Ion pump and method of assembly

Abstract

An ion pump 10 comprises: a pump housing 12 enclosing a pumping chamber: one or more anodes 1; a pair of cathode plates 2; and mounting means 3 for supporting the cathode plates. The mounting means is

Description

FIELD OF THE INVENTION The field of the invention relates to ion pumps and their method of assembly. BACKGROUND Ion pumps are capture type vacuum pumps that may comprise an array of cylindrical anode tubes arranged between cathode plates such that the openings of each anode tube face one of the cathode plates. An electrical potential is applied between the anode and the cathode at the same time as magnets on opposite sides of the cathode plates generate a magnetic field aligned with the axes of the anode cylinders. The ion pump operates by trapping electrons within the cylindrical anodes through a combination of the electrical potential and the magnetic field. When a gas molecule moves into one of the cylindrical anodes, the trapped electrons strike the molecule causing the molecule to ionise. The resulting positively charged ion is accelerated by the electrical potential between the anode and the cathode toward one of the cathode plates leaving the stripped electron(s) in the cylindrical anode to be used for further ionization of other gas molecules. The positively charged ion is trapped by the cathode and thereby removed from the evacuated space. There is a desire for some ion pumps to be made smaller and this makes their assembly more challenging. SUMMARY One aspect provides an ion pump comprising: a pump housing enclosing a pumping chamber: at least one anode; at least one cathode, said at least one cathode comprising a plate; and at least one mounting means configured to support said at least one cathode in a predefined position within said pumping chamber, said at least one mounting means comprising support means defining a straight path, said path comprising an open end such that said cathode can be slid through said open end along said path and into said predefined position. 24 02 25 It is important in ion pumps that the cathode and anode are accurately aligned in order for the pump to function efficiently and thus, they need to be held securely in place. Accessing the interior of the pump housing to secure the cathode in 5 position may be difficult, particularly for smaller ion pumps. Furthermore, the cathode in an ion pump is generally formed of a different material to the pump housing making it hard to weld to the pump housing. The inventors have recognised this problem and have addressed it with a mounting means for the cathode plate(s) that defines a path such that the cathode can be slid into io position and then held firmly without the need to place the cathode in position and then try to weld it or access and manipulate retaining means. The ion pump comprises a pair of said mounting means for each of said at least one cathode plate, said pair of mounting means being configured to support 15 opposing ends of said each of said at least one cathode plate, each of said pair of mounting means comprising support means defining said straight path with said open end. The cathode plate may be held from one side, but in some embodiments it will be 20 held from both sides ensuring that it is held firmly in position. The two sides are parallel to each other and the two paths defined by the respective support means run parallel and straight such that the cathode can be slid into position longitudinally. 25 In some embodiments, said at least one mounting means comprises a rail and said path comprises a groove running along a length of said rail. In other embodiments, said at least one mounting means each comprise a plurality of support means arranged in a line, said line defining said path. In still other embodiments, said at least one mounting means comprises a groove in an inner surface of said pump housing. 24 02 25 The mounting means may have a number of forms provided that it provides an open ended and straight path such that the cathode piate can be s^d into position. It may be a groove within the housing which has the advantage of not 5 requiring additional mounting means and which may be formed either by removing material from the inner surface, or by forming protruding surfaces during manufacture. Alternatively, the mounting means may be be one or more rails mounted within the pump housing or it may be formed of a plurality of discrete elements, perhaps protrusions with recesses in or protrusions that are io offset with each other along the line of the path and on either side of it such that a cathode can be slid into position along the path and held on either side. In some embodiments, the mounting means may be welded to an inner surface of the pump housing. In this regard, the cathode plate is generally formed of a 15 different material to the pump housing and thus, it cannot be welded into position. The mounting means may be made of the same material as the pump housing, perhaps stainless steel and as such can be welded. Alternatively, the mounting means may be held in position by retaining means such as bolts for example. For very small pumps accessing retaining means may be difficult and welding 20 may be a preferable solution. The mounting means may be fixed into position before the pump is assembled. In some example embodiments, said pump comprises two cathode plates mounted at opposing ends of said at least one anode. 25 In some embodiments, said at least one anode comprises a cylinder and said at least one cathode is mounted perpendicular to a longitudinal axis of said cylinder. In some embodiments, said ion pump comprises a plurality of cylindrical anodes 30 mounted in an array, axes of said cylindrical anodes being parallel to each other. 24 02 25 In some embodiments at least one of said at least one cathode plates comprises at least one post extending out of a surface of said at least one cathode plate towards said at least one anode and configured to protrude into said corresponding at least one cylindrical anode. 5 In some cases the cathodes may have posts extending from them. These posts are a source of additional electrons and may improve the performance of the pump. These posts extend into the cylindrical anodes and make assembly of the pump more challenging. One of the advantages of allowing the cathode to be io slid into position is that the anode can be mounted such that the cylinder surrounds one end of the posts and then the anode(s) and cathode plate(s) may be slid into position together, the anode(s) may then be fixed in position perhaps by welding a portion of the anode(s) to the wall of the pump facing the opening. In some embodiments, said ion pump comprises a small ion pump with a nominal 15 pump speed of less than 4 litres / second. Embodiments are particularly useful for small ion pumps such as those with a nominal pumping speed of less than 4 litres per second perhaps a speed of less than 3 litres per second. The pump housing may comprise dimensions of less co than 5cm. In this regard, the pump housing may be rectilinear and each dimension may be less than 5cm. Where the pump is rectilinear the opening through which the cathode plate(s) is slid may be an open side wall of the pump housing and a cover may then be welded across the opening. In other embodiments the pump may be tubular and the opening may be an opening in 25 the tube wall, and a cover may be welded across this opening once the cathode(s) and anode(s) are in position. In some embodiments the cover may be a side wall or portion of a tubular wall, in other embodiments, the cover may comprise an inlet to the pump perhaps surrounded by a flange for attachment to a vacuum chamber for example. 24 02 25 A further aspect provides a method of assembling an ion pump according to one aspect said method comprising: sliding said at least one cathode plate into an opening in a wall of said pump housing and along said path defined by said support means; and fixing said at least one anode to an electrical feed point on a 5 wall of said pump housing facing said opening. In some embodiments, said method further comprises obscuring at least a portion of said opening by welding one of a closure plate or an inlet flange to said pump housing around said opening. io In some embodiments, said pump housing is rectilinear and said closure plate comprises a side wall of said housing. One particular advantage of pumps according to an embodiment is that they are 15 easy to assemble in that the cathode plate may be slid into the pump along the path defined by the support means of the mounting means. The anode may be fixed to an electrical feed point and then an opening through which the cathode was slid may be welded shut or an inlet flange may be welded across it. 20 In some embodiments, the method comprises sliding two cathode plates into said pump housing along paths defined by support means at either ends of said at least one anode. In some embodiments, said method comprises an initial step of arranging said at 25 least one cylindrical anode such that posts extending from said at least one cathode plate extend into said at least one cylindrical anode; and said step of sliding said at least one cathode plate into position comprises sliding said at least one anode and said at least one cathode plate into position together. 30 Where the cathode plate has posts that extend into the cylindrical anode then the cathode and anode may be slid through the open side of the pump housing 24 02 25 together and then the anode may be held in place by fixing the at ieast one anode to the electrical feed point perhaps by welding. Further particular and preferred aspects are set out in the accompanying 5 independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, io it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention will now be described further, with 15 reference to the accompanying drawings, in which: Figure 1 schematically shows the interior configuration of an ion pump according to an embodiment; Figure 2 schematically shows a pump housing according to an embodiment; and Figure 3 shows a flow diagram illustrating steps in a method according to an 20 embodiment. DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. 25 This design aims to simplify installation of the cathode plates in a typical small ion pump. The cathode plates are slid along a path defined by support means, in some embodiments along rails, at the edges of the pump body, these support means then hold the cathode plates in place. This is particularly effective for smaller ion pumps, with pumping speeds of around 3 l / s as the space for mounting the anode and cathode are restricted and 24 02 25 difficult to access making manufacture challenging. This arrangement allows the cathode to slide into the pump body, where it is then held in place which makes it easier to weld the closure / inlet flange across the opening as the cathode should not move during this procedure. Embodiments are concerned with a method of affixing the cathodes to the pump body in a small (nominally 3 litres per second) ion pump. Figure 1 shows an embodiment of an ion pump 10 of the penning style ion with an anode assembly 1 comprising an array of tubes arranged towards the middle of a pump housing 12 and running parallel to each other. Cathode plates 2 are mounted on either side of the anode tubes, perpendicular to them and a distance away from them. The cathodes 2 are typically made of Titanium but can be made of other Ultra High Vacuum safe conductors such as Tantalum or Aluminium. In some embodiments the ion pump comprises a noble diode variation where the plates are made of different materials, one being made of Titanium while the other is made of tantalum. In the figure the pump housing has an opening 14 in a side wall and it is through this opening that the cathode and anodes may be slid to mount them into position. When in position a cover plate, may be welded over the opening to form a vacuum tight seal around the opening. In this embodiment the cathode plates are held on either side of the anodes by support means in the form of rails 3 arranged to support the upper and lower ends of each plate. The cathode plates 2 are plain plates that may or may not have posts 4 extending from them. These posts may be fixed to the plate via welding. The anode 1 and cathode 2 are mounted so that the posts 4 extend through the centre of one or more the anode tubes 1. The posts provide additional electrons particularly when starting the pump at low pressures. As can be seen the posts extending into the tube make the assembly of the pump more challenging. The arrangement of the mounting means such that the cathode 2 may be slid into position along a path defined by the support means 3 24 02 25 allows the anode 1 and cathode 2 to be arranged with the posts extending into the anode tubes outside of the pump housing and the whole arrangement slid into position together. 5 In this embodiment, the rails 3 are tubes with slits the width of the cathode plates cut lengthwise along the rails. The tubes are made of the same material as the pump body to make it easier to weld the rails to the pump body. The rails may be a different shape than a tube. The support means may alternatively comprise a plurality of elements arranged on either side of the cathode plate to form a io straight path along which the cathode plate may be slid. These support means may comprise protrusions extending from one or both of the base and roof of the pump housing. In other embodiments, such as that shown in Figure 2 the mounting means may 15 be formed in the pump body at manufacture by means such as extruding or machining. Figure 2 shows such an arrangement, where grooves 6, 7 in the base and roof of the pump housing 12 are provided in the interior surface of the pump housing for receiving the cathode plates. 20 Figure 3 shows a flow diagram illustrating steps in a method according to an embodiment. At step S10 the anode array is arranged so that posts extending from a cathode plate extend into the anode tubes. This is an optional step and will only occur 25 where the cathode plate has posts extending from it. The anode array and cathode plate are then moved together into position in the pump housing at step S20, the cathode plate being slid along a path defined by support means. Where the cathode plate does not have posts then the anode and cathode may be placed into the pump housing in separate steps. At step S30 a second cathode plate is slid into position along the path defined by support means on the other side of the anode array. These steps may be 24 02 25 performed in a different order, with step S30 being performed before steps S10 and S20. The anode array is then weided to a feedpoint supplying a voltage to the anode 5 array, to hold it in position at step S40. At step S50 a closure plate is welded over the opening in the pump housing through which the anode and cathodes were inserted thereby finishing the assembly of the pump. The closure plate may be a side wall or a portion of a wall io of the pump housing or it may comprise a pump inlet. The welding should provide a vacuum tight seal around the opening. In summary an advantage of embodiments is that the cathode plate(s) are held in position by support means and need not be welded to the pump body. Welding 15 may cause excessive heat and warping in small ion pumps. In addition, as the cathode is made of titanium and the pump body is made of stainless steel, it can be difficult to weld one to the other. Although illustrative embodiments of the invention have been disclosed in detail 20 herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS 1 anode 10 2 cathode plate 3 rail 4 post 6, 7 groove 10 ion pump 12 pump housing 14 opening 24 02 25 24 02 25

Claims

1. An ion pump comprising:a pump housing enclosing a pumping chamber:5 at least one anode;at least one cathode, said at least one cathode comprising a plate; anda pair of mounting means configured to support each of said at least one cathode in a predefined position within said pumping chamber, each of said pair of mounting means comprising support means defining a straight path, said pathio comprising an open end such that said at least one cathode can be slid through said open end along said path and into said predefined position,said pair of mounting means being configured to support opposing ends of said each of said at least one cathode plate.15 2. An ion pump according to claim 1, wherein said at least one mountingmeans comprises a rail and said path comprises a groove running along a length of said rail.

3. An ion pump according to claim 1, wherein said at least one mounting 20 means comprises a plurality of support means arranged in a line, said linedefining said path.

4. An ion pump according to claim 1, wherein said at least one mounting means comprises a groove in an inner surface of said pump housing.

255. An ion pump according to any one of claims 1 to 3, wherein said at least one mounting means is welded to an inner surface of said pump housing.

6. An ion pump according to any preceding claim, wherein said pump30 comprises two cathode plates mounted at opposing ends of said at least one anode.24 02 257. An ion pump according to any preceding claim, wherein said at least one anode comprises a cylinder and said at least one cathode is mounted perpendicular to a longitudinal axis of said cylinder.5 8. An ion pump according to claim 7, said ion pump comprising a plurality ofcylindrical anodes mounted in an array, axes of said cylindrical anodes being parallel to each other.

9. An ion pump according claim 7 or 8, wherein at least one of said at least io one cathode plates comprises at least one post extending out of a surface of said at least one cathode plate towards said at least one anode and configured to protrude into said corresponding at least one cylindrical anode10. An ion pump according to any preceding claim, wherein said ion pup15 comprises a small ion pump with a nominal pump speed of less than 4 litres / second.

11. A method of assembling an ion pump according to any preceding claim, said method comprising:so sliding said at least one cathode plate into an opening in a wall of saidpump housing and along said path defined by said support means; andfixing said at least one anode to an electrical feed point on a wall of said pump housing facing said opening.25 12. A method according to claim 11, said method further comprising obscuringat least a portion of said opening by welding one of a closure plate or an inlet flange to said pump housing around said opening.

13. A method of assembling an ion pump according to claim 11 or 12, said30 method comprising an initial step of:arranging said at least one cylindrical anode such that posts extending from said at least one cathode plate extend into said at least one cylindrical anodes; andsaid step of sliding said at least one cathode into position comprises5 sliding said at least one anode and said at least one cathode into position together.

14. A method of assembling an ion pump according to any one of claims 11 to13, wherein said step of fixing said at least one anode to said electrical feed point io comprises welding said at least one anode to said electrical feed point.24 02 25

Images