US3646775A

US3646775A - Cryostat

Info

Publication number: US3646775A
Authority: US
Inventors: Jacques Bonnerot; Henri Paradan
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1969-03-10
Filing date: 1970-03-09
Publication date: 1972-03-07
Anticipated expiration: 1989-03-07
Also published as: GB1307236A; SE362295B; DE2010967B2; FR2036292A5; DE2010967C3; CH514820A; DE2010967A1; NL7003198A

Abstract

Cryostat for keeping an object at a constant low temperature, comprising an outer container which can be evacuated and in which a liquid container for cryogenic cooling liquid is disposed, the liquid container being annular and closed and substantially surrounding the object to be cooled, an inner container being present in the space between the annular liquid container and the object to be cooled, which inner container likewise substantially surrounds the object to be cooled and is in thermal contact therewith, a system of liquid ducts and a system of vapor ducts present for communicating between said containers with a blowoff valve in a vapor duct opening to the atmosphere outside the cryostat, and a control device present for controlling the pressure in at least the liquid container and the temperature in the inner container.

Description

I United States Patent [151 3,646,775 Bonnerot et al. 51 Mar. 7, 1972 [54] CRYOSTAT 3,424,230 1/1969 Wright ..62/514 [72] Inventors: Jacques Bonnerot, St-Cloud' Henri Primary ExanunerMeyer Perhrl Paradan, Meudon, both of France Attorney prank R Trifari [73] Assignee: U.S. Philips Corporation, New York, N.Y. [22] Filed: Mar. 9, 1970 [57] AB CT Cryostat for keeping an object at a constant low temperature, [2]] Appl' 17560 comprising an outer container which can be evacuated and in which a liquid container for cryogenic cooling liquid is [30] Foreign Application Priority Data disposed, the liquid container being annular and closed and substantially surrounding the object to be cooled an inner Mar. 10 1969 France ..6906690 container being present in the space between the annular liquid container and the object to be cooled which inner con- [52] US. CL ....62/216, 62/514 tamer likewise substantially surrounds the ob ect to be cooled and is in thermal contact therewith, a system of liquid ducts Searc and a system of vapor ducts present for communicating between said containers with a blowoff valve in a va r duct 6 P 1 References cued opening to the atmosphere outside the cryostat, and a control UNlTED STATES PATENTS device present for controlling the pressure in at least the liquid 3 195 620 7H9 St hard: 62/514 container and the temperature in the inner container.

em 3,364,687 1/1968 Kolm ..62/514 10 Claims, 2 Drawing Figures fi o O 12 2 56 2 W 11, o a I o 39 23 i 31 52 O D 27 l c 2 1.1 1 O Y D a 28 1 7' 6 -48 ----3s 3s f 1 42 s g j; u.

o 8 V Q 2 a 9 w z 2 1.7 L 4 O j a s3 22 Z Z o c c o a 0 DD 0 37 -|5 20 3o E m B41. 15 2'9 17 2 a -51 10 CRYOSTAT The invention relates to a cryostat which is suitable for keeping an object at a constant low temperature, and comprises an outer container which can be evacuated and in which a liquid container for cryogenic cooling liquid is arranged.

Numerous electronic measuring instruments must be cooled at a very low temperature, notably to reduce the thermal noise occurring in the electronic circuits. Cryostats are frequently used for this purpose in which a cryogenic cooling liquid, for example, liquid nitrogen or liquid helium, is provided. The known cryostats are not suitable for use in certain investigations, for example, space research where it is required that the cryostat, in addition to an independent self-supporting operation, be able to operate in any position and in a rare atmosphere having a low or no atmospheric pressure, while the weight and the dimensions of the cryostat must remain restricted.

In order to obtain a sufficiently large period of independent operation, the known cryostats having a stationary liquid bath, have comparatively large dimensions, while the cryostats in which the cryogenic liquid is circulated often have a considerable store of gas which necessitates extra space and a fixed position of the cryostat. In the last-mentioned cryostats a pumping device is moreover necessary to maintain the circulation of the cooling medium which also requires considerably extra space.

It is the object of the invention to provide a cryostat which can be operated in any position, which has small dimensions and a low weight, and which can keep an object at a low temperature independently and without further operation with a comparatively small quantity of cryogenic cooling liquid and for a long period of time.

For that purpose, the cryostat according to the invention is characterized in having a liquid container that is annular, and is closed, and mainly surrounds the object to be cooled, and an inner container which also surrounds the cooling object mainly and is in thermal contact therewith being present in the space between the annular liquid container and the object to be cooled; a system of liquid ducts is present within the closed outer container, which system communicates (a) via at least one first and at least one second liquid duct at different levels, with the liquid container, and (b) via at least one third and at least one-fourth liquid duct at different levels, with the inner container. The closed outer container furthermore comprises a system of vapor ducts which communicate on the one hand, via at least one first and at least one second vapor duct at different levels, with the inner container, and on the other hand, via at least one vapor outlet duct in which a blowoff valve is incorporated, opens outside the cryostat; a control device is present for controlling the pressure in at least the liquid container and the temperature in the inner container. Such a cryostat is extremely suitable for use in spacecraft, for example, airplanes, rockets, balloons and the like.

The liquid container and the inner container arranged around the object to be cooled, such as an electronic measuring instrument, both form effective radiation screens against the thermal radiation originating from outside the cryostat, while the cryostat can operate in any position notably by the presence of the closed liquid container and the closed system of ducts. Since the liquid container is annular, the object to be cooled is not fully surrounded by the said container and hence space is available for an entrance window which allows entrance for radiation to the object to be cooled.

The inner container situated between the object to be cooled and the liquid container comprises cryogenic liquid originating from the said liquid container and therefore forms an auxiliary storage container. This arrangement guarantees an excellent transfer of cold from the cryogenic liquid to the object to be cooled.

The quantity of liquid which can flow from the liquid container to the inner container to be evaporated there by thermal contact with the object to be cooled, is determined by controlling the pressure in the said liquid container. It is also ensured that the object to be cooled always remains at a constant low temperature. Since the evaporation of liquid in the inner container is also determined by the pressure in the vapor outlet duct, the outlet valve ensures a control of said pressure and also serves as an extra safety.

In a favorable embodiment of the cryostat according to the invention, the outside of the object to be cooled also forms the boundary of the inner container. A further favorable embodiment of the cryostat according to the invention is characterized in that the first and second liquid duct communicate, via a common communication duct, with the third and fourth liquid duct, and a control valve is incorporated in the common communication duct for controlling the quantity of the cooling liquid flowing from the liquid container to the inner container. This provides an extra possibility of controlling the flow of liquid to the inner container in such manner that the quantity supplied to the inner container corresponds to the quantity evaporated in said container.

in a further favorable embodiment of the cryostat according to the invention the control device is formed on the one hand by at least one pressure gauge which switches on a heating device for supplying thermal energy to the liquid container when the pressure in said container falls below a given minimum value, and on the other hand by a number of temperature-sensitive elements arranged on the inner container and switching off the said heating device when the temperature of the cooling liquid in the inner container exceeds the evaporation temperature which corresponds to a given maximum permissible pressure.

According to the invention, the heating device may be formed by a number of electric resistance elements which are arranged in the liquid container at different levels.

According to the invention the temperature-sensitive elements may be germanium resistance elements. The temperature of the liquid in the inner container depends upon the quantity of cooling liquid supplied to said container, and on the extent of evaporation. A temperature-sensitive element assumes a different temperature according as said element is situated in vapor or in liquid, which enables the determination of the liquid level. The determination is made independent of the position of the cryostat by using several elements preferably placed at some distance from one another.

The temperature-sensitive elements which can switch off the heating device in the liquid container determine the maximum pressure which can occur in the cryostat. Such a control system has minimum dimensions and a minimum response time while notably when helium is used as a cooling medium, the quantity of thermal energy which must be supplied to increase the pressure in the liquid container sufficiently is extremely small. The electric heating resistance elements can be supplied with low voltage and dissipate a low power of, for example, one-tenth of a watt.

In a further favorable embodiment of the cryostat according to the invention a number of radiation screens are present which are arranged within the outer container and at least substantially entirely surround the liquid container, the inner container, and the object to be cooled, the radiation screens being cooled by parts of the system of vapor ducts provided around the radiation screens and constructed as cooling spirals, the cooling spirals which communicate with the vapor outlet duct on their one side communicating on their other side with liquid-vapor separators in the first and the second liquid duct, and with the first and second vapor duct, respectively.

In this manner an extra protection against heat inleak by radiation is obtained with the efficient use of the cold still present in evaporated cooling liquid.

The circumstances in which heat exchange takes place in a cryostat on board a spacecraft vary considerably in the course of an experiment. When the spacecraft returns to the atmosphere it is sometimes necessary that the remaining quantity of cooling liquid in the cryostat is evaporated since this quantity can give a danger of explosion during the landing on earth.

In a further favorable embodiment of the cryostat according to the invention, a communication duct is present between the vapor outlet duct and the vacuum space in the outer container, in which communication duct a control valve is incorporated. By opening the control valve at the suitable instant, for example, via remote control, evaporated cooling liquid flows from the vapor outlet duct to the vacuum space, and ensures in said vacuum space a thermal conductivity which is sufi'lcient to produce a comparatively rapid evaporation of the remaining cooling liquid, without the danger of internal ice formation.

According to a further favorable embodiment of the cryostat according to the invention, the cooling liquid is helium. In this manner, cooling of objects at a very low temperature (4 K.) is possible.

In order that the invention may be readily carried into effect, one embodiment of the cryostat according to the invention will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which FIG. 1 is a cross-sectional view of a cryostat,

FIG. 2 is a cross-sectional view of the cryostat taken on the line ll--Il of FIG. 1.

Reference numeral 1 denotes a recording apparatus 1 having at one end a photocathode 2 facing an entrance window 3, and at the other end a photographic apparatus 4. The cylindrical part of the recording apparatus situated between the photocathode 2 and the photographic apparatus 4 is to be kept at a low temperature. The cylindrical outer wall of the recording apparatus 1 forms the inner wall 5 of the inner container 6 the outer wall 7 of which is also cylindrical.

Around the inner container 6 and the recording apparatus 1 a hermetically sealed annular liquid container 8 is situated which is arranged coaxially with the inner container and has substantially the same height so as to serve both as a radiation screen and as a storage container for cryogenic liquid. The assembly is housed in a hermetically sealed outer container 9 in which the entrance window 3 is incorporated which passes radiation which is of importance for the apparatus 1. Two

radiation screens

10 and 11 are situated between the liquid container 8 and the outer container 9 and fully surround the liquid container 8 and the apparatus 1 with the exception of the passage 12 opposite to the entrance window 3.

The liquid container 8 comprises a tube 13 passed through the wall of said container and having incorporated therein a valve 14, which tube, through the

radiation screens

10 and 11 and through the outer container 9, opens outside the cryostat and is provided there with a stopper 56. This pipe serves for emptying and filling the liquid container 8. The valve 14 is a maximum pressure valve and therefore serves as a safety valve for the liquid container 8. The liquid container 8 furthermore comprises an absolute pressure gauge 15 which comprises a contact which, when the pressure in the liquid container 8 falls below a given adjusted value, switches on

heating coils

52 and 53. The place of the pressure gauge is not critical for the variation in pressure as a result of the variation in the liquid level above the pressure gauge, is nearly always negligible relative to the admissible minimum pressure. Two

separators

16 and 17 in which vapor and liquid are separated from each other and which are situated at oppositely located places, communicate with the liquid container 8. The liquid flows through the ducts l8 and 19, respectively, which open into a control valve 20, after which the liquid is conducted, via

ducts

21 and 22 which open in the inner container 6 at oppositely located

places

23 and 24, respectively, to said container. From the

separators

16 and 17 vapor is conducted, via

ducts

25 and 26, respectively, to cooling

spirals

27 and 28 which cool the outermost radiation screen 11. After leaving the

cooling spirals

27 and 28, the vapor flows, via

ducts

37 and 38, into a vapor outlet duct 29 which, through the

radiation screens

10 and 11 and through the outer container 9, opens outside the cryostat and is there provided with a blowoff valve 30. This blowoff valve is a maximum pressure valve which releases the communication with the ambient air at a pressure in the duct 29 which is slightly higher than the atmospheric pressure.

The vapor which has formed in the inner container 6 is conducted at two oppositely located

places

31 and 32, via

ducts

33 and 34, to two

cooling spirals

35 and 36 to cool the innermost radiation screen 10. From the

cooling spirals

35 and 36 the vapor flows, via

ducts

39 and 40, to the vapor outlet duct 29. Two temperature-sensitive element 41 and 42 are provided on the outer wall 7 of the inner container 6 at oppositely located places. In circumstances, however, it may be of advantage to place the temperature-sensitive elements in the area which is nearest. to those parts of the apparatus 1 which must be kept at the lowest temperature. The temperature-sensitive elements in this case are direct current supplied germanium detectors but may also be semiconductor diodes. The temperature-sensitive elements can switch off the heating coils 52 and 53 by means of relays.

Inside the outer container 9 a vacuum space 43 is present. The vacuum pump can be made to communicate with said space at the area of the stopper 44. The safety mechanism already mentioned for the case in which the cryostat, after having operated in a rare atmosphere, again returns to the normal atmosphere and lands on earth, is formed in this case by a remote control valve 45 which can communicate the vapor outlet duct 29, via blowoff valve 30 and via safety valves 44, with the vacuum space 43, so that said space is then filled with a part of the evaporated liquid. The material and the dimensions of the entrance window 3 are determined by the use for which the apparatus 1 is destined. For the rest, the apparatus may comprise electrodes, which necessitates electric supply. These electrodes can be provided without difficulty on the wall 5; contact pins can then accordingly be placed in an insulated manner in the wall 7 of the inner container 6 and in the wall of the outer container 9.

It is obvious that the liquid container 8, the assembly of apparatus 1 and photographic apparatus 4, as well as the radiation screens 10 and 11, and the various parts of the ducts must be kept in place with suitable means. The necessary supports are preferably formed by thin pipes which have a maximum thermal resistance.

The cryostat can be extended with components which are useful for practical purposes. Therefore H6. 2 shows the control valve 20 provided with a rod 49 which is passed out through the outer container 9. The control valve 20 can then be operated from outside the cryostat at the area 50.

The ducts are mutually provided in such manner that all the necessary detachable connections are situated on the same side of the cryostat corresponding to the plane in which the radiation screens are mounted and to the plane 46 in which the parts of the outer container 9 are combined. As a result of this the connections are easily accessible and detachable.

The cryostat is particularly simple in operation. Filling of the liquid container 8 with cryogenic liquid is effected by siphoning or by means of a transport duct according to conventional methods. The evacuation of the vacuum space 43 is carried out also according to known methods.

During operation the control valve 20 is adjusted so that the supply of liquid corresponds to the quantity necessary to compensate for the losses of the cryostat. Too large a quantity would involve the danger of cryogenic liquid coming in the cooling spirals for the radiation screens, which is unfavorable for the thermal efficiency. Moreover, the valve 20 enables the liquid ducts to be constructed with wide diameters the advantages of which are large heat-exchanging areas and low resistances to flow.

The temperature-sensitive element 41 and 42 ensure that the heating of cryogenic liquid in the liquid container 8 is switched off as soon as the temperature exceeds the evaporating temperature associated with the permitted pressure in the system of ducts. The pressure gauge 15 ensures that, by heating of the liquid in the liquid container 8, said pressure does not fall below the permitted minimum pressure. When the cryostat is filled with liquid helium and an excessive pressure of 0.1 to 2 bar prevails relative to the ambient pressure in the cryostat by controlling the valves, the sensitive components of the apparatus I assume a temperature of approximately 4.3

K. In these circumstances independent operation of the cryostat is possible for more than hours with a liquid container having a capacity of to 30 liters placed in a cylindrical outer container having a diameter of 400 mm. and a power dissipation in the order of 1 watt. The assembly has a light weight by using the light aluminum or magnesium alloys where this is possible.

Although in the embodiment of the cryostat described a recording apparatus having a photocathode and a photographic apparatus are arranged in the cryostat, it is obvious that all kinds of other measuring and recording instruments are possible.

What is claimed is:

l. A cryostat for keeping an object at a constant low temperature comprising an outer container to be evacuated and in which a liquid container for cryogenic cooling liquid is arranged, characterized in that the said liquid container is annular and is closed and substantially surrounds the object to be cooled, an inner container which likewise substantially surrounds said object and is in thermal contact therewith, being present in the space between the annular liquid container and said object, a system of liquid ducts being present in the closed outer container and communicates with the liquid container via at least one first and at least one second liquid duct at different levels, and communicates, via at least one-third and at least one-fourth liquid duct at different levels with the inner container, :1 system of vapor ducts being furthermore present in the closed outer container and communicates via at least one first and at least one second vapor duct at different levels, with the inner container, and at least one vapor outlet duct, a blowoff valve incorporated therein which opens outside the cryostat, and a control device being furthermore present for controlling the pressure in at least the liquid container and the temperature in the inner container.

2. A cryostat according to claim 1 wherein the outside of the object to be cooled also forms the boundary of the inner container.

3. A cryostat according to claim 1 wherein the first and the second liquid ducts communicate, via a common communication duct, with the third and fourth liquid ducts, the apparatus further comprising a control valve for controlling the quantity of cooling liquid flowing from the liquid container to the inner container incorporated in the common communication duct.

4. A cryostat according to claim 1 further comprising heating means for supplying thermal energy to the liquid container, and said control device is formed by at least one pressure gauge which switches on said heating means when the pressure in said container falls below a given minimum value, and the control device further comprises a temperature-sensitive element provided on the inner container for switching off the said heating means when the temperature of the cooling liquid in the inner container exceeds the evaporation temperature which corresponds to a given maximum permissible pres sure.

5. A cryostat according to claim 4 wherein the heating means is formed by electric resistance elements which are arranged in the liquid container at different levels.

6. A cryostat according to claim 4 wherein the temperaturesensitive element is a germanium resistance element.

7. A cryostat according to claim 1 further comprising radiation screens arranged within the outer container and at least substantially entirely surrounding the liquid container, the inner container, and the object to be cooled, the radiation screens being cooled by parts of the vapor duct system provided around the radiation screens and constructed as cooling spirals, the apparatus further comprising liquid-vapor separators incorporated in the first and the second liquid duct, the cooling spirals communicating on their one side with the vapor outlet duct communicating on their other side with said separator and with the first and second vapor duct, respectively.

8. A cryostat according to claim 1 further comprising a communication duct between the vapor duct and the vacuum space in the outer container, said communication duct including a control valve 45.

9. A cryostat according to claim 1 wherein the cooling liquid is helium.

10. A cryostat apparatus for cooling an object to be cooled, the apparatus operable with evacuating means and with cryogenic cooling liquid, and comprising: a closed inner container that surrounds the object to be cooled and is in thermal contact therewith, a closed annular liquid container that generally surrounds the inner container and object to be cooled therein with said cryogenic liquid disposable therein, a closed outer container that surrounds the liquid container with an evacuable space defined within said outer container, first duct means for communicating cryogenic liquid between the outer container and the liquid and inner containers, second duct means for communicating vapor of said cryogenic liquid between the outer and inner containers, valve means for venting the second duct means externally of the apparatus, and means for controlling the pressure in the liquid container and the temperature in the inner container.

Claims

1. A cryostat for keeping an object at a constant low temperature comprising an outer container to be evacuated and in which a liquid container for cryogenic cooling liquid is arranged, characterized in that the said liquid container is annular and is closed and substantially surrounds the object to be cooled, an inner container which likewise substantially surrounds said object and is in thermal contact therewith, being present in the space between the annular liquid container and said object, a system of liquid ducts being present in the closed outer container and communicates with the liquid container via at least one first and at least one second liquid duct at different levels, and communicates, via at least one-third and at least one-fourth liquid duct at different levels with the inner container, a system of vapor Ducts being furthermore present in the closed outer container and communicates via at least one first and at least one second vapor duct at different levels, with the inner container, and at least one vapor outlet duct, a blowoff valve incorporated therein which opens outside the cryostat, and a control device being furthermore present for controlling the pressure in at least the liquid container and the temperature in the inner container.

4. A cryostat according to claim 1 further comprising heating means for supplying thermal energy to the liquid container, and said control device is formed by at least one pressure gauge which switches on said heating means when the pressure in said container falls below a given minimum value, and the control device further comprises a temperature-sensitive element provided on the inner container for switching off the said heating means when the temperature of the cooling liquid in the inner container exceeds the evaporation temperature which corresponds to a given maximum permissible pressure.

6. A cryostat according to claim 4 wherein the temperature-sensitive element is a germanium resistance element.

9. A cryostat according to claim 1 wherein the cooling liquid is helium.