US3442091A - Delivery of coolant to cryostats - Google Patents

Description

y 6, 1969 G. KLIPPING ET AL 3,442,091

DELIVERY OF COOLANT TO CRYOSTATS Filed Nov. 1, 196'? Sheet 1 of s FIG.|.

mvsurons Qusluv Kllppinq Albrecht Eisner 6 Gerd Hildebrand! ATTORNEYS TO CRYOSTAIS Sheet 2 of3 I N VE N TORS Albrecht Eisner 8 Gerd Hildebrundt Gustav Klipping G. KLIPPING ET AL May 6, 1969 DELIVERY OF COOLANT Filed Nov. 1, I967 ATTORNEYS May 1969 G. KLIPPING ET AL 3,442,091

DELIVERY OF COOLANT TO CRYOSTATS Filed Nov. 1, 1967 Sheet 5 of 3 INVENTORS Gustav Klipping Albrecht Eisner 8 Gerd Hildebrundt FIG.3. WM; 7%

ATTORNEYS United States Patent M 7 Int. Cl. F17c 7/02; F25b 19/00 US. Cl. 62--55 19 Claims ABSTRACT OF THE DISCLOSURE Method and apparatus for permitting the continuous replenishment of the helium II working bath of a cryostat while maintaining a stable temperature, particularly in the range below the helium )t-point, by delivering liquid helium to a replenishing bath maintained at a low pressure and held in a container at least one portion of which is constituted by a filter which is at least partially impermeable to gaseous helium and helium I and to helium II at a temperature above that of the working bath, but which passes helium 11 having a temperature equal to or less than that of the working bath, the filter separating the replenishing bath from the working bath.

Background of the invention The present invention relates to the maintenance of extremely low temperatures and is concerned with a method and apparatus for the continuous low temperature COOling of objects in a liquid bath, particularly below 2.17 K. and employing a He-II bath, in which the temperature is set by pressure regulation in the vapor chamber above the helium bath and replenishment is intermittently accomplished via a vacuum-jacketed siphon from a helium storage vessel in dependence on the level of the helium bath.

It is known that temperatures below 42 K. can be generated by evaporation of liquid helium under reduced pressure. Since for many purposes, particularly for scientific experiments, temperatures below 4.2 K. must be maintained constant over long periods of time, devices have been developed in which, for example, the normal boiling helium is replenished by delivering helium through a valve, whereby it undergoes partial evaporation and thus cooling, into the bath which is boiling under a reduced pressure. (See: J. Nicol, H. V. Bohm, Advances in Cryogenic Engineering, vol. 5, p. 332 (Plenum Press, Inc, New York 1960); H. H. Madden, H. V. Bohm, Review of Scientific Instruments, No. 35, p. 1554 (1964)). These devices, however, have the common disadvantage that the establishment of temperatures below the )\-point (2.17 K.) is extremely difficult and that it is practically impossible to maintain a constant temperature in this range.

It is also known that at 2.17" K. helium I changes into helium II which has, in certain respects, completely different properties. Helium II, for example, possesses an abnormally high thermal conductivity which is times as high as that of helium I and ten to one hundred times as high as that of the purest metals. Furthermore, helium II shows the property of superfiuidity not encountered in any other liquid.

The primary reason for the great difficulties in the replenishment of He-II baths is probably that during replenishing it is always warm gas (at the onset of the replenishing process when the siphon must be cooled, the temperature is 4.2 K.; during the process, the temperature is -4.2 K.) which flows into the cryostat. This gas inevitably at least partially condenses on the cold liquid whereby condensation heat is delivered to the bath and its temperature thus rises. In the case of a helium I bath, i.e., at temperatures between 4.2 K. and 2.17 K., the temperature of the entire bath does not increase but rather, due to the poor thermal conductivity of the bath, only a surface layer thereof becomes warmer. Replenishment via an expansion valve is therefore entirely possible while at the same time maintaining the bath temperature constant.

A He-II bath, however, which can reach temperatures of 2.17 K. and less, possesses the already mentioned extremely high thermal conductivity so that any increase in temperature, for example, because of condensation thereinto of a warmer gas, is immediately transferred to all of the liquid. It is therefore impossible to maintain this liquid at a constant bath temperature if replenishment is carried out in the known manner.

Summary of the invention It is a primary object of the present invention to eliminate these drawbacks and difliculties.

It is a more specific object of the present invention to permit the continuous replenishment of a helium bath, which is maintained at temperatures below the )\-point (2.17 K.) by means of pressure reduction, while maintaining the bath temperature relatively constant during continuous operation.

Another object of the present invention is to permit a single cryostat device to operate at temperatures either above or below the helium lt-point.

Yet another object of the present invention is to automatically regulate the delivery of liquid helium to a bath cryostat in dependence on the liquid level in the cryostat.

A further object of the present invention is to permit a bath cryostat to be rapidly brought to the desired low temperature at the start of operation.

These and other objects according to the present invention are achieved by certain improvements in apparatus for the continuous low temperature cooling of objects in a helium II working bath, which apparatus includes a Working bath chamber for containing the Working bath, means for maintaining a reduced pressure in the Working bath chamber, and a vacuum-jacketed siphon for periodically replenishing the working bath from a helium storage vessel. The improvement according to the present invention resides in the provision of a replenishing chamber disposed at least partially in the working chamber above the working bath and arranged for receiving the liquid helium delivered by the siphon. The replenishing chamber according to the present invention includes a filter element forming at least a portion of the lower extremity of the replenishing chamber and separating the interior of the replenishing chamber from the working bath, the filter element being of a material which is at least partially impermeable to gaseous helium and helium I as well as to any helium II having a temperature higher than that of the working bath. The improvement according to the present invention further includes means for maintaining a reduced pressure in the replenishing chamber.

The objects according to the present invention are also achieved by a method for maintaining a helium II working bath at a stable temperature below the helium A-point while periodically replenishing the bath via a siphon in a device which includes a replenishing chamber disposed above the working bath and having at least a portion of its lower extremity composed of a filter element which is at least partially impermeable to gaseous helium and helium I and to any helium II at a temperature higher than that of the Working bath. The method according to the present invention includes the steps of delivering liquid helium via the siphon into the replenishing chamber,

maintaining the interior of the replenishing chamber at a reduced pressure corresponding to a temperature below the X-point for permitting helium II which is at a sufficiently low temperature to pass through the filter element in a vapor-free manner to the working bath, and maintaining the working bath at a reduced pressure corresponding to the desired stable temperature.

The present invention further involves a method for maintaining a low temperature above the helium A-point in low temperature apparatus including a bath cryostat for containing a liquid helium bath, a replenishing chamber whose bottom is provided with a controllable bypass valve and a filter which is at least partially impermeable to gaseous helium and helium I and to helium II above a certain temperature, a siphon whose outlet end is obturated by a controllable expansion valve, and a single pump connected to the cryostat and the replenishing chamber. This method involves the steps of maintaining the bypass valve open for permitting all of the liquid in the replenishing chamber to flow directly to the liquid helium bath, operating the pump for maintaining the cryostat and the replenishing chamber at a predetermined reduced pressure corresponding to the desired low temperature, and opening the expansion valve for delivering liquid helium to the replenishing chamber whenever the liquid helium bath is below a preset level and closing the expansion valve when the helium bath reaches such preset level.

The present invention additionally includes a novel method for beginning operation of ,low temperature apparatus which is to operate at temperatures below the helium A-point and which includes a bath cryostat for containing a helium II working bath, a replenishing chamber whose bottom is disposed above the region to be occupied by the bath and is provided with a controllable bypass valve and a filter which is at least partially impermeable to gaseous helium and helium I and to helium II whose temperature is greater than that of the working bath, and a siphon supplied with liquid helium to be delivered to the replenishing chamber and having its end obturated by a controllable expansion valve. This method involves the sequence of steps of opening both the bypass valve and the expansion valve while maintaining both the cryostat and the replenishing chamber at the same reduced pressure in order to deliver liquid helium from the siphon directly to the cryostat through the replenishing chamber, closing the bypass valve when the helium working bath reaches a predetermined level, and thereafter maintaining the working bath and the replenishing chamber at separately controlled reduced pressures.

The present invention is characterized in that the outlet of the vacuum-jacketed siphon opens into a replenishing chamber which is kept under reduced pressure and which is closed off by a filter element at least partially impermeable to gaseous helium, helium I as well as helium II at a temperature higher than that of the helium II bath and wherein the outlet of this filter element opens into the bath chamber, also kept under reduced pressure and containing the liquid bath used for the cooling process. In such a device it is guaranteed that no vapor created during the replenishing process can condense into the He-II working bath to be collected below the filter element. This vapor cannot pass through the filter element and is extracted from the area above the filter element by means of suction. Furthermore, no liquid which is warmer than the He-II bath can enter the bath since the filter is impermeable thereto also.

It is further proposed in an advantageous embodiment of the present invention to use a porous material with a pore size of less than 1O cm. 1a) as the filter material. Filters with such small pores are generally impermeable to gaseous helium and to helium I, whereas helium II at a sufiiciently low temperature will pass through them without difficulty.

Brief description of the drawings FIGURE 1 is a schematic representation of an apparatus according to the present invention and including a cryostat connected, via a siphon having an expansion valve, to a coolant storage vessel, which cryostat is provided with a partition containing a filter and a bypass valve.

FIGURE 2 is a cross-sectional, elevational view, to an enlarged scale, of one form of construction for the cryostat-filter-siphon assembly according to the present invention.

FIGURE 3 is a cross-sectional detail view of another form of construction of the filter-siphon assembly according to the present invention.

FIGURE 4 is a cross-sectional detail view of a portion of a modified partition according to the invention.

Description of the preferred embodiments FIGURE 1 shows a conventional glass cryostat 1 consisting of a double-walled vacuum-insulated outer container 2 filled with a liquid nitrogen bath 3 and of a double-walled, vacuum-insulated inner container 4 in which is maintained the He-II working bath 5 which is to be replenished at some constant, presettable temperature having a value below 2.17 K.

The inner container 4 is gastightly closed by means of a cover 6. An exhaust line 7 is connected to the cover 6 and is in communication, via a valve 8, with a vacuum pump 9, the pump itself having its high pressure side con nected via a connecting flange 10 to a helium recovery system (not shown).

A cup-shaped partition 11 is suspended in a gastight manner from the interior of cover 6 and is located so that the connection of the exhaust line 7 is outside of the partition 11. At the bottom of the partition 11 there is disposed a filter 12 having very fine pores, preferably with a pore size l0 cm.

According to one suitable embodiment of the present invention, clay can be used as the filter material. Clay members with pore sizes of approximately 10* cm. are available and, since they possess poor thermal conductivity compared to metals, they can serve to minimize heat transfer from the replenishing chamber 16 to the bath 5.

It can further be advantageous to make the filter 12 of a sintered metal body in which the pore size is preferably decreased on one surface by electrodepositing a metal layer thereupon. By electrodepositing metal on one surface of such body it is possible to adjust the pore sizes of commercial sintered metals in an exact and reproducible manner to the required special operational conditions of a given cryostat assembly. Sintered metal members having the required small pore sizes are also available and have the advantage of being easily workable since they can be mechanically treated in the same way as compact metals and can be soldered and shrunk.

The construction of the filter 12 in the form of disc or fiat plate permits a uniform flow of helium II through the filter across the entire area thereof. However, it might also be desirable in certain cases to give the filter the form of a hollow body, or cup. This would permit the effective filter surface area to be increased in those cases where, for construction reasons, the diameter of a filter disc could not be made sutficiently large.

The mounting of filter 12 on partition 11 has the advantage of permitting the filter to be easily installed at the most favorable location in the cryostat.

At the bottom of the partition 11 there is further dis posed a valve 13 for bypassing the filter 12, the movable valve element of valve 13 being connected to an operating element 14, such as a rod, for example, consisting of a material, such as high-grade steel, having poor heatconducting properties and extending in a gastight manner to the outside through the cryostat cover 6, i.e. valve 13 can be operated externally. The provision of the controllable valve 13 permits the cryostat to be rapidly and easily filled at the start of operation so that an economical use is made of the coolant.

It has been found to be advantageous to construct the partition 11 in the form of a cup with the filter 12 and the valve 13 disposed at the bottom of the cup because this represents a simple form of construction in which the partition can be suspended from'a suitable support, such as cover 6, and in which both the filter and the valve are disposed at the lowest point of the cup.

When the partition is mounted on the cover of the cryostat, it is possible to adapt available cryostats in the most simple manner to the requirements of the present invention and to utilize available siphons.

On the other hand, it might also be desirable to mount the partition at the end of the siphon since this enables the present invention to be economically employed in those cases where the cryostat which is to contain the He-II working bath is, for example, part of a larger piece of equipment which is so constructed that it would involve a considerable expense to incorporate the filter element into the cryostat. The mounting of the partition and the filter at the end of the siphon results in a helium II siphon which can be inserted into any desired cryostat in order to achieve the advantage of the present invention, and such a form of construction will be described in greater detail below with reference to FIGURE 3.

The partition 11 and filter 12 define a replenishing chamber 16 which is separated by the partition 11 from the remaining interior of the cryostat 1, and which contains a helium replenishing bath 17. Bath 17 is supplied from storage vessel 20 via the siphon 18 provided with an expansion valve 19. The siphon 18 extends into the cryostat 1 and forms a gastight seal with cover 6 by means of sealing elements 21 which may be of any known type.

The siphon 18 is, as will be described in detail below with reference to FIGURE 2, provided in a known manner with an exhaust gas cooled thermal radiation shield, ie the cold gas evaporating from bath 17 in the replenishing chamber 16 is removed from the cryostat by means of a vacuum pump 22 through an extraction line 24 within the tubular siphon jacket 23 (not shown in FIGURE 1). A valve 25 is disposed in the siphon extraction line 24 and flange 26 serves to connect the output of vacuum pump 22 to a helium recovery system (not shown).

It is desirable to have the evacuation of the replenishing chamber 16 and of the bath chamber separately controllable, preferably through the intermediary of separate valves, either by means of two separate vacuum pumps, as shown in FIGURE 1, or by a common vacuum pump, as desired, because this permits the regulation of either the pressure and thereby the temperature of the He-II bath 5 to be effected at temperatures below 2.l7 K. independently of the pressure regulation of the replenishing chamber 16. On the other hand, during the cooling process, and in the temperature range above 2.17 K., the bath chamber and the replenishing chamber can be connected to a common pump serving as a pressure temperature regulator.

Two electrical level sensors 27 and 28 for liquid helium are disposed in the cryostat 1, sensor 27 being disposed in the bath chamber 15 and sensor 28 being disposed in the replenishing chamber 16. The electrical leads 29 and 30 for the level sensors 27 and 28 are also brought through gastight seals in the cover 6 to the outside and are connected to an electrical switching circuit 31 shown in block form. A control line 32 leads from the circuit 31 to the expansion valve 19, which is preferably of the electromagnetic type. The arrangement of circuit 31, which is composed of wellknown components, will be readily apparent to one skilled in the art.

The electric level sensor 27 for liquid helium, which sensor is adjustable in height and is disposed in the bath chamber 15 below the exit level of the filter, can be employed in cooperation with sensor 28 to cause circuit 31 to synchronize the rate of delivery of liquid helium to chamber 16 with the level of liquid present in the He-II Working bath 5 as well as with the level of liquid in the replenishing bath 17 above the filter, this being accomplished by having circuit 31 suitably control the opening and closing of expansion valve 19 in dependence on the heights of the two liquid levels.

The apparatus of FIGURE 1 further includes a bypass line 66 connected between the extraction, or exhaust, lines 7 and 24 (line 24 not being visible in FIGURE 1) and containing a valve 67. The pressure gauges required for the automatic pressure/temperature regulation of baths 5 and 17, the pressure regulating device, and the control lines to the valves 8 and 25 are not shown for reasons of clarity and because their nature, mode of connection and operation will be readily apparent to one skilled in the art.

In addition, the storage container 20 is constructed in the usual manner as a vacuum-insulated double container having an inner helium container 33 surrounded by an outer container 34 filled with liquid nitrogen. An exhaust gas line for the helium container 33 is connected, via the connecting line 35, to the helium recovery system (not shown).

FIGURE 2 shows a particular embodiment of the present invention and employs the same reference numerals for elements identical with those shown in FIGURE 1. A cryostat 1 with the HeII working bath 5 is provided, as is the cryostat cover 6 provided with a cover seal 36 and the exhaust line 7. The cover 6 is firmly fastened to the inner container 4 by means of conventional fastening elements (not shown). The leads 29 of level sensor 27 are here brought to the outside via the interior of exhaust line 7. The cup-shaped partition 11 having the filter 12 and valve 13 at its bottom here, too, is supported by cover 6 and partition 11 is in the form of a double wall whose annular space 37 is sealed at both ends and can be evacuated via a valve 38. The upper end of partition 11 is gastightly covered by a cover plate 39 having a collar 21 provided with an O-ring into which the siphon 18 is inserted.

The siphon 18 consists, in a known manner, of the evac uable jacket 23 which encloses a liquid-containing inner pipe 41 surrounded by a thermal radiation shielding tube 40, an exhaust line 24 in communication with the radiation shielding tube 40 and arranged for drawing off the exhaust gases coming from the bath 17, and a guide tube 42 for the operating rod 43 of the movable element 45 of expansion valve 19. The inner tube 41 opens into the upper portion of member 44 of the expansion valve 19, which member forms a valve seat for element 45. By displacing the movable valve element 45 in the axial direction, the expansion valve 19 can be opened or closed. The expansion valve 19 is covered at its bottom by a hollow member 46 made of sintered metal and closed on all sides.

The exhaust line 24 in the interior of siphon 18 is connected to the vacuum pump 22 (not shown in FIGURE 2). An exhaust gas line 48 which is additionally inserted into the cover plate 39 and which contains a valve 47 is also in communication with vacuum pump 22. The electrical leads 30 of the level sensor 28 disposed in chamber 16 are brought to the outside through the interior of this exhaust gas line 48.

In addition to the O-ring collar 21 and the exhaust line 48, the operating element 14, here in the form of a rod, for the valve 13 disposed at the bottom of partition 11 is inserted through cover plate 39 by means of a further 0- ring sealing member 49.

Thermal insulation elements 50 and 51 below the cover 6 and the cover plate 39, respectively, serve to reduce the influx of heat into the cryostat from the region thereabove.

FIGURE 3 shows as a further embodiment of the present invention a helium II syphon. The partition 11 is mounted on the end of the siphon to be inserted into the cryostat. The siphon jacket includes a spherical enclosure 52 at the siphon elbow. The exhaust gas line 24, the radiation shielding tube 40 and the liquid-cntainir1g inner tube 41 traverse this enclosure in the elbow region.

FIGURE 3 further shows the guide tube 42 and the operating element 43 for the expansion valve 19, both the tube and the operating element extending to the externally disposed electromagnetic control unit 53 for the expansion valve 19 via the enclosure 52. Unit 53 may be of any well-known type and may be constituted, for example, by a simple solenoid. The partition 11 containing filter 12 and valve 13' is attached to the lower end of the jacket tube 23 of the siphon.

The valve 13' is here constructed as a disc valve and the bottom plate 54 of the partition 11 is provided with a bore 55 which is closed by a valve plate 56 constituting the movable portion of valve 13'. The valve plate 56 is pressed by a spring 57 against the bottom plate 54 when the valve 13 is closed. A housing 58, partially open at the top, surrounding valve 13 and attached to bottom plate 54 serves as the upper abutment for spring 57. The valve plate 56 is connected to a wire 59 (e.g., of highgrade steel) which is brought, together with a guide tube 60 partially disposed in the exhaust gas pipe 24, to the outside to a connecting piece 61 mounted on the enclosure 52.

The guide tube 60 is firmly inserted into an intermediate piece 62 Which is inserted in the connecting piece 61 so that the interior of connecting piece 61 up to the intermediate piece 62 is part of the evacuated interior of the siphon jacket. The portion of the guide tube 60 extending beyond the intermediate piece 62 is brought to an end piece 63 which is slidable on, and coaxial with, tube 60, the wire 59 being rigidly connected to piece 63. A bellows 64 is attached at one end to the intermediate piece 62 and at its other end to the end piece 63, which bellows seals the helium chamber from the atmosphere.

The end piece 63 is slidably mounted in a suitably shaped counterpiece 65 carried by a sleeve 68, which is in turn screwed onto the connecting piece 61. When the sleeve 68 is screwed in or out relative to piece 61, the end piece 63, the wire 59 and the valve plate 56 are moved in an axial direction and thus valve 13 is closed or opened, respectively.

When valve 13' is open, gas or liquid enters into the bath chamber containing the working bath 5, the fluid passing through the upper opening in housing 58 and through the valve opening 55. If necessary, further openings can be provided in the sides of housing 58.

FIGURE 4 shows a modified form of construction for the partition wherein a cup-shaped filter element 12' supporting valve 13 is mounted at the lower end of tubular partition 11'.

A device according to the present invention as shown in FIGURE 1 and having any of the specific forms of construction illustrated is advantageously operated in the following manner: Upon initiation of operation, in order to rapidly cool the cryostat 1, and after evacuation of the various chambers, the valve 13 or 13 in the partition 11 and the expansion valve 19 are opened and, while the pump 9 is operating and bypass valve 67 is open so that pump 9 creates a low pressure in both chambers and 16, helium is sucked from the storage vessel into the cryostat 1. As the cooling of the siphon and the cryostat progresses, the liquid is finally permitted to reach the bath chamber 15 and collects there as the working bath 5, into which, for example, a probe to be examined can be inserted via a conventional lock or other inserting device disposed in the cryostat cover and not shown in the drawings to avoid confusion.

When the liquid in the bath chamber 15 below filter 12 reaches a predetermined level, the expansion valve 19 IS closed, preferably automatically by causing the output of the level sensor 27 to deliver a signal to circuit 31 which causes the circuit to close the expansion valve 19, and the supply of liquid is interrupted. Then the cooldown bypass valve 13 and the bypass valve 67 are closed, the vacuum pump 22 communicating with the replenishing chamber 16 is actuated and, by adjusting the opening of valve 8, the pressure in bath chamber 15 is lowered to a value :below the )t-point corresponding to the nominal temperature of the bath 5 (ie between 37.6 mm. Hg for 2.17 K. and approximately 0.1 mm. Hg for approximately 1 K.). Thus the initial requirements for stable operation below 2.l7 K. are met.

The above-described procedure for beginning operation of the cryostat is characterized by the rapid delivery of all of the coolant to the working chamber 15. This is highly advantageous because it represents an efiicient utilization of the coolant for initially cooling the cryostat and hence involves the use of a minimum amount of coolant for carrying out the initial cooling operation.

During continuous operation it is advantageous for maintaining a stable temperature, to accomplish replenishing automatically. The cool-down bypass valve 13 remains closed and both vacuum pumps 9 and 22 continue opera ing. When the level of the bath 5 liquid, which evaporates particularly due to the heat introduced with the item to be examined, sinks to a value below the level sensed by sensor 27, the expansion valve 19 opens and liquid helium, which is being expanded and cooled to below 4.2 K., enters into the replenishing chamber 16 which is then maintained under a reduced pressure, preferably between 760 and 38 mm. Hg. When the level of the replenishing bath 17 collecting above the still impermeable filter 12 reaches that sensed by sensor 28, the expansion valve 19 is closed again in response to the signal furnished by sensor 28.

As already mentioned above, the temperature of the helium drawn from the storage vessel, and there maintained under a normal pressure corresponding to a temperature of 4.2 K., is already reduced to below 4.2 K. when it passes through the expansion valve 19. Upon closure of expansion valve 19, the pressure, and thus the temperature, of the replenishing bath 17 is reduced, by the action of pump 22 communicating with chamber 16 via conduit 24, to a value which lies several thousandths of a degree below the nominal temperature of the helium II working bath 5. This corresponds to a pressure in the range below 37.6 mm. Hg for temperatures below 2.17 K. Only when this tempera ture is reached, will helium II penetrate filter 12 in substantial amounts (e.g. for a given pore size at a rate of the order of 1 cm. of liquid per sec. per cm. of filter surface area) and how into chamber 15 to replenish the helium II bath 5. This process is periodically repeated. It is advisable to maintain the temperature of the helium bath 5 constant by means of a known pressure regulating device which controls valve 8.

The pressures in chambers 15 and 16 can advantageously be maintained by operating pumps 9 and 22 at a preset speed while automatically regulating the setting of pressure control walves 8 and 25 the operation of the valves possibly being controlled by pressure transducers whose connection to the valves is indicated in FIGURE 1 by the dotted lines.

Although the temperature of the helium II coming out of filter 12 is somewhat lower than the temperature of the helium II bath 5, no reduction in temperature occurs in the bath if care is taken, by appropriate positioning of the level sensors 27 and 28, that the liquid coming through the filter attains the bath 5 temperature before it enters the helium II bath.

Because of the low pressure (and hence low temperature) maintained in chamber 16, the resulting helium II will pass through the filter free of any vapor and will flow into the lower bath chamber 15 which is maintained at a low pressure value corresponding to the nominal temperature by means of the further vacuum pump 9.

This type of replenishment makes it possible to maintain a relatively constant temperature at arbitrarily preset temperatures of between 2.17 K. and approximately 1 K. for the HeII working bath present in the bath chamber, even during the replenishing process.

During continuous operation, a temperature gradient develops in the vertical direction in the gas filling the bath chamber 15 above the bath 5 in the region between the warm cryostat cover 6 and the cold bath 5 in spite of the continuous pumping action. Filter 12 is therefore exposed to gas which is somewhat warmer than the surface of bath 5. Due to the relatively large filtering surface of the filter, and because of the extremely high thermal conductivity of helium II, the temperature balance between the gas surrounding the filter and the liquid coming through the filter is quickly established so that if the space between the bath surface 5 and filter 12 is suitably adjusted, the liquid entering bath 5 will have the same temperature as the bath. The distance between the filter 12 and the bath 5 to be replenished is determined by the position of the two level sensors controlling the replenishing process.

Since the level of the liquid in the bath chamber depends on the setting of the level in the replenishing chamber, and since, as has been mentioned above, a balanced setting, as regards temperature or vapor pressure of the liquid passing through the filter is influenced by the height of the lower surface of the filter above the surface of bath 5, the adjustment of the liquid levels with respect to each other, by properly selecting the vertical positions of sensors 27 and 28, permits the best operating conditions to be obtained for any desired working temperature, thus reducing the consumption of coolant.

According to a further feature of the present invention, it can be useful when it is desired to obtain a working temperature above 2.17 K., to accomplish the replenishment with the valve 13 in the partition 11 opened, with the operation of the expansion valve 19 controlled exclusively by signals from the level sensor 27 in the bath chamber 15 and while evacuating the replenishing chamber 16 and the bath chamber 15 by means of a common pump. For this purpose, valve 67 must be opened to open line 66 and either pump 9 or pump 22 can be used. This method makes possible an uninterrupted transition in the working temperature from temperatures below 2.17" K. to temperatures above 2.17 K.

It is one of the principal advantages of the present invention that the unavoidable increase in pressure, and hence in temperature, occurring when coolant is de livered to the cryostat during the replenishing process cannot be transmitted to the He-II working bath 5 and that no relatively Warm liquid can enter this bath. This assures that the bath temperature can be maintained constant over long periods of time during continuous operation at presettable values of between 2.17 K. and approximately 1 K. Furthermore, the transition to temperatures above 2.17 K. can be accomplished without difliculty.

The present invention also distinguishes itself in that automatic operation can be achieved with simple means and hence a high degree of reliability.

There is the further advantage that a vacuum pump having a relatively low suction power is suflicient to maintain a constant temperature of the working bath 5.

The high degree of efiiciency of the device according to the present invention must be particularly emphasized. If the degree of efliciency is defined as the ratio of the amount of replenishing liquid supplied to the working bath 5 to the amount of liquid taken from the storage vessel 20, the degree of efficiency for maintaining bath 5 at a temperature of 1.7 K., for example, has been found to be of the order of 52% and higher. The maximum theoretical degree of efficiency for isenthalpic expansion to 1.7 K. has a value of 63%.

It will be understood that the above description of the present invention is susceptible to various modifications,

changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

What is claimed is:

1. In apparatus for the continuous low-temperature cooling of objects in a helium II working bath; which apparatus includes a working bath chamber for containing the working bath, means for maintaining a reduced pressure in the working bath chamber, and a vacuum jacketed siphon for periodically replenishing the working bath from a helium storage vessel, the improvement comprising: a replenishing chamber disposed at least partially in said working chamber above the working bath and arranged for receiving the liquid helium delivered by the siphon, said replenishing chamber including a filter element forming at least a portion of the lower extremity of said replenishing chamber and separating the interior of said replenishing chamber from the Working bath, said filter element being of a material which is at least partially impermeable to gaseous helium and helium I, as well as to any helium II having a temperature higher than that of the working bath; and means for maintaining a reduced pressure in said replenishing chamber.

2. An arrangement as defined in claim 1 wherein said filter element is made of a porous material having a pore size of less than 10 cm.

3. An arrangement as defined in claim 2 wherein said filter element is made of clay.

4. An arrangement as defined in claim 2 wherein said filter element is made of a sintered metal. body at least one surface of which is provided with an electrodeposited metal layer to give it the required pore size.

5. An arrangement as defined in claim 2 wherein said filter element is in the form of a disc.

6. An arrangement as defined in claim 2 wherein said filter element is in the form of a hollow body.

7. An arrangement as defined in claim 1 wherein said replenishing chamber further includes: a partition supporting said filter element and made of a material having a poor thermal conductivity; and an externally operable val e for selectively bypassing said filter arrangement.

8. An arrangement as defined in claim 7 wherein said partition is cup-shaped and said filter and said valve are disposed at the bottom of the resulting cup.

9. An arrangement as defined in claim 7 wherein said working bath chamber is provided with a cover to which said partition is attached.

10. An arrangement as defined in claim 7 wherein said partition is attached to the outlet end of said siphon.

11. An arrangement as defined in claim 1 further comprising a first electrical level sensor for liquid helium disposed in said replenishing chamber and a second electrical liquid helium level sensor disposed in said working bath chamber below the exit level of said filter element.

12. An arrangement as defined in claim 11 further comprising an externally operated expansion valve obturating the outlet end of said siphon, and electrical control means connected between said sensors and said. expansion valve for opening said expansion valve in response to signals supplied by said second sensor when the level of said working bath falls below a set value and for closing said valve in response to signals from said first sensor when the liquid in said replenishing chamber rises to a set value.

13. An arrangement as defined in claim 1 wherein said means for maintaining a reduced pressure in said working bath chamber include a first pressure adjusting valve and said means for maintaining a reduced pressure in said replenishing chamber include a second pressure adjusting valve, said arrangement further comprising at least one vacuum pump connected to said two pressure maintaining means for creating the desired reduced pressures.

14. A method for maintaining a helium II working bath at a stable temperature below the helium )vpoint while periodically replenishing the bath via a siphon in a device which includes a replenishing chamber disposed above the working bath and having at least a portion of its lower extremity composed of a filter element which is at least partially impermeable to gaseous helium and helium I, and to any helium II at a temperature higher than that of the working bath, said method comprising the steps of: delivering liquid helium via the siphon into the replenishing chamber; maintaining the interior of the replenishing chamber at a reduced pressure corresponding to a temperature below the \-point for permitting helium II which is at a sufficiently low temperature to pass through the filter element in a vapor-free manner to the working bath; and maintaining the working bath at a reduced pressure corresponding to the desired stable temperature.

15. A method as defined in claim 14 wherein said steps of maintaining the replenishing chamber and working bath at reduced pressures is carried out by placing each in communication with a pump operating at a fixed suction speed via a respective, automatically adjusted pressure control valve.

16. A method as defined in claim 14 wherein said step of delivering liquid helium is carried out by commencing the flow thereof each time the working bath falls below a preset level and then terminating the flow when the liquid helium in the replenishing chamber rises to a preset level.

17. A method as defined in claim 16 comprising the preliminary step of selecting the preset level of liquid in the replenishing chamber as a function of the preset level of the working bath.

18. A method for maintaining a low temperature greater than the helium )\-point in low-temperature apparatus including a bath cryostat for containing a liquid helium bath, a replenishing chamber whose bottom is provided with a controllable bypass valve and a filter which is at least partially impermeable to gaseous helium and helium I and to helium II above a certain temperature, a siphon whose outlet end is obturated by a controllable expansion valve; and a single pump connected to the cryostat and the replenishing chamber, said method comprising the steps of: maintaining the bypass valve open for permitting all of the liquid in the replenishing chamber to flow directly to the liquid helium bath; operating the pump for maintaining the cryostat and the replenishing chamber at a predetermined reduced pressure corresponding to the desired low temperature; and opening the expansion valve for delivering liquid helium to the r plenishing chamber whenever the liquid helium bath iS below a preset level and closing the expansion valve when the helium bath reaches such preset level.

19. A method for beginning operation of low-temperature apparatus which is to operate at temperatures be low the helium k-point and which includes a bath cryostat for containing a helium II working bath, a replenishing chamber whose bottom is disposed above the region to be occupied by the bath and is provided with a controllable bypass valve and a filter which is at least partially impermeable to gaseous helium and helium I and t0 helium 11 whose temperature is greater than that of the working bath, and a siphon supplied with liquid helium to be delivered to the replenishing chamber and having its end obturated by a controllable expansion valve, said method comprising the following steps in the order S t forth: opening both the bypass valve and the expansion valve, while maintaining both the cryostat and the replenishing chamber at the same reduced pressure, for delivering liquid helium from the siphon directly to the cryostat through the replenishing chamber; closing the bypass valve when the helium Working bath reaches a predetermined level; and thereafter maintaining the working bath and the replenishing chamber at separately controlled reduced pressures.

References Cited UNITED STATES PATENTS 3,360,947 1/1968 Fretwell et al. 62-45 WILLIAM E. WAYNER, Primary Examiner.

US. Cl. X.R. 62-514

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