermal insulation between said receptacle and said cooling means
whenever said cooling means is distanced from said probe.
[0019] The invention also provides a method for cooling a superconducting device comprising
the steps of providing a pulse tube, two stage cryocooler having a cooling element
and a tapered cooling probe, connecting said superconducting device with a sleeve
for heat transfer therebetween, said sleeve having a receptacle, a cylinder and a
wall interconnecting said receptacle and said cylinder, joining said sleeve with said
cryocooler to create an enclosed chamber therebetween, pumping helium selectively
into and from said chamber to maintain an operational pressure in said chamber and
establish molecular conduction and to maintain pressure balance between said sleeve
and said cryocooler, and selectively moving said cryocooler relative to said sleeve
between a first configuration wherein said sleeve is engaged with said cryocooler,
where said tapered cooling probe is urged against said receptacle to establish thermal
communication therebetween and said cooling element is positioned in said cylinder
to establish thermal communication therebetween, and a second configuration wherein
said cryocooler is disengaged from said sleeve.
Brief Description of the Drawings
[0020] The invention will now be described in more detail having regard to the accompanying
drawings, in which:
Fig. 1 is a schematic, perspective view of the sleeve assembly of the present invention
engaged with a pulse tube, two stage cryocooler and shown operationally connected
to a superconducting device, with portions broken away for clarity;
Fig. 2 is a perspective exploded view showing the sleeve assembly of the present invention
in its structural relationship with a pulse tube, two stage cryocooler;
Fig. 3A is a cross-sectional view of the sleeve assembly and pulse tube, two stage
cryocooler operationally engaged with each other as would be seen along the line 3-3
in Fig. 1; and
Fig. 3B is a cross-sectional view of the sleeve assembly and pulse tube, two stage
cryocooler as seen in Fig. 3A when they are operationally disengaged from each other
for the purposes of servicing the cryocooler.
Detailed Description of the Drawings
[0021] Referring initially to Fig. 1, a cooling system according to the present invention
is shown and generally designated 10. More specifically, the cooling system 10 includes
a sleeve assembly 12 which thermally interconnects a pulse tube, two stage cryocooler
14 with a superconducting device 16. As also shown, a helium source 18 is connected
via a pipe 19 to the sleeve assembly 12. As intended for the present invention, the
sleeve assembly 12 is an easily operated means for thermally connecting and disconnecting
the cryocooler 14 from the superconducting device 16.
[0022] As shown in Fig. 2, the pulse tube, two stage cryocooler 14 has a valve motor body
17 having a first stage 20 (first cryocooler station) aligned with a second stage
22 (second cryocooler station). A cooling element 24 is disposed between the stages
20 and 22 and is in thermal communication with the first stage 20. As shown, a tapered
cooling probe 26 extends from the second stage 22 and is in thermal communication
with the second stage 22. As intended for the present invention, the second stage
22 maintains a temperature of approximately four degrees Kelvin (4°K) and cools the
tapered cooling probe 26 to that same low temperature. Further, the first stage 20
maintains a temperature of approximately forty degrees Kelvin (40°K) and cools the
cooling element 24 to that same temperature. Preferably, the cooling element 24 and
the tapered cooling probe 26 of the cryocooler 14 can be both made of copper, aluminum
or any other known high thermal conductivity material. A bellows 28 having a flange
29 is shown attached, with the flange 29, to the cryocooler 14. The pipe 19 that interconnects
the helium source 18 with the sleeve assembly 12 is attached through the bellows flange
29 as shown in Fig. 1.
[0023] Still referring to Fig. 2, it will be seen that the sleeve assembly 12 includes a
heat transfer receptacle 30, a heat transfer cylinder 32 and a midsection 34 which
interconnects the heat transfer receptacle 30 with the heat transfer cylinder 32.
It is important for the heat transfer receptacle 30 to be dimensioned to receive the
tapered cooling probe 26 of the cryocooler 14. Similarly, the heat transfer cylinder
32 is dimensioned to receive the cooling element 24 of the cryocooler 14. The details
of the structure of the sleeve assembly 12 can perhaps be best seen in Figs. 3A and
3B.
[0024] In Figs. 3A and 3B, the heat transfer receptacle 30 of the sleeve assembly 12 is
formed with a recess 36 and has an inner surface 38 and an outer surface 40. Importantly,
the inner surface 38 of the heat transfer receptacle 30 that defines the recess 36
is tapered. As also shown in Figs. 3A and 3B, the midsection 34 of the sleeve assembly
12 is hollow and elongated and defines a passageway 42 between the heat transfer receptacle
30 and the heat transfer cylinder 32. The heat transfer cylinder 32 is also hollow
and is annular-shaped, having an inner surface 44 and an outer surface 46. Preferably,
the heat transfer receptacle 30 and the heat transfer cylinder 32 can be made of copper,
aluminum or any other high thermal conductivity material. The midsection 34 of the
sleeve assembly 12 can be made of stainless steel or any other low thermal conductivity
material.
[0025] Referring back to Fig. 1, the sleeve assembly 12 is shown connected to two components
of the superconducting device 16 by a proximal conductor 52 and a distal conductor
54. In more detail, the proximal conductor 52 has a first end 56 and a second end
58 and the distal conductor 54 also has a first end 62 and a second end 64. The first
end 56 of the proximal conductor 52 is attached to the outer surface 46 of the heat
transfer cylinder 32 and the second end 58 is attached to the thermal shield 60 of
the superconducting device 16 as shown in Fig. 1. Similarly, the first end 62 of the
distal conductor 54 is attached to the outer surface 40 of the heat transfer receptacle
30 and the second end 64 is attached to the wire 68 of the superconducting device
16 as shown in Fig.1.
[0026] As shown in Fig. 3A, the flange 29 of expandable bellows 28 joins the room temperature
flange 66 of cryocooler 14 with the heat transfer cylinder 32 of the sleeve assembly
12 by any means known in the art. With this interconnection, an enclosed chamber 50
is created between the sleeve assembly 12 and the cryocooler 14. (see Fig. 3B). Also,
an elongated, thin stainless steel tube 48 is disposed between the bellows 28 and
the heat transfer cylinder 32. Helium gas is pumped from the helium source 18 through
the bellows flange 29 and into the chamber 50. Importantly, the bellows 28, with the
helium gas present in the chamber 50, creates an air-lock seal between the sleeve
assembly 12 and the cryocooler 14 to isolate the external environment from the superconducting
device 16.
[0027] The cooperation of the sleeve assembly 12 of the present invention and the cryocooler
14 can perhaps be best appreciated by cross referencing Figs. 3A and 3B. Specifically,
the cryocooler 14 is moveable relative to the sleeve assembly 12 between a first configuration
wherein the cryocooler 14 is engaged with the sleeve assembly 12 (Fig. 3A) and a second
configuration wherein the cryocooler 14 is disengaged with the sleeve assembly 12
(Fig. 3B). Importantly, the first stage 20 and the second stage 22 of the cryocooler
14 engage and disengage simultaneously with the sleeve assembly 12. It is to be appreciated
that when the cryocooler 14 is engaged with the sleeve assembly 12, the area to gap
distance ratio, A/L, is very big. Specifically, when there is an engagement, the A/L
is typically in the range between approximately 10,000 in
2/in to approximately 50,000 in
2/in and, thus, there is a very small temperature differential ΔT. When the cryocooler
14 is disengaged from the sleeve assembly 12, the A/L will be in the range between
approximately 10 in
2/in to approximately 50 in
2/in. In this case where A/L is small, the ΔT is very big and, as a result, the transfer
of heat, Q, is effectively not accomplished.
[0028] Fig. 3A shows the tapered cooling probe 26 of the cryocooler 14 urged against the
recess 36 of the heat transfer receptacle 30 to establish thermal communication therebetween.
As mentioned above, the heat transfer receptacle 30 is tapered for mating engagement
with the tapered cooling probe 26 with a gap distance 70 between all of their respective
interfacing surfaces. In general, this gap distance 70 between the tapered cooling
probe 26 and the inner surface 38 of the heat transfer receptacle 30 may vary within
a range between zero and approximately two thousandths of an inch (0 - .002 inches).
Importantly, helium molecular/gas or liquid conduction is established through gap
distance 70. Fig. 3A also shows the cooling element 24 of the cryocooler 14 positioned
at a very small gap distance 72 from the inner surface 44 of the heat transfer cylinder
32. It is important for this gap distance 72 to be small enough to establish effective
molecular/gas conduction through helium gas between the cooling element 24 and the
heat transfer cylinder 32. On the other hand, there needs to be sufficient gap distance
72 for the cooling element 24 to be inserted into the heat transfer cylinder 32. As
contemplated for the present invention, this gap distance 72 will vary within a range
between approximately one thousandth of an inch to approximately five thousandths
of an inch (.001 - .005 inches).
[0029] Fig. 3B shows the cryocooler 14 disengaged with the sleeve assembly 12. The bellows
28 allows the cryocooler 14 to be separated from the sleeve assembly 12. There will
be sufficient thermal insulation between the sleeve assembly 12 and the cryocooler
14 to maintain the sleeve assembly 12 at a substantially same low temperature when
the cryocooler 14 is disengaged with the sleeve assembly 12. Meanwhile, the sleeve
assembly 12 will remain in thermal communication with the superconducting device 16.
[0030] In the operation of the sleeve assembly 12 of the present invention, reference is
first made to Fig. 2 wherein the pulse tube, two stage cryocooler 14 is shown being
disposed the sleeve assembly 12. In more detail, as shown in Fig. 3B, the tapered
cooling probe 26 of the cryocooler 14 is passed through the passageway 42 of the sleeve
assembly 12 and is inserted into the recess 36 of the heat transfer receptacle 30
as shown in Fig. 3A. The cryocooler 14 is placed in the sleeve assembly 12 and is
bolted to the bellows flange 29. When the tapered cooling probe 26 contacts the heat
transfer receptacle 30, the second stage 22 of the cryocooler 14 is disposed in the
passageway 42 of the sleeve assembly 12. Furthermore, the cooling element 24 of the
cryocooler 14 is disposed in the heat transfer cylinder 32 of the sleeve assembly
12. Importantly, when the cryocooler 14 is engaged with the sleeve assembly 12, the
A/L is very big. Specifically, A/L is typically in the range between approximately
10,000 in
2/in to approximately 50,000 in
2/in and therefore, the temperature differential, ΔT, between the cryocooler 14 and
the sleeve assembly 12, is very small.
[0031] As shown in Fig. 1, the superconducting device 16 is in thermal communication with
the sleeve assembly 12 which, in turn, is in thermal communication with the cryocooler
14. Stated differently, thermal communication is established between the cryocooler
14 and the superconducting device 16 through the sleeve assembly 12. In more detail,
via the distal conductor 54, the tapered cooling probe 26 will cool the wire 68 of
the superconducting device 16 to approximately four degrees Kelvin (4°K). Similarly,
via the proximal conductor 52, the cooling element 24 of the cryocooler 14 will cool
the thermal shield 60 of the superconducting device 16 to approximately forty degrees
Kelvin (40°K).
[0032] During the engagement or disengagement of the cryocooler 14 with the sleeve assembly
12, helium gas is pumped into the sleeve assembly 12 to establish molecular conduction
between the cryocooler 14 and the sleeve assembly 12. Importantly, helium gas allows
the three orders in magnitude difference in the A/L to act like a switch. This switch
operation, therefore, allows for the engaging and disengaging between the cryocooler
14 and the sleeve assembly 12, as desired. Helium gas will also maintain an operational
pressure between the sleeve assembly 12 and the cryocooler 14 as the cryocooler 14
moves between the first and second configurations.
[0033] To disengage the cryocooler 14 from the sleeve assembly 12 and to disconnect thermal
communication therebetween, the cryocooler 14 is lifted from the sleeve assembly 12
by any mechanical means known in the art. The cryocooler 14, however, is not removed
from the sleeve assembly 12. Instead, the cryocooler 14 is lifted just enough to thermally
disconnect the cryocooler 14 from the sleeve assembly 12. It is important to note
that when the cryocooler 14 is lifted from the sleeve assembly 12, the first stage
20 and the second stage 22 are simultaneously disengaged from their respective positions
in the sleeve assembly 12, which, in turn, are simultaneously disengaged with their
respective thermal communication with the superconducting device 16.
[0034] Upon thermal disengagement between the cryocooler 14 and the sleeve assembly, it
is important to appreciate that the A/L between the two bodies becomes very small.
Specifically, A/L is in the range between approximately 10 in
2/in to approximately 50 in
2/in. As a result, ΔT is very big, and the transfer of heat is relatively insignificant.
[0035] As indicated above, the bellows 28 interconnects the cryocooler 14 with the sleeve
assembly 12 to create a chamber 50 therebetween. Other than the bellows 28, there
is no other mechanical connection between the sleeve assembly 12 and the cryocooler
14. Importantly, when the cryocooler 14 is disengaged from the sleeve assembly 12,
A/L goes from being very large (approximately 10,000 in
2/in - approximately 50,000 in
2/in) to very small (approximately 10 in
2/in - approximately 50 in
2/in). As a result of this, thermal isolation is create. Furthermore, the bellows 28
maintains sufficient thermal insulation between the cryocooler 14 and the sleeve assembly
12 for the sleeve assembly 12 to maintain its substantially same low temperature.
[0036] Upon thermal disconnection between the cryocooler 14 and the sleeve assembly 12,
the cryocooler 14 is warmed to room temperature for servicing. Meanwhile, the sleeve
assembly 12 will remain in thermal communication with the superconducting device 16.
Importantly, the superconducting device 16 will tend to maintain its cold temperature
during disengagement (i.e. 4°Kelvin for the superconducting wires and 40°K for the
thermal shield).
[0037] When the cryocooler 14 is disengaged from the sleeve assembly 12 for servicing, the
cryocooler 14 will tend to expand as it is warmed to room temperature. It is, therefore,
necessary to recool the cryocooler 14 prior to reengaging the cryocooler 14 with the
sleeve assembly 12 in order for the cryocooler 14 to fit into the sleeve assembly
12. To do this, the stages 20 and 22 of the cryocooler 14 will cool the tapered cooling
probe 26 and the cooling element 24 respectively and to their respective low temperatures.
The cooled cryocooler 14 is then reengaged with the sleeve assembly 12 to establish
thermal communication therebetween.
[0038] While the particular Cryocooler Interface Sleeve for a Superconducting Magnet and
Method of Use as herein shown and disclosed in detail is fully capable of obtaining
the objects and providing the advantages herein before stated, it is to be understood
that it is merely illustrative of the presently preferred embodiments of the invention
and that no limitations are intended to the details of construction or design herein
shown other than as described in the appended claims.
1. A method for cooling portions of a superconducting device (16) to temperatures below
approximately six degrees Kelvin, said method comprising the steps of:
providing a cryocooler (14);
joining said cryocooler (14) with a sleeve (12) to create an enclosed chamber (50)
therebetween;
connecting said superconducting device (16) with said sleeve (12) for heat transfer
therebetween; and
selectively juxtaposing said cryocooler (14) with said sleeve (12) in said chamber
(50) to establish thermal communication between said cryocooler (14) and said superconducting
device (16) through said sleeve (12).
2. A method as claimed in claim 1 further comprising the step of pumping helium selectively
into and from said chamber (50) to maintain an operational pressure in said chamber
(50) and establish molecular conduction between said cryocooler (14) and said sleeve
(12).
3. A method as claimed in claim 1 or 2 wherein said sleeve (12) comprises a cylinder
(32), a receptacle (30) and a wall interconnecting said cylinder (32) and said receptacle
(30).
4. A method as claimed in claim 3 wherein said cylinder (32) and said receptacle (30)
are made of copper and said wall is made of stainless steel.
5. A method as claimed in any of claims 1 to 4 wherein said juxtaposing step further
comprises the steps of:
positioning a cooling element (24) of said cryocooler (14) at a first distance from
said cylinder (32) of said sleeve (12); and
urging a cooling probe (26) of said cryocooler (14) against said receptacle (30) of
said sleeve (12) with a second distance therebetween.
6. A method as claimed in any of claims 1 to 5 wherein said connecting step between said
sleeve (12) and said superconducting device (16) is accomplished with a first conductor
(52) being attached to an outer surface (46) of said cylinder (32) and a second conductor
(54) being attached to an outer surface (40) of said receptacle (30), and wherein
each said conductor is attached to said superconducting device (16).
7. A method as claimed in any of claims 1 to 6 wherein said joining step is accomplished
using a bellows (28) attached between said cylinder (32) of said sleeve (12) and said
cryocooler (14) to create said chamber (50).
8. A method as claimed in any of claims 5 to 7 wherein said first distance between said
cooling element (24) and said cylinder (32) is in a range between approximately one
thousandth of an inch to approximately five thousandths of an inch (.001 - .005 inches)
and further wherein said second distance between said cooling probe (26) and said
receptacle (30) varies within a range between zero and approximately two thousandths
of an inch (0 - .002 inches).
9. A method as claimed in any of claims 1 to 8 wherein said cryocooler (14) is a pulse
tube, two stage cryocooler.