US3626706A

US3626706A - Cryostat

Info

Publication number: US3626706A
Application number: US22846A
Authority: US
Inventors: Albrecht Elsner; Gustav Klipping
Original assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Current assignee: Max Planck Gesellschaft zur Foerderung der Wissenschaften eV
Priority date: 1966-12-24
Filing date: 1970-03-26
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14
Also published as: DE1913788A1; FR1551304A; DE1913789B2; FR2038420B1; NL6919393A; GB1312511A; GB1310766A; US3620033A; CH499757A; FR2035082A2; NL7000508A; CH499072A; FR2035082B2; DE1913789A1; CH498351A; NL7000576A; GB1183767A; DE1918624A1; DE1918624B2; GB1229767A

Abstract

LIQUID HELIUM FLOWS DOWNWARDLY THROUGH AN ELEMENT HAVING A PORE SIZE OF LESS THAN 10**4 CM. A CONTROLLED VACUUM IS PLACED AT THE LOWER END OF THE ELEMENT. THE LIQUID HELIUM IS COMPLETELY EVAPORATED AT THIS LOWER END. THE GASES EMITTED AT THE LOWER END HAVE A TEMPERATURE DETERMINED BY THE CONTROLLED VACUUM. IN THIS WAY, A CRYOGENIC COOLING OF A DETERMINED TEMPERATURE IS MADE AVAILABLE.

Description

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United States Patent O U.S. Cl. 62-62 15 Claims ABSTRACT OF THE DISCLOSURE Liquid helium flows downwardly through an element having a pore size of less than cm. A controlled vacuum is placed at the lower end of the element. The liquid helium is completely evaporated at this lower end. The gases emitted at the lower end have a temperature determined by the controlled vacuum. In this way, a

cryogenic cooling of a determined temperature is made available.

BACKGROUND OF THE INVENTION The present invention relates to an apparatus for the low temperature cooling of various objects and to a method of operating such apparatus. Preferably,the temperatures are below 2.17" K. and a replenishable helium bath is used.

Temperatures below 2.17 K. (A-point of helium) have been achieved in the past by controlled lowering of the pressure above a bath of liquid helium. The object to be cooled is generally submerged in the bath. One apparatus using such principles includes the feature of automatic replenishment of a helium-II bath standing under reduced pressure. Two helium baths are situated separated from one another in such a cryostat. One of these baths, the work bath, cools the object to be cooled, while the other bath serves for replenishment of the working bath. Treatment of this apparatus is given in the article of A. Elsner, G. Hildebrandt, and G. Klipping, in Dechema-Monographie, volume 58 (1968), pages 9-16, and in the article of A. Elsner and G. Klipping in Advances in Cryogenic Engineering, 416, volume 14 (1968). Such an apparatus makes it possible to hold objects over extended periods of time at temperatures in the'range below 2.17 K., but involves considerable expense, since two liquid baths are required, as well as two pumps and two control circuits. Accordingly, operation is complicated and there are many possibilities for disturbance of operation.

SUMMARY OF THE INVENTION An object of the present invention therefore is to provide a simpler apparatus for the continuous cooling of objects at temperatures down to below 2.l7 K. using a helium bath. This, as well as other objects which will become apparent in the discussion that follows are achieved according to the present invention by providing the container for the helium bath with an evaporating element situated at the lower end of the container. The evaporating element is made of a porous material having a pore size smaller than 10* cm. Suitable such material is discussed in US. Pat. No. 3,442,091, issued on May 6, 1969, to Gustav Klipping, Albrecht Elsner, and Gerd Hildebrandt for Delivery of Coolant to Cryostats. The length of the evaporating element is chosen such that a complete evaporation of liquid helium moving downwardly through the evaporating element to its external surface located in a vacuum chamber is achieved. The apparatus of the present invention has the 3,625,?05 Patented Dec. 14, 1971 ice advantage that only one helium bath is needed, in order to obtain extended operation at desired low temperatures, and that only one pump is needed. Construction and operation of the apparatus are therefore simplified.

According to a preferred embodiment of the present invention, the length of the evaporating element is greater than 3 cm. This length has been found to give a complete evaporation of the helium flowing downwardly through the evaporating element-even for higher pressure above the helium bath, for example one atmosphere.

According to another preferred embodiment of the present invention, the helium bath is in heat exchanging relationship with the cold gas flowing from the evaporating element. In this way a cooling of the helium bath using the low temperature of the cold gas is achieved. This contributes to economical operation of the apparatus.

In order to achieve a uniform cooling of the object to be cooled, it is advantageous to provide equipment for holding the object in the flow of gas evaporating from the external surface of the evaporating element and in direct heat conducting contact with the outer surface of the evaporating element.

In certain cases, it can be useful if the equipment for holding the object to be cooled is a piece of high heat conductivity material, for example copper, set into the wall of the vacuum vessel in the region of the external surface of the evaporating element of the invention. Besides providing means onto which an object to be cooled can be mounted, this piece of high conductivity material is generally useful as a cooling surface for various other purposes in cryogenics. In this embodiment of the invention, the object to be cooled is separated from the evaporating element. Preferably, the object to be cooled is placed outside of the coolant circulatory system, so that it can be easily gotten at, without necessitating an opening of the portion of the cryostat containing helium. It is furthermore advantageous to make the vacuum vessel, which carries the piece of high heat conductivity, out of a material of poor heat conductivity, preferably steel.

An apparatus according to the present invention has, when compared with known apparatus for the cooling of objects to temperatures below 2.17 K., numerous advantages. It has, for example, a simpler construction,

since only one helium bath is used and one pump suf fices to obtain the desired temperature in the object to be cooled. The temperature of the helium bath (or in other words the pressure above the bath) does not need to be held constant. It can vary between the boiling temperature of helium at one atmosphere pressure (4.2" K.) and lower values without influencing the temperature of the object to be cooled. Likewise, level variations of the helium bath have no influence on the temperature of the object. The pressure, and thus the temperature, control is done with simpler means than those used in previously known apparatus. On the whole, operation of the apparatus of the present invention is simpler and more certain than was the case with apparatus previously used. Furthermore, the apparatus of the present invention is simpler and cheaper to build, because of its inherently less expensive design.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section of a cryostat according to the present invention, the plane of the cross section containing the cylindrical axis of the cryostat.

FIG. 2 is a view as in FIG. 1 showing an alternative embodiment of only the lower part of the cryostat of the present invention.

FIG. 3 is a view as in FIG. 1 showing only the lower part of an alternative embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, the fine-pored evaporating element 1 is set into container 2 for the helium bath 3 in such a manner that the upper end of the evaporating element 1 forms the floor of the container 2. The container 2 is built in the form of a heat exchanger 4 in its portion immediately above the evaporating element 1. This portion is intended to receive the helium bath 3.

The heat exchanger 4 has, in the particular embodiment shown in FIG. 1, the form of a ribbed tube whose ribs 5 are provided with holes spaced around the tube and off-set with respect to one another so that it is not possible to sight through them. According to other embodiments of the present invention, the heat exchanger 4 can, for example, be a porous body made using the techniques of powder metallurgy. Heat exchanger 4 is always arranged so that cold gas streaming from the external surface 6 of the evaporating element 1 flows onto its surfaces. In this way helium bath 3 is cooled by such cold gases.

The container 2 with the evaporating element 1 is suspended from the lid 7 of the cryostat and is surrounded by a vacuum vessel 8 which is also carried by lid 7. A radiation protection shield 9 is attached to the vacuum vessel 8. This shield is cooled to about 100 K. by heat exchange with the cold gas streaming from the external surface 6 through the vacuum vessel 8. It is naturally possible to use a plurality of such radiation protection shields one around the other. Other conventional heat insulation can likewise be used. The apparatus of the present invention is encased in the external cryostat housing 10, which is closed by lid 7 and which can be evacuated through closure valve 11 to provide an insulating vacuum.

An exhaust gas line 13 opens into the vacuum chamber 12 of vacuum vessel 8. Pressure controller 14 is provided for adjusting and controlling the pressure in the vacuum chamber 12. A vacuum pump is connected to exhaust gas line 13 to bring the pressure within the vacuum chamber 12 to that corresponding to the temperature desired for cooling at test object. The constancy of the pressure in the vacuum chamber 12 is maintained using automatic control equipment to operate pressure controller 14. The action of such automatic control equipment on the pressure controller 14 is indicated by dashed control line 15. The automatic control equipment senses the temperature of the test object, for example. The sensing of other parameters is possible for example, pressure itself can be sensed. Various types of pressure controllers 14 With sufficiently high sensitivity can be used. The automatic control equipment is illustrated in FIG. 1 by a labelled rectangular box, since its detailed illustration is not essential for a proper understanding of the invention.

Additionally passing through lid 7 is a vacuum-jacket syphon 16 carrying an expansion valve 17 on its lower end. Suitable examples of such syphons are shown in US. Pat. No. 3,442,091, issued to Gustav Klipping, Albrecht Elsner, and Gerd Hildebrandt on May 6, 1969, for Delivery of Coolant to Cryostats. The uppermost arrow in FIG. 1 indicates the flow of liquid helium down through a pipe in A the center of the vacuum-jacket syphon 16 while the arrow directed upwardly just below the uppermost arrow indicates the flow of exhaust .gas from helium bath 3 flowing through the exhaust gas tube C in the outer jacket D of the vacuum-jacket syphon 16. As the exhaust gas tube in the outer jacket is connected to a radiation shield B surrrounding the liquid carrying pipe A, the radiation shield B thus being cooled, a radiation protection for liquid helium flowing in the central pipe is achieved. Level sensor 18 emits a signal to open expansion valve 17 when the level in helium bath 3 has fallen 4 a predetermined amount and liquid helium is brought into the helium bath 3 from a storage vessel, for example, in the manner shown in U.S. Pat. No. 3,442,091.

Bracket 1% serves for the mounting of test objects to be cooled. The connection of bracket 19a to the external surface 6 of evaporating element 1 is such that heat is easily conducted between the evaporating element 1 and the bracket 19a. Depending upon the particular type of material used to construct evaporating element 1, the mounting of bracket 19a can be effected by gluing, soldering, brazing, etc, To obtain a good heat conducting connection of the test object to the bracket 19a the test object 29 can be screwed, soldered, brazed, or glued to the bracket 19a.

Various materials can be used to make the fine-pored evaporating element 1, provided they have pore sizes less than 10- cm. Examples are aluminum silicate, alumina, coal, fritted glass, and bodies of sintered nickel, silver, and copper powders. The length of the evaporating element 1 from the floor of the helium bath 3 to the external surface 6 depends firstly on the maximum pressure to be expected in container 2 above the helium bath 3 and on the various temperatures desired for the test object. Especially at working temperatures near 217' K., the exceeding of certain pressure values in the container 2 above the bath 3 can lead to an undesired flow of fluid from the external surface 6 of the evaporating element 1, should the length of the evaporating element 1 be too small. Lengths of more than 3 cm. have been found to eliminate this possibility with certainty.

FIG. 2 shows an alternate embodiment of the lower part of FIG. 1. This alternate embodiment provides a different means for holding a test object. Thus, in place of bracket 19a there is provided plate 19b, which is so placed in the floor of vacuum vessel 8, that it is directly in the path of cold gas evaporating from the external surface 6 of evaporating element 1. A test object 30 mounted on the bottom side of this plate 1% in the manner given above for bracket 19a is thus cooled and held at the desired working temperature. Plate 191) is formed from a material having a high heat conductivity, for example copper, while the vacuum vessel 8 itself is made of a poor heat conducting material, for example steel.

In this embodiment of FIG. 2, the radiation protection shield 9 is preferably designed so that it can be removed from the vacuum vessel 8. Then, the test object to be mounted on the plate 19b can be most easily reached by removal of the cryostat housing 10 and the radiation protection shield 9. The parts of the apparatus of the present invention belonging to the helium circulation system do not in this case need to be opened.

FIG. 3 shows a third embodiment of the lower part of the apparatus of the present invention. Plate 20, which forms the floor of vacuum vessel 8 and which is made of copper, is likewise placed in the path of cold gases evaporating from the external surface 6 of the evaporating element 1. Plate 20 is used here to form a cryopump and provides a cold surface for the condensation of gases. Accordingly, the surrounding radiation protection shield 9 is provided in the region of its floor With a Well-known chevron system 21 which is likewise cooled by heat conductivity. A larger receptacle serves as the cryostat housing 10 and this is evacuated with the help of the cryopump action of plate 20.

In operation, the helium bath 3 is filled and replenished by way of syphon 16 from a storage vessel. The expansion valve 17 may be operated by hand or preferably automatically with the help of level sensor 18. The vacuum in vacuum vessel 8 is brought to the particular pressure corresponding to the desired working temperature for the test object and then held constant. For example, for a temperature of T=1.7 K., a pressure of 8.6 mm. of mercury is used. The liquid helium from the bath 3, above which the pressure is higher (for example, pressure=atmospheric pressure) than the pressure present in vacuum chamber 12, flows through the fine-pored evaporating element and completely evaporates on the external surface 6 of the evaporating element 1. As a result of this evaporation, a cooling occurs at the external surface 6 of the evaporating element 1, until the temperature corresponding to the pressure in vacuum vessel 8 is reached. Consequently, a temperature gradient arises in the evaporating element 1 and in the liquid helium held in its pores. The cold helium gas arising at the external surface 6 of the evaporating element 1 streams through the heat exchanger 4, thus cooling the helium bath 3, and finally is pumped out through exhaust line 13. The offset of the holes of heat exchanger 4 relative to one another provides improved contacting of the ribs 5 by the cold gas. The particular working temperature can be held constant as long as desired.

Since the helium bath can be held to a very small volume (for example, to 50 cmfi), the apparatus is no usual bath cryostat. It exhibits the advantages of a helium evaporation cryostat. Upon ending or interrupting operation of the apparatus of the present invention, only minimal coolant losses occur and it is possible to interrupt operation without the time loss determined in bath cryostats by the evaporation of the coolant. Minimum cooling times are achieved and a minimum consumption of coolant for cooling results, since there are no dead volumes to be cooled. Coolant consumption during steady-state operation is correspondingly small. A continual operation of any desired length of time is possible because of the simple yet exact manner in which the apparatus of the present invention operates.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. A method suitable for the cooling of objects at temperatures below 2.l7 K. using a replenishable helium bath, comprising the steps of filling liquid helium into a container to form a bath, the floor of which container is an evaporating element having pores of a size less than 10- cm., creating a vacuum on the side of said evaporating element opposite to that contacting said helium bath, and completely evaporating helium moving downwardly through said element from said container as it passes from said element into said vacuum.

2. A method as claimed in claim 1, further comprising the step of transferring heat from said bath to gaseous helium evaporated from said evaporating element.

3. A method as claimed in claim 1, further comprising the step of placing an object to be cooled in heat conductive contact with the side of said evaporating element opposite to that contacting said helium bath.

4. A method as claimed in claim 1, further comprising the step of bounding the vacuum With a vacuum vessel and with a plate situated in the flow of helium evaporated from said evaporating element and forming one portion of the walls of said vacuum vessel.

5. A method as claimed in claim 4, the heat conductivity of said plate being higher than that of the remainder of said vessel.

6. A method as claimed in claim 5, further comprising the step of mounting an object to be cooled in heat conductive contact With the side of said plate opposite to said vacuum.

7. A method as claimed in claim 5, further comprising the step of condensing gas on the side of said plate opposite to said vacuum, in the presence of the interior of an evacuable housing.

8. Apparatus for the cooling of objects, suitable for temperatures below 2.l7 K. using a replenishable helium bath, comprising a vacuum chamber, a helium bath container extending downwardly into said vacuum chamber, an evaporating element situated within said vacuum chamber and forming the lower end of said helium bath container, said evaporating element having a pore size smaller than 10 cm., the length of said evaporating element from the lower surface of a helium bath in said container to the face of said element exposed to the interior of the vacuum chamber being such that helium moving downwardly through the evaporating element to such face completely evaporates at such face.

9. An apparatus as claimed in claim 8, said length of the evaporating element being greater than 3 cm.

10. An apparatus as claimed in claim 8, further comprising means for transferring heat from a helium bath in said container to helium evaporated at such face.

11. An apparatus as claimed in claim 8, further comprising means for holding an object to be cooled in the gas evaporating from such face and in direct heat conductive contact with such face.

12. An apparatus as claimed in claim 8, further comprising means in the wall of said vacuum chamber situated in the fiow of helium gas evaporating from such face for providing a cold piece.

13. An apparatus as claimed in claim 12, said piece being of material of higher heat conductivity than the remainder of said vacuum chamber.

14. An apparatus as claimed in claim 13, further comprising an object in heat conductive contact to said piece on the side of said piece opposite to the interior of said vacuum chamber.

15. An apparatus as claimed in claim 13, further comprising an evacuable housing and means for mounting said evacuable housing relative to said piece such that the interior of said evacuable housing is in communication with the side of said piece opposite to the interior of said vacuum chamber, whereby a cryopump is created.

References Cited UNITED STATES PATENTS 3,424,230 1/1968 Wright 62-514 3,391,546 7/1968 Campbell 62514 3,410,110 12/1968 Hoyes 62514 3,447,333 6/ 1968 Goodstein 625 14 MEYER PERLIN, Primary Examiner US. Cl. X.R. 62-514