Title:
LOW-TEMPERATURE PUMPING DEVICE
United States Patent 3797264


Abstract:
An improved low-temperature pump and apparatus for producing very high vacua and ultra-vacua with a pumping speed which can be varied at will, comprises a pumping chamber, a gas-evacuation opening formed in the said chamber, a pumping body located at a distance from the said chamber and adapted to form a main low-temperature pumping surface, comprising also means for cooling the said body to a very low temperature and, if so desired, means for forming a head screen at a moderately cold temperature around and at a distance from the said main low-temperature pumping body, such as an internal wall of the heat-insulating chamber and a gas-permeable reflector baffle, the said pump further comprising a wall-screen continuous thermal relation with a cold element of the said low-temperature pump, forming an auxiliary low-temperature pumping surface and disposed in a valve conduit adapted to be controlled to the chamber to be evacuated, said valve being equipped with a clapper device adjustable in position and adapted to close the opening in the said pumping chamber, and also with means for putting the said wall-screen into operation, said latter means being controlled in dependence on the position of the said closure clapper, forming through said wall-screen a passage of minimum conductivity in the minimum opening position of said clapper and with increasing conductivity between the minimum and maximum opening positions of the said clapper. The invention is especially applicable to furnaces or ovens operating under very high vacua.



Inventors:
Thibault, Jean-jacques (Saint Martin d'Uriage, FR)
Boissin, Jean-claude (Montbonnot-Saint-Martin, FR)
Application Number:
05/309701
Publication Date:
03/19/1974
Filing Date:
11/27/1972
Assignee:
L AIR LIQUIDE SA POUR I ETUDE,FR
EXPLOITATION DES PROCEDES GEORGES CLAUDE,FR
Primary Class:
Other Classes:
62/100
International Classes:
F04B37/02; B01D8/00; F04B37/08; (IPC1-7): B01D5/00
Field of Search:
62/55.5
View Patent Images:
US Patent References:



Primary Examiner:
Wye, William J.
Attorney, Agent or Firm:
Young, And Thompson
Claims:
What we claim is

1. A low-temperature pumping device of the kind comprising a pumping chamber, a gas-evacuation opening in said chamber, a pumping body located at a distance from said chamber and adapted to form a main low-temperature pumping surface, comprising also means for cooling said body to a very low temperature and, if so required, means for forming a heat screen at a moderately cold temperature around and at a distance from said main low-temperature pumping body, such as an internal wall of the heat-insulating chamber and a gas-permeable reflector baffle; said device further comprising trapping means adapted to be brought to a cold and possibly very cold temperature and disposed in the vicinity of said chamber opening, together with means for putting said trapping means at least partially into operation at a cold and possibly very cold temperature, said trapping means forming part of said low-temperature pumping surface.

2. A low-temperature pumping device as claimed in claim 1, in which said means for putting said trapping means into operation comprise a continuously cooled shutter adjustable in position across the opening in said chamber.

3. A low-temperature pumping device as claimed in claim 2, in which said adjustable shutter is in thermal contact with a tank adapted to receive a fluid at a moderately cold temperature, such as liquid nitrogen, for example.

4. A low-temperature pumping device as claimed in claim 2, in which said shutter adjustable in position is actuated by control means provided with a passage-bushing fluid-tight to ultra-vacuum, said means acting through the intermediary of a bellows device.

5. A low-temperature pumping device as claimed in claim 4, in which said shutter-actuating means is mechanically operated.

6. A low-temperature pumping device as claimed in claim 4, in which said shutter-actuating means is magnetic.

7. A low-temperature pumping device as claimed in claim 4, in which said shutter-actuating means is adapted to co-operate through said bellows device with a push-rod associated with said shutter.

8. A low-temperature pumping device as claimed in claim 4, in which said shutter-actuating means is adapted to co-operate through said bellows device with a toothed rack and pinion associated with said shutter.

9. A low-temperature pumping device as claimed in claim 1, in which said trapping means comprise an auxiliary trapping surface interposed between said chamber opening and a gas-permeable screen facing the said main trapping surface, and in which said means for putting said trapping device into operation comprise adjustable means for supplying a very cold fluid.

10. A low-temperature pumping device as claimed in claim 9, in which said auxiliary trapping surface has an annular form substantially coaxial with said chamber opening and having substantially the same transverse extension.

11. A low-temperature pumping device as claimed in claim 9, in which said adjustable supply means of a very cold fluid comprise an admission pipe coupled to a low point of a helium tank and a vapour-evacuation pipe, at least one of these pipes being fitted with a regulating valve.

12. A low-temperature pumping device as claimed in claim 9, in which said adjustable supply means of very cold fluid comprise an admission pipe connected to an upper point of a helium tank and a vapour-evacuation pipe, together with means for putting the helium vapour in said tank under a variable pressure.

13. A low-temperature pumping device as claimed in claim 12, in which said pressure-varying means are preferably an electric resistance immersed in said tank.

14. A low-temperature pump of the kind comprising a pumping chamber, a gas-evacuation opening formed in said chamber, a pumping body located at a distance from said chamber and adapted to form a main low-temperature pumping surface, comprising also means for cooling said body to a very low temperature and, if so required, means for forming a heat screen at a moderately cold temperature around and at a distance from said main low-temperature pumping body, such as an internal wall of the heat-insulating chamber and a gas-permeable reflector baffle, said pump further comprising a wall-screen continuously in thermal relation with a cold element of the low-temperature pump, forming an auxiliary low-temperature pumping surface and disposed in a valve conduit adapted to be connected to a chamber to be evacuated, said valve being equipped with a clapper adjustable in position and adapted to close the opening in said pumping chamber, and also with means for putting said wall-screen into operation, in which said means for putting said wall-screen into operation are controlled in dependence on the position of said closure clapper, forming through said wall-screen a passage of minimum conductivity in positions of small opening of said clapper and with increasing conductivity between a small opening of said clapper and a maximum opening position of said clapper.

15. A low-temperature pump as claimed in claim 14, in which the means for putting said wall-screen into operation, controlled by the position of said closure clapper, are constituted by a control rod coaxial with said screen, said rod also carrying said also coaxial clapper, said wall-screen being mounted between said clapper and said main low-temperature pumping surface and being able to pass freely into said pumping chamber opening, and having a diameter slightly less than said opening, said chamber opening serving as a seating for said clapper, said trapping wall-screen being continuously in thermal relation with the cold element of said pump through the intermediary of a member constituted by a flexible and extensible conductor of heat.

16. A low-temperature pump as claimed in claim 15, in which said flexible conductor of heat is a metallic braid and said cold element is a collar coaxial with said chamber opening and having substantially the same diameter as said wall-screen.

17. A low-temperature pump as claimed in claim 15, in which said trapping wall-screen is connected to said clapper by a compression spring mounted on a rod coaxial with said control rod.

18. A low-temperature pump as claimed in claim 15, in which said wall-screen is composed of a plurality of parallel and coaxial discs mounted on said control rod, said discs having a diameter slightly smaller than that of the opening in said pumping chamber.

Description:
The present invention relates to improvements in devices for very low temperature pumping, utilized for very high vacua or ultra-vacua, cooled by liquid helium.

In devices of this kind, the pumping speed obtained is related to the consumption of the refrigerant fluid. As liquid helium is relatively expensive, it becomes necessary to look for a compromise, for apparatus of this kind, between the pumping speed and the consumption of liquid helium.

Up to the present time, low-temperature pumps cooled with liquid helium fall under two types:

1. Those comprising heat screens cooled by liquid nitrogen (this fluid is about 100 times cheaper and is vaporized 60 times less easily then helium).

In this case, the pumping speed available in the vicinity of the low-temperature pump is generally from two to five times lower than that which would be available in the absence of heat screens.

On the other hand, the consumption of liquid helium is generally reasonable, namely from 0.01 to 0.20 litre of liquid helium consumed approximately per hour.

2. Those which do not comprise heat screens cooled by liquid nitrogen. In this case, the pumping speed available in the vicinity of the low-temperature pump is close to the maximum theoretical pumping speed calculated for molecular conditions.

On the contrary, the consumption of liquid helium is generally relatively high (0.5 to 5 litres approximately of liquid helium per hour). In this case, it is strongly recommended to use heat screens cooled by helium vapours produced inside the low-temperature pump.

As a general rule, numerous utilizations of high-vacuum and ultra-vacuum do not constantly require a high pumping speed. This is the case for example in evaporation chambers for the production of thin layers. In addition, when the low-temperature pump is filled with liquid helium, the operations of heating and cooling are costly. It is for this reason that an isolating valve is frequently provided between the low-temperature pump and the chamber to be evacuated. In this case, it is clear that when the low-temperature pump is isolated from the chamber to be evacuated, it will be economical to reduce the consumption of helium and it will cause no difficulty to reduce the pumping speed.

The present invention relates to low-temperature pumps in which the pumping speed can be regulated so as to adapt it to a large number of utilizations, while retaining economy of operation, which is appreciated by users.

According to the invention, the low-temperature pumping device comprises a pumping chamber, a pumping body placed at a distance from the chamber and intended to form a main low-temperature pumping surface, also comprises means for cooling the said body to a very low temperature and, if so desired, means forming a heat screen at a moderately cold temperature around and at a distance from the said main low-temperature pumping body, such as the internal wall of the thermal isolation chamber and/or a reflecting baffle permeable to gases. This device is characterized in that it comprises trap means intended to be brought to a cold and possibly a very cold temperature, and arranged in the vicinity of the said opening, together with means for the possibly partial use of the said trap means at a cold and even very cold temperature, forming a part of the low-temperature pumping surface.

According to the invention, these utilization means comprising a shutter which is constantly cooled and adjustable in position across the said opening, this screen being in thermal contact with a tank intended to receive a fluid at a moderately cold temperature, such as liquid nitrogen, for example.

The adjustable shutter is actuated by mechanical or magnetic means, with a passage fluid-tight to ultra-vacuum.

An alternative form of the invention is characterized in that the trapping means comprise an auxiliary trapping surface interposed between the opening and a gas-permeable screen which faces the main trapping surface, and in that the utilization means comprise regulatable supply means of very cold fluid.

The form of the auxiliary trapping surface is chosen to be substantially annular and coaxial with the said opening, and having substantially the same transverse extension; the very cold fluid adjustable supply means are comprised by a source of liquid or gaseous helium coming from the top or bottom level respectively of a tank containing helium.

The invention also relates to a modified low-temperature pumping device which enables one single operation to be effected by the same actuating means (and trapping means serving as a movable screen and means for putting the chamber to be evacuated into communication with the low-temperature pump).

This arrangement offers an appreciable gain in economy of helium, since it involves a device which utilizes a low-temperature pump of conventional type to which there is adapted, so to speak, a valve-screen.

This arrangement according to the invention is characterized in that the means for utilizing the said wall-screen are controlled in dependence on the position of the shutter-valve, forming between the periphery of the wall-screen and the said conduit, a passage having minimum conductivity in positions of small opening of the said valve and with an increasing conductivity between a position of small opening of the valve and its maximum opening.

A characteristic feature of the low-temperature pump in this arrangement is that the means for utilizing the said wall-screen controlled by the position of the shutter-valve are constituted by a control rod coaxial with the said screen, the said rod also supporting the said valve, which is also coaxial, the said wall-screen being mounted between the valve and the main low-temperature pumping surface and being able to pass freely into the chamber opening and having a diameter slightly less than the said opening, the said chamber opening serving as a valve seating.

Other characteristic features and advantages of the invention will also be brought out in the description which follows below, given by way of example, reference being made to the accompanying drawings, in which:

FIG. 1 is a diagrammatic view in cross-section of a low-temperature pumping device according to the invention;

FIG. 2 is a view in cross-section of a detail of the device shown in FIG. 1;

FIGS. 3 and 4 are diagrammatic views in cross-section of alternative forms of construction of a device according to the invention;

FIG. 5 shows a detail in partial cross-section of the device illustrated in FIG. 1;

FIG. 6 is a diagrammatic view in cross-section of a modified low-temperature pumping device according to the invention;

FIGS. 7 to 10 are enlarged views of the auxiliary low-temperature pumping device with variable operation, according to the various phases of closure and opening of the valve.

Referring now to FIG. 1, a pumping chamber 1 comprises a wall 2 with a tubular opening 3 terminating in a flange 4 which is intended for fluid-tight coupling to a chamber in which it is desired to produce a vacuum (not shown).

Inside the chamber 1, there are successively arranged, vertically above each other, an upper tank 5 of stainless steel, fixed to the chamber and filled with liquid nitrogen, and a second tank 6, also of stainless steel, mounted underneath the tank 5 and forming the fixing surface of the low-temperature pump, arranged in the lower portion of the chamber 1 at a distance from the wall of the chamber.

This tank 6 communicates with the exterior through the intermediary of a vertical tube 7 necessary for the admission and de-gassing of the helium, this tube also permitting the suspension of the tank 6 on the wall 2 by welding. This tube 7 passes through a coaxial well 8 in the tank 5.

A wall-screen shown at 9 and 9' of nickeled copper soldered to the wall of the tank 5, substantially surrounds the tank 6 while forming a clearance between the low-temperature pumping zone of the tank 6 and the flanged opening 3. Another wall-screen 9a is provided between the wall 2 and the screen 9.

A solid circular shutter 10, which may be of nickeled copper, having a polished surface on its main faces, has a diameter in the vicinity of that of the suction flange, and is coupled through the intermediary of a rod 11 to a mechanical control accessible from the exterior of the chamber. This mechanical control consists of a fluid-tight bushing device for ultra-vacuum (of the fluid-tight bushing type for passage in translation or in rotation).

FIG. 2 shows how the circular shutter 10 occupies upper, lower or intermediate positions by moving the rod 11 through the intermediary of the fluid-tight passage device, which may also be a toothed-rack and pinion system.

A metal braid 15 which is a good conductor of heat (for example nickel-plated copper) connects the liquid nitrogen tank 5 to the circular shutter 10 in such manner as to maintain this latter at a temperature in the neighbourhood of about 77° K.

The operation of the device according to the invention is as follows: after having filled the tank 5 with liquid nitrogen through a conduit shown at 17, liquid helium is admitted to the tank 6 through the pipe 7. After having created a primary vacuum in the chamber to be trapped, this latter is put into communication with the low-temperature pump.

If a high pumping speed is desired, it is only necessary to put the circular shutter 10 into the open position (FIG. 2, position I), by actuating the control device with a fluid-tight passage. This results in a relatively large consumption of heluim, and the gases are directly trapped on the low-temperature surface.

On the other hand, if a low pumping speed is desired, it is only necessary to put the circular shutter into the closed position II, shown in broken lines in FIG. 2. The consumption is lower and the low-temperature pump can be used for a longer period.

This method is therefore advantageous when the pumping speed required is low.

The gases such as CO2, H2 O, etc. are condensed or solidified on the shutter 10. The other gases are condensed on the tank 6.

It will readily be understood that any intermediate position of the shutter may be considered, which will parmit a substantial modification of the pumping speed.

For the operation of the shutter 10 shown in FIG. 1, the fluid-tight passage mounted by means of a flange 21 with screws (not shown) on the pumping chamber, is provided a stirrup 22, held in position on the flange 21 by screws 25. A partly threaded rod 23 passes through the flange 21, the flange, the stirrup and the rod being coaxial, as shown in FIG. 5.

Between the flange 21 and the stirrup 22 is provided a knurled screw 24, also coaxial and accessible from the outside, its rotation permitting a forward travel of the thread of the rod 23 by virtue of a bearing 26.

The rod 23 pushes against an end-piece 32 centered on the same axis as the rod, which in turn pushes the rod 11 which serves to displace the circular shutter 10 in accordance with the invention.

On the lateral faces of the said end-piece 20, the extremity of a bellows member 29 is sealed by a collar 28 the other extremity of this bellows being sealed to another collar 31 which rests on a sleeve 27 fixed to the flange 21 but on the face opposite to that facing the knurled screw. The movement of the rod 23 and therefore of the shutter 10 is due to the rotation of the knurled screw 24 through the intermediary of the bearing 26. It is clear that the overall extent of the threads on the rod 23 is chosen to be at least equal to that which is necessary, in order to completely close the pumping opening by the shutter 10, in accordance with FIG. 2.

By way of example, the following Table indicates the performances of a pump of this kind, as compared with a standard pump. ---------------------------------------------------------------------------

Standard Model MOdel modified according to the invention __________________________________________________________________________ Pumping Con- Pumping Con- speed sumption speed sumption for N2 L He for N2 L He 200 ls-1 0.020 l/h 600 l/s 0.4 l/s 400 ls-1 0.025 l/h 1,300 l/s 0.9 l/h 600 ls-1 0.030 l/h 2,500 l/s 1.5 l/h __________________________________________________________________________

According to FIG. 3, it is seen that the regulation of the temperature of the trap 10a may be effected by a circulation of liquid helium coming from the receptacle 6 brought in by the pipe 7 through a coil 11 provided with a valve 12a. This coil is coupled to the bottom of the tank 6.

A fixed screen 8 of the herringbone baffle type is welded to the parts 9 and 9' of the screen surrounding the tank 6. This screen 8 permits the passage of the gases to be pumped at low temperatures while protecting the tank 6 from unfavorable heating by direct radiation through the opening 3. It can be seen that by opening the valve 12a, the liquid helium will circulate by gravity in the coil 11 and will bring the trap 10a to a temperature in the vicinity of 4.2° K.

The gases to be trapped, coming from the chamber to be put under vacuum, will thus be retained on this trap 10a, which will have the effect of accelerating the pumping speed, which will be multiplied by a factor between 2 and 5.

During normal operation, that is to say at a low pumping speed, the valve 12a is closed.

During operation at high pumping speed, the valve 12a is open.

FIG. 4 shows a device similar to that described previously, in which gaseous helium is employed to cool the trap 10a. The circuit further comprises a valve 12 mounted on the pipe 7 and an electric resistance R placed in the bath of helium monitors the vaporization.

The valve 12 being closed, the valve 12a is then opened and a high pumping speed is then obtained.

During normal operation, that is to say at a low pumping speed, the valve 12a is closed and the valve 12 is opened.

It is clear that after a period of operation at high pumping speed, in accordance with FIGS. 3 and 4, if the valve 12a is closed, the gases condensed on the trap 10a will then be rapidly transferred to the fixing surface 6.

The isolating valve between the low-temperature pump and the space to be pumped out may be closed during this operation (with a duration from a few seconds to a few minutes).

Referring now to the accompanying FIG. 6, a pumping chamber 1 comprises a wall 2 with a tubular chamber opening 3 terminating in a flange 4 intended to receive a valve 100 for fluid-tight coupling to a chamber in which the vacuum is to be created (not shown).

In the interior of the chamber 1 there are successively arranged, vertically above each other, an upper tank 5 of stainless steel fixed to the chamber and filled with liquid nitrogen, and a second tank 6, also of stainless steel, intended to receive the liquid helium, placed underneath the tank 5 and forming the main fixing surface of the low-temperature pump, arranged at the lower portion of the chamber 1 at a distance from the chamber wall.

This tank 6 communicates with the exterior by means of a vertical tube 7 necessary for the admission and de-gassing of the helium, this tube also providing the suspension of the tank 6 by welding to the wall 2. This tube 7 passes through a coaxial well 8 into the tank 5.

A screeen of nickel-plated copper, shown at 9 and 9', is soldered to the wall of the tank 5 and substantially surrounds the tank 6 while forming a clearance between the low-temperature pumping zone of the tank 6 and the chamber opening 3. Another screen 9a is placed between the wall 2 and the screen 9.

The valve 100 comprises, as shown in FIGS. 7 to 10, a flange opening 101 adapted by screws 650 to the flange 4. The opening 101 has a shoulder 302, and the valve 100 is also provided with a tubular opening 304 with a flange 102 capable of receiving the chamber to be pumped. This opening 304 is mounted perpendicular to the opening 101 which offers a degree of angular orientation freedom of the chamber with respect to the vertical tube 7, of 180°.

The valve 100 comprises a closure clapper 614 mounted on a central control rod 612 of tubular shape. This clapper 614 has a toric joint 615 and is applied against a joint plane 16 which is suitably machined on the internal face of the flange 101. The clapper 614 is thus capable of closing the chamber opening 3 by means of the flange 101 and is thus adjustable in position.

The rod 612 communicates with the exterior of the valve 100, as shown in FIG. 6, by one of its extremities 612', by any known means, for example a fluid-tight bellows passage (not shown). This control may be mechanical and/or magnetic. The other extremity 612" carries the clapper 614 and also a wall-screeen 100' mounted in front of the said clapper, which means that this wall 100' is located between the clapper and the trapping surface 6. The wall-screen 100' is mounted coaxially with the rod 612, and has a diameter smaller than the chamber opening 3, thus being able to pass freely into the said opening 3, which serves in a way as a seating for the clapper 614, through the intermediary of the flange 101.

In order to carry the clapper 614, the rod 612 comprises at its extremity facing the flange 4, a ring 18 welded on the rod and provided with a collar 18a fitted with a toric joint 19 against which is supported the clapper 614. A washer 20 and a circlip 21 hold the clapper 614 against the joint 19.

The ring 18 is provided axially and facing the opening 3 with a threaded hole coaxial with the rod 612 capable of receiving a sleeve 22 which is also threaded and which has one cylindrical extremity 22a which has a smaller diameter than the rod 612 and which can thus be passed on the said rod after screwing the sleeve 22 into the ring 18 by means of the threaded hole 30. A joint 31 ensures fluid-tightness between the sleeve 22 and the ring 18.

Two blind holes coaxial with the rod 612 are formed on the same side in the sleeve 22, one of smaller diameter but of greater depth 110, the other 111 with a groove 111b of greater diameter but limited in depth. A partly threaded rod 23 slides freely in the holes 110 and 111. A spring 24 is placed on the rod 23 and a washer 24b, welded to the rod 23 and having a diameter slightly less than the hole 111 permits the compression of the spring 24 inside the hole 111 which comes into abutment against the groove 111b.

The threaded portion of the rod 23, external to the holes 110 and 111, carries the wall-screen 10 formed by two discs coaxial with the rod 612, the first 10a having a number of circular perforations 100a, uniformly distributed over the disc, the other 10b being of smaller diameter and interposed between the first and the clapper 614, and separated from the first by a spacer 27.

The discs 10a and 10b are rigidly fixed on the rod 23 by means of two washers 25 and 26 and a blind nut 28 which screws on the threaded extremity of the rod 23. On the flange 18 a threaded plug 29 is mounted on the side of the wall-screen 100'. This plug is bored to a diameter slightly greater than that of the rod 23, so that the rod 23 can circulate freely in the holes 110, 111, within the limits of movement of the washer 24b between the washer 24 and the plug 29.

Thus, the rod 612 serves as a utilization means for the wall-screen 100' and is controlled in dependence on the position of the closure clapper 614.

In the immediate vicinity of the helium tank 6 and facing the opening 3, there is located a mechanical assembly capable of coming into thermal contact with the screen 9 and 9' at the points marked 90 and 90'. This device may be constituted by a ring 200 in which is mounted a ring 201 in two parts. Six screws equally distributed in the ring 201 are screwed into six L-shaped brackets and thus ensure good thermal contact with the ring 200.

On this ring 200 is welded a shell 203 of smaller diameter than that of the flanged opening 3, and the other extremity of which is welded to a second shell 205 facing the clapper 614, having a diameter slightly greater than that of the shell 203, but less than that of the opening 3. The shell 203 is chosen with a diameter close to that of the disc, the edges 100b of which are deliberately curved back so as to be applied against the said shell.

Thus, the shells or collars 203 and 205 are brought up to the temperature of the liquid nitrogen by means of the contacts 90 and 90' and form the cold element of the low-temperature pump. Furthermore, the arrangement of the collar 205 is chosen in such manner that it serves as an optical screen for the helium tank 6.

A constant heat relation exists between the wall-screen 10 and the collars 203, 205, which serve as a cold element of the low-temperature pump, and this relation is ensured by a member constituted by a flexible and extensible heat conductor, such as for example a metallic braid 500 of nickel-plated copper having a length sufficient to permit movement of the rod 612 over the whole length of the valve 100. This valve 100 has a conduit 299 which communicates on the one hand through the opening 304 with the chamber to be evacuated, and on the other hand through the opening 3 with the low-temperature pumping surface.

Depending on the position of the rod 612 inside the conduit 299, the periphery of the wall-screen 100' will describe a displacement contour-envelope such as shown in broken lines in FIGS. 9 and 10, and thus the edges 100b of the disc 10a describe a cylinder of revolution having an axis identical with that of the rod 612, and between the wall-screen 100' and the conduit 299, the rod 612 forms a clearance passage which increases the conductivity and permits access of the gas to be trapped on the main low-temperature pumping surface 6. The wall-screen 100' being provided with circular perforations 100a, the gases can pass through this wall-screen so as to reach the surface 6.

By virtue of the spring device 24, 24b, the wall-screen 100' can remain forced against the collar 203 while the clapper 614 moves away from its joint plane 16. The clearance passage is then shown in FIG. 8 by the references 303, 303', 305 and 305', as shown by the arrows.

Referring now to FIG. 7, the clearance passage is zero, that is to say the conductivity is a minimum when the valve 614 is totally closed and it may also be as low as possible (see FIG. 8) when the said clapper is lifted away from its joint plane 16, the gases then following the path 303, 303' and 305, 305' while being capable of being partly trapped on the wall-screen 100' (H2 O, CO2, for example).

This clearance passage remains as small as possible, as long as the spring 24 is not de-compressed. Then, by continuing to operate the rod 612, the wall-screen 100' is separated from the collar 203 and the clearance passage becomes increasingly large (see the arrow and the reference 301, 301', 303, 303', 305 and 305' shown in FIGS. 9 and 10), and the gases pass directly on to the surface; the conductivity is then increasing. As and when the clapper 614 is moved away from the seating plane 16, the clearance passage increases and the conductivity becomes increasingly large.

The operation of the device according to the invention is as follow:

After having filled the tank 5 with liquid nitrogen through a conduit shown at 17, liquid helium is admitted to the tank 6 by the pipe 7. After having produced a primary vacuum in the chamber to be evacuated, this latter can be put into communication with the low-temperature pump.

As shown in FIG. 7, the clapper 614 is then supported at the level of the joint 615 on the seating plane 16, and the wall-screen 100' closes the collar 203 by means of the disc 10a.

According to FIG. 8, by means of the external control device (not shown), the clapper 614 is opened and moves away from its seating plane 16. The assembly of the plug 29, flange 22, ring 18 and clapper 614 being rigidly fixed on the operating rod 22 moves, but the wall-screen 100' remains closed, since the rod 23 slides freely in the holes 110 and 111, and the spring 24b is still sufficiently compressed to hold the wall-screen 100' against the collar 203. At that moment, certain gases such as CO2 and H2 O become condensed or solidified on the wall-screen 100' and also on the collars 203 and 205. The pumping speed concerned is low. The consumption of liquid helium is lower and the low-temperature pump can work for longer periods.

If a high pumping speed is desired, it is only necessary to put the screen 100' in the open position (see FIGS. 9 and 10) while continuing to actuate the control device. This results in a relatively large consumption of helium. The gases are directly trapped on the liquid helium low-temperature surface 6.

In order to do this, the control rod 612 must be pulled in such manner that the spring 24 is compressed by the washer 24b when the plug 29 comes into abutment against it. It is clear that any intermediate position of the screen may be contemplated, which permits a substantial modification of the pumping speed, since the overall displacement of the rod 612 may be equal to the length of the valve 100.

When the pumping operation on the chamber to be evacuated is completed, it is only necessary to re-close the valve by means of the external control device, which may, as has previously been stated, be of the mechanical and/or magnetic type, so that the clapper 614 becomes applied against the seating plane 16 and that the screen 100' comes into contact with the collar 203 at the level of the disc 10a.

It is thus easily possible to limit the consumption of liquid helium while maintaining the low-temperature pump in communication with the chamber to be evacuated through the intermediary of the valve, this chamber being utilizable as an evaporation chamber for the production of thin layers.

The low-temperature pumps thus described may advantageously be provided with a device for the introduction of argon, as described in U.S. Pat. application Ser. No. 198,178, filed Nov. 12, 1971 now U.S. Pat. No. 3,769,806.

Similarly, the chamber 1 may be adapted for example to an evaporation chamber for the production of thin layers or to a furnace under high vacuum or ultravacuum.