US3338063A - Cryopanels for cryopumps and cryopumps incorporating them - Google Patents

Description

g- 29, 1967 w. H. HOGAN ETAL' 3,338,063

CRYOPANELS FOR CRYOPUMPS AND CRYOPUMPS INCORPORATING THEM Filed Jan. 17, 1966 3 Sheets-Sheet 1 CALCULATED N CRYOPUMPING SPEED PER WATT OF REFRIGERATION AT TEMPERATURES BELOW 20K I I I I I0" I0 Io IO'3 Io STARTING PRESSURE (TORR) Fig.2

CRYOPUMPING SPEED (LlTER/SEC/WATT) 6 INVENTOR. Wulter H. Hogan Raymond W. Moore,Jr.

L44 att ozn ey Aug. 29, 1967 w. H. HOGAN ETAL 3,338,063

CRYOPANBLS FOR CRYQPUMPS AND CRYOPUMPS INCORPORATING THEM Filed Jan. 17, 1966 3 Sheets-Sheet E v 28 Q 27 \l Fig. 4

a (1 Q Q W E I 34 I5 l6 l8 1 l ROUGHING l? PUMP OIL Fug. 3

DIFFUSION 1 PUMP IQ, 'VVN- 7 INVENTOR.

Walter H. Hogan Raymond W. Moore AttoFn ey a Aug. 29, 1967 w, HOGAN ETAL 3,338,063

CRYOPANELS FOR CRYOPUMPS AND CRYOPUMPS INCORPORATING THEM Filed Jan. 17, 1966 3 Sheets-Sheet 3 RADIATION SHIELD TEMPERATURE so L REGION OF 5 como ucnow so I O F (1:125

40 5 X IO 4 Torr L I v 20 l X IO 4 forr REGION OF CRYOPUMPING O 23 /7 !2? 28 CRYOPANEL LOCATION F I 2 lIvvEIv'roR.

Walter H. Hogan Raymond W. Moore,Jr.

ABSTRACT OF THE DISCLOSURE The cryopanels for cryopumps disclosed herein are .formed of a plurality of separate cryopanels which are connected by temperature discontinuity means serving as thermal resistances. These thermal resistances are so designed as to effect automatic engagement of succeeding cryopanels as the pressure within the cryopump decreases. This in turn permits optimum use of the refrigerator means.

cryopumps have within the past few years received considerable attention since they offer a number of advantages over other means for attaining ultrahigh vacuums, e.g., in the pressure range from about 10- to 10- torr. Basically, cryopumping provides a refrigerated surface on which gas molecules are condensed or sorbed. Vacuum systems incorporating cryopumps are normally designed so that the gross pumping, i.e., down to 10- or 10- torr is achieved through the use of mechanical vacuum pumps. With the attainment of this pressure range, it is then practical to begin cryopumping and to supplement the cryopumping with a small ion or diffusion pump which removes the noncondensables, i.e., neon, hydrogen, and helium.

Refen'geration to the cryopanel is normally supplied by circulating a cryogenic fluid (liquid hydrogen, or

"liquid helium) in thermal contact with the cryopanel to provide the necessary cryopumping surface. In such cases, the amount of refrigeration delivered can be fairly well controlled by controlling the mass flow of the cryogenic liquid. Such cryopumps are now widely used in large space simulation chambers where pumping speeds ranging from 100,000 to several million liters per second are required. However, cryopumping has not gained much acceptance for applications Where pumping speeds in the 100 to 10,000 liter per second range are required. This has been due to a lack of suitable mechanical refrigerators and to the inconveniences inherent in handling liquid hydrogen or liquid helium. There is, however, a need for vacuum test chambers requiring these lower pumping speeds and the cryopanels of this invention make it practical to construct and operate them using recently developed small cryogenic refrigerators. Thus, the apparatus described herein is concerned primarily with cryopumps which can be built into and integrated with small, continuously-operating, cryogenic refrigerators. Miniature cryogenic refrigerators constructed in accordance with the teachings of US. Patents 2,906,101, 2,966,035, and 3,151,466 have been found to be particularly well adapted to be built into small space simulation chambers and test chamber apparatus requiring extremely good vacuum. Although such refrigerators may be built in a wide variety of sizes to deliver a wide range of refrigeration, they can conveniently be sized to deliver in the order of 1 watt of refrigeration below 20 K. This represents the refrigeration load normally associated with a cryopump having a pump speed of a few thousand liters per second. However, it is not meant to limit the cryopanel of this invention to any size range of the cryopump.

United States Patent The refrigeration load placed upon the refrigerator used to cool a cryopanel must be determined both with respect to that required to effect the pumping to reduce the gas pressure to a desired level and to that required to maintain it over a period of time at that pressure a a given temperature, it has been usual in such cryopumpsv to employ a cryopanel, the surface area of which represented a compromise between that which was optimum for pumpdown and that which was optimum for sustained operation. The compromise was between a minimum surface area which could-be initially cooled to start cryopumping and a maximum surface area which was desirable when the pressure of the system had dropped and continuous operation at minimum pressure was desired. Thus, cryopumps constructed as integral parts of a refrigerator have previously suffered from this need to compromise and have therefore not been as efiicient in capturing and immobilizing gas molecules as might be desired.

We have now found that it is possible to provide a novel cryopanel for a cryopump, incorporating a refrigerator, which eliminates any necessity for such a compromise in design. Thus, the cryopump of this invention is so constructed as to be able to make optimum use at all times of the refrigeration furnished, while at the same time attaining satisfactory pumping speeds.

It is therefore a primary object of this invention to provide an improved cryo ump incorporating a cryogenic refrigerator which does not require the usual compromise in cryopumping surface area. It is another object of this invention to provide a cryopump of the character described which exhibits pumping speeds up to about 10,000 liters per second and a high capture coeflicient over its entire pumping range. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the features of construction, combinations of elements, and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

In the apparatus described herein, the necessity for compromise with regard to the cryopanel surface area is eliminated by matching optimum refrigerator operation with optimum cryopanel performance. This is accomplished by using a relatively small cryopanel surface area at the high gas pressures and then increasing this surface area as the pressure of the surface decreases. This permits a material reduction in refrigeration load when the cryopump takes over from the mechanical pump, the reduction being such that the load is not substantially greater than for continuous low-pressure operation. The engagement of additional cryopanel condensing surfaces into the system as part of the refrigeration load can not be achieved through externally manipulated means for such means would require mechanical connections between the cryopanel within the cryopump and the ambient atmosphere. Such mechanical connections in turn would provide heat leak paths which can not be tolerated in an efiicient cryopump.

To effect the required automatic and internally controlled engagement of one or more additional increments larger than, the condensation surface of the primary cryopanel, and one or more thermal resistances between the cryopanels. The thermal resistances provide a temperature discontinuity between the primary and secondary panels such that the primary panel which is in direct thermal contact with the refrigerator is always at a lower temperature than the secondary panel or panels. The temperature difference which defines the temperature discontinuity is a function of the gas pressure in the cryopump; and when this pressure drops to a level at which the cryopump load on the secondary panel is acceptable to the refrigerating means, the secondary panel is engaged, cooled and becomes cryopumping.

Although the cryopanel system of this invention will be described in terms of its use with a cryogenic refrigerator, it is to be understood that it functions equally well when refrigeration is delivered to the primary panel by contacting it with a circulating cryogenic fluid.

For a fuller understanding of the nature and objects of the invention, reference should be had to be following detailed description taken in connection with the accompanying drawings in which FIG. 1 is a schematic plot of cryopump pressure versus cryopumping surface area or pumping speed for two difierent levels of refrigeration;

FIG. 2 is a plot illustrating the calculated relationship between cryopumping speed for nitrogen per watt of refrigeration and the starting pressure of the cryopump;

FIG. 3 is a cross-section of a cryopump incorporating the cryopanel system of this invention;

, FIGS. 4-8 are top plan views of four different cryopanel system configurations using a wire as the thermal resistance connecting the cryopanel sections;

FIG. 9 is a side view, partially in cross-section, illustrating the use of a single panel having the thermal resistance the thermal load requirements of the cryopanel. This thermal load originates from three main sourcesradiation, condensation or absorption, and conduction. Radiation is independent of pressure and is normally minimized by the use of cooled radiation shielding. The thermal load is therefore primarily a function of gas pressure-the higher. the pressure, the higher the thermal load. This means that for a given refrigeration capacity of a'cryogenie refrigerator in the cryopump the higher pressures in the system (i.e., the greater number of gas molecules which must be cooled and immobilized) require smaller cryopanel areas and lower pumping speeds. As the pressure within the system decreases, the cryopanel surface area can be increased.

The problem confronted in the design of a cryopump, in which a cryogenic refrigerator is incorporated, can best be described with reference to the schematic plot presented in FIG. 1. Assume that the cryopanel is in direct thermal contact with the coldest end of a small refrigerator. Once the cryopanel is cooled down to temperature, it will provide a given surface area with which the gas molecules will come in contact through their natural motion within the sytem. It will be appreciated that the greater the pressure in the system, the more gas molecules are present in a given volume so that the number of gas molecules striking the cryopanel surface is directly portional to the pressure. Also, this number is proportional 4- to the cryopanel area surface. It is therefore possible to consider either cryopumping surface area or pumping speed as one parameter of the system as indicated in the ordinate of FIG. 1. Cryopumping must begin at a fixed pressure within the system which is determined by the capabilities of the mechanical roughing pump. Since pumping capacity is limited by the capacity of the refrigerator, the torr liters which can be pumped is therefore limited. Assume, for example, that there is one unit of refrigeration available from the refrigerator to cool the cryopanel surface. Then it will be seen from the schematic plot in FIG. 1 that a pumping speed of 10 can be attained or a cryopumping surface area of 10 can be used if the pressure in the cryopump, e.g., at the time the mechanical pump is shut down, is at the arbitrary value of 1. If the pressure in the cryopump at the time cryopumping begins is an order of magnitude greater, i.e., 10, then with the fixed refrigeration of one unit the cryopumping speed is only one on this arbitrary scale or the cryopumping surface area must be small. Thus, the need for compromise in the usual design of the cryopanel is immediately apparent; for although pumping speeds and cryopumping surface areas are advantageously fairly large at the very low pressures, they must be fairly small as cryopumping is begun when the pressure is an order of magnitude greater. This may be stated in another way; that is, of cryopumping is to begin at 10 on this scale and is to achieve a relatively good cryopumping speed of 10, then ten units of refrigeration must be furnished--a solution which would require a refrigerator ten times larger than would be required for normal operation at a pressure of one or less.

The apparatus of this invention, by making it possible automatically to engage additional increments of cryopanel surface area as the system pressure decreases thus makes it possible to attain maximum performance throughout the entire cryopumping period. Returning to FIG. 1, assume that cryopumping is begun when the pressure in the system is 10 on the arbitrary scale. For a fixed refrigeration capacity of one unit, it is possible to cool a cryopanel area of one. When the pressure has dropped to one on this scale, then it is possible with the same refrigeration capacity to cool a cryopanel area of 10 and raise the pumping speed by a comparable factor. Thus, the operational compromise dictated by a fixed cryopanel surface area need no longer be made.

FIG. 2 is a plot of the calculated nitrogen cryopumping speed attainable per watt of refrigeration capacity at temperatures below 20 K. using a liquid nitrogen-cooled radiation shield. It will be seen that one watt of refrigeration would be sufiicient to maintain a nitrogen pumping speed of about 100,000 liters per second up to a pressure of 10- torr. However, mechanical pumps used for initial evacuation of a vacuum system typically provide a lower limit of 10% torr for a single-stage pump, to 10' torr for a two-stage pump in good condition. This would limit the size of cryopump to take over from the mechanical pump to 200 liters per second at 10- torr to 15,000 liters per second at 10* torr. Since a direct relationship exists between cryopumping speed and cryopanel surface area, the desirability of having a small surface area at the higher pressures becomes apparent. The apparatus of this invention supplies the means for increasing the cryopanel surface area with decreasing system pressure. Thus, when cryopumping is first begun a relatively small cryopanel surface area is engaged and is cooled by the refrigerator. As the gas pressure within the system decreases through the immobilization of gas molecules on the cryopanel surface, additional cryopanel surface area is engaged, thus providing optimum conditions throughout the cryopumping cycle.

Before describing the actual operation of the cryopanel system of this invention it will be helpful to describe a typical cryopump and to examine various modifications and embodiments of the cryopanel system.

FIG. 3 illustrates in cross-section what might be com sidered to be a typical cryopump incorporating the novel cryopanel of this invention. The cryopump consists of an upper working or test section joined to a lower pumping section 11 through a suitable joining member 12. Within upper section 10 is a working volume 13 available for experimental purposes. Within lower section 11 is a volume 14 containing the cryopumping mechanism. Inasmuch as the evacuation process begins with mechanical pumping, there is supplied a conduit 15 controlled by a valve 16 which leads to a mechanical roughing pump (not shown). There is also provided a branch conduit 17 controlled by valve 18 which leads to an oil diffusion or ion pump (not shown). In keeping with normal cryopumping practice, the roughing pump is used to lower the pressure within the apparatus to about 10- torr; while the oil diffusion pump is used to remove the residual noncondensables, i.e., neon, hydrogen, and helium.

The cryopumping mechanism in the arrangement shown in FIG. 3 derives its refrigeration from a refrigerator 20 which is shown to be integrally incorporated into the cryopump. The coldest end 21 of the refrigerator provides the refrigeration directly to the cryopanel generally indicated by the numeral 25. The cryopanel comprises a primary panel 26 which is in direct thermal contact with the cold end 21 of the refrigerator and a secondary panel 27 having an area equal to or greater than the primary panel and joined to it through a thermal resistance 28 which gives rise to a temperature discontinuity between the panels. As will be seen in the following detailed description, the primary and secondary panels may take various forms and the thermal resistance may also take a variety of forms.

In order to reduce the heat leak from the ambient atmosphere to the cryopanel, there is provided around the cold end of the refrigerator and the cryopanel suitable radiation shielding means which in FIG. 3 takes the form of a combination of a cylindrical radiation shield 30 and chevron shields 31 positioned above the cryopanel. In order to make the radiation shield more effective, it is cooled with liquid nitrogen which is circulated through cooling coils 32 which are in turn in direct thermal contact with the wall of the radiation shield 30 and with the chevrons 31. Liquid nitrogen, introduced into the cryopump through a suitable inlet conduit 33, circulates first through coils 32 associated with the chevrons and then with the cooling coil wound around the cylindrical radiation shield 30. The nitrogen is withdrawn from the system through conduit 34.

FIGS. 4-11 illustrate a number of forms which the cryopanel system of this invention may take. In FIG. 4 it is seen that the primary panel 26 is in the form of a thin circular plate having a single condensing surface 27 surrounding the circular plate. The thermal resistance joining these two panels comprises four wires 28. It is preferable, although not necessary, that the wires 28 are formed from a metal which exhibits an increasing thermal conductivity with decreasing temperature. As an example of such metals we may cite silver which is 99.999% pure, high-purity copper, coalesced copper, and single crystal aluminum. Extremely pure silver, has, for example, a thermal conductivity of 9 watts/cm. K at 40 K. and reaches a maximum of about 180 watts/cm. K. at about 6 K. High-purity copper has a thermal conductivity of about 20 watts/cm. K. at 40 K. and reaches a maximum thermal conductivity of about 140 watts/cm. K. at about 15 K. (See for example FIG. 10.7 in Cryogenic Engineering, by R. B. Scott, P. Van Nostrand Company, Inc., Princeton, 1959.)

The direct thermal contact between the cold end 21 of the refrigerator and the small primary cryopanel 26 elfects the necessary cooling of the condensing surface 22. The temperature of surface 22 is soon low enough to capture and immobilize a portion of the gas molecules striking it. However, because the secondary panel 27 is connected to the primary panel 26 only through one or more wires which are thermal resistances, the heat transferred to the secondary panel is transferred to the panel through a relatively large temperature difierence. This temperature difference maintains the secondary panel at a temperature too high to condense gas molecules. The heat transferred to the panels is only that due to radiation and gas conduction, which may be a small fraction of the possible gas condensation load if the secondary and primary panels were at the same temperature. This condition permits the optimum utilization of the refrigeration delivered and pumping to continue. As the pressure decreases within the system, the heat conduction to the secondary panel is continued and finally a point is reached where the secondary panel becomes cold enough to capture and immobilize gas, i.e., it becomes a cryopumping surface 23 (see FIG. 3).

FIG. 12, which is a schematic, somewhat stylized diagram represents a typical situation, using a cryogenic refrigerator such as shown in US. Patent 3,151,466 and helium as the refrigerant. It will be seen that, at a gas pressure of 5 10- torr, the gas conduction heat transfer to the secondary panel from the radiation shield is enough to maintain that panel at a temperature much higher than that of the primary panel, because of the thermal resistance joining the two panels. Under these conditions, a major heat load is applied to the primary panel as it captures and immobilizes gas molecules received from a gas at a pressure of 5 10- torr; a minor heat load by gas conduction from the radiation shield is received by the secondary panel and conducted through a thermal resistance to the primary panel. At a gas pressure of l l0 torr the heat loads on both the primary and secondary panels are reduced and consequently the temperature difference between them is reduced. In the example shown, even at a gas pressure of 5Xl0 torr, a sharp temperature discontinuity between the primary and secondary panels exists to maintain the secondary panel at a temperature above which the major load of cryopumping can be effected. At a pressure of 1 l0 torr, or lower, the heat load to both panels is such that both can be cryopumping. There will, however, always be a small temperature discontinuity because of the manner in which the panels are thermally connected.

If, in the embodiment shown in FIGS. 4-8, the thermal resistance is a wire formed of a metal which displays an increasing thermal conductivity with decreasing temperature the engagement of the secondary panel is more rapid at the time such engagement is desired.

The modification shown in FIG. 5 illustrates the use of two secondary panels, the outer one designates as 27 which is connected to the inner secondary panel by a thermal resistance 28'. This cryopanel system functions in the same manner described for the system of FIG. 4. The thermal resistances 28 and 28', which are connected in series, may be of equal or different thermal resistance values.

FIG. 6 shows a construction'in which the circular configuration is replaced by a polygonal configuration which, of course, may be in the form of a square, rectangle, or other shape.

In FIG. 7 the larger secondary panel 37 is placed at one side of the smaller primary panel 36 and joined by a wire 38 which may be coiled or formed into a serrated configuration to increase its length without materially increasing the distance between the primary and secondary panels.

In FIG. 8, two secondary panels 37 and 39 are connected in parallel to the primary panel 36 through thermal resistances 38 and 38'. The resistances are of difierent magnitudes to exhibit different values of resistance. Thus, these two resistances efiect different cut-in levels. The resistances may also, of course, be formed of two different materials to achieve this result.

FIG. 9 is a cross-sectional detail of another form of the cryopanel system of this invention. In this structure the cryopanel 40 is formed of a single piece of material,

e.g., copper, and the thermal resistance is created by milling a deep groove 44 in the panel 'to'join the primary panel section 41 to the secondary panel section 42 by a narrow section 43. It will be appreciated that the thermal resistance of the narrow section 43 is considerably greater than that exhibited by the total thickness of the cryopanel 40.

FIGS. 10 and 11 illustrate mechanical ways in which thermal contact may be affected between the primary and secondary panel. In these cases, the atmosphere, which is actually gas at very low pressure, separates these two I panels and forms the thermal resistance. By making actual thermal contact through the mechanical actuation process, the thermal resistance of the atmosphere is broken but a temperature discontinuity will continue to exist due to the fact that the thermal contact can not be absolutely perfect.

In FIG. 10 the primary panel 45 is cooled directly by the cold end 21 of the refrigerator 20. A bimetallic strip 47, which may be narrow compared to the lateral dimensions of either of the panels, is permanently affixed to the primary panel 45. The bimetallic strip is formed of two joined metals 48 and 49. These two metals have different thermal properties at the temperatures involved. Thus, for example, the metal strip shown at 49 is one which may have a greater coeflicient of thermal expansion than metal 48. As bimetallic strip 47 is cooled by virtue of the cooling of the primary panel 45 from the center out to itsside, the metal 4 will contract and bend the bimetallic strip downward forcing it into physical contact with the surface of the secondary panel 46.

In the modification shown in FIG. 11, the primary panel 50 is supported on a fluid-actuated expansible member 51, e.g., a bellows, which defines within it a fluid volume 52. Within the fluid volume 52 is a fluid such as helium or hydrogen. As the fluid within volume 52 is cooled by contact with the cold end 21 of the refrigerator, it will, of course, contract in volume and the expansible member will tend to move downwardly in the direction of the arrows. Contacting members 53 attached to the bottom section of the primary panel 50 will then make contact with the surface of the secondary panel 55 thus providing thermal contact between the two panels. In order to impart a positive downward motion to the central. primary panel 50, there is provided a spring 56 which is grounded to a suitable ground 57.

Thus, in each of these embodiments and modifications, as the pressure in the system decreases a point is reached where an additional increment of cryopanel surface area is engaged. Since the time of such engagement coincides with that time when the heat load on the panel through gas conduction is decreased, it is possible to furnish the refrigeration necessary to bring the additional increment of cryopanel surface area down to cryopumping temperature without increasing the overall load on the cryogenic refrigerator. This, in turn, means that a fixed amount of refrigeration may be used without having to employ a fixed and compromising cryopanel surface area. The cryopanel of this invention therefore makes it possible to provide a cryopump integrated with a cryogenic refrigerator. Such a cryopump is particularly well adapted for relatively small vacuum test chambers and for pumping speeds up to 10,000 liters per second.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and, since certain changes may be made in the above construction 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.

We claim: 1. A cryopanel array suitable for incorporation into a cryopump, comprising in combination (a) a primary cryopanel in thermal contact with refrigerating means; (b) at least one secondary cryopanel having a condensation surface area at least as great as that of said primary cryopanel, and separated from said primary cryopanel by (c) temperature discontinuity means responsive to a decrease in pressure within said cyropump whereby said temperature discontinuity means becomes sufiiciently small in effect when said pressure has dropped to a predetermined level to permit said secondary cryopanel to attain cryopumping temperature. 2. A cryopanel array in accordance with claim 1 wherein said temperature discontinuity means comprises metal wire in thermal contact with said cryopanels.

3. A cryopanel array in accordance with claim 2 wherein said metal is characterized by being one which exhibits an increasing thermal conductivity with decreasing temperature.

4. A cryopanel array in accordance with claim 1 having a plurality of secondary cryopanels connected in series.

5. A cryopanel array in accordance with claim 1 having a plurality of secondary cryopanels connected in parallel with said primary cryopanel. I

6. A cryopanel array in accordance, with claim 5 where'- in said temperature discontinuity means separating said secondary cryopanels from said primary cryopanel exhibit different thermal resistance values thereby to effect different engagement levels for said secondary cryopanels.

7. A cryopanel array in accordance with claim 1 wherein said temperature discontinuity means comprises a thin metal section between said first and second cryopanels.

8. A cryopanel array in accordance with claim 1 wherein said temperature discontinuity means is a small physical gap between said primary and secondary cryopanels, said physical gap being closable through metallic heat conductive means thereby to effect thermal contact between said cryopanels to permit said secondary cryopanel to attain said cryopumping temperature. a

9. A cryopanel array in accordance with claim 8 wherein said metallic heat conductive means in a bimetallic strip, the metals of which exhibit diflerent coefiicients of thermal expansion.

10. A cryopanel array in accordance with claim 8 wherein said metallic heat conductive means is fluidactuatable.

11. A cryopump incorporating therein a cryopanel array, comprising in combination (1) housing means defining a working section and a pumping section;

(2) cryopumping means located within said pumping section and comprising (a) refrigerating means;

(b) a primary cryopanel in thermal contact with refrigerating means;

(c) at least one secondary cryopanel having a condensation surface area at least as great as that of said primary cryopanel, and separated from said primary cryopanel by (d) temperature discontinuity means responsive to a decrease in pressure within said cyropump whereby said temperature discontinuity means becomes sufficiently small in effect when said pressure has dropped to a predetermined level to permit said secondary cryopanel to attain cryopumping temperature.

12. A cryopump in accordance with claim 11 wherein said refrigerating means is a mechanical cryogenic refrigerator.

13. A cryopump in accordance with claim 11 wherein 9 10 said cryopanels are copper and said temperature discon- 3,130,562 4/1964 Wood et a1. 6255.5 finvlty means higlrpurity pp 3,137,551 6/1964 Mark 62-555 References Cited 7 3,220,167 11/1965 Ster et a1. 6255.5

UNITED STATES PATENTS 5 LLOYD L. KING, Primary Examiner.

3,122,896 3/1964 Hickey 62-555

Claims (1)

1. A CRYOPANEL ARRAY SUITABLE FOR INCORPORATION INTO A CRYOPUMP, COMPRISING IN COMBINATION (A) A PRIMARY CRYOPANEL IN THERMAL CONTACT WITH REFRIGERANT MEANS; (B) AT LEAST ONE SECONDARY CRYOPANEL HAVING A CONDENSATION SURFACE AREA AT LEAST AS GREAT AS THAT OF SAID PRIMARY CRYOPANEL, AND SEPARATED FROM SAID PRIMARY CRYOPANEL BY (C) TEMPERATURE DISCONTINUITY MEANS RESPONSIVE TO A DECREASE IN PRESSURE WITHIN SAID CYROPUMP WHEREBY SAID TEMPERATURE DISCONTINUITY MEANS BECOMES SUFFICIENTLY SMALL IN EFFECT WHEN SAID PRESSURE HAS DROPPED TO A PREDETERMINED LEVEL TO PERMIT SAID SECONDARY CRYOPANEL TO ATTAIN CRYOPUMPING TEMPERATURE.

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