BACKGROUND OF THE INVENTION
Field of the Invention:
[0001] The present invention relates to Joule-Thomson cryostats. More specifically, the
present invention relates systems and techniques for improving the performance of
Joule-Thomson cryostats.
[0002] While the present invention is described herein with reference to illustrative embodiments
for particular applications, it should be understood that the invention is not limited
thereto. Those having ordinary skill in the art and access to the teachings provided
herein will recognize additional modifications, applications, and embodiments within
the scope thereof and additional fields in which the present invention would be of
significant utility.
Description of the Related Art:
[0003] A cryostat is an apparatus which provides a localized low-temperature environment
in which operations or measurements may be carried out under controlled temperature
conditions. Cryostats are used to provide cooling of infrared detectors in guided
missiles, for example, where detectors and associated electronic components are often
crowded into a small containment package. Cryostats are also used in superconductor
systems where controlled very low temperatures are required for superconductive activity.
[0004] A Joule-Thomson cryostat is a cooling device that uses a valve (known in the art
as a "Joule-Thomson valve") through which a high pressure gas is allowed to expand
via an irreversible throttling process in which enthalpy is conserved, resulting in
lowering of its temperature.
[0005] The simplest form of a conventional Joule-Thomson cryostat typically had a fixed-size
orifice in the heat exchanger at the cold end of the cryostat such that cooling by
the cryostat was unregulated. The input pressure and internal gas flow dynamics established
the flow parameters of the coolant through the cryostat. Although the conventional
Joule-Thomson cryostat is a simple apparatus in that it has no moving parts, the inherent,
uncontrolled flow characteristics make the fixed-orifice type cryostat unsuitable
for many applications where rapid cool-down and long cooling durations from a limited
size gas supply source are required. Rapid cool-down requires high rate gas flow and
a large size orifice, while long cooling durations require low gas flow rates and
a small size orifice. These two conditions cannot be simultaneously met in a fixed
orifice cryostat.
[0006] Since approximately the 1950's, demand-flow Joule- Thomson cryostats with internal,
passive, thermostatic control of variable orifice size have been used. These cryostats
have the ability to start cool-down with the maximum orifice size, thereby providing
high rate gas flow and refrigeration for rapid cool-down. After cool-down is achieved,
the orifice size is reduced for minimal gas flow rate and refrigeration necessary
for the thermal load. A thermostatic element within the mandrel of the apparatus provides
self-regulation of gas flow based upon the temperature in and around the gas plenum
chamber. The cooling rate is proportional to the mass flow rate of gas through the
cryostat. The thermostatic element, which can be a gas-filled bellows or a segment
of material which contracts or expands based upon temperature, is coupled to a demand-flow
needle valve mechanism. As the temperature drops, the bellows is adapted to contract
and cause the needle to extend into and partially close the Joule-Thompson orifice.
At the predetermined critical temperature, the bellows thermostat mechanism can close
the needle valve entirely. As the temperature rises, the bellows expands again and
actuates the valve mechanism, allowing new coolant flow through the orifice and ultimately
to the heat load.
[0007] While the self-regulating demand-flow cryostat provides control over coolant flow,
there are still limitations which can make it unacceptable in systems where temperature
fluctuations can be critical to operations.
[0008] The thermostat bellows mechanism (and similar substitute devices that rely upon expansion
and contraction of materials, such as certain plastics) are inherently subject to
large performance tolerances. This can result in instabilities and fluctuations in
the critical on-off temperature point for the needle valve. Therefore, such cryostats
have been found to be non- proportional in their gas flow regulation over the expected
full range of operation.
[0009] Additionally, the same cryostat will react differently using different cryogens.
Thus, the system performance will be dependent upon the specific cryogen in use during
any single operation. If a substitute coolant is used in place of that for which the
thermal contraction link is designed, the cryostat will seek the design set temperature
and may repeatedly fail.
[0010] Furthermore, temperature waves within the expansion chamber can result in thermal
cycling of the bellows mechanism. Moreover, in rocketry environments, the mechanism
will be subject to the effects of acceleration and deceleration. In infrared detector
cooling, instability in the detector due to fluctuations in refrigeration by the cryostat
may result in a type of thermal noise, known as thermophonics, produced in a coupled
video display.
[0011] Pooling of the liquid gas when a saturated condition within the plenum chamber is
reached can also seriously affect the performance of the cryostat needle valve operation
if the liquid pool moves under the influence of environmental forces. This can result
in a rapid vaporization or impedance to gas flow in the low pressure side of the heat
exchanger and consequent liquid vapor pressure changes and a sudden opening of the
needle valve. Another operational limitation is encountered when an extended thermal
load is used. The temperature of the gas plenum chamber cools down before the remote
extremities of the thermal load and causes the needle valve to prematurely close before
the load is entirely cooled down.
[0012] Any of these exemplary conditional aspects can result in a premature closing or undesired
variations in the needle valve positioning and gas flow rate. This forces a manufacturing
design of a multiplicity of Joule-Thomson cryostat components tailored to both heat
load factors and coolant parameters.
[0013] Thus, there is a need for an active mechanism for cryostats which can use the sensed
temperature of the thermal load, and not just the gas plenum chamber temperature,
to adjust the gas expansion valve orifice size setting from maximum opening during
cool-down to a reduced minimal opening for sustained cooling of the thermal load.
Furthermore, there is a need for an active mechanism for cryostats to provide stable,
time-averaged, proportional control of the gas expansion valve orifice size for stable
gas flow and, therefore, stable refrigeration rate and temperature control. Ideally,
this control should be able to adjust to changing heat load and input pressure conditions,
as well as changing thermal environments in a continuous, gradual and non- abrupt
manner, which maintains a stable cold temperature. Ideally, this control should be
able to work with multiple coolant gases and provide a stable cold temperature and
optimal performance. In addition, there is a need for an active mechanism for cryostats
that minimizes susceptibility to temperature fluctuations caused by outside environmental
effects, such as inertial accelerations, or to internal thermal waves, or to pressure
variations within the plenum chamber.
SUMMARY OF THE INVENTION
[0014] The need in the art is addressed by the present invention which provides an active
gas throttling mechanism for a cryostat, using a needle valve controlled, Joule-Thomson
effect cryostat to cool a heat load. The inventive apparatus includes an actuator
device connected, to a needle valve, for controlling the flow of a coolant therethrough.
A temperature sensor is connected to the heat load in proximity thereto. The sensor
provides a signal indicative of sensed temperature. A servo-controller receives the
signal and regulates the flow of coolant to the actuator device in response thereto.
[0015] In operation, the invention provides a closed-loop method for controlling refrigeration
of a thermal load by a Joule-Thomson effect cryostat comprising the steps of: (a)
sensing proximate temperature of the thermal load, (b) transmitting a first signal
related to the proximate temperature, (c) converting the signal to a second signal
for regulating flow of coolant in the cryostat, and (d) adjusting the flow of coolant
in direct relation to the first signal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1A is a cross-sectional, plan (side) view of a typical conventional cryostat.
[0017] FIG. 1B is a cross-section, plan view (side) of a typical demand-flow needle valve
as utilized in the typical conventional cryostat of FIG 1A.
[0018] FIG. 2A is a depiction of the present invention in a cross-section, plan view (side)
as incorporated in a Joule-Thomson cryostat.
[0019] FIG. 2B is a detail depiction of a portion of the present invention as shown in FIG.
2A.
[0020] FIG. 3 is a schematic diagram of the present invention as shown in FIG. 2A.
[0021] FIG. 4 is a schematic flow chart diagram of the operation of the present invention
as shown in FIGS. 2 and 3.
[0022] FIG. 5 is an alternative embodiment of the present invention as shown in FIG. 2A.
[0023] FIG. 6 is a schematic flow chart diagram of the operation of the alternative embodiment
of the present invention as shown in FIG. 5.
DESCRIPTION OF THE INVENTION
[0024] A typical conventional Joule-Thomson cryostat 10 is shown in FIGS. 1A and 1B. A coolant,
such as high pressure argon or nitrogen gas or even air, is introduced through a gas
inlet fitting 1 into a recuperative heat exchanger 3 that encompasses a support mandrel
5 inside a cold finger section 7 of a dewar package 9. The heat exchanger 3 basically
comprises counterflow finned metal tubing 4 wrapped around the mandrel 5, that allows
the high pressure gas to cool significantly as it moves toward the lower end of the
cold finger section 7. The heat exchanger tubing 4 terminates in an orifice 16 at
the lower end of the mandrel 5, commonly referred to as the cold end of the cryostat.
The orifice 16 acts as a Joule-Thomson gas throttling valve. As the gas passes through
the orifice 16 and enters the surrounding gas plenum chamber 29, it expands to a low
pressure gas and creates a liquid form. The evaporated liquid and low pressure gas
are used to cool the thermal load 15 which is located adjacent the dewar window 11.
The cooling of the load is accomplished by a liquid coolant spray from the orifice
16 onto the portion of cold finger 7 positioned in contact with thermal load 15. The
gas from the chamber 29 is recycled through another low pressure branch of the heat
exchanger 3 before exiting into the atmosphere through exit port 13 at the upper,
of warm end, of the cryostat 10.
[0025] As mentioned above, the simplest form of a conventional Joule-Thomson cryostat typically
had a fixed-size orifice 16 in the heat exchanger 3 at the cold end of the cryostat
such that cooling by the cryostat was unregulated. The input pressure and internal
gas flow dynamics established the flow parameters of the coolant through the cryostat.
While a simple apparatus in that it has no moving parts, the inherent, uncontrolled
flow characteristics make the fixed-orifice type cryostat unsuitable for many applications
where rapid cool-down and long cooling durations from a limited size gas supply source
are required. Rapid cool-down requires high rate gas flow and a large size orifice,
while long cooling durations require low gas flow rates and a small size orifice.
These two conditions cannot be simultaneously met in a fixed orifice cryostat.
[0026] Since approximately the 1950's, demand-flow Joule- Thomson cryostats with internal,
passive, thermostatic control of variable orifice size have been used as shown in
FIG. 1B. These cryostats have the ability to start cool-down with the maximum orifice
size, thereby providing high rate gas flow and refrigeration for rapid cool-down.
After cool-down is achieved, the orifice size is reduced for minimal gas flow rate
and refrigeration necessary for the thermal load.
[0027] A thermostatic element within the mandrel 5 provides self-regulation of gas flow
based upon the temperature in and around the gas plenum chamber 29. The cooling rate
is proportional to the mass flow rate of gas through the cryostat. The thermostatic
element, which can be a gas- filled bellows or a segment of material which contracts
or expands based upon temperature, is coupled to a demand-flow needle valve mechanism
19. As the temperature drops, the bellows is adapted to contract and cause the needle
to extend into and partially close the Joule-Thomson orifice 16. At the predetermined
critical temperature, the bellows thermostat mechanism 17 can close the needle valve
19 entirely. As the temperature rises, the bellows expands again and actuates the
valve mechanism 19, allowing new coolant flow through the orifice and ultimately to
the heat load 15.
[0028] As mentioned above, while the self-regulating demand-flow cryostat provides control
over coolant flow, there are still limitations which can make it unacceptable in systems
where temperature fluctuations can be critical to operations.
[0029] The thermostat bellows mechanism 17 (and similar substitute devices that rely upon
expansion and contraction of materials, such as certain plastics) are inherently subject
to large performance tolerances. This can result in instabilities and fluctuations
in the critical on-off temperature point for the needle valve. Therefore, such cryostats
have been found to be non- proportional in their gas flow regulation over the expected
full range of operation.
[0030] Additionally, the same cryostat will react differently using different cryogens.
Thus, the system performance will be dependent upon the specific cryogen in use during
any single operation. If a substitute coolant is used in place of that for which the
thermal contraction link is designed, the cryostat will seek the design set temperature
and may repeatedly fail.
[0031] Furthermore, temperature waves within the expansion chamber can result in thermal
cycling of the bellows mechanism. Moreover, in rocketry environments, the mechanism
will be subject to the effects of acceleration and deceleration. In infrared detector
cooling, instability in the detector due to fluctuations in refrigeration by the cryostat
may result in a type of thermal noise, known as thermophonics, produced in a coupled
video display.
[0032] Pooling of the liquid gas when a saturated condition within the plenum chamber 29
is reached can also seriously affect the performance of the cryostat needle valve
operation if the liquid pool moves under the influence of environmental forces. This
can result in a rapid vaporization or impedance to gas flow in the low pressure side
of the heat exchanger 3 and consequent liquid vapor pressure changes and a sudden
opening of the needle valve 19.
[0033] Another operational limitation is encountered when an extended thermal load 15 is
used. The temperature of the gas plenum chamber 29 cools down before the remote extremities
of the thermal load 15 and causes the needle valve 19 to prematurely close before
the load is entirely cooled down.
[0034] Any of these exemplary conditional aspects can result in a premature closing or undesired
variations in the needle valve positioning and gas flow rate. This forces a manufacturing
design of a multiplicity of Joule-Thomson cryostat components tailored to both heat
load factors and coolant parameters.
[0035] Thus, there is a need for an active mechanism for cryostats which can use the sensed
temperature of the thermal load, and not just the gas plenum chamber temperature,
to adjust the gas expansion valve orifice size setting from maximum opening during
cool-down to a reduced minimal opening for sustained cooling of the thermal load.
[0036] Furthermore, there is a need for an active mechanism for cryostats to provide stable,
time-averaged, proportional control of the gas expansion valve orifice size for stable
gas flow and, therefore, stable refrigeration rate and temperature control. Ideally,
this control should be able to adjust to changing heat load and input pressure conditions,
as well as changing thermal environments in a continuous, gradual and non- abrupt
manner, which maintains a stable cold temperature. Ideally, this control should be
able to work with multiple coolant gases and provide a stable cold temperature and
optimal performance.
[0037] Furthermore, there is a need for an active mechanism for cryostats that minimizes
susceptibility to temperature fluctuations caused by outside environmental effects,
such as inertial accelerations, or to internal thermal waves, or to pressure variations
within the plenum chamber.
[0038] The present invention addresses these needs. The present invention is adaptable to
a configuration of a cryostat of the type shown in FIG. 1A. FIG. 2A shows the invention
as incorporated into such a Joule-Thomson cryostat.
[0039] The invention is contained within the cold finger 7 of the vacuum dewar container
9 of like manner to the prior art of FIG. 1A.
[0040] Referring back to FIG. 2A, a mandrel cap member 21 seals a mandrel chamber 23. Generally,
the cap member 21 comprises an encapsulated thermal foam insulation 25, such as polyurethane,
to prevent thermal leakage from the chamber 23.
[0041] A sealed sheath tubing 27, preferably made of stainless steel or the like, runs the
from the cap member 21 the length of the mandrel chamber 23 and openly connects within
a coolant plenum chamber 29 at the cold end of the cryostat. The coolant plenum chamber
29 houses a fixedly mounted bellows spring 31. In the preferred embodiment, a low
mass, high spring constant device is used for the bellows spring 31.
[0042] Within a cap chamber 33 of the mandrel cap 21 is seated an electrical feedthrough
connector 35. In the preferred embodiment, this is a glass-to-metal feedthrough connector
35. The feedthrough connector 35 has an electrical lead 55 terminating externally
of the mandrel cap 21. A bellows spring actuator mechanism is terminated at a first
end 39 in the feedthrough connector 35. The bellows spring actuator comprises a first
section, which, in the preferred embodiment, is a biometal wire rod 41. The rod 41
extends through the length of the mandrel cap 21 toward the sheath tubing 27 and is
coupled at an electrical return junction 43 to a wire rod 45, in the preferred embodiment
a steel wire rod. Note that with a relatively sturdy rod 45, the sheath 27 may be
eliminated. An electrical return wire 55' coupled to the junction 43 also leads externally
of the mandrel cap 21.
[0043] The biometal rod 41 acts as a transducer. In the preferred embodiment, the biometal
wire rod 41 is a type of titanium-nickel shape memory alloy which contracts during
heating when an electrical current is passed through it (known in the art as martensitic-austensitic
phase transitioning). Either a small proportional direct current or a pulse-width
modulated electric current may be provided through the rod 41 via the pair of electrical
leads 55, 55'. The rod 41 thus acts as a miniature transducer for translating motion,
using the attached steel wire rod 45, to a needle valve 19 in the coolant plenum chamber
29. Alternatives to the biometal could be a solid-state electromagnetic device, such
as a solenoid or magnetostrictive alloy, or another solid-state electromechanical
device, such as a piezoelectric transducer. The application of the cryostat may dictate
which alternative is most appropriate. In FIG. 2A, electrical current is applied to
the wire rod 41 through feedthrough connector 35 (FIG. 3). Current is also supplied
by wires 62 and 63 to a heating element 64.
[0044] Referring also to FIG. 2B, the steel wire rod 45, connected to the biometal wire
rod 41 at the electrical return junction 43, runs through the sheath tubing 27 to
a bellows compression plate 47 connected to the movable end of the bellows spring
31.
[0045] The bellows spring 31 is connected to a needle valve mechanism 19 within the coolant
plenum chamber 29 via bellows pressure plate 47. The combination of spring 31 and
pressure plate connected through the steel wire rod 45 to the biometal rod 41 forms
a valve actuator device. As shown in FIG. 2B, the needle valve mechanism 19 and connection
bellows spring 31 is similar or identical to that recognized by a person skilled in
the art as exemplified in FIG. 1B.
[0046] Returning to FIG. 2A, a thermal sensor 51 is mounted on, or adjacent, a thermal load
mounting platform 15' (which may be integral with the particular load). In the preferred
embodiment, a silicon diode temperature sensor 51 is fixedly mounted on the heat load
platform 15' in close proximity to the load 15.
[0047] A commercially available sensor such as the IN914 diode manufactured by Texas Instruments
is suitable for use in the present invention. Transistor-type sensors may alternatively
be employed in the invention. The sensor 51 is mounted so as to be sensitive to the
ambient temperature of the load environment rather than the temperature within the
plenum chamber 29.
[0048] Referring now to FIG. 3, the invention is represented in a schematic diagram to assist
understanding of its operation.
[0049] The heat load 15 that requires temperature control, for example, a missile infrared
detector or focal plane array, is fixedly mounted on a thermal load platform 15'.
The platform 15' is in turn located adjacent to the cryostat near an orifice 16 as
shown in FIG. 2A.
[0050] A servo-controller actuator 53 is coupled by electrical leads 55, 55'(return) to
the bellows spring actuator 57 and thermal sensor 51. As shown by arrows designated
Qx, there are various sources of heat comprising the load. The load 15 and its platform
15', Qfpa and other load environment heat contributors are shown as Qrad, Qcond and
even Qvalve, representing heat generated from the valve actuator itself Heat to be
extracted by the cryostat is shown a -Qcryo. The temperature sensor 51 provides feedback
from the load 15, 15' environment to a servo-controller 53 of the valve control actuator
57. The operational parameters of the servo-controller 53 are designed to control
the dimensional length of the biometal rod 41 based upon the sensed temperature. Servo-controller
actuator 53 current, being coupled to the biometal wire rod 41 by the electrical leads
55, 55' drives the transducer action of the biometal wire rod 41. In turn, the steel
wire rod 45 attached to the biometal wire rod 41 controls the bellows spring 31 and,
thus, the Joule-Thomson orifice of the needle valve 19 and, therefore, the flow of
the coolant to the load 15.
[0051] The method of operation is depicted in the block diagram designated FIG. 4. Designated
as reference numeral 101, the servo-controller 53 of the valve control actuator 57
is activated at the time when the coolant gas is input through the gas inlet fitting
1 into the heat exchanger 3. The current to the biometal rod 41 is turned on to reach
a predetermined temperature which is just below the phase transition temperature of
the preselected biometal material used in rod 41. During the cool down phase of a
refrigeration cycle, the needle valve 19 is normally wide open to maximize gas flow
and refrigeration effectiveness. Designated by reference numeral 103, when the load
temperature approaches its predetermined reference value, the temperature sensor 51
provides a temperature error feedback signal to the servo-controller 59 of the valve
control actuator 57, as shown by reference numeral 105. The servo-controller 59 in
turn begins to generate an additional current into the biometal wire rod 41 via electrical
lead 55 and feedthrough connector 35. As the rod 41 heats, the biometal material contracts.
This contraction, transmitted through the steel wire rod 37 and bellows pressure plate
47 throttles the coupled needle into the valve orifice and reduces gas flow and, therefore,
the Joule-Thomson cryostat refrigeration effect.
[0052] The gas flow cut back continues until the load temperature rises above the reference
level which is detected by the thermal sensor 51. As a result, the current to the
biometal wire 41 is cut back, causing the reverse reaction and opening the orifice
of the needle valve 19 to produce more refrigeration. The temperature may oscillate
but only until the temperature error signal is properly nulled to zero when the feedback
through the valve control actuator 57 has adjusted the needle valve 19 orifice to
a size where the refrigeration rate matches the thermal load within the desired reference
temperature range.
[0053] FIG. 5 shows an alternative embodiment of the valve control actuator 57 to include
a fine servo-controller 61 coupled to the temperature sensor 51 by electrical lead
pair 56. Also shown, the fine servo-controller element 61 drives a heating element
64 located inside the cold end of the cryostat chamber 23 or, as shown on FIG. 5,
is alternatively mounted near the heat load 15. The function of the second fine servo-controller
element 61 is to add any additional necessary heat to maintain the load temperature
within a finer tolerance reference temperature range for the specific load.
[0054] Similarly, the operation of the alternative embodiment of FIG. 5 is depicted in the
block diagram of FIG. 6. Thus, a secondary servo loop with feedback uses small amounts
of electrical heater power added to the thermal load to maintain the temperature constant
within tighter tolerances.
[0055] In this manner, active, servo-controlled cryogenic refrigeration to maintain a substantially
steady-state temperature environment for a thermal load is accomplished with a non-saturated
cryogen operating mode. In the non-saturated cryogen operating mode, excess liquid
cryogen is not produced and, therefore, there is no pooling of liquid cryogen in the
plenum chamber 29. As the present invention operates in the non-saturated mode, the
invention should operate over a wide range of ambient environmental pressure, including
in space vacuum, since there is no dominant dependency on the cryogen boiling pressure
or temperature as long as the reference temperature is higher than the range of cryogen
boiling points.
[0056] This invention is also capable of operating in the saturated to near-saturated operating
mode. For example, by using a servo-control reference temperature slightly above the
liquid cryogen boiling temperature, but converting the servo-controller from a null
search mode to a mode that delivers a fixed predetermined output current when the
threshold temperature is reached, the cryostat would be made to operate with the orifice
reduced but fixed, during sustained cooling, to produce a slight excess of liquid
cryogen.
[0057] Because of these features, the invention is useful in Joule-Thomson cryo-systems
which provide fast cool down and long duration cooling from minimal size and weight
gas supply systems, for example, in missile systems.
[0058] The present invention has been described herein with reference to a particular embodiment
for a particular application. Those having ordinary skill in the art and access to
the present teachings will recognize additional modifications, applications and embodiments
within the scope thereof.
[0059] It is therefore intended by the appended claims to cover any and all such applications,
modifications and embodiments within the scope of the present invention.