US3256706A

US3256706A - Cryopump with regenerative shield

Info

Publication number: US3256706A
Application number: US434232A
Authority: US
Inventors: Hansen Siegfried
Original assignee: Hughes Aircraft Co
Current assignee: Raytheon Co
Priority date: 1965-02-23
Filing date: 1965-02-23
Publication date: 1966-06-21
Anticipated expiration: 1983-06-21

Description

June 2l, 1966 s, HANSEN 3,256,706

GRYOPUMP WITH REGENERATIVE SHIELD .lune 21, 1966 s, HANSEN 3,256,706

CRYOPUMP WITH REGENERATIVE SHIELD www June 21, 1966 s. HANSEN 3,256,706

GRYOPUMP WITH REGENERATIVE SHIELD Filed Feb. 12,3/V 1965 3 Sheets-Sheet 3 Arme/V554 United States Patent O 3,256,706 CRYOPUMP WITH REGENERATIVE SHIELD Siegfried Hansen, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Filed Feb. 23, 1965, Ser. No. 434,232 Claims. (Cl. 62--55.5)

This invention relates generally to a high vacuum pump and relates more specifically to an ultra high vacuum cryogenic pump.

To achieve ultra high vacuum it is necessary to remove substantially all of the gas molecules from free space within a closed vacuum chamber. Ultra high vacuum pumps have been developed which condense and solidify the different gases of the air by freezing them to extremely low temperatures (4.2 K.). At this temperature all of the different gases, except helium, that make up air condense and solidify against the cold surfaces, whereupon substantially all of the gas molecules are removed from the free space and prevented from freely traveling therein. Several vacuum pumps that operate at this low temperature are the National Research Corporation models 3601 and 3602, Vacuum Cryopumps, and Union Carbides Linde 2300-H-10, Cryogenically Pumped Ultra High Vacuum Chamber. These pumps Iconsist basically of two separately refrigerated surfaces, one which is refrigerated by liquid nitrogen (77 K.) and the other which is refrigerated by liquid helium (4.2" K.).

Cryogenic pumping with refrigeration at 4.2 K. is expensive in terms of the amount of cryogen required to pump a certain volume chamber to a high vacuum and to maintain the Vacuum at this low level. One reason for this low efficiency is that heat radiated from the warmer surfaces of the pump to the colder (42 K.) surfaces of the pump can cause a substantial amount of the cryogen (liquid helium) to boil off.

Structurally, the above referenced -cryopumps both use planes of baffles or louvers positioned adjacent the coldest surface olf the cryopump to block radiated heat without substantially restricting the flow of gas molecules to the coldest surface. In the NRC cryopump, pumping efficiency has been increased by-cooling the louvers with a feedback heat exchange of refrigeration available from the boiled olf nitrogen. In the Linde cryopump, pumping efficiency has been increased by cooling the louvers in the plane furtherst away from the cold surface with a heat exchange of refrigeration available from the boiled off nitrogen and by cooling the louvers in the plane nearest the cold surface with a heat exchange of refrigeration available from the boiled olf helium.

An object of this invention is to provide an improved cryopump in which the pumping efficiency is substantially increased.

Still another object is to provide an improved cryopump in which the pumping speed is increased and the pumping cost is decreased.

The above and other objectives of this invention can be attained by optically isolating the coldest surface from radiant heat with a baffle assembly and support assembly and using the refrigeration available from both the nitrogen and the boiled off helium to cool a plurality of adjacent planes of louvers mounted at progressive distances from the side wall of the coldest surface so that the temperature from the louvers in the plane nearest the cold surface (4.2 K.) to the louvers in the plane furthest away from the cold sur-face (77 K.) varies in substantially uniform increments.

Other objects, features and advantages of this invention will become apparent upon reading t-he following ,the gas molecules from free space.

ice

detailed description of one embodiment and referring to the accompanying drawings in which:

FIG. 1 is a schematic diagram illustrating the operation of the cryogenic pumping circuit;

FIG. 2 is a side view partly in' cross section illustratin'g the structural relationship of a louver array that encloses a cold plate; and

FIG. 3' is a cross sectional top plane View of the Vacuum pump taken along the line 3 3y of FIG. 2.

Referring now to the drawings, FIGS. 1 and 2 are closely interrelated to one another in that FIG. 1 illustrates the operational aspects of the vacuum pump and FIG. 2 illustrates the structural details.

Structurally, the vacuum pump includes an outer housing of a hollow cylindrical housing 12 having an end cap 13 secured across one end and a flange 14 formed about the rim of an open end. The housing 12 and end cap 13 are made of a high strength material, such as stainless steel, and can be polished to be highly reflective. A vacuum chamber or bell jar (not shown) can be secured to the flange 14 in gas communication with the open end and evacuated by means of the following pump structure.

Supported within the housing 12 is a hollow cylindrical shield 16 that .blocks heat radiated into the interior of the vacuum pump and that is cooled by liquid nitrogen to operate as a cold surface for removing a portion of The cylindrical shield 16 is supported in annular spaced-apart relationship within the outer housing 12 by -means of a plurality olf radially extending angularly displaced support members 17. To increase thermal isolation between the refrigerated cylindrical shield 16 and the warmer outer housing 12, the support members 17 have a tubular portion 18, with a re-entrant chamber formed therein connected to the outer housing 12. A hollow pipe 19 projects into the pump chamber along the axis of the tubular portion 18 to provide a'support bea-m so that the cylindrical shield'16 can be clamped between a pair of flanges 21 and 22 at the end of lthe pipe 19. To increase thermal isolation and structural strength, the support member 17 is made of stainless steel.

In order to provide an efhcient thermal circuit, the cylindrical shield 16 is made of a good thermal conductor .19 and downward through a hollow pipe 24. The liquid nitrogen reservoir 23 is secured to extend across the bottom end of the cylindrical shield 16. Structurally, the reservoir 23 includes an upper and a lower dome-

like member

26 and 27, which are fastened to one another at their rims and fastened to the shield 16 to provide a sealed chamber 28 for holding a supply of liquid nitrogen while vproviding a continuous thermal circuit between the nitrogen reservoir and the radiation shield 16. Thus, in operation, the shield 16 is continuously cooled by the liquid nitrogen to a temperature not lower than 77 K.

As heat is transferred to the liquid nitrogen from the radiation shield 16 and from the dome-

like members

26 and 27, a certain portion of the liquid nitrogen boils olf to form nitrogen gas. This nitrogen gas is vented to the atmosphere from the upper portion of reservoir chamber 28 through a vent pipe 29 and through one of the support members 17.

Although refrigeration available from the liquid nitrogen will condense and solidify those gases having freezing points above 77 K., it should be remembered that air consists of many gases, some of which have a freezing point lower than 77 K. In order to condense and freeze out these low freezing point gas molecules, it is necessary to provide an ultra cold refrigerated surface.

To obtain an ultra low temperature refrigeration, a hollow rectangular box 31 is supported symmetrically within and thermally isolated from ythe cylindrical shield 16 and the liquid nitrogen reservoir 23. Two

flat side walls

32 and 33 of the rectangular box 31 form cold plates which readily transmit refrigeration from liquid helium contained within the rectangular box 31. For efficient refrigeration the rectangular box 31 is made of some high thermal conductivity material, such as copper.

Liquid helium -is fed into the rectangular box 31 from outside the vacuum pump through a pipe 36 and a pipe 37. As the liquid helium within the hollow rectangular box 31 receives heat conducted through .the

cold plates

32 and 33, it boils, forming helium gas in a plenum space at the upper end of the box 31. This helium gas is vented to the atmosphere after it is used to refrigerate the fins of louvers or a baffie array, as will be explained in more detail shortly.

Radiated heat is blocked from the

cold plates

32 and 33 without substantially impeding migration of gas molecules thereto by means of an optically isolating baffle assembly having a plurality of ranks of finned louvers; plates 38 are positioned in adjacent planes parallel to and spaced from the

cold plates

32 and 33 at a progressive distances. Structurally, the louvers each include a plurality of equally spaced fins 39 which are supported to extend laterally across the cold platesby pairs of vertically extending support bars 41 and 42 (FIG. 3). The support bars 41 and 42 are formed with a plurality of equally spaced slots formed along one edge thereof for receiving the ends of the fins 39. The fins 39 are blackened and have a curved cross section which increases the structural strength and reduces impedance of the migration of gas molecules toward

cold plates

32 and 33. In order to efficiently cool the baffle assembly, the support bars 41 and 42 and the fins 39 are all made of a material having a good thermal conductivity, such as copper.

The planes or ranks of louvers are supported by means of a plurality of nested rectangular box frames 46 through 49 each having windows 53 formed through the wall thereof.

structurally, each of the rectangular box frames 46 through 49 includes two rectangular, pan-like members 51 and 52 of copper which are placed in back-to-back and edge-to-edge relationship so that L-like cross sectional walls of the rectangular members define spaces of progressively increasing volume. tangular box frame 46 defines the space of smallest Volume, whereas, the outermost rectangular box frame 49 defines the space of greatest volume. The side wall of each rectangular box frame is formed with a rectangular window 53 in which one of the louvers 38 is mounted, thus permitting gas molecules to migrate to the

cold plates

32 and 33.

Each of the rectangular box frames 46 through 49 are supported, one within the other, by means of a plurality of nylon support members 57. Each nylon support member 57 has a conical form with the base against the outer rectangular support box frame and the apex against the inner adjacent rectangularbox frame. With this arrangement, heat transfer from the outer rectangular box frames to the inner rectangular box frames is substantially reduced. Thus, each of the box frames 46 through 49 is substantially thermally isolated from the others.

The fins 39 of the louvers 38 extend across the windows 53 to block radiant heat from the

cold plates

32 and 33 without substantially impeding the flow of gas molecules therethrough. The vertically extended support bars 41 and 42 are fastened to `the vertically extending edge of each rectangular member 51 and 52 adjacent the vertical edge of the rectangular windows 53. Appropriate fastening means would be a solder or a'weld, which provides That is, the innermost recgood thermal conductivity between the rectangular box frame and the support bar.

It has been discovered that by cooling the ranks of louvers at increasing temperature increments from the innermost to the outermost ranks, the operation and efficiency of the vacuum pump can be greatly improved. Accordingly, refrigeration available from the helium gas that is boiled off in .the rectangular box 31 is used to refrigerate the innermost three ranks of louvers, whereas, refrigeration available from the nitrogen is used to cool the outermost louver rank or plane.

In operation, boiled off helium gas is ven-ted from the top of the rectangular box 31 by means of hollow tubes 58 connected at each upper corner thereof. The helium gas is transferred through a stainless steel hairpin elbow section of tubing 59 and down along the innermost pair of .support bars 41 and 42 through

copper tubes

60 and 61. The

copper tubes

60 and 61 are secured in thermal contact with the support bars 41 and 42 and the rectangular box frame 46 to refrigerate the innermost rank or plane of louvers 38 and the box frame. After flowing through a lower hairpin elbow section of stainless steel tubing 62, the helium gas is transferred up along the next outermost pair of support bars 41 and 42'through sections of

copper tubing

63 and 64 to cool the second rank of louvers 38. Thereafter, the helium gas refrigerates the third rank of louvers after flowing through a stainless steel hairpin elbow 66 and down through the

copper tubing

67 and 68 along the next outermost pair of support bars 41 and 42, respectively. By placing a hairpin bend section of stainless steel tubing between the vertical sections of copper tubing, thermal isolation is achieved between adjacent ranks or planes of louvers since a significant amount of heat will not be conducted upstream of the ow of helium along the tubing. As a result of 'the thermal isolation between ranks of louvers and the heat absorbed by the helium gas at each rank or plane of louvers, the temperature of each rank of louvers increases in substantially equal increments with the innermost rank of louvers being at the lowest temperature, and the third rank of louvers being at a higher temperature.

Prior to venting Athe helium gas to the atmosphere through tubing 70, it first ows through another hairpin elbow section of stainless steel tubing 69 and thence to the atmosphere.

The outermost rank of louvers is cooled by refrigeration of the nitrogen in the reservoir 23. To accomplish this cooling, the outermost box frame 49 is supported in thermal contact with the upper surface of the nitrogen reservoir 23 and in thermal contact, at its vertical corners, with the cylindrical shield 16. As a result of this contact with the thermal circuit of the nitrogen, the outer rank of louvers is cooled to a temperature not lower than 77 K.

By first evacuating the vacuum pump chamber to a low pressure with a back-up pump 71 it is possible to prevent a substantial layer of solidified and condensed gases from forming on the refrigerator or cold surfaces of the cryopump. One type of back-up vacuum pump that could be used is a mechanical pump of the two-stage rotary piston type, which would be connected 4to communicate with the interior of the vacuum cryopump through a pipe fitting 72 extending through the side wall of the housing 12. Any oil vapors that tend to travel upstream into the cryopump chamber through the pipe fitting 72 would condense and freeze against the exterior surface of the cylindrical shield 16. As a result, the interior of the cryogenic vacuum pum-p would remain substantially free of contaminants.

With this efficient pumping operation, it may be possible to initially fill the reservoir 23 with liquid nitrogen and to fill the hollow rectangular box 31 with liquid helium. Once the reservoir 23 and the box 31 are filled, the supplies of liquid cryogens are cut off and any boiled off gas is prevented from escaping by the check valve 73.

As a result, no external connections are required during pumping, and -there are only relatively small refrigeration losses from the liquid cryogen to the atmosphere.

While salient features have been illustrated and described with respect tto a panticular embodiment, it should be readily apparent that modifications can be made within the spirit and scope of the invention, and it is therefore not desired to limit the invention to the exact details shown and described.

What is claimed is:

l. In a vacuum pump of the -type including an outer housing, the housing having an open end, a hollow shield of high thermal conductivity material mounted within and in thermal isolation lfrom the outer housing, the shield having an open end, a first supply of cryogen coupled to refrigerate the shield, a cold plate means mounted within and thermally isolated from the hollow shield, a second supply of cryogen coupled to refrigerate (the cold plate, the cryogen of the second supply having a lower boiling point than the cryogen of the rst supply, and the combination therewith of: a bafiie assembly having a plurality of louver plates mounted in adjacent planes at progressive distances from the cold plate for blocking radiant heat; a plurality of heat transfer means each connected to an individual one of said louvers, each of the said heat transfer means being thermally isolated from :the said heat transfer means of the adjacent louvers; and a gas transfer means connected to Vent boiled off cryogen gas of the second cryogen supply from the vacuum pump, said gas transfer means being coupled to transfer avalaible refrigeration n from the cryogen gas to the louvers in a serial sequence from the inner plane of louvers to the adjacent outer plane of louvers.

Z. In a vacuum pump of the type including an outer housing, the housing having an open end, a hollow shield of high thermal conductivity material mounted within and in :thermal isolation from the outer housing, the shield having an open end, a first supply of cryogen coupled to refrigerate the shield, a cold plate means mounted within and thermally isolated from the hollow shield, a second supply of cryogen coupled to refrigerate the cold plate, the cryogen of the second supply having a lower boiling point than the cryogen of the first supply, and the combination therewith of a batiie assembly having a plurality 'of louvers mounted in adjacent planes at progressive distances from the cold plate for blocking radiant heat; a plurality of heat transfer means each connected to an individual one of said louvers, each of said heat transfer means being thermally isolated from the said heat transfer means of the adjacent louvers, the said heat transfer means connected Ito the outermost plane of louvers-being coupled to receive available .refrigeration from the first supply of cryogen; and a gas transfer means connected to vent boiled otf cryogen gas of the second cryogen supply from the vacuum pump, said gas transfer means being coupled .to transfer available refrigeration from the cryogen gas to the louvers in a serial sequence from the inner plane of louvers [to the adjacent outer plane of louvers.

3. In a vacuum pump of the type having an outer housing and having an open end, a hollow shield of `high thermal conductivity material mounted within and in thermal isolation from the outer housing, the shield having an open end, a first supply of cryogen coupled to refrigerate the shield, a cold plate means mounted within and thermally isolated from the hollow shield, and a second supply of cryogen coupled to refrigerate the cold plate, the cryogen of the second supply having a lower boiling point than the cryogen of the first supply, the combination therewith of: a baffie assembly having a plurality of louvers mounted in adjacent planes at progressive distances from the cold plate for blocking radiant heat; heat transfer means-each connected to an individual one of said louvers, each of the said heat transfer means being thermally isolated from the said heat transfer means of the adjacent louvers; and a gas transfer means connected to vent boiled off cryogen gas from the second cryogen supply, said gas transfer means being coupled to transfer available refrigeration from the cryogen gas to the louvers in a serial sequence from the inner plane 'of louvers to the adjacent outer plane of louvers, said gas transfer means having thermal isolation sections between planes of louvers.

4. In a vacuum pump of the type comprising a housing enclosing a first cryogenic refrigeration circuit and a second cryogenic refrigeration circuit, the second cryogenic refrigeration circuit being at a lower temperature than the first cryogenic refrigeration circuit, the refrigerator circuits being operable to condense and freeze gas molecules from space within a vacuum chamber, in combination therewith: a means for supporting the second cryogenic refrigeration circuit in thermal isolation from the first cryogenic refrigeration circuit and in optical isolation from the surrounding environs except for a window portion; a bafiie assembly comprising a plurality of louvers each mounted in adjacent planes, each plane being at a progressive distance from the second cryogenic refrigerator circuit, said baffle assembly being operable to block heat radiated through the window portion and to permit migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration from the second cryogenic refrigerator circuit to the louvers for refrigerating the louvers,

5. In a vacuum pump of the type comprising a housing enclosing a first cryogenic refrigeration circuit and a second cryogenic refrigeration circuit, the second cryogenic refrigeration circuit being at a lower operating temperature than the first cryogenic refrigeration circuit, the refrigeration -circuits being operable to condense and freeze gas molecules from space within a vacuum chamber, in combination therewith: a means for supporting the second cryogenic refrigeration circuit in thermal isolation from the first cryogenic refrigeration circuit and in optical isolation from surrounding surfaces except for a window portion; a baffle assembly comprising a plurality of louvers each mounted in adjacent planes at progressive distances from the second cryogenic refrigeration circuit for blocking heat radiated through the window portion and for permitting migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration from the second cryogenic refrigeration circuit to the said louvers for refrigerating the said louvers in a series sequence from the said plane of louvers near the second refrigeration circuit to the said planes of louvers further away from the second refrigeration circuit,

6. In a vacuum pump of the type comprising a housing enclosing a first cryogenic refrigeration circuit and a second cryogenic refrigeration circuit, the secon-d refrigeration circuit havin-g a lower operating temperature than the first cryogenic refrigeration circuit, the refrigeration circuits being operable to condense and freeze gas molecules from space within a vacuum cham-ber, in combinati-on therewith: a means .for supporting the second cryogenic refrigeration circuit in thermal isolation 4from the first cryogenic refrigeration circuit and in optical isolation therefrom except lfor a window portion; a bafiie assembly comprising a plurality of louvers each mounted in adjacent planes at progressive distances from the second cryogenic refrigeration circuit for blocking heat radiated through the window portion and for permitting migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration lfrom the second cryogenic refrigeration circuit to the said louvers for refrigerating the said louvers in a series sequence from the said plane of louvers near the second refrigeration circuit to the said planes of louvers further away.

7. In a vacuum pump of the type comprising an outer housing enclosing a first cryogenic refrigeration circuit and a -second cryogenic refrigeration circuit, the second cryogenic refrigeration circuit having a lower operating temperature than the first cryogenic refrigeration circuit, the refrigeration circuits being operable to condense and freeze gas molecules from space within a vacuum chamber, and the combination therewith of: support means for holding the second cryogenic refrigeration circuit in thermal isolation from the first cryogenic refrigeration circuit, said support means including a plurality of nested frame members 4mounted in thermal isolation from one another, each frame member having Window portions formed therein; a bafiie assembly comprisingr a plurality of louvers, each louver being secured in thermal contact to individual frame members and being disposed in adjacent planes at progressive distances ifrom the second cry-ogenic refrigeration circuit for blocking heat radiated Y through the window portion and for permitting migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration from the second cryogenic refrigeration circuit to the said louvers for refrigerating the said louvers in series `sequence from the said plane of louvers near the second refrigeration circuit to the said plane olf louvers further away.

8. ln a vacuum pump of the type comprising an outer housing enclosing a first cryogenic refrigeration circuit and a second cryogenic refrigeration circuit, the second cryogenic refrigeration circuit having a lower operating temperature than the first cryogenic refrigeration circuit, the refrigeration circuits being operable to condense and freeze gas molecules from space within a vacuum chamber, and the combination therewith of 2 support means for holding the second cryogenic refrigeration circuit in thermal isolation from the first cryogenic refrigeration circuit, said support means including a plurality ofnested frame members mounted in thermal isolation from one another with the outermost frame member being supported in thermal contact with the first said cryogenic refrigeration circuit, each of the said frame members having window portions formed therein; a baffle assembly comprising a plurality of louvers, each louver being secured in thermal contact to individual frame members and being disposed in adjacent planes at progressive distances from the second cryogenic refrigeration circui-t for blocking heat radiated through the window porti-on and for permitting migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration from the second cryogenic refrigeration circuit to the said louvers for refrigerating the said louvers in ser-ies sequence from the said plane of louvers nearest the second refrigeration circuit to the said plane of louvers adjacent the outermost plane of louvers.

9. In a vacuum pump of the type comprising an outer housing enclosing a first cryogenic refrigeration circuit and a second cryogenic refrigeration circuit, the second cryogenic refrigeration circuit having a lower operating temperature than the first cryogenic refrigeration circuit, the refrigeration circuits being operable to condense and freeze gas molecules from space w-ithin'a vacuum chamber, and the combination therewith of: support means for holding the second cryogenic refrigeration circuit in thermal isolation from the first cryogenic refrigeration circuit, said support means including a plurality of nested frame members mounted in thermal isolation from one another, each frame member having window portions formed therein whereby the second said refrigerati-on circuit is optically isolated from the surrounding environs except at the window portions; a baffie assembly comprising a plurality of louvers, each louver being secured in thermal contact to individual frame members and being disposed in adjacent planes at progressive distances from the second cryogenic refrigeration circuit for blocking heat radiated through the window portion and for permitting migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration from the second cryogenic refrigeration circuit to the said louvers for refrigerating the said louvers in series sequence from the said plane of louvers near the second refrigeration circuit to the said plane of louvers further away.

1f). In a vacuum pump of the type comprising an outer housing enclosing a first cryogenic refrigeration circuit land a second cryogenic refrigeration circuit, the second cryogenic refrigeration circuit having a lower operating temperature than the first cryogenic refrigeration circuit, the refrigeration circuits being operable t-o condense and freeze gas molecules from space Within a vacuum chamber, and the combination therewith of: support means for holding the second cryogenic refrigeration circuit in thermal isolation from the first cryogenic refrigeration circuit, said support means including a plurality of nested frame members mounted in thermal isolation from one another, with the outermost frame members being supported in thermal contact with the first said refrigeration circuit, each frame member having window portions formed therein whereby the second said refrigeration circuit is optically isolated from the surrounding pump environs except at the window portions; a baffle assembly comprising a plurality of louvers, each louver being secured in thermal contact to individual frame members and being disposed in adjacent planes at progressive distances from the second cryogenic refrigeration circuit for blocking heatradiated through the window portion and for permitting migration of gas molecules therethrough; and heat transfer means for transferring available refrigeration from the second cryogenic refrigeration circuit to the said louvers for refrigerating the said louvers in series sequence from the said plane of louvers near the second refrigeration circuit to the said plane of louvers further away.

References Cited by the Examiner UNlTED STATES PATENTS 2,565,722 8/1951 Dawley et al. 62-55.5 3,081,068 3/1963 Milleron 62-55.5 3,188,785 6/1965 Butler 62-555 ROBERT A. OLEARY, Primary Examiner'. L. L. KNG, Assistant Examiner.

Claims

1. IN A VACUUM PUMP OF THE TYPE INCLUDING AN OUTER HOUSING, THE HOUSING HAVING AN OPEN END, A HOLLOW SHIELD OF HIGH THERMAL CONDUCTIVITY MATERIAL MOUNTED WITHIN AND IN THERMAL ISOLATION FROM THE OUTER HOUSING, THE SHIELD HAVING AN OPEN END A FIRST SUPPLY OF CRYOGEN COUPLED TO REFRIGERATE THE SHIELD, COLD PLATE MEANS MOUNTED WITHIN AND THERMALLY ISOLATED FROM THE HOLLOW SHIELD, A SECOND SUPPLY OF CRYOGEN COUPLED TO REFRIGERATE THE COLD PLATE, THE CRYOGEN OF THE SECOND SUPPLY HAVING A LOWER BOILING POINT THAN THE CRYOGEN OF THE FIRST SUPPLY, AND THE COMBINATION THEREWITH OF: A BAFFLE ASSEMBLY HAVING A PLURALITY OF LOUVER PLATES MOUNTED IN ADJACENT PLANES AT PROGRESSIVE DISTANCES FROM THE COLD PLATE FOR BLOCKING RADIANT HEAT; A PLURALITY OF HEAT TRANSFER MEANS EACH CONNECTED TO AN INDIVIDUAL ONE OF SAID LOUVERS, EACH OF THE SAID HEAT TRANSFER MEANS BEING THERMALLY ISOLATED FROM THE SAID HEAT TRANSFER MEANS OF THE ADJACENT LOUVERS; AND A GAS TRANSFER MEANS CONNECTED TO VENT BOILED OFF CRYOGEN GAS OF THE SECOND CRYOGEN SUPPLY FROM THE VACUUM PUMP, SAID GAS TRANSFERMEANS BEING COUPLED TO TRANSFER AVAILABLE REFRIGERATION FROM THE CRYOGEN GAS TO THE LOUVERS IN A SERIAL SEQUENCE FROM THE INNER PLANE OF LOUVERS TO THE ADJACENT OUTER PLANE OF LOUVERS.