GB2058934A - Cryopumps - Google Patents

Abstract

A cryopump includes a chamber having an opening at one end for the entry of gases. A heat radiation shield is provided adjacent the opening which shields a cryoplate assembly mounted within the chamber from at least a major portion of direct heat radiation. The cryoplate assembly includes at least one member 20 of wedge-shaped configuration which provides a large totally available surface areas through which gases can readily find their way and a large subtended surface area at the aperture of the chamber all, of which enables the cryopump to perform with large pumping capacity and fast pumping speeds. <IMAGE>

Description

SPECIFICATION Improvements in cryopumps The present invention relates to cryopumps.

It is known for a cryopump to include a chamber open at one end for entry therein of gases. Within the chamber there is positioned a cryoplate assembly in thermal contact with the second stage of a two-stage cryogenerator. Adjacent the open end of the chamber there is arranged a heat radiation shield in the form of a louvre assembly including a plurality of baffles. The louvre assembly is in thermal contact with the first stage of the two-stage cryogenerator.

Now a cryopump works on the principle of providing a vacuum or low pressure zone by condensing gases on the cooled surfaces of the cryoplate assembly contained within the chamber. The rate of condensation of a gas or pumping speed depends, amongst other factors, upon the subtended surface area of the cryoplate assembly at the open end of the chamber. Furthermore, the pumping capacity of the cryopump depends upon the total available surface area which the gases can readily reach. It follows therefore that the greater the 'subtended' area and the 'total available' area of a cryoplate assembly the greater will be the pumping speed and capacity of the cryopump.

A factor that limits the effective 'subtended' area of a cryoplate assembly is the need to prevent or at least minimise direct heat radiation through the open end of the chamber on to the cryoplate assembly. For this reason the heat radiation shield is provided in the known cryopump with baffles that are of such size and so arranged that they prevent direct heat radiation onto the cryoplate assembly. However, there is a limit to the number and size of the baffles in such an arrangement since the heat radiation shield must permit the passage therethrough without unnecessary restriction of the gases to be condensed.

It follows therefore that for optimum pumping efficiency a cryopump should: (a) have a cryoplate assembly with a large subtended surface area at the open end of the chamber containing the cryoplate assembly; (b) have a large total available surface area to which gases can readily find their way; (c) have a heat radiation shield which pre vents at least a major portion of direct heat radiation onto the cryoplate but at the same time permits the passage of gasses through the shield in an unim peded fashion; (d) be economic to manufacture and main tain.

It is an aim of the present invention to meet these desiderata. According to the present invention, a cryopump comprises a chamber having an opening at one end for the entry of gases, a heat radiation shield adjacent the opening which includes a plurality of baffles so arranged that they provide shielding for a cryoplate assembly mounted within the chamber in thermally conducting contact with a cooling head, from at least a major portion of direct heat radiation, the cryoplate assembly including at least one member of wedgeshaped configuration having its apex adjacent the opening in the chamber and its base arranged in a plane drill parallel to the plane containing the opening, and means for removing and/or adsorbing uncondensed gases from the interior of the chamber.

Preferably, the wedge-shaped member is hollow, at least a portion of the interior surface of the hollow wedge-shaped member being provided with a layer of adsorption material and means for permitting uncondensed gases entry into the interior of the hollow wedge-shaped member.

Preferably, the hollow wedge-shaped member is made from sheet metal and includes two spaced triangular side walls interconnected by two rectangular walls which meet at the apex of the hollow wedge-shaped member, the base being open to permit uncondensed gases entry into the interior of the hollow wedge-shaped member.

In a preferred embodiment, two hollow wedge-shaped members are provided, the hollow members being interconnected by a bracket joined to the cooling head and connecting a triangular side wall of one hollow wedge-shaped member with a triangular side wall of the other hollow wedge-shaped member.

An embodiment of the invention will now be described, by way of example, reference being made to the Figures of the accompanying diagrammatic drawings in which: Figure 1 is a section through a cryopump; and Figure 2 is an exploded perspective view of a cryoplate assembly forming art of the cryopump of Fig. 1.

As shown, a cryopump 1 includes a chamber 2 contained within a vessel 3, the space 4 between the chamber 2 and vessel 3 being evacuated. At one end 5, the chamber 2 is open for the entry into the chamber of gases.

Immediately adjacent the end 5, there is provided a heat radiation shield 6 which includes a plurality of baffles 7 mounted on a louvre assembly 8. The heat radiation shield 6 is supported by two spaced pillars 9 which extend through the chamber 2 and are supported on and are in good thermal contact with a flange 10 forming part of the first stage of a two-stage cooling head 11 which also cools the chamber 2. On the second stage of the cooling head 11 there is provided a flange 1 4 to which is bolted an inverted open channel-shaped bracket 1 4 forming part of a cryoplate assembly.

The bracket 1 5 interconnects two hollow wedge shaped members 20. Each hollow wedge-shaped member 20 has two spaced triangular side walls 21, 22 interconnected by two rectangular walls 23, 24 which meet at the apex of the hollow member 20. As shown, each hollow member has a wedge shaped configuration and is arranged with the chamber 2 such that its base is contained in a plane parallel to the plane of the open end 5.

Each hollow member 20 is made from sheet metal which has been stamped, formed and soldered to achieve the wedge-shaped configuration. The base of each hollow member 20 is open but one triangular side wall 22 has an extension 26 which is formed at right angles to define a flap 27 parallel with the plane containing the base of the hollow wedgeshaped member. The interior surfaces of each hollow member 20 are provided with a layer of adsorption material, for example, charcoal.

In operation, during pumping, gas is passed into the chamber 2 via the open end 5 and between the baffle 7 of the louvre assembly 8. Some gas species such as nitrogen and argon condense on the external surfaces of the hollow members 20. Those gas species which are not condensed pass over the flaps 27 of each hollow member 20 into the interior of the hollow member 20 where they are adsorbed by the adsorbent material.

The purpose of the flaps 27 is to shield the charcoal adsorbent from surfaces at temperatures substantially above that of the cryoplate assembly.

An advantage of the embodiment described above is that the hollow wedge-shaped members 20 provide a large subtended surface area at the open end 5 of the chamber 2, a major portion at least of which is protected from direct heat radiation by the shield 6. The large subtended surface area permits of a high pumping speed. Furthermore, the large overali surface area of the hollow wedge-shaped members 20 permits of a large pumping capacity. The hollow members are easy to manufacture from sheet metal and are virtually maintenance free. Yet a further advantage of the hollow wedge-shaped members 20 is that they allow the adsorbent material to be used more effectively. That is to say, it has been found that the wedge-shaped configuration causes substantially all af the adsorbent material to adsorb uncondensed gases.In known cryoplate assemblies it has been found that frequently only some of the adsorbent material adsorbs uncondensed gases whilst the remainder is not effective.

The pillars 9 provide a short thermally conducting link between the heat radiation shield 6 and the flange 10 forming part of the first stage of the cooling head 11.

Although reference has been made in the above described embodiment to the hollow wedge-shaped members which are provided with a layer of adsorption material on their interior surfaces, the wedge-shaped members need not be hollow and no adsorbent material need be provided. In its place, the interior of the chamber 2 can communicate with a high vacuum pump such as a vapour diffusion pump, ion pump or a turbomolecular pump at the base of the chamber 2 to remove any uncondensed gases such as hydrogen.

It is possible that the hollow wedge shaped members 20 together with the adsorbent material on their inner surfaces can be used in conjunction with a high vacuum pump.

Although in the above described embodiment the hollow wedge-shaped members 20 are of generally pyramidal form it is forseen that hollow wedge-shaped members of generally conical form could be used especially with a heat radiation shield which includes a plurality of baffles arranged in a series of concentric circles.

Although the heat radiation shield 6 is shown and described in the preferred embodiment supported by the pillars 9, it is possible for the shield 6 to be supported from and cooled by contact with the end 5 of the chamber 2. Furthermore, the shield 6 and/or the chamber 2 can be cooled independently of the first stage of the cryogenerator by means of cryogenic fluid such as liquid nitrogen.

Claims (10)

1. A cryopump comprising a chamber having an opening at one end for entry of gases, a heat radiation shield adjacent the opening which includes a plurality of baffles so arranged that they provide shielding for a cryoplate assembly mounted within the chamber in thermally conducting contact with a cooling head, from at least a major proportion of direct heat radiation, the cryoplate assembly including at least one member of wedgeshaped configuration having its apex adjacent the opening in the chamber and its base arranged in a plane parallel to the plane containing the opening, and means for removing and/or absorbing uncondensed gases from the interior of the chamber.

2. A cryopump as claimed in claim 1, in which the wedge shaped member is hollow, at least a portion of the interior surface of the hollow wedge shaped member being provided with a layer of adsorption material and means for permitting unconderised gases entry into the interior or the hollow wedge shaped member.

3. A cryopump as claimed in claim 1 or 2, in which the hollow wedge-shaped member is made from sheet metal and includes two spaced triangular side walls interconnected by two rectangular walls which meet at the apex of the hollow wedge-shaped member, the base being open to permit uncondensed gases entry into the interior of the hollow wedgeshaped member.

4. A cryopump as claimed in claim 3, in which two hollow wedge-shaped members are provided, the hollow members being interconnected by a bracket joined to the cooling head and connecting a triangular side wall of one hollow wedge shaped member with a triangular side wall of the other hollow wedge shaped member.

5. A cryopump as claimed in claim 4, in which one triangular side wall of each hollow member has an extension which is formed at right angles to define a flap parallel with the plane containing the base of the hollow member, the flap protecting the interior of its hollow member from heat radiation.

6. A cryopump as claimed in any preceding claim, in which the interior of the chamber is in communication with a high vacuum pump for removing uncondensed gases from the interior of the chamber.

7. A cryopump as claimed in any preceding claim, in which the heat radiation shield is supported and thereby cooled by two spaced pillars extending through the chamber, the pillars being supported in turn on a flange forming the first stage of the cooling head.

8. A cryopump as claimed in any one of claims 1 to 6, in which the heat radiation shield and/or the chamber is cooled by means of a cryogenic fluid independently of the cooling head.

9. A cryopump as claimed in any one of claims 1 to 6, in which the heat radiation shield is supported and cooled by contact with said one end of the chamber having the opening.

10. A cryopump constructed and arranged substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2 of the accompanying drawings.