CA1317469C

CA1317469C - Method of removing helium in a compound cryopump

Info

Publication number: CA1317469C
Application number: CA000516217A
Authority: CA
Inventors: Albert G. Tobin
Original assignee: Grumman Aerospace Corp
Current assignee: Grumman Corp
Priority date: 1985-08-16
Filing date: 1986-08-18
Publication date: 1993-05-11
Anticipated expiration: 2010-05-11

Abstract

ABSTRACT OF THE DISCLOSURE Helium pumping elements as cryopanels for compound cryopumps, cryopumps containing such elements, a process for using such pumps in the removal or pumping of helium from mixtures thereof with deuterium and tritium and methods for preparing such pumps including a method comprising the steps of cleaning chemically a cryogenic support, rinsing the cryogenic support, placing an inorganic bonding agent onto the cryogenic support, sprinkling a granular charcoal sorbent onto the inorganic bonding agent, and curing the inorganic bonding agent.

Description

` 1317469 6383 HELIUM PUMPING ELE~IENT, METHOD OF
FORMING S~ME AND METHOD OF USING SAME
IN_REMOVING HELIUM IN A COMPOUND CRYOPUMP

The present invention relates to helium pumping 5 elements adapted to be used as cryopanels in cryopumps, a method for making such pumping elements, compound cryopumps made with such pumping elements and a method of removing, with such compound cryopumps, helium from gaseous mixtures containing the same, and particularly such gaseous mixtures 10 as may be generated during operation of a fusion reactor burning deuterium-tritium fuel.
Fusion reactors of the future will generate copious quantities of helium since the fusion of deuterium and tritium will generated helium and an energetic neutron. The 15 helium generated must be removed efficiently since the buildup of helium concentration in the reacting plasma will significantly reduce reactor operating efficiency. Various attempts at removing helium via liquid argon cryotrapping and the use of molecular sieves have met with limited success and 20 do not promise to provide the helium pumping performance needed for large scale commercial operation. Cryopumping using activated charcoal sorbents is an effective way of removing helium. ~H.C.Hseuh and H.A.Worwetz, J. Vao.Sci.
TechnG1. Vol. 18, No. 3, April 1981) However, no effort has 25 been made to optimize the helium pumping performance of such a cryopump and thus only limited improvements in helium pumping performance are achieved with the prior art devices.
~ U.S. Patent No. 3,713,790 to Takamori et al relates to a joined body comprising a pyrolytic graphite member and a 3 metallic member and a method of joining those members into a joined body which is particularly useful at elevated _~ -2- 1 3 17 ~69 temperatures of at least about 700C. The patent to Takamori et al. teaches pyrolytic graphite to mean the known polycrystalline material formed by pyrolyzing one or more different types of carbonaceous gases (i.e., gases containing carbon) at a reduced or normal pressure or in the presence of another gas or gases, to the extent that the deposition of the carbon is caused onto a heated surface of a substrate.
However, the patent to Takamori et al teaches the use of bonding only pyrolytic graphite and only in layer form. The layer form offers less absorption surface area 1 than granules. The joining of the layers must be performed in a non-oxidizing atmosphere such as in a vacuum, instead of in air. The suggested utilizations are only for electrical and high temperature applications, wherein the container is designed to hold materials that are either corrosive to the metal or which are at a temperature near or above the melting point of the metal in the container, and thus is not designed for cryogenic applications.
U.S. Patent No. 3,981,427 to Brookes relates to a method of laminating graphite sheets to a metallic substrate by use of a low melt metal coating applied to the metallic substrate as the means for adhesive for bonding the graphite sheets to the substrate to form a composite laminate. The patent to Brookes teaches that the metallic substrate is provided with a coating of low melt metal which forms an intermetallic phase or zone at the interface between the substrate and coating. The coated substrate, while in a solidified condition, is placed in a press between a pair of graphite sheets. Heat is applied to the substrate and the graphite sheets, while under pressure, to a temperature 3 slightly above the melting point of the substrate coating but below the temperature that will destroy the intermediate 3 1317~69 interface. The substrate and graphite sheets are cooled while maintained under pressure in the press with the graphite sheets becoming firmly bonded to the metallic substrate by the resolidified metal coating.
However, the patent to Brookes teaches the use of bonding only graphite sheets and requires that the adhesive and metallic substrate must be heated, and then subjected to pressure to achieve a bond, instead of just heated without pressure. The sheet form of graphite offers less absorption surface area than granules.
The present invention, in one particular aspect, thus includes a helium pumping element or panels adapted for use in a compound cryopump and comprising particles of activated charcoal bonded to an inorganic cryogenic support with an inorganic bonding agent. The present invention, in another particular aspect, includes a compound cryopump adapted to cryogenically pump helium and which employs one or more of such panels as helium pumping panel means in such pumps. In yet another particular aspect, the present invention includes a process for cryogenically pumping helium in a compound cryopump at temperatures of 4.2 to 20K by means of one or more of such panels as helium pumping panel means in such pumps. Still another particular aspect of the present invention includes a more general method of a forming such panels by chemically bonding particles of the activated charcoal with an inorganic bonding agent to an inorganic cryogenic support. Additional details of all of said aspects of the present invention are disclosed hereinafter.
In accordance with one embodiment of the present invention, there is provided a method of removing, in a 4 1~7~
compound cryopump, helium ~rom a mixture o~ gases containing the helium, while employing helium sorbent panels in the cryopump, the improvement which comprises employing panels formed by a process comprising the steps of: a) cleaning chemically a cryogenic support;
b) rinsing the cryogenic support; c) placing an inorganic bonding agent onto the cryogenic support;
d) sprinkling a granular charcoal sorbent onto the bonding agent; and e) curing the bonding agent.
In accordance with another embodiment of the present invention, there is provided a method of fabricating a helium cryopump employing helium sorbent panels therein, the improvement which comprises employing panels formed by a process comprising the steps of: a) cleaning chemically a cryogenic support;
b) rinsing the cryogenic support; c) painting an inorganic copper-based low temperaturecuring adhesive onto the cryogenic support; d) sprinkling a granular charcoal sorbent onto the inorganic copper-based low temperature curing adhesive; and e) curing the inorganic copper-based low temperature curing adhesive.
In accordance with yet another embodiment of the present invention, there is provided a method of forming a helium pumping element adapted for use in a compound cryopump comprising the steps of chemically bonding particles of activated charcoal with an inorganic bonding agent to an inorganic cryogenic support, wherein the inorganic bonding agent is a copper based bonding agent or a silver-copper-phosphorous braze alloy.
Still further, another embodiment of the present invention provides a helium pumping element adapted for use in a compound cryopump and comprising particles of activated charcoal bonded to an inorganic cryogenic 13~7~
support with an inorganic bonding agent which is a copper based bonding agent or a silver-copper-phosphorous braze alloy.
One further feature of a preferred embodiment of the present invention resides, briefly stated, in a method of removing, in a compound cryopump, helium generated during the operation of a fusion reactor burning deuterium-tritium fuel while employing helium sorbent panels in such pump and wherein such panels are formed in a process which includes and comprises the steps of cleaning chemically a cryogenic support, rinsing the cryogenic support, placing an inorganic bonding agent onto the cryogenic support, sprinkling a granular charcoal sorbent onto the bonding agent, and curing the bonding agent, wherein the bonding agent is either a braze alloy or an inorganic copper-based low temperature curing adhesive.
In accordance with another feature of a preferred embodiment of the present invention the step of cleaning chemically a cryogenic support includes cleaning chemically a cryogenic support that is a copper substrate.
Another feature of a preferred embodiment of the present invention is that the step of cleaning chemically the cryogenic support includes cleaning chemically a cryogenic support that is an aluminum substrate when the bonding agent is the inorganic copper-based low temperature curing adhesive.
Still another feature of a preferred embodiment of the present invention is that the step of cleaning chemically the cryogenic support includes cleaning chemically a cryogenic support that is 3 to 6 mm thick.

6 13~7l~
Yet another feature of a preferred embodiment of the present invention i9 that the step of cleaning chemically the cryogenic support includes cleaning chemically the cryogenic support with nitric acid.
Still yet another feature of a preferred embodiment of the present invention is that the step of rinsing the cryogenic support includes rinsing the cryogenic support with methanol and distilled water.
Still another feature of a preferred embodiment of the present invention is that the step of placing a braze alloy onto the cryogenic support includes placing a braze alloy onto the cryogenic support that is thick enough to successfully bond the granular sorbent to the cryogenic support while permitting sufficient exposure of the granular sorbent.
Yet still another feature of a preferred embodiment of the present invention is that the step of placing a braze alloy onto the cryogenic support includes placing a 150~ m layer of a silver-based braze alloy onto the cryogenic support.
Another feature of a preferred embodiment of the present invention is that the step of painting an inorganic copper-based low temperature curing adhesive onto the cryogenic support includes painting a 25 to 125 ~m layer of inorganic copper-based low temperature curing adhesive onto the cryogenic support.
Still another feature of a preferred embodiment of the present invention is that it further comprises the step of heating the cryogenic support in air, in a vacuum, or in an inert gas, until the braze alloy melts and flows over the copper substrate, the heating step occurring after the placing step.

6A 1317 ~
Yet another feature of a preferred embodiment of the present invention is that the step of heating the cryogenic support includes heating the cryogenic support to the braze liquidus point.
Still yet another feature of a preferred embodiment of the present invention is that the step of sprinkling a granular sorbent includes sprinkling a granular sorbent that is either coconut based charcoal granules or coal based charcoal granules.
Yet still another feature of a preferred embodiment of the present invention is that the step of sprinkling a granular sorbent includes sprinkling a granular sorbent that is charcoal granules 600 to 1000~ m in size.
Another feature of a preferred embodiment of the present invention is that it further comprises the step of tamping down the charcoal granules with a ceramic brick when the bonding agent is the braze alloy, the tamping step occurring after the sprinkling step.
Still another feature of a preferred embodiment of the present invention is that the step of curing the braze alloy includes curing the braze alloy by air cooling to room temperature.
Yet another feature of a preferred embodiment of the present invention is that the step of curing the inorganic copper-based low temperature during adhesive includes curing the inorganic copper-based low temperature curing adhesive, partially, at ambient temperature for 1 hour.
Still yet another feature of a preferred embodiment of the present invention is that the step of curing the inorganic copper-based low temperature curing adhesive includes curing the inorganic copper-based low 6B 1 ~1 7~
temperature curing adhesive, finally, in a drying oven at 90 to lOOC for 3 to 4 hours.
~inally, a further feature of a preferred embodiment of the present invention is that it further comprises the step of shaking off excess particles of the charcoal granules from the cryogenic support so that approximately 90~ of the surface area of the cryogenic support is covered with charcoal granules, the shaking step occurring after the curing step.
The novel features which are considered characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional advantages thereof, will be best understood from the following description of specific embodiments.

13~7~

1 Charcoal sorbents are an effective means of removing helium from a gas stream because they possess a large internal surface area and an open pore size distribution that is easily accessible to the impinging helium. When the charcoal is cooled into the temperature range of 4.2 to 20K, the impinging helium will adsorb onto the charcoal surface and diffuse into the micropores of the charcoal until "saturation" occurs at all internal surfaces.
At that point, the pumping speed will drop off. The probable mechanism of desorption is due to weak Van der Waals forces between the carbon and the helium and come into play when thermal vibrations are weak enough to permit interactions between the two molecular species.
It was found that particulate coconut-based charcoals offer the optimum helium pumping performance in the particle size range of 600-1000 ~m size range, although particulate coal based charcoals may also be applicable in the 600-1000 ~m size range. Two metallic bonding agents provide optimum helium pumping performance: a silver-copper-phosphorous braze alloy and an inorganic copper-based low temperature curing adhesive.
It was concluded that the thermal conductivity of the bonding agent between the charcoal and the substrate exerted a strong influence on the helium pumping performance of the charcoal. In particular, bonding materials exhibiting a high thermal conductivity appear to yield the highest pumping performance. On this basis several commercially available silver-based braze alloy compositions and a copper cement were evaluated for compatibility with both the charcoal and copper substrate.
3o -1317~

1 The selection of a silver-copper-phosphorous braze alloy was based upon the "self-fluxing" action of the phosphorous additive while the selection of the other braze compositions was based upon the presence of a manganese additive which is a strong carbide former and which might enhance "wetting" of the charcoal by the braze alloy.
To evaluate the various braze alloy compositions, a specimen of each braze alloy was placed on a copper substrate and heated in air with a propane torch. Where applicable, the copper substrate may be heated in a vacuum or in an inert gas. A fluxing agent was painted on the surface before heat application. With the exception of the silver-copper-phosphorous braze, severe oxidation of the braze alloys and substrate led to poor braze flow and coverage. As a result, these brazes were eliminated from further consideration. On the other hand, the silver-copper-phosphorous braze alloy melted readily on the copper substrate and yielded complete coverage. It was possible to apply the charcoal granules to the liquid silver-copper-phosphorous braze during the heating operation and obtain their retention during the cooldown phase. This led to approximately 95-98~ of the effective surface area being covered with charcoal granules.
In the case of the inorganic adhesive bonding agents, a copper-based cement was identified as the baseline adhesive bonding agent. The charcoal granules applied to the cement layer lead to 98% effective area coverage.
To identify the effects of bond thickness upon the helium pumping performance, 100 mm diameter copper substrates were prepared for three different thicknesses of braze layer.

3o 13~7,~

l The braze alloy was provided in sheet form as a 75 ~m x 25 mm wide strip. Specimens were prepared by melting one, two and three layers of braze on the copper substrate to yield bondline thicknesses of 75 l~m, 150 ~m and 225 ~ , respectively The 150 ~m bond layer yielded the best "overall" pumping performance, although the 75 ~m bond layer has comparable behavior. A significant reduction in pumping performance was observed for the 225 ~m braze layer. This reduction was accompanied by the observation that numerous charcoal particles "sheared off" at the charcoal/braze bonding during cooldown from the brazing temperature, presumably from stresses developed due to the large expansion mismatch of the two materials. Thus, the effective mass of charcoal/unit area may have been greatly reduced.
Three thicknesses of copper substrate were evaluated for brazing compatibility: 3.2 mm, 4.8 mm and 6.3 mm. Specimens were prepared with each thickness using a silver-copper-phosphorous braze and 60~ to 1000 ~m size particulate coconut charcoal. Although all these substrate thicknesses allowed excellent bonding of the charcoal, significant differences were observed amongst them. For example, the 3.2 mm copper substrate buckled during the brazing operation while the 6.3 mm substrate required an excessive heat input to melt the braze and bond the charcoal.
On this basis, it was decided that the 4.8 mm thick copper substrate would be optimum.
The braze alloy has been optimized for charcoal retention and the charcoal particule size distribution to - yield helium pumping performances that are reproducible and will meet anticipated fusion reactor requirements for helium 3 cryopumping. A copper cement bond/charcoal combination has been identified which produces comparable performance and offers the manufacturing flexibility expected with a cement as compared to a braze.

-` --10--1317~6~

1 Conventional brazing techniques are suitable for producing large flat charcoal pumping surfaces made of 180 cm2 tiles~ However, scale-up to larger tile sizes is expected with improvements in brazing practice and equipment.
Brazing methods would be difficult to apply to large complex shapes with curvilinear contours because access and control of heat transfer to such shapes would be difficult to maintain. In such cases, the copper cement approach appears to be a preferred solution.
Since the bonding interface between the charcoal and the bonding agent will influence the heat transfer characteristics as well as determine the durability of the charcoal cryopanel, one would anticipate that panels demonstrating high performance would also have good durability.
A copper or aluminum substrate is used as the cryosupport for the coconut granules. A charcoal cryopanel of a compound cryopump is fabricated in the following preferred manner:

a) The copper substrate is prepared by chemical cleaning in nitric acid followed by a methanol and distilled water rinse;
b) One layer (150 ~m) of silver-copper-phosphorous braze alloy is placed on the copper substrate;
c) Using a propane torch, the copper substrate is heated in air until the silver-copper-phosphorous braze alloy melts and flows over the copper surface (about 705C);
d) The coconut charcoal granules, in the size range of 600-1000 W~ are slowly sprinkled onto the molten 3 braze and tamped down with a ceramic brick; and . ..;.

1317~69 e) The torch is removed and the specimen is allowed to air cool to room temperature and excess particles are shaken off the copper substrate. Approximately 95-98% of the copper surface is covered with the retained charcoal.
In the case of the inorganic copper-based low temperature curing adhesive, a similar preferred procedure is utilized, except a cleaned copper or aluminum substrate is painted with a thin layer (25-125 ~m) of uncured inorganic copper-based low temperature curing adhesive and the coconut charcoal granules are sprinkled on the uncured layer. The uncured layer is partially cured at ambient temperature for 1 hour and finally cured in a drying oven at 90-100C for 3 to 4 hours. Excess charcoal granules are then shaken off.
Approximately 98% of the copper or aluminum surface is covered.
Test results of helium cryopump elements prepared in these preferred manners indicate that helium pumping speeds of 10 to 12.5 liter/s-cm2 at helium pumping capacities exceeding 2 Torr-liter/cm2 at temperatures of 4.2-6K (cooled with liquid helium) are achievable.
With brazed panels, scale-up to any size is achievable by designing a network of interlocking tiles.
With the inorganic copper-based low temperature curing adhesive approach there is theoretically no limit to the size and shape of the cryopump that could be fabricated.
The 150 ~m braze represents the optimum for helium pumping performance. Helium pumping tests run with braze alloy thickness of 75 ~m and 225 ~m showed poorer helium pumping performance, although the 75 ~m braze layer was only slightly poorer than the 150 ~m braze layer. The most likely 3 explanation for this behavior is that the thermal contact . .

-12- 1317~9 between the charcoal granules and the liquid helium cooled 1 cryosupport structure is extremely critical and the bonding agent supplies this contact. Too thin a layer of bonding agent will give insufficient thermal contact. Too thick a bonding layer will cause the charcoal particles to become too deeply embedded in the braze alloy and will lead to large thermal stresses at the braze/charcoal interface on cooldown and subsequent mechanical failure of the charcoal particles at the braze line. This results in a large loss in helium pumping speed capacity for the cryopump. The key is thus a bonding agent thickness that will allow the charcoal granules to be cooled to below 20K since the helium pumping speed of charcoal falls off sharply above 20K.
For the case of the inorganic copper-based low temperature curing adhesive, the thermal conductivity of the cured copper layer is as critical as in the case of the braze. However, the thickness of the layer is not as critical since the inorganic copper-based temperature curing adhesive cures at a low temperature (200F) and charcoal debonding will not occur as in the case of the braze.
Therefore, a range of adhesive layer thicknesses will give a good result. Thus, the formulation of the cement must consist of constituents that will (1) bond the charcoal particles, (2) bond to the substrate material, (3) be inorganic for fusion compatability, and (4) possess a high thermal conductivity on subsequent curing at low temperature.
Helium cryopumps using activated coconut charcoal granules as the sorbent have thus been shown by the present - inventor to meet anticipated fusion reactor requirements for helium generation and removal. Materials processing parameters have been optimized to yield the highest specific -13- 1 3 ~

helium pumping speeds reported. Scale-up of a charcoal bonded panel to 40 cm in diameter has been demonstrated and reproducibility of results has been achieved. Silver-based braze alloys and cooper-based adhesives have been shown to offer the optimum means of joining the charcoal granules to the liquid helium cooled cryopanel.
It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of methods and constructions differing from the types described above.
While the invention has been illustrated and described as embodied in a method of removing helium in a compound cryopump, it is not intended to be limited to the details above, since various modifications and changes may be made without departing in any way from the spirit of the present invention.
What is claimed is new and desired to be protected by Letters Patent is set forth in the appended claims.

3o -

Claims

1. In a method of removing, in a compound cryopump, helium from a mixture of gases containing the helium, while employing helium sorbent panels in said cryopump, the improvement which comprises employing panels formed by a process comprising the steps of:
a) cleaning chemically a cryogenic support;
b) rinsing the cryogenic support;
c) placing an inorganic bonding agent onto the cryogenic support;
d) sprinkling a granular charcoal sorbent onto the bonding agent; and e) curing the bonding agent.

2. A method as defined in claim 1, wherein said step of cleaning chemically a cryogenic support includes cleaning chemically a cryogenic support that is a copper substrate.

3. A method as defined in claim 1, wherein said step of cleaning chemically the cryogenic support includes cleaning chemically a cryogenic support that is 3 to 6 mm thick.

4. A method as defined in claim 1, wherein said step of cleaning chemically a cryogenic support includes cleaning chemically the cryogenic support with nitric acid.

5. A method as defined in claim 1, wherein said step of rinsing the cryogenic support includes rinsing the cryogenic support with methanol and distilled water.

6. A method as defined in claim 1, wherein said step of placing the inorganic bonding agent onto the cryogenic support includes placing the inorganic bonding agent in sufficient thickness on the cryogenic support as to successfully bond the granular sorbent to the cryogenic support while permitting sufficient exposure of the granular sorbent.

7. A method as defined in claim 1, wherein said step of placing an inorganic bonding agent onto the cryogenic support includes placing a 150 µm layer of a silver-based braze alloy as said bonding agent onto the cryogenic support.

8. A method as defined in claim 7; further comprising the step of heating the cryogenic support until the braze alloy melts and flows over the cryogenic support, said heating step occurring after said placing step.

9. A method as defined in claim 8, wherein said step of heating the cryogenic support includes heating the cryogenic support in air.

10. A method as defined in claim 8, wherein said step of heating the cryogenic support includes heating the cryogenic support in a vacuum.

11. A method as defined in claim 8, wherein said step of heating the cryogenic support includes heating the cryogenic support in an inert gas.

12. A method as defined in claim 8, wherein said step of heating the cryogenic support includes heating the cryogenic support to the braze liquids point.

13. A method as defined in claim 1, wherein said step of sprinkling a granular sorbent includes sprinkling charcoal granules that are 600-1000 µm in size.

14. A method as defined in claim 13, wherein said step of sprinkling a granular sorbent includes sprinkling charcoal granules that are coconut based charcoal granules.

15. A method as defined in claim 13, wherein said step of sprinkling a granular sorbent includes sprinkling charcoal granules that are coal based charcoal granules.

16. A method as defined in claim 1; further comprising the step of tamping down the granular sorbent with a ceramic brick, said tamping step occurring after said sprinkling step.

17. A method as defined in claim 8, wherein said step of curing the inorganic bonding agent includes curing the braze alloy by air cooling to room temperature.

18. A method as defined in claim 1; further comprising the step of shaking off excess particles of the granular sorbent from the cryogenic support so that approximately 95 to 98% of the surface area of the cryogenic support is covered with the granular sorbent, said shaking step occurring after said curing step.

19. In a method of fabricating a helium cryopump employing helium sorbent panels therein, the improvement which comprises employing panels formed by a process comprising the steps of:
a) cleaning chemically a cryogenic support;
b) rinsing the cryogenic support;
c) painting an inorganic copper-based low temperature curing adhesive onto the cryogenic support;
d) sprinkling a granular charcoal sorbent onto the inorganic copper-based low temperature curing adhesive; and e) curing the inorganic copper-based low temperature curing adhesive.

20. A method as defined in claim 19, wherein said step of cleaning chemically a cryogenic support includes cleaning chemically a cryogenic support that is a copper substrate.

21. A method as defined in claim 19, wherein said step of cleaning chemically a cryogenic support includes cleaning chemically a cryogenic support that is an aluminum substrate.

22. A method as defined in claim 19, wherein said step of cleaning chemically a cryogenic support includes cleaning chemically a cryogenic support that is 3 to 6 mm thick.

23. A method as defined in claim 19, wherein said step of cleaning chemically a cryogenic support includes cleaning chemically the cryogenic support with nitric acid.

24. A method as defined in claim 19, wherein said step of rinsing the cryogenic support includes rinsing the cryogenic support with methanol and distilled water.

25. A method as defined in claim 19, wherein said step of painting an inorganic copper-based low temperature curing adhesive onto the cryogenic support includes painting a 25-125 µm layer of inorganic copper-based low temperature curing adhesive onto the cryogenic support.

26. A method as defined in claim 19, wherein said step of sprinkling a granular sorbent includes sprinkling a granular sorbent that is charcoal granules that are 600-1000 µm in size.

27. A method as defined in claim 26, wherein said step of sprinkling a granular sorbent includes sprinkling charcoal granules that are coconut based charcoal granules.

28. A method as defined in claim 26, wherein said step of sprinkling a granular sorbent includes sprinkling charcoal granules that are coal based charcoal granules.

29. A method as defined in claim 19, wherein said step of curing the inorganic based low temperature curing adhesive includes curing the inorganic based low temperature curing adhesive partially at ambient temperature for 1 hour.

30. A method as defined in claim 19, wherein said step of curing the inorganic based low temperature curing adhesive includes curing the inorganic based low temperature curing adhesive finally in a drying oven at 90° to 100°C for 3 to 4 hours.

31. A method as defined in claim 19; further comprising the step of shaking off excess particles of the granular sorbent from the cryogenic support so that approximately 95 to 98% of the surface area of the cryogenic support is covered with granular sorbent, said shaking step occurring after said curing step.

32. A method as in claim 1 in which said helium is removed from a gaseous mixture thereof with tritium and deuterium formed during the operation of a fusion reactor burning deuterium and tritium.

33. A method of forming a helium pumping element adapted for use in a compound cryopump comprising the steps of chemically bonding particles of activated charcoal with an inorganic bonding agent to an inorganic cryogenic support wherein said inorganic bonding agent is a copper based bonding agent or a silver-copper-phosphorous braze alloy.

34. A method as in claim 33 in which said cryogenic support is a panel of copper or aluminum.

35. A method as in claim 34 in which said inorganic bonding agent is a copper based bonding agent.

36. A method as in claim 35 in which said cryogenic support is about 3 to 6 mm thick.

37. A method as in claim 36 in which said cryogenic support is about 4.8 mm thick.

38. A method as in claim 36 in which said bonding agent is used in a thickness of about 25-150 µm.

39. A method as in claim 33 in which said charcoal particles are about 600 to 1000 µm in size.

40. A method as in claim 39 in which said charcoal particles cover about 95-98% of the surface of said panel.

41. A method as in claim 33 in which said charcoal is derived from coconut.

42. A method as in claim 33 in which said charcoal is derived from coal.

43. A helium pumping element adapted for use in a compound cryopump and comprising particles of activated charcoal bonded to an inorganic cryogenic support with an inorganic bonding agent which is a copper based bonding agent or a silver-copper-phosphorous braze alloy.

44. A pumping element as in claim 43 in which said cryogenic support is a panel of copper or aluminum.

45. A pumping element as in claim 44 in which said inorganic bonding agent is a copper based bonding agent.

46. A pumping element as in claim 45 in which said cryogenic support is about 3 to 6 mm thick.

47. A pumping element as in claim 46 in which said cryogenic support is about 4.8 mm thick.

48. A pumping element as in claim 46 in which said bonding agent is used in a thickness of about 25-150 µm.

49. A pumping element as in claim 43 in which said charcoal particles are about 600 to 1000 µm in size.

50. A pumping element as in claim 49 in which said charcoal particles cover about 95-98% of the surface of said panel.

51. A compound pump adapted to cryogenically pump helium by means of cryogenic helium pumping panels therein wherein one or more of said panels are panels as claimed in claim 43.

52. In a process for cryogenically pumping helium in a compound pump at a temperature of 4.2 to 20K by means of cryogenic helium pumping panels therein wherein one or more of said panels are panels as claimed in claim 43.

53. A process as in claim 52 wherein said pump is operated at a helium pumping speed of 10 to 12.5 liters/s-cm2 at helium pumping capacities of >2 Torr-liter/cm2.

54. A process as in claim 53 which is conducted at a temperature of 4.2-6K.

55. A process as in claim 52 in which said helium is being removed from an admixture thereof with deuterium and tritium.

56. A pumping element as in claim 43 in which said charcoal is derived from coconut.

57. A pumping element as in claim 43 in which said charcoal is derived from coal.