US3485054A

US3485054A - Rapid pump-down vacuum chambers incorporating cryopumps

Info

Publication number: US3485054A
Application number: US590061A
Authority: US
Inventors: Walter H Hogan
Original assignee: Cryogenic Technology Inc
Current assignee: Cryogenic Technology Inc; Azenta Inc
Priority date: 1966-10-27
Filing date: 1966-10-27
Publication date: 1969-12-23
Anticipated expiration: 1986-12-23
Also published as: GB1170824A; DE1628440A1

Description

Dec. 23, 1969 w. H, HOGAN 3 ,0

RAPID PUMP-DOWN VACUUM CHAMBERS INCORPORATING CRYOPUMPS Filed Oct. 2'7, 1966 2 Sheets-Sheet 1 6O Wolrer H. Hogan INVENTOR.

/EWMJ a Arwrney Dec. 23, 196-9 I w, H. HOGAN 3,485,054

RAPID PUMP-DOWN VACUUM CHAMBERS INCORPORATING CRYOPUMPS Filed Oct. 27, 1966 2 Sheets-Sheet 2 88 Walter H. Hogan INVENTOR. Fig '6 Z United States Patent 3,485,054 RAPID PUMP-DOWN VACUUM CHAMBERS INCORPORATING CRYOPUMPS Walter H. Hogan, Wayland, Mass., assignor to Cryogenic Technology, Inc., Waltharn, Mass., a corporation of Delaware Filed Oct. 27, 1966, Ser. No. 590,061 Int. Cl. F25b 19/00 US. Cl. 62-555 19 Claims ABSTRACT OF. THE DISCLOSURE A rapid pump-down vacuum chamber suitable for rapidly reducing pressures in a test volume'from atmospheric to about 10 torr. A cryopump chamber containing a refrigerated thermal mass is associated with a work chamber. By effecting fluid communication between the two chambers, the pressure rapidly reduced in the system through the mechanismof cryopumping.

This invention relates to vacuum chambers and more particularly those which are designed to operate in the region between atmospheric and torr or lower,

and which employ cryopumping.

Many different systems are available for providing relatively high vacuums for industrial processing and for testing chambers. In some of these systems Which require the attainment of a high vacuum, the time consumed in pumping down the vacuum chamber is not of great importance. However, there are a number of applications for vacuum chambers where it is desirable, if not necessary, to be able to achieve very rapid pumpdown to the vacuum level desired. For example, in space simulation chambers there is a need to be able to simulate the rapid pressure change that space vehicles experience as a consequence of rapid ascent in going from atmospheric pressure to pressures in the range of 10- torr. Ideally, this decrease in pressure should be attained in about four minutes accurately to simulate the ascent of a space vehicle. In some special industrial processes, it is necessary to treat items in an evacuated atmosphere, the steps being to place them in a work chamber, evacuate, treat and then remove. This means that the major amount of time is spent in evacuating the work chamber. As an example, many plastic items are metallized on the surface; and this is done by the process of vacuum deposition. In such cases, the time consumed in evacuating the working chamber represents a major portion of the time required to process the items. If the pumpdown time of the chamber can be materially reduced, the cost of such operation can also be materially reduced.

Mechanical pumps exist which are capable of achieving rapid pumpdown from atmospheric pressure to several torr (mm. of Hg) of pressure. There are also many pumping means for achieving rapid exhaust from the sub-torr level. These latter include diffusion pumps which operate from 10 torr, ion pumps from 10- and sublimation pumps from 10 torr. However, there is no known mechanical device which is capable of achieving rapid pumpdown in the pressure range between several torr and about 10 torr.

A great deal of effort has been devoted in recent years to the development of cryopumps in which there is provided a refrigerated surface on which gas molecules are condensed and sorbed. Recently, cryopumping has been used to pump below 10" or 10 torr and supplemented with ion or diffusion pumps to remove the noncondensables such as neon, hydrogen and helium. Cryopumps are usually sized for the pumping speed required "ice in the ultimate perssure required, and little, if any, consideration is given to attaining a rapid pump-down. This in turn means that the refrigerator or the refrigerating system used with the cryopump is sized for the load at the highest starting pressure. For example, a suitably sized cryopump for 10,000 liters/ second starting pumpdown at 10 torr will have a radiation load of about 0.05 watt and a condensation load of about 0.05 watt-a total of 0.10 watt. Starting at 10' torr, the pump will be the same physical size but the loads will be 0.05 watt for radiation and 5.0 watts for condensation, or a total of 5.05 watts refrigeration load. This refrigeration load is raised to 50.05 watts if the pump must begin to operate at about 10 torr. In patent application Ser. No. 521,082, filed in the names of Walter H. Hogan. and Raymond W. Moore, Jr., and assigned to the same assignee as the present application, the problems noted above which are inherent in cryopumps have been materially overcome by providing an automatically actuatable, variable-size cryopanel. This in turn effects a reduced load and pumping speed at higher pressures to match the refrigerating load to the torr-liter/ second load rather than to the liter/ second load. The panels for such a cryopump are typically made as light in weight as possible to reduce the mass and thus the cool-down load. Cryopanels of this type make possible the use of small cryogenic refrigerators in small cryopumps having pumping speeds in the range of to 100,000 liters/second. However, none of the mechanical or cryogenic devices used to attain high vacuums have been directed to, or solved the problem of, very rapid pump-down.

It is therefore a primary object of this invention to provide improved vacuum apparatus capable of achieving rapid pump-down. It is another object of this invention to provide vacuum apparatus of the character described which is particularly suitable for space simulation chambers and some industrial processes. It is another primary object of this invention to provide an improved method of evacuating a chamber at a very rapid rate, the method comprising a combination of mechanical and cryogenic pumping steps. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combinations of elements and arrangement of parts which are adapted to effect such steps, all as exemplified in the following detailed disclosure, and the scope of the invention will be indicated in the claims.

In contrast to previous cryopump designs, the vacuum system of this invention incorporates a cryopump having a relatively large thermal mass which is matched to the cryopumping load. A separate cryopumping chamber and a working chamber are provided; and they are in fluid communication. They are, however, completely controllably isolated as will be apparent in the following description. In the cryopumping chamber, there is provided a large thermal mass, cooled by appropriate refrigerating means, which is characterized by having an extensive heat transfer surface area. After the working chamber has been evacuated to a level of several torr (e.g., 1 to 10 mm. Hg) by appropriate mechanical means, the valve which controls the communication between the two chambers is opened and gas from the working chamber enters the cryopumping chamber and is immediately condensed upon the large surface area of the thermal mass thus rapidly reducing the pressure. The temperature of the thermal mass is raised by several degrees but not to the extent that any appreciable quantity of the condensed gases boils olf. Refrigeration to the cryopumping surface is continued and external pumping may also be continuued for as long as desired. Prior to raising the pressure in the work chamber for withdrawal of test specimens, work pieces or the like, the valve between the work chamber and cryopump chamber is closed. The thermal mass is cooled again to its lower temperature level in preparation for re-evacuation of the work chamber, Periodically, the cryopump chamber may be warmed up and pumped out by means of a suitable mechanical pump.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which:

FIG. 1 is a longitudinal cross section through one embodiment of a vacuum chamber system of this invention having a refrigerator built into the system;

FIG. 2 is a cross section along line 22 of FIG. 1;

FIG. 3 is a cross section along line 3-3 of FIG. 1;

FIG. 4 is a fragmentary cross section of the lower part of the cryopumping chamber showing the use of externally supplied refrigerants in place of a refrigerator;

FIG. 5 is a longitudinal cross section of another embodiment of the vacuum system of this invention; and

FIG. 6 is a diagrammatic sketch of a vacuum system employing multiple work chambers with a single cryopumping chamber.

The embodiment shown in FIG. 1 has the working chamber in line with the cryopump chamber, and a builtin refrigerator. It also illustrates the use of a solid material such as lead as the thermal rnass. In FIG..1 the vacuum-tight housing 10 having an attaching flange 11 defines a working chamber 12. It will be appreciated that the working chamber 12 may be of any size and configuration, the choice being determined by the use of the vacuum system. Its representation in FIG. 1 as an essentially hemispherical chamber is only exemplary and is to be considered as illustrative only. A fluid exhaust conduit 13 communicates, through a branch fluid conduit 14, with a mechanical vacuum pump 15; and, through a branch fluid conduit 17, with a vacuum pump 18 suitable for pumping in the sub-torr range. The pump 18 may be a diffusion, ion, or sublimation pump.

Valves

16 and 19 are located in

branch fluid conduits

14 and 17, respectively. Main vacuum valve 20 controls the flow of fluid between the working chamber 12 and the cryopump 26. This may be a gate valve, or any other suitable valve used in vacuum systems. The valve 20 is located within a valve housing 21 which has a flange 22 for achieving a fluidtight seal with flange 11 of the vacuum-tight housing 10 through the use of suitable means such as screws 23. The valve housing 21 is also equipped with a lower flange 24 for attachment to the cryopump 26, and the opening and closing of valve 20 is achieved through suitable actuating means shown schematically at numeral 25.

It will be appreciated that throughout the description of the apparatus of this invention it is necessary always to form vacuum-tight seals between the various components. Since there are many ways in which this may be done, e.g., elastomeric O-rings, welding, etc., such seals will not be shown in detail.

The cryopumping section 26 is seen to comprise an outer cryopump housing 28 which defines within it a chamber 29 in which the main cryopump is located, Within this outer cryopump housing 28 is located the cryopump chamber housing preferably constructed of a material such as thin stainless steel. It defines the actual cryopump chamber 36, and it is spaced from and affixed to the outer housing 28 and the valve body by a suitable spacing ring 37. A radiation shield 38 surrounds the lower portion of the cryopump chamber housing 35 and with it defines the lower portion to the cryopump chamber 36, there being fluid communication through openings 39. The cylindrical wall 35 defining the upper portion of the cryopumping chamber has a bottom plate 40 which serves as a portion of the thermal mass and which supports the remaining portion of the thermal mass illustrated here as comprising extended heat-exchange surfaces in the form of concentric cylindrical configurations 41 (see also FIG. 2).

The cryopumping surfaces 40 and 41 and the radiation shield 38 are cooled by suitable refrigeration means which in FIG. 1 is shown to comprise a refrigerator'45 built into the cryopumping section. A suitable refrigerator for this purpose is described in US. Patent 3,218,815 (seein particular, FIG. 6 of that patent). Such a refrigerator, shown as a stepped

refrigerator comprising sections

46 and 47 in FIG. 1, is capable of delivering refrigeration at two temperature levels, corresponding to heat

stations

68 and 85 of the refrigerator of FIG. 6 of US. Patent 3,218,815.'Refrigeration at the lower temperature level (e.g., from 14 to 20 K.) is provided at the end of section 46 which is bonded to bottom plate 40 and is iii-direct heat exchange contact with plate 40 and through it to the heat-exchange surfaces 41. Inasmuch as the refrigerator is not part of the invention, the auxiliary equipment of the refrigerator (driving means, refrigerating fluid source, etc.) are diagrammatically represented byfthe numeral 48.

The radiation shield 38, as illustrated inFIG. 1, has a bottom plate 49 which serves as the lower wall of the cyropumping chamber and as the thermal connection to the refrigerator. Refrigeration to the radiation shield is delivered at the higher temperature level (e.g., about 70 to 80' K.). Using the refrigerator of FIG. '6 of US. Patent 3,218,815 as a typical suitable refrigerator, the radiation shield would be bonded in heat exchange relationship to heat station 68 which draws its refrigeration from expansion chamber 50. The remaining portion of the radiation shielding comprises a cylindrical section 50, an upper annular ring 51 which passes through cryopump housing wall 35 and makes a fluid-tight seal therewith, and an inclined ring 52 which extends within the cryopump chamber 36. The radiation shield is closed by means of a top plate 53 supported on the inclined section 52 through heat-conducting supports 54 (see also FIG. 3).

Inasmuch as it may be necessary periodically to pump out the cryopump chamber 36, a mechanical vacuum pump 42 may be connected to the cryopump chamber 36 through a fluid conduit 43, controlled by valve 44.

That volume which is defined between the inside wall of the outer housing 28 and the outer walls of the inner housing 35 and radiation shielding bottom plate 47 and walls and 51 serves as insulation for the cryopump chamber. As such it may be evacuated, or it may be filled with finely divided particulate insulation or with a foamed-in-place insulation. Many suitable insulating systems are known for this purpose.

It is also within the scope of this invention to provide refrigeration to the cryopumping thermal mass and to the radiation shielding by the circulation ofcryogenic fluids, such as in heat-exchang relationship with

bottom plates

40 and 49. This is illustrated in FIG. 4. Coils 55, made of a suitable materials such as stainless steel, are bonded in heat exchange relationship with bottom plate 40 of the thermal mass. Helium gas at temperature of about 15 K. to 20 K., or liquid helium, is then introduced into the coils by means of line 56 and withdrawn through line 57. These lines are seen to be insulated with a suitable insulation 58. In a similar manner liquid nitrogen is circulated in coils 60 which are bonded in heat exchange relationship to bottom plate 49 of the radiation shielding. The liquid nitrogen is brought in through linlet conduit 61 and withdrawn as gaseous nitrogen through conduit 62, inlet conduit .61, having a suitable insulation 63.

FIG. 5 illustrates another embodiment of the apparatus of this invention in which the working chamber is distinct from the cryopump chamber and is connected through an appropriate fluid conduit with the valve being located within this fluid conduit. In FIGS. 1 and 5 like elements are referred to by like numerals. In the apparatus of FIG. 5 the working chamber 65 has a flange 66 which is designed to provide the necessary fluid-tight and vacuum-tight seal with an end cover 67 which has a' corresponding flange 68 suitabl for bolting to flange 66 through screws 69. The end cover 67 may be so designed as to swing away from the working chamber 65 or otherwise be removed therefrom in order to permit access to the working chamber 70. As in the apparatus of FIG. 1, the working chamber 70 is adapted to be evacuated by a mechanical pump 15 and by a pump 18 suitable for pumping in the sub-torr range.

The working chamber 70 is in fluid communication with the cryopump chamber through the conduit 73 which contains vacuum valve 20. Valve 20, in turn, is connected to the cryopump 26 which in the embodiment of FIG. 5 comprises an outer housing 75 defining an outer chamber 76 which contains the inner cryopump chamber wall 78 extending therein. As in FIG .1, a radiation shielding 38 is used and forms part of the housing for the cryopump chamber 36. The thermal mass of the embodiment of FIG. 5 is high-pressure helium (20,000 to 200,000 p.s.i.). This is contained in a plurality of thinwall stainless tubes 80 communicating with a suitable manifold 81. Refrigeration is supplied to the manifold 81, and hence to the thermal mass of high-pressure helium, and to the radiation shielding by a refrigerator as in FIG. 1. Alternatively, the'tubes 80 may contain a fluid having a heat capacity which derives from a phase change, e.g., from a liquid to a gas. Neon may be cited as an example of such a fluid.

FIG. 6 illustrates diagrammatically how more than one working chamber may be connected to and served by a single cryopumping system 26. The cryopumping system 26 may be of any of the embodiments illustrated and described and the working

chambers

85, 86 and 87 may be of any desired configuration. It is to be understood that mechanical vacuum pumps and vacuum pumps capable of pumping in sub torr ranges are to be supplied as discussed in detail in th description of FIG. 1. A single main fluid conduit 88 and

branch conduits

89, 90 and 91 provide the necessary fluid communications which are controlled by

main valves

92, 93 and 94, respectively. In the apparatus of FIG. 6 the working

chambers

85, 86 and 87 may be connected in sequence with the cryopump 26, or they may b served simultaneously. The use of a multiplicity of separate and distinct working chambers permits more flexibility in operation and provides, in general, more efficient and continuous use of the cryopump.

The operation of the vacuum system of this invention may be described with reference to the apparatus of FIG. 1. To begin the operation, valve 20 is closed, thus closing off the fluid communication between the working chamber 12 and the cryopump. This then permits access to the working chamber and the insertion of test specimens or items to be processed within the chamber. Subsequent to the effecting of a fluid-tight seal between the work chamber and the valve body, valve 16 is opened to the mechanical vacuum pump 15 and the work chamber is evacuated to a pressure ranging between a few torr and a fraction of a torr. Valve 16 is then closed, and .main vacuum valve 20 is opened. At this point in the process, the thermal mass comprising the bottom plate 40 and the extended surfaces 41 has been cooled by refrigeration to about 15 to 20 K. Only a few seconds are required to condense a large portion of the gaseous molecules (which have entered the cryopump 36 from the work area 12) on the extended surfaces of the thermal mass thus reducing the pressure within the entire system to about torr or even lower. In this condensation the temperature of the thermal mass is raised to about 30 K. Once this pressure level of about 10 torr has been reached, the heat load on the thermal mass is small and the refrigeration delivered by the refrigerator may be used to cool down the thermal mass from its upper temperature of about 30 K. to

to 20 K. If pressures of less than about 10* torr are required in the working chamber 12 then valve 19 is opened and pumping is continued by the pumping means 18 which, as explained above, may be an ion pump, a diffusion pump or a sublimation pump. In this operation it will be seen that each of the different types of vacuum pump means has been employed over that range in which it operates most efficiently and most rapidly. For example, the mechanical pump 15 is capable of pumping out the work volume .12 to a few torr in a matter of a few seconds or a few minutes. The cryopump then reduces the pressure of the order of l0 torr very rapidly and finally the ion pump or diffusion pump can remove the noncondensables, e.g., hydrogen and neon, from the work area at a relatively rapid rate.

At the end of the test period or processing period, valve 20 is again closed, air or some other gas is introduced into the work chamber 12 and it is returned to atmospheric pressure. The cryopump, on the other hand, remains at reduced pressures and the cryopumping thermal mass is returned to its lowest temperature if this has not already been accomplished.

Two typical applications of this vacuum system may be given to illustrate its operation and use.

Assume in the first example that a work chamber having a volume of 44,000 cubic feet is to be used to simulate the very rapid ascent of a space vehicle and that it is required to pump this chamber down to about 10" torr in four minutes. Rough evacuation to a fraction of a torr, e.g., 0.38 torr, or /2000 atmosphere, can be achieved by mechanical pumps in approximately two minutes, leaving two minutes to pump down to 10* torr. This, in effect, requires at this stage the attainment of a pumping speed of about 100,000 liters per second. N

If a conventional, or prior art, cryopump were to be used to provide cryopumping at this required rate, it would be necessary to use a cryogenic refrigerator having a capacity of several thousand watts. However, once steady cryopumping at 10* torr, or lower, is attained, the refrigeration requirement drops drastically to only a few watts which means that the remaining capacity of the refrigerator is in effect wasted.

In contrast to the refrigerator capacity demanded by prior art cryopumps to attain the rapid pump-down, a vacuum system constructed according to this invention would require a refrigerator having a capacity of about 120 watts if as little as one hour is allowed between successive pump-downs. This is due to the fact that the refrigeration required over the two-minute pump-down from a fraction of a torr to 10 torr is furnished by the thermal mass, and not by the refrigerator. To condense the air in the work chamber of this example from, say, 0.38 torr (V atmosphere) requires refrigeration of 370 B.t.u. or 108 watt hours. The heat capacity of lead (the thermal mass) between15 K. and 30 K. is about 0.37 B.t.u. per pound, so that about 1,000 pounds of lead initially at 15 K. (a little over a cubic: foot), presenting a surface area between 40 and square feet, would warm up to 30 K. while condensing a mass of air equivalent to the 22 standard cubic feet which must be removed to attain 10 torr in this chamber. The initial heat flux would be about 20,000 watts and the average over the two-minute pump-down would be about 3,000 watts. The heat load at the end of this period, when the pressure is down to 10- torr, is less than 1 watt due to condensation and radiation; and it may be about 10 watts due to conduction heat leaks down the support structure for the thermal mass. Since the initial heat load in condensing out the condensable constituents of the gas at 0.38 torr is taken by the heat capacity of the thermal mass, the refrigerator used need only be sized for the average duty placed upon it to cool down the thermal mass within a specified time period and to maintain steady state cryopumping, i.e., that which is required by virtue of heat leaks into and radiation heat transfer within the cryopump.

In the vacuum system of this example, assume further that it is required to repeat the pump-down of the working chamber from atmospheric pressure to 10- torr every hour. Thermodynamic calculations show that 108 Watts of refrigeration would be required to cool the thousand pounds of lead from 30 K. to 15 K. in the hour interval and that an additional one to 10 watts are needed to meet steady-state load requirements over that hour. 'Hence, the refrigerator must have a capacity of about 120 watts. If it were necessary to repeat this pumpdown only once every ten hours, then similar calculations show that a refrigerator having a capacity of from about 10 to 20 watts is adequate.

For many uses, it may not be necessary to use mechanical pumping at all. As a second example of the operation of the vacuum system of this invention, assume that a small working chamber is to be rapidly evacuated. Typically, such a chamber is a bell jar which may have a volume of about two cubic feet. Using the figure of 16.8 B.t.u./ standard cubic foot as the energy required to condense the condensable constituents in air to attain a vauum of about l torr, it follows that the total refrigeration load is 33.6 B.t.u. This could be adequately provided by about one hundred pounds of lead having a surface area of about one square foot. The pump-down, once the main vacuum valve is opened, is almost instantaneous and the noncondensable residual gases can be rapidly removed by suitable pumping means well known in the art.

Once the choice of the thermal mass material (e.g., lead, copper, high-pressure helium or a fluid capable of experiencing a phase change in the cryogenic temperature range involved) has been made, the weight of the thermal mass may be readily calculated from its known physical properties to meet any desired performance characteristics. In general, the thermal mass may be defined as that quantity which is required to provide essentially all of the refrigeration required for the transient pump-down condensation to about torr whether it begins at atmospheric pressure or at a pressure in the range of a fraction of a torr to a few torr. For all practical purposes, this amount of thermal mass will be at least 10 times that which would be required if the refrigeration were supplied directly from the refrigerating means rather than stored in the thermal mass.

Finally, the operation of the vacuum system of this invention may be described in terms of a comparison with the prior art approach as it is described in the literature. (See for example The Journal of Vacuum Science and Technology, 3, No. 5: 252257 (September-October 1966).) From this reference, temperature levels, refrigeration capacity and cryo-array geometry requirements can be determined. For example, a 100,000 liter per second pump to operate at a steady state pressure of 10- torr would require about 40 square feet of condensing surface. Panels making up square feet arranged to condense on both sides could effect this speed. In order to minimize temperature differences from one part to another of this panel, and to insure the panel is adequately rigid, a reasonable choice for panel material would be to ,4 inch thick high purity copper. The weight of these panels would be only 55 to 110 pounds, with a heat capacity between 15 K. and K. of 4.4 B.t.u. to 8.8 B.t.u. If this pump were used in the transient mode as described in the first example of the operation of the vacuum system of this invention, it would, if the cryopanels were initially cooled to 15 K., be able to pump down the system to 10 torr only if cryopumping began at a pressure no higher than 5 10- torr to 10* torr, depending on the mass used, rather than from 0.38 torr as shown in the example. This, of course, does not make it possible to achieve the rapid pump-down through the diflicult region between about 1 torr and 10 torr which is achieved through the use of a thermal mass.

It will be seen from the above description and detailed discussion that there is provided method and apparatus for rapidly attaining vacuums in the range of about 10" torr. Moreover, with additional vacuum pumping means the pressures may be lowered to 10 The apparatus is flexible in its operation, relative simple to construct and in some modifications can be used continuously.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efliciently attained; and since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A vacuum system, comprising in combination:

(a) a work chamber;

(b) a cryopump chamber containing thermal mass means in the form of extended surface area, said thermal mass means being present in at least that quantity which is capable of providing essentially all of the refrigeration required for the transient pump-down from more than about 10 torr, through condensation of gases, to about l0 torr;

(c) refrigerating means associated with said thermal mass means and adapted to cool it to a temperature below that at which the major constituents of said gases condense and solidify;

(d) fluid conduit means connecting said Work chamber and said cryopump chamber; and

(e) valve means located in said fluid conduit means and adapted to control the flow of fluid between said Work chamber and said cryopump chamber.

2. A vacuum system according to claim 1 wherein said thermal mass means is at least ten times that required in normal cryopumping wherein essentially all of the refrigeration required for said condensation is supplied directly from said refrigerating means.

3. A vacuum system according to claim 1 wherein said thermal mass means is formed of a metal having a high heat capacity at cryogenic tempera-tures.

4. A vacuum system according to claim 3 wherein said metal is lead.

5. A vacuum system according to claim 1 wherein said thermal mass means is a fluid capable of undergoing a phase change within the temperature change range experienced by said thermal mass means.

6. A vacuum system according to claim 1 wherein said thermal mass means is high-pressure helium contained in thin walled tubing.

7. A vacuum system according to claim 1 wherein said refrigerating means is a cryogenic refrigerator forming an integral part of said cryopump.

8. A vacuum system according to claim 1 further characterized by having mechanical vacuum pumping means in controllable fluid communication with said work chamher.

9. A vacuum system according to claim -8 including vacuum pumping means capable of pumping in the subtorr region in controllable fluid communication with said work chamber.

10. A vacuum system according to claim 1 further characterized by having mechanical vacuum pumping means in controllable fluid communication with said cryopump chamber.

11. A vacuum system, comprising in combination:

(a) a work chamber;

(b) a cryopump chamber containing thermal mass means in the form of extended surface area, said thermal mass means being present in at least that quantity which is capable of providing essentially all of the refrigeration required for the transient pumpdown from more than about 10- torr, through condensation of gases, to about 10- torr;

(c) radiation shielding means surrounding at least that portion of said cryopump chamber containing said thermal mass means;

((1) outer housing means surrounding said cryopump chamber and said radiation shielding means and adapted to provide thermal insulation therefor;

(e) refrigeration means adapted to cool said thermal mass means to a temperature below that at which the major constituents of said gases condense and solidify and to cool said radiation shielding means;

(f) fluid conduit means connecting said work chamber and said cryopump chamber; and

(g) valve means located in said fluid conduit means and adapted to control the flow of fluid between said work chamber and said cryopump chamber.

12. A vacuum system according to claim 11 wherein said refrigerating means is a cryogenic refrigerator capable of delivering refrigeration at two temperature levels, the lower level being used to refrigerate said thermal mass means and the upper level to refrigerate said radiation shielding means.

13. A vacuum system according to claim 11 including mechanical vacuum pumping means in controllable fluid communication with said work chamber.

14. A vacuum system according to claim 12 including vacuum pumping means capable of pumping in the subtorr region in controllable fluid communication with said work chamber.

15. A vacuum system according to claim 11 including mechanical vacuum pumping means in controllable fluid communication with said cryopump chamber.

16. A vacuum system, comprising in combination:

(a) a plurality of work chambers;

(b) a cryopump chamber containing thermal mass means in the form of extended surface area, said thermal mass means being present in at least that quantity which is capable of providing essentially all of the refrigeration required for the transient pumpdown from more than about torr, through condensation of gases, to about 10- torr;

(-c) refrigerating means associated with said thermal mass means and adapted to cool it to a temperature below that at which the major constituents of said gases condense and solidify;

(d) fluid conduit means connecting each of said work chambers with said cryopump chamber; and

(e) valve means located in said fluid conduit means and adapted to control the flow of fluid between each of said work chambers and said cryopumping chamber.

17. A method of rapidly reducing the pressure of a gas mixture within a chamber from atmospheric down to about 10- torr, characterized by the step of causing the condensable constituents in said gas mixture suddenly to contact previously cooled thermal mass means whereby said condensable constituents are solidified on the surface of said thermal mass means, said thermal mass means being cooled to a temperature sufliciently low so that its temperature subsequent to the solidification of said condensable constituents remains below that at which said constituents are solidified.

18. A method in accordance with claim 17 wherein said thermal mass means is cooled to between 15 and 20 K. prior to said sudden contacting and rises to a temperature no higher than about 30 K. during the solidification of said condensable constituents thereon.

19. A method in accordance with claim 17 wherein said step of suddenly contacting is preceded by mechanically pumping to reduce the pressure in said chamber down to a range between a few torr and a fraction of a torr.

References Cited UNITED STATES PATENTS 3,144,200 8/ 1964 Taylor 62-555 3,168,819 2/1965 Santeler 62-555 3,252,652 5/ 1966 Trendelenburg 62-555 MEYER PERLIN, Primary Examiner US. Cl. X.R. 62-268