REDUCING TEMPERATURE STABILIZATION TIME OF AN IR DETECTOR

IN THE JOULE-THOMSON CRYOGENIC COOLING PROCESS

Field of the Invention

The invention relates to the field of cryogenic cooling. In particular the invention is a method and system for reducing the temperature of sensor elements from ambient temperature and quickly stabilizing it at a cryogenic temperature.

Background of the Invention

Many applications in modern technology, such as systems comprising electro-optical elements, require cooling to cryogenic temperatures. A typical application is the Forward Looking Infrared Radiation (FLIR) systems used for navigation and target identification purposes in airplanes and missiles. It is not possible for some cooled FLIR systems to produce a picture unless its IR matrix detector (sensor) is maintained at a stable temperature of about 77 - 90 ⁰ K. One of the technical problems to be overcome is the length of time required to reduce the temperature of the sensor fronr ambient to the stable boiling temperature of the liquid nitrogen or other cryogenic gas used as the refrigerant. Shortening this length of time can be of critical importance. For example in an anti-tank missile used for ground combat, the limited amount of cryogenic gas does not allow the FLIR system to be operational continuously and the cooling system is only activated during the immediate preparations for launching the missile. In such a situation, in which seconds and even fractions of a second can determine the outcome, the necessity of reducing the period of time until the FLIR system is operational is obvious.

It is therefore a purpose of the invention to provide a method and system that allows cooling of an object from ambient to a stable cryogenic temperature in a period of time that is significantly shorter than the time taken by prior art cryogenic coolers and FLIR structures.

Further purposes and advantages of this invention will appear as the description proceeds.

Summary of the Invention The invention is a system and method for quickly cooling an object to a stable cryogenic temperature. The method is based on the fact that the boiling point of a liquid is dependent on the pressure of the ambient gas surrounding the boiling liquid and the system is designed to control that pressure.

In a first aspect, the invention is a system for quickly cooling an object to a stable cryogenic temperature. The system comprises a container containing refrigerant, a refrigerant container valve, a transfer line, a DDC (Dewar Detector Cooler), and one or more pressure release valves. The cooler is located in a Dewar adjacent to the object-to be-cooled. The- J-T cooler comprises a heat exchanger, an expansion orifice, and a manifold with a high pressure input and a low pressure output tube. The refrigerant is a cryogenic gas at high pressure and the transfer line transfers the cryogenic gas to the cooler when the refrigerant container valve is opened. The refrigerant that passes through the cooler passes into an enclosed volume that surrounds and/or is in good thermal contact with the IR matrix. The pressure release valves are adjusted to release gas from the interior of the enclosed volume when the pressure inside the enclosed volume rises above a predetermined pressure.

The system of the invention is characterized in that it additionally comprises a temperature measuring device to measure the temperature of the object, an open/closed valve for controlling the passage of the gas from the interior to the exterior of the enclosed volume, and a controller that opens the open/closed valve when the refrigerant container valve is opened to release the refrigerant from the container. The controller closes the open/closed valve when the temperature-measuring device indicates that the temperature of the object has been lowered to a predetermined temperature.

In one embodiment of the system of the invention the enclosed volume is a sealed chamber, which surrounds at least the J-T cooler and the object to be cooled. In another embodiment the enclosed volume is the volume that surrounds the heat exchanging manifold of the J-T cooler.

In preferred embodiments of the system of the invention, the cryogenic gas is nitrogen, the predetermined pressure is 6 psig, and the predetermined temperature is 8O ⁰ K.

In another aspect ^~ the invention is. a ^~ jnethod of quickly cooling an object to a stable cryogenic temperature. The method comprises the steps of:

— Providing a container containing cryogenic gas at high pressure, a refrigerant container valve, a transfer line, and a J-T cooler comprising an orifice. The cooler is located adjacent to the object to be cooled and the transfer line transfers the cryogenic gas to the cooler when the refrigerant container valve is opened.

— Providing an enclosed volume that surrounds and/or is in good thermal contact with the object to be cooled and into which the cryogenic gas which passes through the J-T cooler enters. — Providing one or more pressure release valves adjusted so that they release gas from the interior of the enclosed volume when the

pressure inside the enclosed volume exceeds a predetermined pressure.

— Providing a temperature-measuring device to measure the temperature of the object. - Providing an open/closed valve for allowing or preventing the passage of gas between the interior and the exterior of the enclosed volume.

— Providing a controller.

The controller activates the open/closed valve turning it to the open position when the refrigerant container valve is opened to allow the cryogenic gas to flow to the cooler. This allows gas to pass out of the enclosed volume and maintains the pressure inside the sealed enclosed volume at 0 psig. The controller activates the open/closed valve turning it to the closed position when the temperature-measuring device indicates that the object has been cooled to a predetermined temperature. When the open/closed valve is in the closed position gas is prevented from passing out of the enclosed volume and the pressure inside the sealed enclosed volume rises to the predetermined pressure. The pressure inside the enclosed volume is maintained at the predetermined pressure by the action of the one or more pressure release valves.

In preferred embodiments of the method of the invention, the cryogenic gas is nitrogen, the predetermined pressure is 6 psig, and the predetermined temperature is 80 ⁰ K.

All the above and other characteristics and advantages of the invention will be further understood through the following illustrative and non-limitative description of preferred embodiments thereof, with reference to the appended drawings.

Brief Description of the Drawings

— Fig. 1 schematically shows a prior art scanner/seeker head for a missile;

— Fig. 2 shows a typical cooling curve;

— Fig. 3 schematically shows a scanner/seeker head for a missile according to the present invention;

— Fig. 4 shows the results of an experiment that compares the time of cooling a sensor using the system of the prior art to the time required using the present invention; and

— Fig. 5 schematically shows a preferred embodiment of the system of the invention.

Detailed Description of Preferred Embodiments

In order to illustrate the technical problem addressed by the present invention, a prior art scanner/seeker head for a missile is schematically shown in Fig. 1. In the nose of the missile is located seeker head 10, which is comprised of sealed chamberl2. At the front of chamber 12 is a dome or shield 14, which is transparent to the wavelengths of the detectors and/or other optical- elements located inside the chamber. Inside the chamber 12 is a suitable optical system (not shown), which focuses an image onto the surface of an infrared matrix sensor 16. Ceramic plate 18 provides physical support to the sensor 16. In order to cool sensor 16, a Joule-Thomson (J-T) cooler 21 is provided. The J-T cooler 21 is of the open cycle type, which means that the refrigerant is not recycled through a compressor and reused. The cooling system basically comprises a refrigerant container 24, which contains a cryogenic gas at very high, pressure and a Dewar Detector Cooler (DDC) 20 comprising the J-T cooler -21, the sensor 16, and ceramic plate 18. The refrigerant can be any cryogenic gas, e.g. nitrogen, Argon, air, or mixed gases. For illustrative purposes only nitrogen will be considered herein. Skilled persons will easily be able to make the necessary changes to the description of the invention in order to modify it for use with any other cryogenic gas.

When the command is given to activate seeker head 10, then refrigerant container valve 26 is opened, the high pressure nitrogen passes through transfer tube 22 and as it exits through cooler 21 the rapidly expanding gas cools and a small puddle of liquid nitrogen is formed. The liquid nitrogen absorbs heat from the ceramic plate 18. As a result the liquid nitrogen boils, i.e. undergoes a phase change and becomes nitrogen gas. Eventually a thermal equilibrium is reached and ceramic plate 18 and the sensor 16 are at the temperature of the liquid nitrogen. At the same time as the sensor is being cooled, nitrogen gas fills the interior of the sealed chamber 12. In order to prevent the pressure inside sealed chamber 12 from rising to the point at which the chamber explodes or the dome 14 is damaged or "blows out" of the nose of the missile, one or more pressure relief valves 28 are provided. Valve 28 is a one-way valve that allows gas to flow out of chamber 12 but not into it. Valve 28 is designed to open when the interior pressure is a predetermined amount (typically 1 to 3psig) greater than the outside pressure.

The cooling of the elements mounted on ceramic plate 18 takes place in two distinct stages. Initially the temperature drops very rapidly and nearly linearly to about 80 ⁰ K. At this point, the rate of cooling slows and it takes on the order of two or three times as much time to cool the sensor the final 3 ⁰ K from 80 ⁰ K to 77 ⁰ K as it did to cool the 20O ⁰ K from ambient temperature down to 80 ⁰ K. This effect is shown in Fig. 2, which shows a typical cooling curve 30. The vertical axis represents the temperature in degrees Kelvin and the horizontal axis shows time in seconds, with tτ<κ) representing the time required after the refrigerant container valve 26 is opened for the sensor to reach temperature T°(K).

Fig. 3 schematically shows a scanner/seeker head for a missile according to the present invention. The system shown in Fig. 3 is similar to that of the prior art with one exception and the addition of two new components. The exception is that in the system of the present invention, the pressure release valves 28 are adjusted so that they release gas from the interior of sealed chamber 12 only when the internal pressure exceeds 6 psig. The additional components are a two-position open/closed valve 32, which controls the pressure inside of sealed chamber 12 by allowing or preventing the passage of gas between the interior and the exterior of the chamber; and a temperature diode 34, located on the ceramic plate 18 and the sensor 16.

When the command is given to activate scanner head 10, then refrigerant container valve 26 is opened allowing the high-pressure nitrogen gas to pass through transfer tube 22 and exit through the expansion valve of cooler 21. Simultaneously with the opening of refrigerant container valve 26, valve 32 is also opened so that the pressure inside the sealed chamber 12 is kept equal to that outside of the chamber (0 psig). At this stage, the conditions within chamber 12 are equivalent to those of the prior art and the temperature rapidly falls towards 8O ⁰ K as shown in the first stage of Fig. 2. At this point however, the method of the invention deviates from that of the- prior art. When the temperature diode indicates that the temperature has reached 80 ⁰ K, the valve 32 is closed. The pressure inside chamber 12 continues to rise to 6 psig and is maintained at this level by the action of pressure relief valves 28. As the pressure inside chamber 12 increases the temperature of the boiling point of liquid nitrogen also rises and stabilizes at 8O ⁰ K when the pressure stabilizes at 6psig

Valve 32 is preferably an electric valve that can be activated by means of an electric signal thereby allowing the system to be controlled automatically, for example by use of the control system connected with the detector device or by a dedicated microprocessor, which opens valve 32 when the signal is

given to begin the cooling operation and automatically closes it when the temperature diode sends a signal indicating that the temperature has been reduced from surrounding temperature to 8O ⁰ K.

Fig. 4 shows the results of an experiment that compares the time of cooling a sensor using the system of the prior art to the time required using the present invention. The example is provided merely to illustrate the invention and is not intended to limit the scope of the invention in any manner. The experiment was carried out at an ambient temperature of about 60 ⁰ C using a 450cc container 24, which contained nitrogen at a pressure of 10,000psi. In Fig. 4 the horizontal shows time in seconds and the vertical axis the temperature in degrees Kelvin.

Curve I shows the results using the system of the invention, i.e. Fig. 3. In this experiment, valve 32 was closed at point to on the time scale (when the temperature reached near 8O ⁰ K, about 58 seconds after opening refrigerant container valve 26). The temperature continued falling while the pressure inside the chamber was rising until the pressure stabilized at 6psig. At this point, about 7 seconds (ti) after to, the temperature stabilized at a steady temperature near 8O ⁰ KT Curve II shows the results obtained using the system of the prior art, i.e. Fig. 1. In this case, the temperature reached near 80 ⁰ K about 58 seconds after opening refrigerant container valve 26 and finally stabilized at steady temperature near 77 ⁰ K about 17 seconds later fø). Thus by using the system and method of the invention the second stage of the cooling process, i.e. the time it takes to go from 80 ⁰ K to a stable minimum temperature, has been reduced by 60%.

Considering the experimental results shown in Curve I of Fig. 4 and the explanation given hereinabove, it can be understood that the length of time ti is dependent upon the time it takes for a sufficient volume of nitrogen to boil to raise the internal pressure inside of sealed chamber 12 to the desired

pressure. In other words one of the principal factors that determine ti is the internal volume of the chamber.

Fig. 5 schematically shows a preferred embodiment of the invention that greatly reduces the time necessary to attain stabile temperature conditions by severely reducing the size of the chamber and thereby reducing its influence on ti. The embodiment shown in Fig. 5 can be used to rapidly cool the sensor to stabile cryogenic temperature both if the sensor is inside of a sealed chamber and also if it is not.

In this embodiment an exit manifold 36 is attached to the expansion valve of cooler 21 and valves 28 and 32, such that the nitrogen gas can escape from the expansion valve only through one of the two valves. As in the embodiment described hereinabove, valve 28 is a one-way pressure valve adjusted to open when the backpressure reaches 6psig and valve 32 is an open/closed valve, preferably electrically activated.

When the command is given to activate scanner head 10, then refrigerant container valve 26 is opened allowing the high-pressure nitrogen to pass through transfer tube 22 and enter manifold 36 through the expansion -valve of cooler 21. Simultaneously with the opening of refrigerant container valve 26, valve 32 is also opened so that the backpressure inside manifold 36 and the expansion valve is kept equal to that of the surroundings (0 psig). At this stage the temperature rapidly falls towards 8O ⁰ K. When the temperature diode 34 indicates that the temperature has reached 80 ⁰ K, the valve 32 is closed. The back pressure inside the expansion valve and manifold 36 rises rapidly to 6 psig and is maintained at that level by the action of pressure relief valve 28. Because of the very small internal volumes of the expansion valve and manifold 36 the pressure stabilizes at 6psig and the temperature of the sensor 16 stabilizes at 8O ⁰ K almost instantaneously.

Although embodiments of the invention have been described by way of illustration, it will be understood that the invention may be carried out with many variations, modifications, and adaptations, including using other refrigerants, other temperatures, other pressures and in systems having other configurations without departing from its teachings or exceeding the scope of the claims.